Membrane structure can be investigated using a technique known as freeze fracture.
Cells are frozen and then split. Fracturing often occurs between the two phospholipid layers of membranes in the cell. An electron micrograph of such a fractured membrane is shown.
[Source: © Science Photo Library]
\nUsing the scale bar, calculate the magnification of the image.
\nExplain how electron micrographs such as this helped to falsify the Davson–Danielli model of membrane structure.
\nExplain how the amphipathic nature of phospholipids allows them to form bilayers.
\n✔
\na. the Davson–Danielli model proposed two layers of protein on either side of a lipid bilayer ✔ OWTTE
b. micrographs illustrate proteins in and/or crossing the membrane ✔
a. amphipathic means that they are both hydrophilic and hydrophobic ✔
\nb. the outside hydrophilic parts are exposed to water ✔
\nc. hydrophobic parts are away from water in the inside ✔
\nWater or lack of needs to be mentioned for mpb
\nTwo marks were awarded for an answer within the given range, since many candidates did not show working, and the question did not request it. Many candidates measured the whole diagram for the numerator instead of the scale bar, and others inverted the formula. A formula in words that appeared correct was not credited if the numbers then negated this. Errors in unit conversion were frequent, but credit was given for working.
\nDescriptions of protein layers either side of the phospholipid bilayer in the Davson-Danielli model were unclear. Embedded or integral proteins were frequently stated for the second marking point.
\nThe terms hydrophilic and hydrophobic were commonly stated, although sometimes assigned to heads and tails incorrectly; however this was not penalised. Orientation towards or away from water was less often explained.
\nWhen feed for animals is stored, moisture levels need to be kept low to prevent the growth of fungi. Feed was exposed to 15 % moisture and placed into a respirometer. O2 consumption was monitored using the respirometer over a period of six days.
\n[Source: © International Baccalaureate Organization 2019]
\nOutline how O2 consumption is measured using a respirometer.
\nSuggest one conclusion that can be drawn from the results shown in the graph.
\nOther than humidity, suggest a variable that would need to be controlled in this experiment.
\nOutline the role of fungi in nutrient cycling.
\na. a CO2 absorber/KOH is added ✔
\nb. a bubble/starting water level is measured ✔
\nc. «the rate of» movement of the bubble/pressure reduction is a measure of O2 consumption ✔
\na. as fungus grows, the rate of oxygen consumption increases ✔
\nb. around day 4/5 fungus begins to grow/no growth in days 1–3 ✔
\nc. the rate of fungus growth/rate of respiration is highest after 5 days ✔
\ntemperature/amount or type of feed «in the respirometer»/amount of CO2 absorber
OR
other reasonable suggestion ✔
Do not accept light
\na. act as saprotrophs/decomposers ✔
\nb. rot/feed on/break down organic matter/food/organisms
OR
return nutrients trapped in organic matter to the cycle/soil ✔
Do not accept detritivores
\nVery few candidates achieved a mark. Some confused a respirometer with a spirometer; others mentioned data logging probes. Credit was given for mention of a water column. Only the highest scoring candidates understood that a carbon dioxide absorber is required to allow pressure changes to be measured.
\nMany candidates described what was shown by the graph rather than making a conclusion related to the growth of fungi. Thus “oxygen consumption increased after day 4” or references to the feed respiring were common answers.
\nOften more than one answer was given but only the first was marked. Temperature and amount of feed were the most common answers.
\nTwo models of plasma membrane structure are shown.
\n[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson,
The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
Phospholipase C is an enzyme that digests the polar heads of phospholipids. Scientists used phospholipase C to test these models of membrane structure. They found that the enzyme could digest the heads of phospholipids in the plasma membranes of red blood cells.
\nState the scientists who proposed model A.
\nLabel the model A diagram to show a region of protein.
\nLabel the model B diagram to show a phospholipid.
\nDeduce one conclusion about the structure of the plasma membrane reached by the scientists from their results.
\nSuggest one reason for maintaining a pH of 7.5 throughout the experiment.
\nState one technological improvement, other than enzymatic digestion, that led to the falsification of previous models to determine the current model of membrane structure.
\nDavson–Danielli ✔
\n[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson, The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
\nAccept label to top protein.
\n[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson, The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
\nAccept a label to any part of any phospholipid
Accept different form of labelling that clearly indicate the phospholipids.
a. phospholipids on outside/exposed ✔
\nb. impossibility of continuous protein layer «of Davson–Danielli/model A» ✔
\nc. supports idea of mosaic pattern of membrane
OR
supports model B ✔
Award any other valid conclusion.
\nDo not accept \"membrane is made of phospholipids\".
\na. pH values away from optimum pH affect enzyme
OR
so enzyme can function properly
OR
pH 7.5 is the optimum pH for the enzyme ✔
b. sketch of enzyme activity versus pH ✔ Sketch of enzyme activity needs labels.
c. change in pH affects 3D structure of protein/active site
OR
change in pH denatures the enzyme / protein ✔
d. substrate does not fit in active site
OR
interaction of substrate and active site affected ✔
a. scanning electronmicrography / SEM ✔
\nb. freeze fracture/etching ✔
\nc. X-ray diffraction
OR
crystallography ✔
d. fluorescent antibody / marker tagging ✔
\nDo not accept electron microscope
\nAccept description of process
\nThere were a variety of suggestions for the names of the scientists with Singer and Nicolson, as given in the question paper, being a common incorrect answer.
\nMost students could label the areas with protein and phospholipid.
\nMost students could label the areas with protein and phospholipid.
\nMany candidates answered that the enzyme acted on the phospholipids but failed to make a deduction about the structure of the membrane.
\nMost candidates answered correctly that a specific pH was necessary for the enzyme to function properly and that deviation from this pH could denature the enzyme.
\nStating a technological improvement that led to the currently accepted model of the cell membrane proved more challenging and only the better candidates answered correctly.
\nA survey was made of the intertidal zone at Butter Lump Bay, Great Cumbrae, Scotland. The three species of barnacle found were Elminius modestus (EM), Chthamalus montagui (CM) and Semibalanus balanoides (SB). The kite diagram shows the vertical distribution of these three species from the low tide mark at 0 m to 2.25 m above low tide.
\n[Source: reprinted from Estuarine Coastal and Shelf Science, 152, M C Gallagher, et al., The invasive barnacle species,
Austrominius modestus: Its status and competition with indigenous barnacles on the Isle of Cumbrae, Scotland,
pages 134–141, 2014 with permission from Elsevier]
Outline how the data could have been obtained.
\nDescribe the distribution of C. montagui and S. balanoides barnacles in Butter Lump Bay.
\nE. modestus is an invasive barnacle while the others are native species. Analyse the data to show how it supports this statement.
\nState one abiotic factor that could have determined the distribution of barnacles.
\nBarnacles are sensitive to pollution. Outline how it might be possible to use these organisms as indicator species.
\na. transect across area to be studied ✔
\nb. count/record barnacles «per species» in quadrats at regular intervals ✔
\na. both species present throughout the range ✔
\nb. C. montagui has small number of individuals «throughout»
OR
C.montagui occupies «mostly» upper shore/intertidal zone
c. S. balanoides «mostly» occupies low tide area ✔
OR
S.balanoides has large number of individuals «throughout» ✔
OWTTE referring to maximum numbers at specific heights.
\na. native species/C. montagui and S. balanoides have niches that don’t overlap much / are distinct
OR
range of E. modestus overlaps with both native species ✔
b. niches of native species «which don’t overlap much» shows competition between native species
OR
E.modestus invades habitats of C. montagui/S. balanoides resulting in competition with «both» native species ✔
c. EM has a wide niche/higher tolerance/covers entire «intertidal» range making it easier to invade the habitat ✔
\na. exposure/tides/waves ✔
\nb. temperature ✔
\nc. surfaces «of attachment» ✔
\nd. resource availability/nutrients ✔
\ne. pH ✔
\nf. light ✔
\ng. salinity ✔
\nDo not accept biotic factors eg: “competition or predation”
\na. indicator species need particular environmental conditions
OR
indicator species tolerate only certain environmental conditions ✔
b. increase/decrease in population size «over time» shows effect of environmental conditions ✔
\nc. used to calculate biotic index/index of cleanliness ✔
\nd. index of 10/high index number indicates totally unpolluted
OR
index of 2 or 1/low index number indicates severe pollution ✔
The data in this question was a kite diagram showing the distribution of three barnacle species. In part (a) most strong candidates could outline how the data was obtained though many failed to use specific vocabulary and did not mention transects or quadrats.
\nCandidates were able to describe the distributions of the barnacles very well in part (b).
\nCandidates had difficulty finding data to support E.modestus being aninvasive barnacle in part (c).
\nIn part (e) some candidates chose to describe invasive species in general (without using the data) and others described the data but were not able to attach this to the idea of an invasive species. Many repeated the information given in the question.
\nA study was conducted on 25 healthy, non-smoking males to look at the effect of exercise and altitude on ventilation rate. Subjects were first asked to rest in a sitting position for six minutes. They then pedalled for three periods of six minutes at increasing exercise intensity: at 20 %, 30 % and 40 % of their maximal aerobic power. The entire study was conducted either in normal sea level oxygen conditions or in lower oxygen conditions simulating an altitude of 4000 m. The results are shown in the bar chart.
\n[Source: E Hermand, et al., (2015), Periodic breathing in healthy humans at exercise in hypoxia,
Journal of Applied Physiology, 118, pages 115–123. https://doi.org/10.1152/japplphysiol.00832.2014]
State one other variable that should have been controlled in this study.
\n\n
Compare and contrast the effect of increasing exercise intensity at sea level and at an altitude of 4000 m.
\nOutline how ventilation rate could have been monitored in this study.
\nage/height/fitness level/weight/room temperature/rest in between tests/model or type of bike ✔
\nOther valid factor. Only mark first factor listed.
\nDo not accept sex, health, smoking, oxygen level or altitude as this already listed.
\na. in both sea level and 4000m ventilation rate while exercising «at all intensities» is «significantly» more than at rest
OR
both sea level and 4000m show an increase in ventilation rate «dm3 min–1» as exercise intensity increased ✔
b. ventilation rate at 4000m «slightly» higher than at sea level for all conditions
OR
higher ventilation rate at 4000m not «significantly» different as error bars overlap ✔
Accept positive correlation.
\na. «data logging» with spirometer
OR
chest belt ✔
b. «tidal» volume recorded for a given period of time
OR
average «tidal» volume found and multiplied by number breaths per minute ✔
Do not accept confusion with respirometer (measuring oxygen consumption or CO2 release).
\nMust include a reference to time.
\nIn this question the candidates were asked to look at data on the effect of exercise and altitude on ventilation rate. Most candidates could state one other variable that should have been controlled in this study.
\nIn this question the candidates were asked to look at data on the effect of exercise and altitude on ventilation rate. Most candidates could state one other variable that should have been controlled in this study and could compare and contrast the effect of increasing exercise intensity in the two groups.
\nQuestion (c) was poorly answered. Many candidates thought a respirometer would be used to measure ventilation rate while other candidates gave a description of how to measure the number of breaths per minute. This was considered inadequate as the Y axis of the graph indicated that ventilation rate was volume per minute, so reference to volume and time were required in the response.
\nScientists investigated how relative brain mass varied in primate evolution. The graph shows the relative brain mass for species belonging to different primate groups against time of divergence from the common ancestor of modern humans in million years ago (Mya), 0 being the present era.
\n[Source: S Herculano-Houzel and J H Kaas, (2011), Brain, Behavior and Evolution, 77, pages 33–44.
© 2011 Karger Publishers, Basel, Switzerland]
State the trend in relative brain mass in primates other than humans according to their time of divergence from humans.
\nSuggest one reason that the relative brain mass of Homo is different from Parapithecus and Australopithecus.
\nDeduce, with a reason, whether the relative brain mass is a good indicator of brain development.
\nPrimates belong to the phylum chordata. The neural tube of chordates is formed by the infolding of the ectoderm followed by the elongation of the tube. Outline the process of formation of neurons from this neural tube in primates.
\nthe longer ago they diverged from humans, the greater the relative brain mass ✔
\nAccept inverse.
\nDo not accept negative correlation.
\na. human cerebral cortex has extensive folding producing a greater mass ✔
\nb. better diet/more protein allowed increase in «relative» brain mass in humans ✔
\nc. others had larger body to protect them from predators «without increase in brain mass» ✔
\na. not a good indicator because it depends on body mass ✔
\nb. not a good indicator as less developed organisms show a larger relative brain mass ✔
\nAccept answers referring to specific organisms shown
\na. neurons formed by a process called neurulation ✔
\nb. neurons are «initially» produced by differentiation «in the neural tube» ✔
\nc. immature neurons migrate to a final location ✔
\nd. an axon grows from each immature neuron in response to chemical stimuli ✔
\ne. some axons extend beyond the neural tube to reach other parts of the body ✔
\nf. a developing neuron forms multiple synapses ✔
\nThis question concerned how relative brain mass varied in primate evolution. Overall it was not well answered. The decreasing scale on the X-axis caused confusion for a lot of candidates in part (a).
\nIn parts (b) and (c) few candidates referred to ‘relative’ brain mass thereby providing incorrect responses.
\nIn parts (b) and (c) few candidates referred to ‘relative’ brain mass thereby providing incorrect responses.
\nIn Part (d) many candidates were familiar with the process of neuron formation in primates though some may have been confused by the introduction to the question.
\nEbola virus disease (EVD) is the disease in humans and other primates that is caused by the Ebola virus. Fruit bats are the reservoir for the virus and are able to spread the disease without being affected. Humans can become infected by contact with fruit bats or with people infected by the virus, their body fluids or equipment used to treat them.
\nThe stacked bar graph shows the epidemiological data for the EVD cases in Conakry, the capital city of Guinea, surrounding suburbs and rural areas in Guinea from the beginning of January 2014 to the end of March 2015.
\n[Source: Adriana Rico, et al. “Epidemiology of Epidemic Ebola Virus Disease in Conakry and Surrounding Prefectures,
Guinea, 2014–2015.” Emerging Infectious Diseases 22.2 (2016): 178–183. PMC. Web. 23 Mar. 2017.
https://wwwnc.cdc.gov/eid/article/22/2/15-1304_article]
The table summarizes epidemiological data from Guinea during the Ebola outbreak in 2014. The data are based on figures supplied by Ebola treatment centres. The last column refers to people who died in places other than Ebola treatment centres.
\n[Source: Adriana Rico, et al. “Epidemiology of Epidemic Ebola Virus Disease in Conakry and Surrounding Prefectures,
Guinea, 2014–2015.” Emerging Infectious Diseases 22.2 (2016): 178–183. PMC. Web. 23 Mar. 2017.
https://wwwnc.cdc.gov/eid/article/22/2/15-1304_article]
An antiviral drug, T-705, was tested in order to establish whether it has potential to treat EVD. The graph shows the data from an in vitro trial of T-705 on cells that had been infected with Ebola virus five days previously. Virus concentration and live cells are shown as a percentage of the control.
\n[Source: Oestereich, Lisa & Rieger, Toni & Neumann, Melanie & Bernreuther, Christian & Lehmann, Maria & Krasemann,
Susanne & Wurr, Stephanie & Emmerich, Petra & de Lamballerie, Xavier & Ölschläger, Stephan & Günther, Stephan. (2014).
Evaluation of Antiviral Efficacy of Ribavirin, Arbidol, and T-705 (Favipiravir) in a Mouse Model for Crimean-Congo
Hemorrhagic Fever. PLoS neglected tropical diseases. 8. e2804. 10.1371/journal.pntd.0002804.]
In 2015, an experimental vaccine was trialled in Guinea in an area where new Ebola cases continue to develop. Among the nearly 6000 people who accepted the vaccine, no cases were recorded after vaccination. In comparison, there were 23 cases among those who did not accept the vaccine.
\nIdentify the week and year in which the first cases were recorded in the suburbs.
\nWeek:
\nYear:
\nBased on the graph, compare and contrast the progress of the epidemic in the suburbs and rural areas.
\nSuggest two reasons for the overall decline in the epidemic after week 51.
\nCompare and contrast the data for Conakry with the data for the three suburbs.
\nSuggest reasons for the high percentage of fatal cases at Ebola treatment centres.
\nBased on these data, outline the evidence that T-705 has potential to be used as a treatment for EVD.
\nExplain how vaccination can lead to the production of B cells specific to the Ebola virus.
\nSuggest possible reasons for the difficulty of preventing or controlling a viral epidemic such as the 2014 EVD epidemic in a remote rural region.
\nweek 34 AND 2014 ✔
\nboth needed
\na. start of epidemic/first cases in rural areas
OR
epidemic spread to suburbs later ✔
b. higher maximum number of cases/greater increase in rural areas
OR
converse for suburbs ✔
c. increase came earlier in rural areas «than suburbs»
OR
number of cases peaked earlier in rural areas
OR
more cases in rural areas «than suburbs» in 2014 ✔
d. decrease came earlier in rural areas «than suburbs»
OR
decreasing in rural areas but not in suburbs in 2015/by end of study period
OR
more cases in suburbs than rural areas in 2015 ✔
e. «large» fluctuations in both ✔
\na. «overall decline due to» fewer cases in rural areas ✔
\nAnswers relating to people who died from the disease or develop immunity to it:
b. fewer cases due to deaths of people who had the disease/people recovering
OR
more people vaccinated/became immune/made antibodies/were not vulnerable to infection ✔
Answers relating to health care workers or availability of resources:
c. more doctors/nurses/medical equipment/treatment centers/hospitals/spending/aid/NGOs ✔
Answers relating to medical techniques used to tackle the epidemic:
d. better treatments/infection control/hygiene/quarantine/new vaccine/new antiviral drugs ✔
Answers relating to the public and patients:
e. education/better awareness/avoidance of infection/taking precautions/vaccination accepted ✔
Answers relating to reservoirs of infection:
f. fewer infected people «who could spread infection»/fewer bats/less contact with bats ✔
differences:
a. Conakry has more cases than any of the suburbs
OR
more cases in total in the suburbs than in Conakry ✔
b. more male cases in Conakry whereas more female cases in suburbs ✔
\nc. higher «% of» fatal cases at Ebola treatment centers in suburbs than in Conakry ✔
\nsimilarity:
d. in both Conakry and suburbs «% of» fatal cases in treatment centers is higher than outside ✔
a. most serious cases are in/are taken to treatment centers
OR
treatment centers are set up where there are most cases/most serious cases ✔
b. long time/distance to travel between contracting disease and arrival at treatment center
OR
travel to treatment center weakens/upsets/harms the patient ✔
c. Ebola is a virulent disease/Ebola virus mutated «to become virulent»
OR
little known about Ebola/new disease so treatments not yet developed ✔
d. no/not enough vaccine/antiviral drug available «in 2014/15»
OR
antibiotics do not work against viral diseases ✔
e. secondary infections/Ebola patients infected with other diseases/other Ebola strains
OR
ineffective hygiene/cleaning/sterilization/use of contaminated equipment/disposal of corpses ✔
f. small number of staff relative to patients/treatment centers overcrowded/swamped with patients
OR
insufficient equipment/supplies for large number of patients/with the rapid rise in patients ✔
g. better reporting at Ebola centers/deaths due to Ebola not reported in rural areas ✔
\na. cells not killed/few cells killed «even at high concentrations» ✔
\nb. «T-705» effective/viruses reduced/viruses killed at 100 μM
OR
«T-705» very effective/viruses much reduced/nearly all viruses killed at 1000 μM ✔
c. virus concentration decreases as T-705 concentration increases ✔
\nd. drug has «high» potential for treatment «at high enough concentration» ✔
\na. vaccine contains Ebola antigens ✔
\nb. vaccine «could» contain weakened/attenuated/dead/killed form of «Ebola» virus/virus genetically modified to express an Ebola/viral protein ✔
\nc. phagocyte/macrophage engulfs the antigen/presents the antigen to T cell ✔
\nd. antigen recognized by «specific» T cells/binds to T cells ✔
\ne. «activated» T cells activate «specific) B cells ✔
\nf. «activated» B cells make the antibodies «against Ebola» ✔
\ng. B cells divide forming «clone of» plasma cells/producing more B cells specific to Ebola ✔
\na. poor transport infrastructure/poor communication/bad roads/difficult access/no maps/support slow arriving/scattered population ✔
\nb. poor education/understanding of disease amongst health workers/local population
OR
continued contact with infected people / other example of unsafe actions ✔
c. more sources of infection such as bats/difficult to find sources of infection ✔
\nd. lack of/limited access to medical care/doctors/health care workers ✔
\ne. lack of/no access to/unaffordability of treatment centers/medicalsupplies/equipment/antivirals/drugs/vaccine/treatments ✔
\nf. refusal/reluctance in local population to be vaccinated
OR
difficult to find/reach everyone to vaccinate them/repeat the vaccination ✔
g. migration of people spreads the infection ✔
\nh.poor sanitation/lack of clean water ✔
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nNearly all candidates read from the graph the week and year in which the first cases were recorded in the suburbs.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nThe skill required in this question was picking significant trends out of data with a great deal of noise. Only one similarity was accepted – the large week-to-week fluctuation in number of cases. The differences between the rural areas and the suburbs were that Ebola epidemic started earlier in rural areas, rose to higher peaks and started declining earlier.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nA wide range of possible reasons for the decline in the epidemic was accepted here and most candidates scored both marks. The commonest type of misunderstanding was that there might be few people left to become infected. Given that the total number of cases was only in the thousands and the population of Guinea must be in the millions, that explanation for the decline was implausible. As this was a suggest question and candidates are not required to have specific knowledge of the Ebola epidemic, any reasonable answer was accepted even if it was not historically true.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nThis was another question where the skill was in picking out significant trends. Differences between individual suburbs were not significant so all the answers accepted were either similarities or differences between the capital city and the suburbs generally. Most but not all candidates made two or more statements about the data that were significant enough to score marks. Some students treated percentages as though they were absolute numbers and for example added the percentage of fatal cases in and outside treatment centres together, which does not give a meaningful total.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nThis was a more difficult question. We might expect fewer fatalities in percentage terms at treatment centres than elsewhere, but the data showed that there were more. Not all candidates realised that the figures given indicated the percentage of people diagnosed with Ebola that died, not the percentage of the population as a whole. As in 1(c) a wide variety of answers was accepted. Some of the answers related to Ebola having a high mortality rate generally, but to score three marks it was necessary to find reasons for treatment centres in particular having high rates of death. One suggestion commonly given but not accepted was that patients became infected after arrival at treatment centres – this would not necessarily give a high fatality rate or percentage and also uninfected people are unlikely to be brought to treatment centres. Some students thought that the more infected people a patient came into contact with, the more infected they would be. This ignores the potential for the Ebola virus to multiply exponentially once inside a patient. Material shortages were only accepted as a reason if this was combined with the idea that treatment centres were swamped with patients.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nThis was generally very well answered, with candidates correctly concluding that the drug had potential as a treatment because at a high enough dose it reduced the number of viruses without harming a significant number of cells.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nVaccination is a major focus on 11.1 of the programme so it was perhaps surprising that answers here were not better. Given the current outbreaks of measles and other diseases that can be prevented by vaccination, this topic should be given greater prominence in some schools. There were a few impressive answers giving the details of the steps that lead up to production of antibodies against a specific pathogen, but less than a quarter of candidates scored all three marks. There were many misconceptions. The terms ‘resistant’ and ‘immune’ have different meanings in biology, which should be carefully distinguished. Some candidates think that a vaccine contains a small amount of the pathogen. Would those candidates be happy to be infected with Ebola viruses as long as it was only a few? There was too much talk of cells ‘remembering’ the antigen or the virus. Memory cell is perhaps an unfortunate term. They are only memory cells in the sense that if an antigen binds to the antibodies they display, after cell multiplication more of those antibodies are produced and they will bind to the same antigens on the surface of an invading pathogen. This is just a series of molecular processes, not true memory.
\nThis was a timely question with Ebola epidemics again causing great problems in parts of Africa. Students were expected to have had some practice at analysing data relating to an epidemic because of this Skill in the programme: Analysis of epidemiological data related to vaccination programmes. It was assumed that candidates would understand terms such as ‘case’, ‘fatal case’, ‘infect’ and ‘treat’.
\nThis was a third question where a very wide range of answers were accepted. Even so, not all candidates scored both marks. The commonest weakness was vague answers such as “shortage of resources”. The best answers gave common-sense reasons for special difficulties in controlling an epidemic in the population of a remote rural region.
\nThe karyogram shown is for the African marsh rat (Dasymys incomtus). In this species, sex is determined by X and Y chromosomes. Females are XX and males are XY.
\n[Source: adapted from https://embryology.med.unsw.edu.au]
\nThomas Hunt Morgan established that genes for body colour and wing size in Drosophila are autosomally linked. The allele for grey body (b+) is dominant over that for black body (b) and the allele for normal wing size (vg+) is dominant over that for vestigial wing (vg).
\nDistinguish between the structure of the chromosomes of prokaryotes and eukaryotes.
\nOutline the causes of sickle cell anemia.
\nIdentify, with a reason, the sex of this individual.
\nState the haploid number for this nucleus.
\nA fly that is homozygous dominant for both body colour and wing size mates with a fly that is recessive for both characteristics. In the table, draw the arrangement of alleles for the offspring of this mating and for the homozygous recessive parent.
\nThe offspring, which were all heterozygous for grey body and normal wings, were crossed with flies that were homozygous recessive for both genes. The table shows the percentages of offspring produced.
\nExplain these results, based on the knowledge that the genes for body colour and wing size are autosomally linked.
\na. prokaryotes have circular DNA/chromosome but eukaryote chromosomes linear/OWTTE ✔
OR
eukaryotes have telomeres/centromeres whereas prokaryotes do not ✔
b. some prokaryotes have plasmids whereas eukaryotes do not ✔
\nc. eukaryotes have multiple chromosomes whereas prokaryotes «typically» have only one ✔
\nd. histones/nucleosomes/proteins associated with DNA in eukaryotes but not in prokaryotes/naked DNA in prokaryotes
OR
eukaryote DNA can coil/supercoil/condense «due to histones» but not prokaryote DNA ✔
a. genetic disease/caused by a gene
OR
inherited «from parents»
OR
caused by mutation «of a gene» ✔
b. base substitution
OR
GAG → GTG ✔
c. hemoglobin gene mutated / different allele/form/version of hemoglobin gene
OR
HbA → HbS ✔
d. leads to change in amino acid sequence «in hemoglobin»
OR
glutamic acid → valine ✔
e. only homozygotes have full disease/sickled cells / heterozygote has milder form
OR
hemoglobin crystallizes at low oxygen concentration ✔
f. «selected for/spreads in population» as it gives resistance to malaria ✔
\nmale because «X and» Y chromosome present
OR
male because sex chromosomes/last two chromosomes/pair 21 are unpaired/different «from each other»/not homologous ✔
The answer must include “male” and the reason.
\n21
\na. not a 1:1:1:1 ratio «because of linkage»
OR
not independent assortment
OR
grey normal and black vestigial types/parental combinations/double dominant and double recessive were commoner than 25 %/commoner than expected ✔
b. «linked genes» so were on the same chromosome ✔
c. grey body vestigial wing and black body normal wing are recombinants
OR
2 % plus 3 % of the offspring are recombinants ✔
d. recombinants due to crossing over/exchange of genes between «non-sister» chromatids
OR
2 % and 3 % of offspring were due to crossing over
OR
genes inherited together unless separated by crossing over ✔
e. crossing over between the two loci/between the two genes on the chromosomes ✔
\nf. few recombinants/not much crossing over because genes/gene loci close together ✔
\nAccept any of these points from an annotated diagram.
\nAnswers to this question were variable. Some candidates wrote about cell structure rather than chromosome and many wrote about the location of chromosomes in the cell rather than their structure. Weaker candidates did not know the difference between prokaryotes and eukaryotes and some think that prokaryotes are plants and eukaryotes are animals. The term ‘naked’ was often used for DNA not enclosed in a nuclear membrane, when in this context it should be reserved for DNA that is not associated with histone proteins.
\nThis question also elicited a wide range of answers. Some were accurate and detailed but some showed no knowledge of this genetic disease. There was lack of clarity in many answers, for example the disease was described as the mutation and sickling of cells was known but not linked to haemoglobin. The weakest answers tended to describe anemia in general, with iron deficiency sometimes given as the cause.
\n90 % of candidates correctly identified the individual as male, because the two sex chromosomes were different in size so must be an X and a Y. The other 10% mostly thought that there were two X chromosomes present, perhaps because pair 20 were X shaped.
\nSlightly more than 50 % of candidates got this right. A common wrong answer was 42 – diploid number. There were a variety of other answers, some of which must have been guesses from candidates who did not understand the term haploid.
\nThis was generally well answered, with more than two thirds of candidates getting at least one of the genotypes correct. The best answers gave the alleles on lines that symbolised linked genes on a chromosome, but there was a potential pitfall here. If the alleles were arranged in a way that was impossible, given the nature of the cross, one mark was deducted. For example, each chromosome must have one of each gene, not two copies of one gene.
\nMost candidates found it very hard to explain the non-Mendelian ratio. A good start would have been to state the expected Mendelian ratio for unlinked genes in this type of cross and then compare this with the actual percentages. Answers tended to state which traits tended to be inherited together rather than explain the mechanism. An obvious answer was that that the two genes are located on the same chromosome but only a minority of candidates stated this and even fewer made the point that crossing over between the two gene loci results in the small percentage of recombinants. Many students referred to the 9:3:3:1 ratio, even though the cross performed in this case would not have given this ratio, even if there had not been gene linkage. Also many candidates claimed that more individuals showed dominant phenotypes than recessive, which was not supported by the percentages.
\n\n Deduce from the results of the study whether HGH improves strength.\n
\n\n [1]\n
\n\n Suggest\n \n one\n \n reason that it is difficult to detect illegal use of HGH to enhance athletic performance.\n
\n\n [1]\n
\n\n the drug does not appear to improve strength as less mass can be lifted «by arms and legs» ✔\n
\n\n a. occurs naturally so hard to tell whether it has been injected ✔\n
\n\n b. HGH has very short half life ✔\n
\n\n While this was generally done well, those who missed this mark chose to write about muscle growth rather than the mass being lifted.\n
\n\n Students were unable to state that the detection of illegal use of HGH was due it already being naturally present in the body.\n
\n\n State\n \n one\n \n condition which would require the use of an artificial pacemaker.\n
\n\n [1]\n
\n\n Describe briefly how an artificial pacemaker works.\n
\n\n [2]\n
\n\n a. heartbeat too slow/fast/irregular/tachycardia/fibrillations ✔\n
\n\n b. sinoatrial node is malfunctioning ✔\n
\n\n c. pathway that conducts electrical impulses generated by the sinoatrial node is impaired ✔\n
\n\n \n Do not accept heart attack\n \n
\n\n a. a pacemaker contains a battery and pulse generator\n
\n \n \n OR\n \n \n
\n it is connected to the heart by wires/cables ✔\n
\n b. it detects that the heart’s natural rhythm is incorrect ✔\n
\n\n c. it sends electrical impulses to correct the heartbeat/it replaces sinoatrial node ✔\n
\n\n d. provide a regular impulse/constant rhythm ✔\n
\n\n This was generally well answered, with a few students identifying generic (and incorrect) medical problems such as heart attacks and hypertension.\n
\n\n A majority of the candidates stated that the pacemaker sends electrical impulses or replaces the SA node, and they achieved 1 mark. Few were able to state that the pacemaker is connected to the heart by wires or that it monitors the heart rate itself.\n
\nThe electron micrographs show a typical prokaryote and a mitochondrion.
\nCompare and contrast the structure of a typical prokaryotic cell with that of a mitochondrion.
\nExplain how mitochondria could have been formed from free living prokaryotes.
\ndifferences
\na. prokaryote has cell wall but mitochondrion does not ✔
\nb. mitochondrion has double membrane whereas prokaryote has single membrane
OR
«Gram negative» bacteria have cell wall between two membranes whereas mitochondria has intermembrane space between two membranes ✔
c. mitochondrion has cristae/invaginations of inner membrane but prokaryote does not
OR
prokaryote «may have» flagella/pili/«slime» capsule which mitochondria do not have ✔
similarities
\nd. 70S ribosomes in both ✔
\ne. DNA in both / loop of DNA in both / naked DNA in both ✔
\nf. shape similar/both rod shaped/OWTTE
OR
size of both is similar/both about 3 μm long ✔
g. both are membrane-bound/OWTTE ✔
\na. endocytosis/engulfing of prokaryote by a larger/another/anaerobic prokaryote/cell ✔
\nb. double membrane of the mitochondrion is the result of endocytosis
OR
inner membrane of mitochondrion from engulfed cell and outer from food vacuole ✔
c. «engulfed prokaryotic cell» was aerobic/respired aerobically/consumed oxygen
OR
«engulfed prokaryotic cell» provided energy/ATP ✔
d. «engulfed prokaryotic cell» not destroyed/not digested
OR
«endo»symbiotic/mutualistic relationship developed ✔
e. «engulfed prokaryotic cell» had its own DNA/own «70S» ribosomes ✔
\nDo not award mpc for “mitochondrion makes ATP”.
\nAnswers were spread over the whole of the four-mark range. The scale bars allowed the sizes of both to be calculated – they were close, but even so a substantial number of candidates claimed that one or other was significantly larger.
\nThis was generally well answered with candidates explaining how endocytosis would have got a free-living prokaryote into a larger cell and because it made itself useful, there was a selective advantage in developing a mutualistic relationship rather than digesting the engulfed cell. Some candidates gave valid evidence for endosymbiosis – presence of DNA, 70S ribosomes or the double membrane.
\nThe micrograph shows a vascular bundle from the stem of a barley plant.
\n[Source: Copyright Carolina Biological Supply Company. Used by permission only.]
\nXylem and phloem contain structures that are adapted for transport. Outline the differences between these structures in xylem and phloem.
\nExplain how the properties of water allow it to move through xylem vessels.
\nOutline how the structure of cellulose makes it suitable as a component of cell walls.
\na. plasma membrane in phloem/sieve tubes but not in xylem/vessels
OR
xylem/vessels dead/acellular and phloem/sieve tubes alive ✔
b. xylem vessels have thicker walls «than phloem» ✔
\nc. xylem «vessel» walls are lignified «but phloem walls are not» ✔
\nd. phloem vessels have sieve plates «whereas xylem vessels have no cross walls» ✔
\ne. xylem/vessels are wider/larger than phloem/sieve tubes ✔
\nf. companion cells in phloem «but not in xylem» ✔
\na. water is polar/a dipole/oxygen slightly negative and hydrogen slightly positive ✔
\nb. polarity results in hydrogen bonds/attraction between water molecules ✔
\nc. hydrogen bonding/polarity causes cohesion of water «molecules» ✔
\nd. cohesion/hydrogen bonding allows water to withstand tension/withstand low pressure/be pulled «upwards»/moved against gravity ✔
\ne. cohesion/hydrogen bonding prevents column of water «in xylem» from breaking/column of water is maintained ✔
\nf. adhesion of water to xylem/vessel walls «due to hydrogen bonds» ✔
\na. chains of glucose/1-4 glycosidic linkages/covalent bonding between glucose ✔
\nb. beta glucose so alternating orientation of glucose units
OR
beta glucose forms straight chains ✔
c. forms microfibrils/long and thin/thin fibres/parallel bundles of cellulose molecules
OR
hydrogen bonding/cross linkage between cellulose molecules holds them together ✔
d. high tensile strength/rigid/doesn’t stretch so provides support/allows turgidity ✔
\nMany candidates knew nothing about the structure of the conducting tissues of plants and instead gave some rudimentary information on their functions. A common misconception is that phloem sieve tubes are wider than xylem vessels. This may have come from a failure to identify correctly the xylem and phloem tissue correctly in the micrograph. The most commonly known feature of xylem was the lignified wall and if phloem was the presence of companion cells.
\nThis was better known, but many candidates still confuse adhesion with cohesion, or do not distinguish between them. Cohesion is far more significant in xylem transport because it explains how tensions can be resisted. Adhesion of water to the wall of xylem vessels is only significant when air-filled xylem vessels are refilling with aqueous sap under positive pressures. If this happens at all in a plant, it will generally only be once per year and yet adhesion to the xylem was the most common answer given. In some answers cause and effect had been confused so cohesion was given as the cause of hydrogen bonding rather than the effect.
\nMany candidates found this question the hardest in Section A. There were a few knowledgeable accounts that easily scored the two marks, but far more revealed little or no understanding of the structure of cellulose. It might be thought to be an obscure topic, but cellulose may be the most abundant biochemical on Earth. It is the key to how plants support themselves and grow. The paper that candidates wrote their answers on was mostly cellulose and nearly all candidates will have been wearing clothes composed largely of it. One misunderstanding that was often evident was that cellulose is freely permeable. The cell wall as a whole has this property, but it is due to the gaps between cellulose microfibrils, not to the cellulose itself. Another common mistake was to state that cellulose is made of starch, so contrasts between these two polysaccharides should be emphasised more widely.
\nOutline the functions of rough endoplasmic reticulum and Golgi apparatus.
\nOutline the control of metabolism by end-product inhibition.
\nExplain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane.
\na. ribosomes on RER synthesize/produce polypeptides/proteins ✔
\nb. proteins from RER for secretion/export/use outside cell/for lysosomes ✔
\nc. Golgi alters/modifies proteins/example of modification ✔
\nd. vesicles budded off Golgi transport proteins «to plasma membrane»
OR
exocytosis/secretion of proteins in vesicles from the Golgi ✔
Accept “for use inside and outside the cell” for mpb.
\na. metabolism is chains/web of enzyme-catalyzed reactions
OR
metabolic pathway is a chain of enzyme-catalyzed reactions ✔
b. end product/inhibitor is final product of chain/pathway ✔
\nc. inhibits/binds to/blocks the first enzyme in chain/pathway ✔
\nd. non-competitive inhibition ✔
\ne. end-product/inhibitor binds to an allosteric site/site away from the active site ✔
\nf. changes the shape of the active site/affinity of the active site «for the substrate» ✔
\ng. prevents intermediates from building up
OR
prevents formation of excess «end» product/stops production when there is enough
OR
whole metabolic pathway can be switched off ✔
h. negative feedback ✔
\ni. binding of the end product/inhibitor is reversible
OR
pathway restarts if end product/inhibitor detaches/if end product concentration is low ✔
j. isoleucine inhibits/slows «activity of first enzyme in» threonine to isoleucine pathway ✔
\nAllow mark points shown in clearly annotated diagrams.
\nTo gain mpd, mpe and mpf the answer must be in the context of end-product inhibition, not enzyme inhibition generally.
\na. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔
\nb. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔
\nc. phospholipid bilayer in water/in membranes ✔
\nd. hydrophilic heads «of phospholipids» face outwards/are on surface ✔
\ne. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔
\nf. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔
\ng. some amino acids are hydrophilic and some are hydrophobic ✔
\nh. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔
\ni. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔
\nj. integral proteins are embedded in membranes due to hydrophobic properties/region
OR
transmembrane proteins have a hydrophobic middle region and hydrophilic ends ✔
k. peripheral proteins on are on the membrane surface/among phosphate heads due to being «entirely» hydrophilic
OR
«carbohydrate» part of glycoproteins is hydrophilic so is outside the membrane ✔
l. pore of channel proteins is hydrophilic ✔
\nAllow mark points shown in clearly annotated diagram.
\nIn any part of the answer, accept polar instead of hydrophilic and non-polar or apolar instead of hydrophobic.
\nMost candidates had some knowledge and some had broad understanding of this topic. A common mistake was to think that the rough endoplasmic reticulum makes ribosomes. Many candidates stated that the Golgi apparatus packages proteins but they did not always mention that the packages are vesicles. If this was all that the Golgi did, vesicles from the rough ER could be used to secrete proteins. The role of the Golgi apparatus therefore involved processing or modification of proteins rather than just packaging of them.
\nAnswers to this question were rather polarised. Candidates who had studied this topic and understood it had no difficulty in scoring high marks but other candidates struggled to include any useful ideas in their answers. Weaker answers tended to exclude the ideas of enzyme catalysis, pathways of reactions and the mechanisms involved in non-competitive enzyme inhibition. Diagrams to illustrate the process were a valuable part of some answers.
\nCandidates struggled to explain the meaning of the term hydrophobic. Many stated the literal meaning – water-fearing, hence repelled by water. This was not accepted as hydrophobic molecules aren’t repelled by water. They appear to be, because water is more attracted to polar or ionic substances than to apolar/hydrophobic substances, but there is no chemical mechanism for repulsion and of course molecules do not fear each other. Many focused only on phospholipids, but stronger answers also included information on proteins and how the positions they occupy within the membrane depend on their hydrophobic and hydrophilic properties.
\nOutline the process of inspiration in humans.
\nDescribe the functions of valves in the mammalian heart.
\nExplain how blood solute concentrations are kept within narrow limits in the human body.
\na. diaphragm and external intercostal muscles contract ✔
\nb. diaphragm moves down/becomes flatter
OR
external intercostals raise the ribcage/move the ribcage up/out ✔
c. muscles/diaphragm/intercostals increase volume of thorax/expand the thorax
OR
muscles/diaphragm/intercostals decrease pressure in the thorax ✔
d. as volume «of thorax/lungs» increases the pressure decreases ✔
\ne. air enters «lungs» due to decreased pressure/higher pressure outside body ✔
\nf. air flows to lungs through trachea and bronchi/bronchioles ✔
\nAccept thoracic cavity or chest cavity in place of thorax in any part of the answer.
\nDo not allow “oxygen” instead of air in mpe or mpf.
\na. prevents backflow/ensures one-way flow/controls direction of flow ✔
\nb. open valves allow blood to flow through
OR
opening and closing of valves controls timing of blood flow «during cardiac cycle» ✔
c. closed «semilunar» valves allow ventricles/chambers to fill with blood
OR
closed «semilunar» valves allow pressure in ventricles to rise «rapidly» ✔
d. valves open when pressure is higher upstream/OWTTE/converse for closed valves ✔
\ne. AV/bicuspid/tricuspid/mitral valves prevent backflow from ventricle to atrium
OR
AV/bicuspid/tricuspid/mitral valves open when pressure in atrium is higher «than in the ventricle»/when atrium is pumping/contracting ✔
f. semilunar/aortic/pulmonary valves prevent backflow from artery to ventricle
OR
semilunar/aortic/pulmonary valves open when pressure in ventricle is higher «than in the artery»/when ventricle is pumping/contracting ✔
Allow mpa, mpb, mpc or mpd if the point is made through the example of one specific valve.
\na. solute concentration of blood monitored by the brain/hypothalamus ✔
\nb. pituitary gland secretes ADH ✔
\nc. ADH secreted when solute concentration/osmolarity is too high/a person is dehydrated/OWTTE ✔
\nd. collecting duct more permeable to water ✔
\ne. «more» aquaporins/opens aquaporins «in the plasma membrane of collecting duct cells» ✔
\nf. «more» water reabsorbed «into the medulla» ✔
\ng. medulla is hypertonic/hyperosmotic «so water can be reabsorbed from filtrate» ✔
\nh. small volume of urine/concentrated urine produced «with ADH» ✔
\ni. no/little/less ADH secreted if «blood» solute concentration is too low ✔
\nj. collecting duct less permeable to water/less water reabsorbed/large volume of urine produced/dilute urine produced «with low/no ADH» ✔
\nk. insulin causes blood glucose «concentration» to be reduced ✔
\nl. glucose stored as glycogen in the liver ✔
\nm. glucagon causes blood glucose «concentration» to be increased ✔
\nn. negative feedback ✔
\nAccept hypertonic for solute concentration too high and hypotonic for too low.
\nThe mechanisms used to cause ventilation of the lungs are poorly understood by many candidates, despite them being used throughout our lives. Cause and effect are often confused – air rushes in to cause a volume increase in the lungs for example.
\nMost candidates got the general idea of valves in the heart preventing backflow, but many did not make any other valid points. A clear statement of what is achieved by valves opening and by valves closing was expected and then comments on the specific roles of the atrio-ventricular and the semi-lunar valves.
\nThis was another question where many candidates struggled. Sub-topic 11.3 of the programme is titled ‘The kidney and osmoregulation’ but perhaps the connection was not made. A significant minority wrote only about regulation of blood sugar levels Others gave an exhaustive account of kidney function, but mostly did not have time left to write enough about osmoregulation. A small proportion of candidates gave a convincing account of the negative feedback mechanisms that keep blood solute concentrations within narrow limits.
\n\n Using the climograph, identify the relationship between maximum temperature and rainfall.\n
\n\n [1]\n
\n\n Using the climograph, outline the pattern of rainfall.\n
\n\n [1]\n
\n\n Natural forests in the area around Mangalore contain hardwood trees such as teak,\n \n Tectona grandis\n \n , which are deciduous, shedding their leaves once a year. Suggest, with a reason, when this might happen.\n
\n\n [2]\n
\n\n maximum temperature occurs just when rainfall begins/at the onset of the rainy season/monsoon\n
\n \n \n OR\n \n \n
\n negative relationship «as maximum temperature drops, rainfall increases» ✔\n
\n \n OWTTE\n \n
\n\n rainfall concentrated between April to December/peaks in June-August «followed by months with little/no rainfall» ✔\n
\n\n \n OWTTE\n \n
\n\n a. dry season/Jan/Feb ✔\n
\n\n b. «drop leaves» to prevent water loss/transpiration «since no rainfall for almost four months» ✔\n
\n\n Most outlined correctly the pattern of rainfall, but irrelevant answers were seen.\n
\n\n Many provided correct answers, but also an important number could not relate the process to prevention of water loss during the dry months.\n
\n\n Describe the structure of cardiac muscle cells.\n
\n\n [2]\n
\n\n State the name of the valves that prevent blood flowing from the arteries back into the heart.\n
\n\n [1]\n
\n\n Distinguish between the different phases of the cardiac cycle in the atria and in the ventricular muscle.\n
\n\n [2]\n
\n\n Sketch the pattern seen in a typical electrocardiogram (ECG) trace for one complete cardiac cycle, including labels of the main features.\n
\n\n [3]\n
\n\n a. cardiac muscle cells are branched ✔\n
\n\n b. rich in mitochondria ✔\n
\n\n c. rich in glycogen granules ✔\n
\n\n d. formed by short cylindrical cells ✔\n
\n\n e. contains\n \n intercalated\n \n discs ✔\n
\n\n f. has gap junctions ✔\n
\n\n g. «intercalated discs are» transverse cross-bands which represent the attachment site between adjacent cells ✔\n
\n\n \n Do not accept myogenic as it is not a structure.\n \n
\n\n semilunar / sigmoid / pulmonary\n \n and\n \n aortic valve ✔\n
\n\n a. action potential of atrium precedes the ventricle\n
\n \n \n OR\n \n \n
\n the phases happen later in ventricle\n
\n \n \n OR\n \n \n
\n atrium contracts before the ventricle ✔\n
\n b. atrium has a shorter phase 2/longer phase 2 in ventricle\n
\n \n \n OR\n \n \n
\n atrium falls abruptly in phase 2/ventricle shows a plateau in phase 2 ✔\n
\n c. phase 3 is more distinct/falls more abruptly in ventricular action potential ✔\n
\n\n d. ventricular phase is overall longer than atrial phase ✔\n
\n\n a. sketch with the correct shape ✔\n
\n\n b. P, Q, R, S and T indicated ✔\n
\n\n c. atrial contraction/systole/depolarisation labelled ✔\n
\n\n d. ventricular contraction/systole/depolarisation labelled ✔\n
\n\n e. ventricular relaxation/diastole/repolarization labelled ✔\n
\n\n \n Correct shape should show peaks at P R and T and dips at Q and S\n \n
\n\n \n
\n The stronger candidates could describe the structure of cardiac muscle cells.\n
\n\n Most could name the semilunar valves.\n
\n\n Part (c) confused many candidates and they did not realise they had to refer to the graphs in their answers. Most scored one mark for stating ventricular contraction occurs after atrial contraction.\n
\n\n In part (d) the better candidates could sketch the pattern seen in a typical electrocardiogram. Some made errors in the shape or letters but could still achieve marks for showing the areas of contraction or relaxation.\n
\nDNA methylation has a critical role in gene regulation by affecting transcription. Samples were taken from two colon cancer tumours (T1 and T2) and two normal colon samples (N1 and N2). A particular gene was implicated as a possible cause of cancer. The promoter of this gene was cloned (A–J). The data show the DNA methylation patterns from these samples. The numbers (32–269) represent different markers in the promoter.
\n[Source: Philipp Schatz, Dimo Dietrich & Matthias Schuster. Rapid analysis of CpG methylation patterns using RNase T1
cleavage and MALDI-TOF. Nucleic Acids Research (2004) 32 (21): e167, doi:10.1093/nar/gnh165.
Reproduced by permission of Oxford University Press]
Identify the stage of mitosis labelled X in the image, giving a reason.
\n[Source: Copyright 2002, The Trustees of Indiana University]
\nOutline what is indicated by the mitotic index of tissue taken from a tumour.
\nDNA has regions that do not code for proteins. State two functions of these regions.
\n1.
\n2.
\n\n
Outline the difference in methylation pattern between tumorous and normal tissue samples.
\n\n
Suggest a way methylation may affect tumour cell genes.
\ntelophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔
OWTTE
\na. mitotic index is an indication of the ratio/percentage of cells undergoing mitosis/cell division ✔
\nb. cancer cells «generally» divide much more than normal «somatic» cells ✔
\nc. a high/elevated mitotic index in tumours / possible diagnosis of cancer /measure of how aggressive/fast growing the tumour is ✔
\na. promoters / operators / regulation of gene expression/transcription ✔
\nb. telomeres/give protection to the end of chromosomes «during cell division» ✔
\nc. genes for tRNA/rRNA production ✔
\nd. other valid function for non-coding sequence ✔
\nDo not accept stop codon, accept centromeres (connecting sister chromatids).
\na. «overall» much more methylation in the colon tumour samples than normal ✔
\nb. tumour and normal samples the markers 258 and 269 similar degree of methylation/fewer differences ✔
\nc. degree of methylation on certain markers may correlate with the presence of cancer / correct example of a marker only methylated in tumour cells eg marker 32 ✔
\na. «DNA» methylation may inhibit transcription of genes that would prevent cancer/tumor formation ✔
\nb. «DNA» methylation may increase mitosis/cell division leading to tumor formation ✔
\nDo not accept discussion of histone methylation.
\nBoth anaphase and telophase were accepted as answers, but students had difficulty providing a reason that was an appropriate justification; for example, students might say telophase and then support this with the statement because chromatids are seen moving to opposite poles. Students sometimes referred to chromatids as homologous chromosomes. As this is an image of plant cells, students would be mistaken by referring to a cleavage furrow. The mechanism of cytokinesis involves the formation of a cell plate.
\nMany students could accurately define the term mitotic index. An area of misunderstanding was to categorize the mitotic index as a rate rather than a ratio.
\nThis question was well answered. The occasional answer made reference to processes associated with translation.
\nMany students detected that greater degrees of methylation in tumour samples. Fewer could accurately summarize the specific differences between methylation in the two samples.
\nStudents lacked the understanding of epigenetics necessary to make reasonable suggestions.
\n\n State\n \n one\n \n function of the atrioventricular node.\n
\n\n [1]\n
\n\n Describe how the structure of cardiac muscle cells allows them to carry out their function.\n
\n\n [3]\n
\n\n a. relays signal from SAN to ventricles ✔\n
\n\n b. causes ventricular systole ✔\n
\n\n c. delays signal enabling both ventricles to contract simultaneously\n
\n \n \n OR\n \n \n
\n delays signal so the atria empty before ventricular systole ✔\n
\n \n
\n \n Not necessary to present answer in a table.\n \n
\n\n \n Award\n \n [1]\n \n for each set of corresponding structure and function.\n \n
\n\n A large number could state one correct function of the atrioventricular node, mainly that it causes ventricular systole.\n
\n\n Many could provide a correct description including many elements, but many discussed the structure of the heart as a whole, rather than the cellular structure of cardiac muscle cells or discussed at length the myogenic nature of cardiac tissue.\n
\nThe graph shows blood pressure changes on the left side of the heart during one heartbeat. Identify the two parts of the circulatory system that produce traces I and II on the graph.
\n[Source: © International Baccalaureate Organization 2019]
\nOutline the actions taken by the body to avoid infection when the skin is cut.
\nHormones are distributed throughout the body by the blood. Outline the roles of two reproductive hormones during the menstrual cycle in women.
\nI. aorta ✔
\nII: «left» atrium ✔
\na. platelets/cut tissues release clotting factors ✔ Mp a requires student to identify source of clotting factors.
\nb. «clotting factors» activate thrombin «from prothrombin» ✔
\nc. thrombin converts fibrinogen to fibrin ✔
\nd. «fibrin» forms a clot/scab/mesh that seals the cut ✔
\ne. phagocytic white blood cells ingest pathogens ✔
\na. FSH/follicle stimulating hormone stimulates the development of follicles/follicle cell division in the ovary «to produce eggs» ✔ Two different hormones must be identified.
\nb. LH/luteinizing hormone triggers ovulation/development of the corpus luteum ✔
\nc. estrogen stimulates development of the uterine lining/endometrium ✔ Description of role required as well as name of hormone.
\nd. progesterone maintains the uterine lining/endometrium
OR
inhibits other hormones by negative feedback eg, FSH ✔
e. HCG stimulates ovary to produce progesterone «in early pregnancy» ✔
\nf. other verifiable hormone and roles relevant to the menstrual cycle ✔
\nThis question was poorly answered with most students earning either one mark or no marks. It was more common for candidates to correctly identify the atrium rather than the aorta. ß
\nThis question about clotting was well answered with most being able to correctly identify the roles of clotting factors, fibrinogen and thrombin, though in a number of cases, specific details were lacking.
\nStudents were able to correctly identify hormones that played a role in the menstrual cycle and were often successful in outlining their function. A common problem was distinguishing the role of progesterone from that of estrogen.
\nThe images show parts of plants belonging to two different phyla.
\nState the phylum of plant X and of plant Y.
\nX:
\nY:
\nSome plant families, such as the figwort family, have been reclassified on the basis of evidence from cladistics. Explain the methods that have been used recently to reclassify groups of plants.
\nSuccessful sexual reproduction in flowering plants depends on several essential processes. Outline the role of pollination and seed dispersal.
\nPollination:
\n\n
Seed dispersal:
\n\n
\n
X: Filicinophyta ✔
\nY: Coniferophyta/Conifera/Gymnosperms ✔
\na. «previous» classification used to be based on the appearance/structures of the plant/leaves/flowers/seeds/analogy/phenotype ✔
\nb. «modern cladistics uses» RNA/DNA nucleotide/base sequencing/amino acid sequencing/homology ✔
\nc. DNA mutation occurs at a relatively constant rate allowing estimation of when species diverged ✔
\nd. a shared/common derived characteristic places organisms in the same clade ✔
\ne. the number of changes in sequences indicates distance from common ancestor
OR
the fewer the differences «in sequences» means the closer the relationship ✔
pollination:
transfer/dispersal/movement of pollen from anther/stamen to stigma
OR
transfer/dispersal/movement of pollen between plants/flowers prior to/allowing fertilization ✔
seed dispersal:
«strategy of» distribution of seeds so that new plants have space/nutrients to develop/avoid competition/colonize new habitats ✔
Accept any other valid role.
\nAccept OWTTE. Answers must be about the role.
\nThis question requiring students to identify plant phyla from images was successfully answered by most. It was more common for students to correctly identify Plant Y as a member of the Coniferophyta phylum with a greater number being unable to identify Plant X as a member of the Filicinophyta phylum.
\nMany students were able to identify molecular sequencing as a method used to re-classify plants. Fewer were clear on the notion of original classification being mistakenly based on analogous features.
\nMost students were able to outline the role of seed dispersal in minimizing competition between parent and offspring. Slightly fewer were successful in correctly outlining the role of pollination in sexual reproduction with a number being unable to correctly draw upon the necessary vocabulary.
\nOutline energy flow through a food chain.
\nDraw a fully labelled graph of the action spectrum for photosynthesis.
\nExplain Calvin’s experiment and what was discovered about photosynthesis through his work.
\na. energy from the sun/light energy is converted to chemical energy by photosynthesis ✔
\nb. «chemical» energy flows through the food chains by feeding ✔
\nc. energy is released «from carbon compounds» by respiration
OR
energy from respiration is used by living organisms and converted to heat ✔
d. heat is not recyclable / heat is lost from food chains
OR
heat cannot be converted to other forms of energy ✔
e. energy is lost in excretion/uneaten material/egestion/feces ✔
\nf. energy losses between trophic levels limits the length of food chains
OR
energy transfer is only 10 % between trophic levels ✔
a. axes correctly labelled «wavelength and rate of photosynthesis» ✔ Accept rate of oxygen production for rate of photosynthesis.
\nb. 400 and 700 nm as limits ✔
\nc. correct shape of curve involving two peaks at the correct places, broader in the blue-violet range not starting at zero and a narrower peak in the orange-red range with the trough in the green range that does not reach zero ✔
\nd. peaks of activity at 430 nm AND at 660 nm ✔
\ne. peaks indicated as «violet» blue light AND peak indicated as «orange» red light ✔
\n\n
a. Calvin cycle is light-independent ✔
\nb. carbon fixation
OR
carboxylation of ribulose bisphosphate/RuBP occurs ✔
c. algae placed in thin glass container/“lollipop” apparatus ✔
\nd. given plenty of light and bicarbonate/ CO2 ✔
\ne. at start of experiment algae supplied radioactive carbon/HCO3-/14C ✔
\nf. samples taken at intervals / heat/alcohol killed samples ✔
\ng. C-compounds separated by chromatography ✔
\nh. 14C/radioactive-compounds identified by autoradiography ✔
\ni. showed that RuBP was phosphorylated ✔
\nj. after five seconds/immediately more glycerate-3-phosphate/3-PGA labelled than any other compound ✔
\nk. shows glycerate-3-phosphate/3-PGA first «carboxylated» compound/the first stable product ✔
\nl. next compound to be detected containing radioactive carbon was triose phosphate/G3P/glyceraldehyde 3 phosphate ✔
\nm. showed that a wide range of carbon compounds was quickly made in sequence ✔
\nn. showed that a cycle of reactions was used to regenerate RuBP ✔
\nWas well answered with most students being knowledgeable about ecology.
\nA number of students made errors in their sketches. Axes were commonly mis-labelled. The colors were commonly presented in the reverse order with red at the left end and blue at the right end. Showing red as a higher peak was another common error. The overall shape was often correctly drawn.
\nThis question was commonly answered poorly with students showing a lack of knowledge of both the Calvin cycle as well as the Calving experiment.
\nEbola virus disease (EVD) is the disease in humans and other primates that is caused by the Ebola virus. Fruit bats are the reservoir for the virus and are able to spread the disease without being affected. Humans can become infected by contact with fruit bats or with people infected by the virus, their body fluids or equipment used to treat them.
\nThe table shows data for four African countries that were affected by the 2014–2015 Ebola outbreak.
\n[Source: adapted with permission, from Ebola Situation Report, figure 1, http://apps.who.int/ebola/current-situation/ebolasituation-
report-2-march-2016, March 2016, and from Successful treatment of advanced Ebola virus infection with T-705
(favipiravir) in a small animal model, Oestereich, L. et al, 2014, under CC BY 3.0]
The graphs show the progress of the EVD epidemic in Guinea and Liberia for the period April 2014 to May 2015.
\n[Source: Ebola Situation Report 2 March 2016 and data from International Journal of Infectious Diseases, 38,
Ligui Wang et al, Epidemiological features and trends of Ebola virus disease in West Africa, 52-53.,
Copyright 2015, with permission from Elsevier]
An antiviral drug, T-705, was tested in order to establish whether it has potential to treat EVD. The graph shows the data from an in vitro trial of T-705 on cells that had been infected with Ebola virus five days previously. Virus concentration and live cells are shown as percentage of the control.
\n[Source: Oestereich, Lisa & Rieger, Toni & Neumann, Melanie & Bernreuther, Christian & Lehmann, Maria & Krasemann,
Susanne & Wurr, Stephanie & Emmerich, Petra & de Lamballerie, Xavier & Ölschläger, Stephan & Günther, Stephan. (2014).
Evaluation of Antiviral Efficacy of Ribavirin, Arbidol, and T-705 (Favipiravir) in a Mouse Model for Crimean-Congo
Hemorrhagic Fever. PLoS neglected tropical diseases. 8. e2804. 10.1371/journal.pntd.0002804.]
Identify the country with the largest number of Ebola cases.
\nIdentify the country with the largest number of deaths.
\nAnalysis of the data suggests that the number of deaths from EVD is not related to the total population size. State one piece of evidence from the data that would support this analysis.
\nBased on the mode of transmission of the Ebola virus, suggest a possible reason for the relationship between population density and the number of Ebola cases in these four countries.
\nBased on the data, compare and contrast the progress of the epidemic in Liberia and Guinea.
\nSuggest two possible reasons for the drop in the daily numbers of newly infected cases after October 2014 in Liberia.
\nBased on these data, outline the evidence that T-705 has potential to be used as a treatment for EVD.
\nDistrict administrators combatting the 2014 Ebola epidemic in West Africa were assisted by international organizations such as the World Health Organization, who provided data on the progress of the epidemic. Suggest one other way in which international organizations can assist with combatting an epidemic of Ebola.
\nSierra Leone ✔
\nLiberia ✔
\ncountry with biggest population/Mali has lowest number of deaths
OR
country with smallest population/Liberia has biggest number of deaths ✔
OWTTE
\ngreater density means more frequent contact with infected people/animals ✔
\nNeed both greater density and frequency of contact
\na. overall pattern similar in both/both show a rise and a fall in the infections ✔
b. both countries show an increase during 2014
OR
neither country shows an increase in 2015 ✔
c. both show a sudden drop at one point
OR
sudden drop earlier «Oct–Nov 14» in Liberia than in Guinea «Dec 14, Jan 15» ✔
d. Guinea fluctuates whereas Liberia rises to a peak and then decreases/no fluctuations ✔
\ne. epidemic starts earlier «in April 14» in Guinea than in Liberia «in June 14» ✔
\nf. epidemic peaks earlier «Sept 14» in Liberia than in Guinea «Dec 14» ✔
\ng. epidemic lasts longer in Guinea than it does in Liberia
OR
last case recorded in Liberia Feb 15 while cases continue «at least» until May 15 in Guinea ✔
h. numbers of cases in Guinea generally lower than in Liberia
OR
number of cases higher in Liberia than in Guinea ✔
a. improved medical care/support/supplies/equipment/training of staff/hygiene/distribution of vaccine ✔
b. improved understanding of how to avoid infection «amongst public»/greater awareness in society/better education✔
c. rise in number of deaths means fewer infectious individuals ✔
\nd. impact of disease control measures/control policies/quarantine/isolation ✔
\ne. drop in the number of fruit bats ✔
\nf. maybe seasonal changes/weather changes ✔
\ng. people may have left the area ✔
\nh. international aid arrives ✔
\nOWTTE
\na. cells not killed/few cells killed «even at high concentrations» ✔
\nb. «T-705» effective/viruses reduced/viruses killed at 100 μM
OR
«T-705» very effective/viruses much reduced/nearly all viruses killed at 1000 μM ✔
c. virus concentration decreases as T-705 concentration increases ✔
\nd. drug has «high» potential for treatment «at high enough concentration» ✔
\nraise awareness/provide information for local population/supply health workers/equipment/ train local staff/share expertise/provide financial support/provide vaccine/travel ban alert to affected country ✔
\nThis set of data-based questions produced the best showing among candidates. Teachers are to be commended for developing these skills in their students. Many candidates showed confidence in looking at unfamiliar data and formulating answers. Some weaknesses included not being careful with what they were writing when there were similar aspects in the data set (total population size and population density), comparing and contrasting - many students finding this difficult and simply writing descriptions of the data set rather than focussing on similarities and differences. Also giving information on part of a data set where some aspects of the data set may be similar in name but fundamentally different (e.g. Total population and population density) – Some candidates used incorrect parts of data sets rather than those that had been asked about - for example, talking about Ebola cases rather than Ebola deaths, total population size rather than population density.
\nMost candidates correctly used high density and frequency of contact to describe the relationship. Very few responses discussed high density but did not relate it to the frequency of contact. Others focused on modes of transmission alone and did not relate it to density.
\nData is described rather than interpreted e.g. epidemic starts in April for Guinea and in June for Liberia instead of epidemic starts earlier in Guinea than in Liberia
\nThis was a well answered response where almost all students got full marks. Students were able to identify more than three marking points here mp a, b, c and d were most common. Some students provided mp e and f. Few used mp g and h.
\nGraph was well read and understood by most. The question was answered with more insight than expected, especially when alluding to the implications of live cells at high drug concentrations
\nAgain, most students got a mark here for correct reasoning. Those who lost a mark made references to research. Research was not accepted as it might have future benefits, but not likely to help in the immediate epidemic.
\n\n State the trophic level of the sea urchin.\n
\n\n [1]\n
\n\n With respect to this food chain, outline what is meant by a keystone species.\n
\n\n [3]\n
\n\n herbivore/primary consumer ✔\n
\n\n \n Do not accept second trophic level\n \n
\n\n a. keystone species have a disproportionate effect on the biological community ✔\n
\n\n b. removal of the sea bream «due to fishing» ✔\n
\n\n c. results in more sea urchins ✔\n
\n\n d. which significantly reduce the producers/seagrass ✔\n
\n\n \n Do not accept first trophic level for mpd\n \n
\n\n Primary consumer was frequently stated and second trophic level also seen, although not credited as the syllabus demands the former term.\n
\n\n The syllabus definition of a keystone species was not often given, so alternative wording had to be judged on merit. The keystone species was not always recognized as the sea bream, but if it was, three marks were easily achieved.\n
\n\n Yeast cells,\n \n Saccharomyces cerevisiae\n \n , were incubated with and without glucose at three different temperatures for a period of four minutes, during which the rate of CO\n \n 2\n \n production was measured with a CO\n \n 2\n \n sensor.\n \n
\n [Source: © International Baccalaureate Organization 2019]\n
\n\n
\n\n What conclusion can be drawn from the results of this experiment?\n
\n\n A. Yeast uses lipids rather than glucose in respiration at low temperatures.\n
\n\n B. Addition of glucose has a greater impact on rates of cell respiration at lower temperatures.\n
\n\n C. Rates of cell respiration increase with temperature.\n
\n\n D. More glucose is produced at higher temperatures.\n
\n\n [1]\n
\n\n From the data, identify the depth along the transect where the greatest species richness is observed.\n
\n\n [1]\n
\n\n Outline the relationship between\n \n Zooxanthellae\n \n and reef-building coral reef species.\n
\n\n [2]\n
\n\n Suggest\n \n one\n \n way in which depth may act as a limiting factor for coral.\n
\n\n [1]\n
\n\n C\n
\n\n 6.5 m ✔\n
\n\n \n Unit required\n \n
\n\n a. a symbiotic/mutualistic relationship ✔\n
\n\n b.\n \n Zooxanthellae\n \n obtain shelter/habitat/exposure to light ✔\n
\n\n c. coral obtains energy/food through photosynthesis of the\n \n Zooxanthellae\n \n ✔\n
\n\n low light levels/lower temperatures cannot support growth/metabolism ✔\n
\n\n \n Requires the explanation for the mark\n \n
\n\n Richness appeared well understood but the depth unit was sometimes omitted.\n
\n\n This was a well-answered question with many candidates knowing the term symbiosis or mutualism. Protection provided by coral was often stated but the role of\n \n Zooxanthellae\n \n in providing nutrients from photosynthesis was less often seen.\n
\n\n The reduction in light or temperature with greater depth was recognized but not often linked to growth or photosynthesis.\n
\n\n Explain what is meant by competitive exclusion, with respect to a\n \n named\n \n example of an invasive species.\n
\n\n [4]\n
\n\n Describe the relationship between rock phosphate production and world population.\n
\n\n [2]\n
\n\n Phosphate can be lost from agricultural land in several different ways. State\n \n one\n \n of these.\n
\n\n [1]\n
\n\n Describe how the changes in world rock phosphate production after 1985 may have affected world food supplies.\n
\n\n [2]\n
\n\n a. when two species with similar niches occupy the same habitat ✔\n
\n\n b. the competition increases\n
\n \n \n OR\n \n \n
\n reduction in resources ✔\n
\n c. the population of one species will die/be excluded ✔\n
\n\n d. an invasive species «often» lacks predators ✔\n
\n\n e. the endemic species is usually the one that cannot compete ✔\n
\n\n f.\n \n eg\n \n : grey squirrel/Japanese knot weed/cane toad ✔\n
\n\n \n \n [3 max]\n \n \n \n if the invasive species is not named.\n \n
\n\n \n Do not allow humans.\n \n
\n\n a. as population increases so does phosphate production/positive correlation ✔\n
\n\n b. since 1985 phosphate production has not risen while population has continued to increase ✔\n
\n\n leaching/soil erosion, run-off/removed by the harvesting of agricultural crops ✔\n
\n\n a. phosphorus is important as a fertilizer ✔\n
\n\n b. a drop in phosphate could lead to less agricultural output ✔\n
\n\n c. «this could mean» less food available for increasing population ✔\n
\n\n This was generally well answered. There were a wide variety of verifiable invasive species and only a few confused examples. The most common example was the cane toad introduced into Australia. Candidates understood increased competition and lack of predation.\n
\n\n Many candidates were able to score 1 mark here for seeing the positive correlation. However, students continue to have problems determining when changes shown on graphs are significant. Although the final mark scheme allowed for credit to be given if a student wrote that the phosphate levels off/fluctuates/drops and rises while population has continued to increase, many candidates were still unable to draw correct conclusions about the final years shown on the graph.\n
\n\n Most students obtained the mark here, possibly because the mark scheme allowed for a variety of answers for the mark.\n
\n\n Students who struggled to read the graph correctly had problems with this question. Very few scored the full marks here, as only a small number of students clearly stated that rock phosphates were used as fertilisers to increase plant growth. Few students composed coherent answers that tied together fertiliser use, agricultural output and food.\n
\n\n Distinguish between levels of predation in marmots born in the wild and those born in captivity.\n
\n\n [1]\n
\n\n Suggest reasons for the differences in predation.\n
\n\n [2]\n
\n\n predation was greater in those born in captivity ✔\n
\n\n a. the marmots have experience with/recognize predators ✔\n
\n\n b. parents shield them from predators\n
\n \n \n OR\n \n \n
\n parents teach them about predators ✔\n
\n c. those born in the wild are favoured in\n \n natural selection\n \n ✔\n
\n\n This question was generally well answered.\n
\n\n Most candidates were able to score at least 1 mark here by commenting on either the greater experience with predators for marmots in the wild, or the role of learning. However, some examiners noted that students tended to use anthropomorphisms when discussing animals in this option. Also, it was surprising that very few students were able to achieve the marking point that those born in the wild were favoured by natural selection, as natural selection is one of the central ideas in biology.\n
\n\n Outline the changes of distribution of the grey squirrel from 1945 to 2010.\n
\n\n [2]\n
\n\n Suggest\n \n two\n \n possible reasons for the change in distribution.\n
\n\n 1.\n
\n\n 2.\n
\n\n
\n\n [2]\n
\n\n The niches of the red and grey squirrels overlap. Explain the concept of competitive exclusion with respect to the changes in squirrel distribution shown in the maps.\n
\n\n
\n\n [3]\n
\n\n a. overall the grey squirrel distribution has increased «and the red squirrel decreased» ✔\n
\n\n b. in 2010 the grey squirrel was found in areas where it was previously not found ✔\n
\n\n c. grey squirrel has dominated/red squirrel virtually eliminated ✔\n
\n\n \n OWTTE\n \n
\n\n a. no predators ✔\n
\n\n b. ample food supply/habitats ✔\n
\n\n c. few competitors ✔\n
\n\n d. resistant to disease ✔\n
\n\n e. high reproductive rate ✔\n
\n\n f. no physical barrier to prevent spread of grey squirrels on mainland ✔\n
\n\n a. competitive exclusion states two species that occupy a similar niche in the same location cannot coexist ✔\n
\n\n b. one of the two competitors will always have an advantage over the other ✔\n
\n\n c. leads to extinction/displacement/evolution of the second competitor ✔\n
\n\n d. grey squirrels have replaced/occupied niches formerly occupied by red squirrels\n
\n \n \n OR\n \n \n
\n habitats favour the grey squirrel in competition for the niche ✔\n
\n e. the niche of one competitor/both competitors becomes narrower ✔\n
\n\n While most candidates were able to achieve a mark by noting that the grey squirrel “dominated” in the second map, students again lost marks due to an inability to communicate concepts clearly. A large number of students confused distribution with population size, an idea not addressed in the maps.\n
\n\n This was answered well by only a few students even though the mark scheme allowed for a wide range of answers.\n
\n\n This question was yet another instance where weaker students were disadvantaged by their inability to access the appropriate subject-specific vocabulary. They tended not to use the term “niche” with this question, and too frequently “niche” and “habitat” were confused.\n
\nIdentify which electron micrograph shows a mitochondrion, providing one observation to support your choice.
\nDiscuss the evidence for the theory that mitochondria may have evolved from free-living prokaryotes by endosymbiosis.
\n«micrograph» C cristae/double membrane is visible/«folds of» membranes inside ✔
\nMust state observation
\na. «double» membrane may have formed when engulfed ✔
\nb. replicate by binary fission like free-living prokaryotes
OR
reproduce separate from «host» cell replication ✔
c. they have their own «circular» DNA AND reproduce on their own ✔
\nd. they have «70s» ribosomes AND can manufacture «their own» proteins ✔
\ne. have organelle«s» similar to free-living prokaryotes ✔
\nf. similar in size to free-living prokaryotes ✔
\ng. mitochondrial inner membranes manufacture ATP like bacterial membranes ✔
\nh. currently there is no free-living prokaryote like a mitochondrion ✔
\nMost candidates identified the correct micrograph but for the wrong reason such as 70S ribosomes visible, has a matrix, or correct shape. Only starch granules were visible in the image, not ribosomes.
\nThe command term for this question was “discuss”. Connections were not made as many candidates just gave a list of features of mitochondria with no attempt at explanation or discussion. There was some confusion between the words endocytosis and endosymbiosis. In spite of the many shortcomings, there were a few brilliant responses
\nThe diagram shows the human heart.
\nOn the diagram, label the aorta.
\nOn the diagram, label the right atrium.
\nExplain how valves control the flow of blood through the heart.
\nOutline the causes and consequences of blood clot formation in coronary arteries.
\nOutline the role of lymphocytes in defence against disease.
\nboth labelled clearly as in diagram ✔
\n[Source: BlueRingMedia/Shutterstock]
\nboth labelled clearly as in diagram ✔
\n[Source: BlueRingMedia/Shutterstock]
\na. valves open and close in response to changes in blood pressure/heart contraction/pumping ✔
\nb. valve prevents backflow/maintains direction of blood flow ✔
\nc. valves allow heart chambers to fill/to empty ✔
\na. coronary heart disease/CHD/coronary artery disease/CAD occurs when there is reduction of oxygen to the heart muscle ✔
\nb. high ratio of LDL to HDL/fatty diet leads to plaque formation in arteries ✔
\nc. plaque breaks off causing damage that activates blood clot formation ✔
\nd. clots «in the bloodstream» may block a coronary artery/coronary thrombosis reducing blood flow/oxygen
OR
clots can cause heart attack/muscle death ✔
e. sickle cell anemia «crisis» produces blood clots «that can cause coronary/arterial blockage» ✔
\na. produce antibodies ✔
\nb. memory cells confer immunity ✔
\nc. specific immunity results from production of antibodies specific to a particular antigen ✔
\nd. recognize pathogens ✔
\ne. destroy foreign cells/cancer cells ✔
\nLabelling the heart was generally ok, though the rest of this question was weakly answered.
\nLabelling the heart was generally ok, though the rest of this question was weakly answered.
\nMost candidates knew that heart valves prevent backflow of blood but that was all. There were a variety of misconceptions such as that valves pump blood or that the SA node sends signals to the valves to make them open/close.
\nClots were related to a fatty diet or cholesterol but rarely to plaque build-up. The idea that plaque breaking could activate clot formation was seldom mentioned. Without really giving any accurate information, candidates got a mark just by linking blood clots to heart attack. There were many misconceptions and confusion. The first confusion was that blood clots lead to strokes. Quite a few responses focused on seizures and strokes as a possible consequence. In fairness to candidates, there is ambiguity in the way we use terminology in the common parlance. Cardiac arrest, cardiac failure, CHD and heart attack are technically not synonyms, though they are often used interchangeably. A student of biology is expected to understand the differences.
\nMany candidates correctly stated that lymphocytes produce antibodies. Lymphocytes were often mistaken for phagocytes and lengthy descriptions of engulfing of diseases were given.
\nThe diagram shows the structure of an alveolus and an adjacent capillary.
\n[Source: © International Baccalaureate Organization 2019]
\nOutline the functions of type I and type II pneumocytes.
\nExplain how gases are exchanged between the air in the alveolus and the blood in the capillaries.
\ntype I:
\na. carry out gas exchange
OR
diffusion of gases/CO2/O2 ✔
type II:
\nb. secrete fluid/surfactant ✔
\na. O2 concentration in alveolar air greater than in capillary/blood «prior to gas exchange»
OR
hemoglobin in blood binds oxygen maintaining the concentration gradient ✔
b. O2 gas dissolves in water lining the alveolus ✔
\nc. O2 diffuses through wall of alveolus and capillary into blood ✔
\nd. CO2 concentration in blood greater than in alveolar air «prior to gas exchange» ✔
\ne. CO2 diffuses through wall of capillary and alveolus into alveolar airspace ✔
\nWeak knowledge of pneumocytes from many candidates and whilst many showed an understanding of gas exchange they failed to articulate it properly using the correct phrasing and terminology.
\nThe command term was “explain”. Only a few candidates actually explained the process. Candidates gave very general responses without reference to diffusion or the correct concentration gradients. Very often diffusion was referred to in general for gas exchange and it was not linked to concentration gradients. Some students got max 2 for correct references to diffusion with reference to both gases. Students getting max 3 were able to include references to correct concentration gradients as well.
\nThe table gives common names and binomial names for some mammals.
\nState one feature that characterizes these species as mammals.
\nIdentify the two species most closely related.
\n1.
\n2.
\n\n
Identify two species from the list that are classified in different genera.
\n1.
\n2.
\n\n
fur/hair/mammary glands/feed young with milk/three inner-ear bones/lungs ventilated by diaphragm/lungs contain alveoli ✔
\nCanis aureus/golden jackal AND Canis lupus/grey wolf ✔
\nBoth needed
\nHapalemur aureus/Golden bamboo lemur AND Canis aureus/golden jackel/Canis lupus/grey wolf/Vulpes vulpes/red fox
OR
Vulpes vulpes/red fox AND Canis aureus/golden jackal/Canis lupus/grey wolf/Hapalemur aureus/golden bamboo lemur ✔
The most common correct answers were having fur/hair or providing milk. Warm-blood or live birth were often given but were not accepted as other classes of animals exhibit these characteristics.
\nWell done but some thought the species aureus meant they were similar rather than choosing the genus name.
\nThe image shows the chromosomes from a body cell of an adult human.
\n[Source: http://www.ornl.gov/sci/techresources/Human_Genome/graphics/slides/elsikaryotype.shtml,
U.S. Department of Energy Human Genome Program.]
Identify, with a reason, the sex of this individual.
\n\n
Identify the chromosome that is affected by a trisomy in this individual, naming the condition that this trisomy gives rise to.
\nChromosome number:
\nName of condition:
\nmale because Y chromosome present
OR
male because sex chromosomes/last two chromosomes/pair 23 are unpaired/different «from each other»/not homologous ✔
21 AND Down syndrome/trisomy 21 ✔
\nBoth needed
\nCandidates knew this!
\nThe idea of 21 trisomy and Down Syndrome was very common. Rarely students made references to autism or other conditions after correctly identifying 21 trisomy. Rarely other chromosome numbers with incorrect conditions were referred to
\nOutline the stages in the production of mRNA by transcription.
\nDescribe the functions of proteins in cell membranes.
\nExplain how natural selection can lead to speciation.
\na. DNA is unwound/strands are separated «by RNA polymerase» ✔
\nb. new nucleotides attached to template strand «by RNA polymerase» ✔
\nc. complementary base pairing/base pairing with an example
OR
adenine with thymine/uracil with adenine/cytosine with guanine/guanine with cytosine ✔
d. mRNA detaches from template ✔
\ne. DNA rewinds ✔
\na. facilitated diffusion by channel proteins ✔
\nb. active transport by protein pumps
OR
protein pumps eg sodium-potassium ✔
c. cell recognition by glycoproteins/protein receptors ✔
\nd. communication/receptors for hormones/signal molecules ✔
\ne. cell adhesion ✔
\nf. allow up to one additional mark for AHL material ✔
\na. natural selection is caused by selection pressures in the environment
OR
example of a selection pressure ✔
b. natural selection requires that variation exists within a species ✔
\nc. variation arises randomly due to mutation
OR
variation is enhanced by meiosis/sexual reproduction ✔
d. over-production of offspring promotes selection
OR
natural selection occurs when there is competition/overpopulation/predators/environmental changes/changes in selection pressures ✔
e. well adapted individuals/individuals with best variations survive to reproduce/survival of fittest ✔
\nf. «frequency of» genes/alleles conferring an advantage are selected for
OR
genes/alleles conferring a disadvantage are selected against ✔
g. genetic divergence/difference increases
OR
natural selection «genetically» isolates members of a species so eventually they can no longer produce fertile offspring ✔
h. genetic divergence» leads to reproductive isolation ✔
\ni. geographical/behavioural/ecological factors may lead to «reproductive» isolation ✔
\nj. prolonged «reproductive» isolation leads to speciation ✔
\nk. up to one additional mark for AHL information ✔
\nAlmost all students had a clear idea of complementary base pairing and that A pairs with U in RNA. However, this question revealed weakness in the understanding of transcription and the ability to distinguish transcription from DNA replication and RNA translation. Often times candidates had hybrid answers containing information from at least two, but sometimes all three processes. Many candidates were able to gain a couple of the easier marks in 7a on transcription despite having a poor understanding of the process.
\nA wide variety of fairly well-developed answers. Often, they did not couple a type of protein with its function or missed marks by being too general. Saying “transport proteins” was not enough. Often the terms integral and peripheral proteins were mentioned without going beyond that. Channel proteins and facilitated diffusion was most commonly given. The mistaken idea that proteins offer structural support or strengthen the cell membrane was seen repeatedly.
\nThere were some excellent explanations given in well-structured responses. Many candidates avoided talking about natural selection in genetics terms, losing marks in the process. Darwin’s finches, industrial melanism and the giraffe were common examples. Some students invented their own creative examples.
\nThe idea that natural selection limits biodiversity appeared a few times. The flawed case of the peppered moth was given to exemplify speciation, however Darwin's finches were the most frequent example.
\nDescribe how detritivores obtain nutrition and the effects they have in ecosystems.
\nOutline the role of amylase in digestion in humans.
\nExplain how plants capture and use light in photosynthesis.
\na. «detritivores» obtain nutrition from detritus/waste/dead bodies ✔
\nb. are heterotrophic ✔
\nc. removes large waste/cleans up the ecosystem
OR
helps control spread of disease ✔
d. facilitates further decomposition ✔
\ne. contribute to the supply of «inorganic» nutrients for autotrophs/nutrient cycling
OR
improve soil conditions/aeration ✔
a. amylase is an enzyme ✔
\nb. secreted by salivary glands/pancreas ✔
\nc. active/released into the mouth/small intestine ✔
\nd. acts on starch/polysaccharides ✔
\ne. breaks «glycosidic» bond by hydrolysis/adding water ✔
\nf. converts insoluble/large molecule to soluble/small molecules ✔
\ng. product is maltose/disaccharide/sugar molecule ✔
\na. plants convert light energy into chemical energy by photosynthesis ✔
\nb. photosynthesis takes place in chloroplasts ✔
\nc. chloroplasts «are organelles that» contain the pigment chlorophyll ✔
\nd. chloroplasts/chlorophyll «in plants» absorb sunlight ✔
\ne. «chlorophyll» absorbs red AND blue light most effectively ✔
\nf. light causes photolysis/splits water molecule ✔
\ng. carbon dioxide AND water are reactants «in photosynthesis» ✔
\nh. glucose AND oxygen are products «of photosynthesis» ✔
\ni. light intensity is a limiting factor for the rate of photosynthesis ✔
\nj. organic/carbon compounds/glucose provide food/stored energy «for plant itself, animals,food chains» ✔
\nk. up to one additional mark for an accurate detail from AHL ✔
\nMost candidates got max 2 for mp a (nutrition of detritivores) and mp e (recycling of nutrients). Candidates getting high marks were able to provide correct examples of detritivores and correctly link to their role in enriching the soil and remove dead material. The biggest problem was the notion that detritivores recycle energy.
\nThe name, location and correct function of the enzyme was very common making 3 max the most common marks. Students who got fewer marks gave generic explanations of the process of digestion. Some students made incorrect references to amylase digesting proteins. A few candidates ignored amylase and discussed digestion in general.
\nGenerally well done with some very knowledgeable answers from the Higher Level programme sometimes to the detriment of the more basic answers required by the standard level examination. Many candidates who attempted this response got more than 4 marks for mp b, d, g and h which were the most common. Students getting higher marks were able to focus on correct conversion of energy, efficiency of red and blue light absorption, and photolysis. Some egregious misconceptions exist among candidates such as plants are green because they absorb green light. The idea that during photosynthesis light energy is converted to chemical energy was frequently neglected.
\nMice (Mus musculus) have various defence systems against predators such as foxes (Vulpus vulpus) or stoats (Mustela erminea). The mice release specific alarm compounds when under threat that serve as danger signals for other mice. Predators also release scents that the mice can detect.
\nIn one study, mice were exposed to paper soaked in compounds taken from other mice, foxes or stoats in a test chamber. The scientists then measured the reaction of three different groups of neurons used in smelling: G1, G2 and G3, as shown in the diagram.
\nThe percentage of G1, G2 and G3 neurons responding to the mouse, fox and stoat compounds, as well as a control compound, are shown in the chart.
\nPercentage of response of neurons in mice
\n[Source: Julien Brechbühl, et al., (2013), PNAS, 110 (12), pages 4762–4767]
\nThe scientists recorded the neural traces (action and resting potentials) of six G1 neurons exposed to the compounds from the different animals. The horizontal bar beneath each trace corresponds to the time of exposure to the test compound.
\n[Source: adapted from J Brechbühl, et al., (2013), PNAS, 110 (12), pages 4762–4767]
\nAnother study was carried out on the defence systems in aphids (Myzus persicae) which feed on thale cress plants (Arabidopsis thaliana). Individual aphids release an alarm compound containing the chemical (E)-β-farnesene (EBF) when attacked by a predator. Other aphids are repelled and leave the thale cress plant or hide.
\nThe gene for EBF was inserted to produce transgenic (T) thale cress plants to test aphid reaction to EBF over several generations. Aphids were raised on wild type (W) thale cress and then allowed to feed for three generations on either W or T thale cress. Aphids were tested in each generation for their reaction when EBF was added to a choice chamber. The percentages of aphids attracted to or repelled by EBF and those which made no choice were recorded.
\n[Source: M De Vos, et al., (2010), PNAS, 107 (33), pages 14673–14678]
\nState the percentage of G2 neurons in the mice that respond to the fox scent.
\n. . . . . . . . . . . . . . . . . . . .%
\nUsing evidence from the chart, identify, giving a reason, which group of neurons responded most to the chemicals.
\nState the name of the instrument used to make these traces.
\nDistinguish between the effects of the mouse alarm compound and the effects of the control compound on the G1 neurons.
\nCompare and contrast the effects of the mouse alarm compound and stoat scent on the G1 neurons.
\nDeduce whether there is a correlation between the neural traces and the percentage of responding G1 neurons.
\nThe molecular structures of the mouse alarm compound and fox scent are very similar. Suggest an evolutionary advantage for this chemical similarity.
\nIdentify, with a reason, the aphids that were most strongly repelled by EBF.
\nOutline the conclusions that can be drawn from the data in the graph for the group of aphids fed for three generations on transgenic thale cress plants (T).
\nDiscuss whether natural selection would favour the transgenic EBF-producing thale cress plants if they were released into the wild.
\n10–25 «%» ✔
\nG1 always respond more than 25% «except control», while G2 and G3 always respond 25% or less ✔
\nG1 always responds more than G2 and G3/all of the others ✔
\nOWTTE
\noscilloscope ✔
\nmouse chemicals cause action potentials «in all six neurons» while control ones cause none «remain in resting potential»/mouse chemicals cause greater responses ✔
\nOWTTE
\na. both chemicals cause action potentials
OR
both chemicals respond in the majority of/five/most neurons ✔
b. stoat scent causes a higher action potential/longer/bigger response than mouse alarm compound «in each neuron»
OR
neuron 2 reacts strongly to the stoat scent but has a minimal/no response to the mouse alarm compound ✔
a. there is a positive relationship/correlation between the size of neural traces and the percentage of responding G1 neurons Accept vice versa
OR
the chemicals that cause stronger/higher neural traces also cause the greatest percentage of responding G1 neurons ✔
b. fox and stoat scents have «approximately» the same/similar neural traces and the same percentage of responding neurons/>75 % ✔
\nc. mouse alarm compounds cause smaller neural traces and smaller percentage of responding neurons/25–75 % ✔
\nd. control chemicals have no response in both cases
OR
no percentage of «G1 neurons» response and no action potential «in neural traces» ✔
the mice would have the same response to another mouse’s danger signal as to the actual presence of the predator/fox
OR
adaptation to fool predator by producing a scent similar to predator’s own scent
OR
allows a group response to a predator/fox/danger when just one mouse detects the danger ✔
Accept any other feasible answer.
\nthe aphids that were fed on wild-type thale cress/W as they had 80 % «or more» repelled each generation/ always had the higher % response
OR
the aphids that fed on wild-type thale cress plants/W of G3 as they had «about» 85 % repelled/had the higher % response ✔
Answer should refer to a percentage.
\na. «over the generations» fewer are repelled by EBF ✔
\nb. «over the generations» more are attracted to EBF ✔
\nc. by G3 a «slight» increase in no choice ✔
\nd. aphids respond less to EBF/alarm compound if they feed on plants that produce it/exposed to it constantly ✔
\ne. mutant aphids with attraction to transgenic plants can arise from aphids with no attraction or repulsion to transgenic plants
OR
aphids with no attraction or repulsion to transgenic plants may produce new type of aphids with attraction to transgenic plants ✔
a. mutant aphids/varieties may be indifferent to/attracted to transgenic plants as these do not present a hazard «not favour» ✔
b. initially/for limited time the plants would thrive as the aphids would be «largely»repelled and thus not eat the plants «so natural selection would favour them» ✔
c. over time/in a few generations, the aphids population become more resistant/more attracted/less repelled to EBF and return to feed on the plants so long-term benefit very limited «so natural selection would not favour them» ✔
\nd. the aphids resistant to EBF would not respond to other aphid alarms and «likely»be more readily eaten by predators «so the long-term benefit to plants could be supported by natural selection» ✔
\nThe answers must indicate whether natural selection would support or not for each statement.
\nNearly all were able to state the answer of 10-25%.
\nmany lost the mark by not stating the reason that the G1 responses were higher than the others (G2 and G3)
\nFewer than 1 in 10 were able to name the oscilloscope. There were some G2 comments about this, but the students should have seen traces like this and should know how they were obtained.
\nMany missed the mark in d as they did not state that the control remained at zero.
\nAnswers to e were encouraging as most were able to give both the similarity and the difference.
\nSimilarly, in 1f most could see the positive correlation, albeit not stating it, between the size of the trace and the percentage of responding G1 neurons.
\nIn g, better candidates were able to suggest an evolutionary advantage either from the point of view of one mouse being able to bring about a group response, or from the point of view that the fox could not distinguish between mice and other foxes.
\nThere was some discussion in the G2 forms about the amount of reading before points h, I and j. Good candidates could be seen to have read it carefully, underling the key words. Weaker candidates not so. In 1h a reason in terms of the highest percentage was looked for and was missed by weaker candidates.
\nThere was some discussion in the G2 forms about the amount of reading before points h, I and j. Good candidates could be seen to have read it carefully, underling the key words. Weaker candidates not so. In i. a large proportion gained both marks for stating that the percentage of attraction had increased and the percentage of repulsion had decreased.
\nThere was some discussion in the G2 forms about the amount of reading before points h, I and j. Good candidates could be seen to have read it carefully, underling the key words. Weaker candidates not so. Very few candidates scored both marks in j as they just wrote about one possibility. Perhaps the answer box should have been larger.
\n\n Explain the factors that can contribute to the exponential growth phase in a sigmoid population curve.\n
\n\n [3]\n
\n\n Outline the effect of carrying capacity on the growth of a population.\n
\n\n [2]\n
\n\n a. ideal environment/unlimited resources/below carrying capacity ✔\n
\n\n b. little disease/few predators ✔\n
\n\n c. high natality/birth rate\n \n \n AND\n \n \n immigration ✔\n \n Both needed.\n \n
\n\n d. natality and immigration\n \n greater\n \n than mortality and emigration ✔\n
\n\n a. carrying capacity is maximum population size/number of individuals that environment can support\n
\n \n \n OR\n \n \n
\n carrying capacity varies with abundance of limiting resources ✔\n
\n b. population growth slows/fluctuates as the carrying capacity of environment reached ✔\n
\n\n The vast majority of candidates could provide part of an explanation; most mentioned resources, but for many the effect of natality, mortality, immigration and emigration (both not well distinguished for some) was obscure.\n
\n\n Most candidates had an idea about carrying capacity, but there was only one element in the outline of many.\n
\nIdentify the stage of mitosis labelled X in the image, giving a reason.
\n[Source: Copyright 2002, The Trustees of Indiana University]
\nOutline the use of a karyogram during pregnancy.
\ntelophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔
OWTTE
\na. is a photograph/diagram of homologous pairs of chromosomes that can be analysed ✔
b. information may be used to determine other chromosome abnormalities/changes in chromosome numbers/possible birth defects ✔
c. Down syndrome/trisomy can be detected if there are three copies of a chromosome Not just “Down syndrome”.
OR
accept any other valid example ✔
d. other missing or extra pieces of chromosomes can be detected ✔
\ne. sex can be determined as the Y chromosome is shorter than the X ✔ Or correct ref to X and Y.
\nMost were able to correctly state (late) anaphase or telophase but did not give a reason as clearly asked. Many students seem to have excellent eyesight as they claimed to see that the chromosomes were unwinding and that the nuclear membranes had formed. Please encourage the students not to go beyond what can be seen.
\nMost knew what a karyogram was. However, the question was about use, which does not involve a detailed description, taking up half the space about how the sample was obtained and the risks involved. Most knew that it was to do with the chromosomes, few stated that it was a diagram or a picture of the homologous pairs. An explanation of how it is used to confirm sex was needed.
\nState two causes of the decrease of biomass along food chains in terrestrial ecosystems.
\nThe table shows the global carbon budget over two decades; the years 1990 to 1999 and 2000 to 2009.
\n[Source: © International Baccalaureate Organization 2019]
\n\n
Using the table, explain causes of the changes in carbon flux over the two decades.
\nSuggest how climate change can influence the natural selection of organisms that live in the Arctic oceans.
\na. «cell» respiration/loss of CO2/biomass consumed to provide/as a source of energy ✔
\nb. loss of energy «as heat» between trophic levels means less energy available for building biomass ✔
\nc. waste products «other than CO2»/loss of urea/feces/egesta ✔
\nd. material used/CO2 released by saprotrophs ✔
\ne. undigested/uneaten material «teeth, bones, etc»/detritus buried/not consumed
OR
formation of peat/fossils/limestone ✔
a. increased CO2 flux to the atmosphere due to increased burning of fossil fuels by industry/transportation / cement production ✔
\nb. «land use change leading to» decreased rate of forest burning
OR
better fire suppression leading to decrease in CO2 release
OR
example of land use changes that uses less fossil fuel
OR
increase in land covered by forests/plants / forests recovering from historical forestry
OR
any other reasonable explanation of land use change that would lead to decreased rate of carbon flow to atmosphere ✔
c. carbon storage in land decreased as less photosynthesis due to fewer forests/more construction
OR
release of methane due to «drying of» wetlands/sealing of land with concrete/buildings/roads ✔
d. carbon storage in ocean increased due to more photosynthesis/algae/greater concentration of CO2 in the atmosphere
OR
increased diffusion/rate of dissolving of CO2 into ocean from the atmosphere
OR
limestone/carbonate accumulation «more snails» ✔
a. individuals in a population will show a variation of adaptations to climate change ✔
b. organisms that resist temperature changes
OR
current changes of the ocean/melting ice/more acidity/changes in food chains will survive better ✔
c. reproduce more and pass on their characteristics ✔
\nd. organisms with less adaptation will disappear with time ✔
\ne. example «eg polar bears have less ice to be able to catch prey/seals and are starving the ones that manage to find other food sources will survive» OWTTE ✔
\nf. changes will occur within species
OR
new species may appear «over time» ✔
Accept any valid example of an Arctic ocean organism.
\nA large proportion of students looked at this question and immediately reeled off an answer in terms of energy (as in many previous years). Few made the connection between biomass and energy, for example in respiration CO2 is lost from the food chain during respiration, or that urea has mass etc.
\n3a(ii) was the question most discussed on the G2 comments. The expectation was that the students would address each of the four arrows, with an explanation for each. Good candidates did this. Others just combined everything into a general essay on climate change. Weaker candidates just restated the figures without any explanation of them at all. Whilst it is appreciated that none of the candidates was around between 1990 and 1999, references to the Industrial revolution were out by well over a century. This question would have benefitted from a larger answer box.
\nIn 3b weaker candidates seemed to think that they needed to give a detailed description of climate change and filled up most of the box with it. Better candidates were able to state that animals who could cope with warmer surroundings, would survive to breed and pass on their characteristics, while those who could not would perish, and give an example such as the polar bear. There were some G2 comments that Arctic foodwebs were not on the syllabus. It is doubted that any IB student does not know that it is cold in the Arctic, but because of climate change it is warming up slowly.
\nThe images show parts of plants belonging to two different phyla.
\nState the phylum of plant X and of plant Y.
\nX:
\nY:
\n\n
Explain one cause of mutation.
\n\n
Outline the types of evidence that can be used to place a species in a particular clade.
\nThe cladogram includes four marsupial (non-placental mammal) families.
\n[Source: Koala image: Quartl, https://commons.wikimedia.org/wiki/Phascolarctos_cinereus#/media/
File:Friendly_Female_Koala.JPG; Wombat image: JJ Harrison, https://en.wikipedia.org/wiki/Wombat#/
media/File:Vombatus_ursinus_-Maria_Island_National_Park.jpg; Marsupial lion: Nobu Tamura,
https://en.wikipedia.org/wiki/Marsupial_lion#/media/File:Thylacoleo_BW.jpg;
Diprotodontoidea image: Anne Musser]
\n
Deduce the family that is most closely related to the Diprotodontoidea.
\nX: Filicinophyta ✔
\nY: Coniferophyta/Conifera/Gymnosperms ✔
\nNot chromosomal.
\na. a clade is a group of organisms that have evolved from a common ancestor ✔
\nb. identify the base sequences of a gene ✔
\nc. identify amino acid sequence of a protein ✔
\nd. comparing homologous structures ✔
\ne. the fewer the differences, the closer they diverged in time from a common ancestor ✔ Accept vice versa.
\nVombatidae/wombats ✔
\nThose who had been taught it could identify the filicinophyte and the coniferophyte, albeit with a variety of spellings.
\nMany were confused by a relatively simple question, but a large proportion gained both marks by linking radiation or carcinogens to changes in the DNA base sequence.
\nThose who had been taught it gained at least two marks for evidence and clades, with comparing base sequences of genes and amino acid sequences of proteins amongst the most common correct answers.
\nNearly everyone spotted the wombat correctly.
\nOutline four types of membrane transport, including their use of energy.
\nDraw the structure of a dipeptide.
\nExplain the action of enzymes in digestion and the different roles of at least two named enzymes that are produced in the pancreas.
\na. simple diffusion is passive movement of molecules/ions along a concentration gradient ✔
\nb. facilitated diffusion is passive movement of molecules/ions along a concentration gradient through a protein channel «without use of energy» ✔
\nc. osmosis is the passage of water through a membrane from lower solute concentration to higher ✔ OWTTE
\nd. active transport is movement of molecules/ions against the concentration gradient«through membrane pumps» with the use of ATP/energy ✔ Active transport requires mention of the use of energy.
\ne. endocytosis is the infolding of membrane/formation of vesicles to bring molecules into cell with use of energy
OR
exocytosis is the infolding of membrane/formation of vesicles to release molecules from cell with use of energy ✔
f. chemiosmosis occurs when protons diffuse through ATP synthase «in membrane» to produce ATP ✔
\nThe description of each type of transport should include the name and brief description.
\nmpa, mpb and mpc require reference to concentration.
\n\n
a. two amino acids, one with NH2/NH3+end and one with COOH/COO– end ✔
\nb. peptide bond between C=0 and N—H correctly drawn ✔
\nc. «chiral» C with H and R group on each amino acid ✔
\nd. peptide bond labelled/clearly indicated between C terminal of one amino acid and N terminal of the second amino acid ✔
\nLabels not required for amino group and carboxyl group.
\na. enzymes catalyse/speed up chemical reactions/lower the energy needed ✔ OWTTE
\nb. have specific active sites to which specific substrates bind ✔
\nc. enzyme catalysis involves molecular motion and the collision of substrates with the active site ✔ OWTTE
\nd. enzymes break macromolecules into monomers/smaller molecules indigestion ✔
\ne. smaller molecules/monomers more readily absorbed ✔
\nf. <<pancreas>> secretes enzymes into the «lumen of» small intestine ✔
\ng. the small intestine has an alkaline pH ✔
\nh. enzymes have maximum action at specific pHs
OR
enzymes can be denatured at other pHs ✔
i. amylase breaks down starch into sugars/disaccharides ✔
\nj. lipase breaks lipids/triglycerides into monoglycerides/fatty acids and glycerol ✔
\nk. endopeptidase/protease breaks «peptide» bonds in proteins/polypeptides ✔
\nl. accept any other valid pancreatic enzyme, substrate and product ✔
\nAward [6 max] if there is no mention of two specific groups of enzymes.
\nMembrane transport – In spite of the question asking about energy, many lost easy marks by forgetting to refer to it. Others did not mention the gradients. In osmosis, please stress to the students which concentration they are talking about. They should be talking clearly about water potential or solute concentration.
\nStructure of the dipeptide – candidates who also do chemistry were at an advantage here. Diagrams went from the superb to something Picasso might have produced.
\nEnzymes in digestion. A disturbing number of candidates saw the word pancreas and wrote about insulin and glucagon. These are hormones, produced by ENdocrine glands not enzymes produced by EXocrine glands. Better candidates scored nearly full marks, by describing enzyme action and correctly describing the three main enzymes from the pancreas. Weaker candidates could not string the correct sequence together and scored very few marks. Better candidates wrote some sort of plan before starting, using the space below the instructions for Section B. This should be encouraged.
\nDescribe briefly the endosymbiotic theory.
\nOutline how photosynthesis produces glucose.
\nDiscuss the control of blood glucose levels and the consequences if they are not maintained.
\na. eukaryotes evolved from prokaryotes ✔
\nb. prokaryotes engulfed other prokaryotes without digesting them ✔
\nc. engulfed aerobic cell/prokaryote became mitochondria ✔
\nd. engulfed photosynthetic cell/ prokaryotes became chloroplasts ✔
\ne. these organelles have a double membrane «due to the engulfing process» ✔
\nf. mitochondria/chloroplasts contain DNA/small ribosomes/70S ribosomes ✔
\na. solar/light energy is converted to chemical energy ✔
\nb. energy needed to produce glucose ✔
\nc. only specific wavelengths are absorbed by chlorophyll
OR
red and blue absorbed most strongly.
OR
chlorophyll is the pigment that absorbs light energy ✔
d. H(+)/electrons from water are used to reduce compounds ✔
e. CO2 is absorbed/used/reduced to produce carbohydrates ✔
\nf. correct word/balanced symbol equation of photosynthesis ✔
\nAccept correct reference to NADPH/ATP from AHL.
\ncontrol: [6 max]
a. homeostasis is the maintenance of a constant internal environment ✔
b. the pancreas produces hormones that control the levels of glucose ✔
\nc. if glucose levels in blood are high, beta-cells «of the pancreas» produce insulin ✔
\nd. «insulin» causes the cells to take up /absorb glucose ✔
\ne. liver stores excess glucose as glycogen ✔
\nf. if glucose levels in blood are low, alpha-cells «of the pancreas» produce glucagon ✔
\ng. «glucagon» causes the liver to break down glycogen into glucose ✔
\nh. «glucagon» increase levels of glucose in the blood ✔
\ni. negative feedback controls the glucose levels ✔ OWTTE
\nconsequences:
\nj. if the pancreas produces little/no insulin a person can develop type I diabetes ✔
\nk. a person with type I diabetes «usually» needs/is dependent on injections of insulin ✔
\nl. type II diabetes occurs when the body becomes resistant to insulin/cells do not respond to insulin ✔
\nm. type II diabetes can «sometimes» be controlled by diet and exercise ✔
\nn. named consequence of having diabetes «eg: eye damage» ✔
\nAward [6 max] if no consequences are given.
Endosymbiotic theory. Those who had been taught it gained at least one mark. Few mentioned that the cell that became the mitochondrion was aerobic, and that the one that became the chloroplast was photosynthetic.
\nGlucose and photosynthesis. Again, this was an example of leaping into an answer without planning. Despite G2 protestations, HL material was not expected. Better candidates were able to gain full marks by stating that the light was trapped by chloroplasts, processes converted this into chemical energy and energy was needed to make glucose and included a correct equation ( if a symbol equation, it needed to be balanced).
\nControl of blood glucose levels. Weaker candidates tied themselves in knots trying to explain. Weak candidates also described insulin and glucagon as enzymes, not hormones. Other common mistakes were Alpha cells and Beta cells do not move, they just produce the hormones and glucagon and insulin do not have any direct effect on the glycogen molecules. The causes and treatment of type 2 diabetes were quite well known. A large number of students used the word ‘digest’ incorrectly, stating that glucose was ‘digested’ to glycogen.
\n\n Distinguish between the structures of DNA and RNA.\n
\n\n [3]\n
\n\n Mendel found the same pattern of inheritance in all the crosses that he performed. Outline, with examples, different types of inheritance that produce non-Mendelian ratios.\n
\n\n [4]\n
\n\n Explain the cause of sickle cell anemia and how this disease affects humans.\n
\n\n [8]\n
\n\n \n
\n \n A table format is not required but clear distinctions must be apparent.\n \n
\n\n \n The full names of the bases must be given.\n \n
\n\n a. some traits may involve many genes/be polygenic\n \n eg\n \n : height, skin colour «correct example required» ✔\n \n Accept any verifiable examples of these types of inheritance.\n \n
\n\n b. linked genes/alleles of different genes on same chromosome ✔\n
\n\n c. «small numbers of» recombinant phenotypes due to crossing over «between linked genes» ✔\n
\n\n d. co-dominance of specific alleles/intermediate forms\n \n eg\n \n : pink flowers «from red and white ones»/blood groups «correct example required» ✔\n
\n\n e. sex-linked effects\n \n eg\n \n : colour blindness «correct example required» ✔\n
\n\n f. environmental influence on inheritance/epigenetics/methylation ✔\n
\n\n g. any other example of non-Mendelien inheritance with a specific example ✔\n
\n\n
\n\n a. caused by a single nucleotide/base substitution mutation/GAG to GTG ✔\n
\n\n b. «mutation of» a gene of β-globin/a subunit of hemoglobin ✔\n
\n\n c. mRNA copies the mutation of DNA and substitutes an amino acid in hemoglobin «subunit» ✔\n
\n\n d. glutamic acid is substituted by valine ✔\n
\n\n e. sickle cell anemia involves distorted hemoglobin protein/HbS ✔\n
\n\n f. «distorted HbS causes» distortion/sickling/shape change of red blood cells ✔\n
\n\n g. «distorted/sickled red blood cells» block capillaries/blood flow ✔\n
\n\n h. HbS/sickled red blood cells cannot carry enough oxygen «for the body»/leads to fatigue ✔\n
\n\n i. low oxygen concentration seriously affects structure of HbS ✔\n
\n\n j. homozygous «HbS/HbS» state causes severe anemia/death at low oxygen concentrations ✔\n
\n\n k. heterozygous state has less anemia/minor effects/less effect of structure of hemoglobin\n
\n \n \n OR\n \n \n
\n heterozygous state only affected at high altitude/extreme exercise/low levels of oxygen ✔\n
\n l. «heterozygous state» provides protection against malaria parasite/selective advantage in malaria areas ✔\n
\n\n \n OWTTE\n \n
\n\n This question was well answered by most. Students are cautioned against representing bases with letter symbols such as A,C,T,G and U. The expectation of the mark scheme is that students write out the full names.\n
\n\n This question was less well answered. Students commonly indicated linked genes and sex-lined genes as examples of non-Mendelian inheritance. Accurate discussion of co-dominance was less frequent.\n
\n\n This question was well answered. The mutation and its impact was well understood by most. The genetic causes were less frequently discussed.\n
\n\n Outline energy flow through a food chain.\n
\n\n [4]\n
\n\n Draw a fully labelled graph of the action spectrum for photosynthesis.\n
\n\n [3]\n
\n\n a. energy from the sun/light energy is converted to chemical energy by photosynthesis ✔\n
\n\n b. «chemical» energy flows through the food chains by feeding ✔\n
\n\n c. energy is released «from carbon compounds» by respiration\n
\n \n \n OR\n \n \n
\n energy from respiration is used by living organisms and converted to heat ✔\n
\n d. heat is not recyclable / heat is lost from food chains\n
\n \n \n OR\n \n \n
\n heat cannot be converted to other forms of energy ✔\n
\n e. energy is lost in excretion/uneaten material/egestion/feces ✔\n
\n\n f. energy losses between trophic levels limits the length of food chains\n
\n \n \n OR\n \n \n
\n energy transfer is only 10 % between trophic levels ✔\n
\n a. axes correctly labelled «wavelength and\n \n rate\n \n of photosynthesis» ✔\n \n Accept\n \n \n rate\n \n \n of oxygen production for\n \n \n rate\n \n \n of photosynthesis.\n \n
\n\n b. 400 and 700\n \n nm\n \n as limits ✔\n
\n\n c. correct shape of curve involving two peaks at the correct places, broader in the blue-violet range not starting at zero and a narrower peak in the orange-red range with the trough in the green range that does not reach zero ✔\n
\n\n d. peaks of activity at 430 nm\n \n \n AND\n \n \n at 660 nm ✔\n
\n\n e. peaks indicated as «violet» blue light AND peak indicated as «orange» red light ✔\n
\n\n
\n\n Was well answered with most students being knowledgeable about ecology.\n
\n\n A number of students made errors in their sketches. Axes were commonly mis-labelled. The colors were commonly presented in the reverse order with red at the left end and blue at the right end. Showing red as a higher peak was another common error. The overall shape was often correctly drawn.\n
\nAn experiment was carried out on osmosis in carrot (Daucus carota) root tissue and potato (Solanum tuberosum) tuber tissue. Similar sized pieces of tissue were cut and soaked in different sucrose solutions for 24 hours. The results are shown in the graph below.
\nUsing the graph, estimate isotonic sucrose solutions for potato tissue and carrot tissue.
\nPotato:
\nCarrot:
\nSuggest a reason for the difference in the isotonic points for the potato and the carrot tissues.
\nFrom the evidence provided by the graph, evaluate the reliability of these data.
\nExplain one reason for calculating the percentage change in mass.
\npotato: 0.26 mol dm–3 ✔ Allow a range of 0.22–0.32
carrot: 0.50 mol dm–3 ✔ Allow a range of 0.45–0.55 (Allow 1 max if no units or either unit is omitted)
different dissolved solutes/sugars/sucrose/salts/molarities ✔
OR
may have been grown in different soils giving their tissues different contents ✔
OR
may have been stored under different conditions ✔
OR
may be more dehydrated / different water content ✔
OR
different types of tissue / different age ✔
Do not accept starch.
\na. the data show clear trends
OR
a trend line could be drawn through these data
OR
there are no outliers ✔
b. the error bars/standard deviations shown
OR
the error bars/standard deviations vary with concentration ✔
c. sample size is unknown ✔
\na. the change in mass indicates whether the tissue has gained/lost water ✔
\nb. the pieces of tissue will not be the same mass «at the beginning of the experiment» ✔
\nc. to compare the relative changes in mass ✔
\nThe enzyme beta-galactosidase hydrolyses lactose to release glucose and fructose. A study was carried out to determine how acidity affects the activity of a beta-galactosidase enzyme, extracted from the fungus Penicillium simplicissimum.
\n[Source: Cruz R, et al. Properties of a new fungal β-galactosidase with potential application in the dairy industry.
Revista de Microbiologia 30: 265–271, 1999]
State another independent variable that would affect the activity of this enzyme.
\nOutline the measurements which would need to be taken to determine the activity of the beta-galactosidase at different pH values.
\ntemperature
OR
lactose/substrate concentration
OR
Inhibitors / cofactors ✔
Do not accept pH or acidity.
\na. test samples for the concentration/amount of products/glucose/fructose/substrate/lactose ✔ Accept use Benedict/Fehling to see change in amount of reducing sugar
\nb. take samples of the reaction mixture at regular/timed intervals ✔ Must indicate that samples are taken at timed intervals
\nc. repeated measurements «at the different pH values» ✔
\nd. measure independent/controlled/standardised/variables / temperature ✔
\n\n Estimate the solute concentration of the zucchini cells.\n
\n\n [1]\n
\n\n If a zucchini is allowed to dry in the open air, predict how the osmolarity of the zucchini cells would change.\n
\n\n [1]\n
\n\n Explain\n \n one\n \n reason for calculating the percentage changes in mass.\n
\n\n [2]\n
\n\n Predict what would happen to a red blood cell placed in distilled water.\n
\n\n [1]\n
\n\n 0.36 mol dm\n \n -3\n \n /M ✔\n
\n\n \n Units required\n \n
\n \n Allow a range of 0.35–0.37 mol dm\n \n -3\n \n /M\n \n .\n
\n osmolarity will increase «because the cells become dehydrated»\n
\n \n \n OR\n \n \n
\n the cells become hypertonic ✔\n
\n \n Accept water potential of the tissue decreases.\n \n
\n \n Do not accept \"change\" instead of \"increase\".\n \n
\n a. the change in mass indicates whether the tissue has gained/lost water ✔\n
\n\n b. the pieces of tissue will not all be the same mass «at the beginning of the experiment» ✔\n
\n\n c. to compare the relative changes in mass ✔\n
\n\n a. water would move into the red blood cells ✔\n
\n\n b. it would lyse\n
\n \n \n OR\n \n \n
\n swell\n
\n \n \n OR\n \n \n
\n burst ✔\n
The black-legged tick (Ixodes scapularis) is an arthropod which sucks blood from humans and other mammals. It is encountered mainly in wooded and semi-wooded areas. Some ticks can be infected by the bacterium Borrelia burgdorferi. When a tick bites a human, the bacterium is often introduced, causing Lyme disease. Lyme disease is a public health problem in North America and, if left untreated, can cause important neurological impairment. The diagram represents the two-year life cycle of a tick.
\n[Source: Cary Institute of Ecosystem Studies / Leslie Tumblety]
\nScientists fear that global warming will change the distribution range of ticks.
\nThe graphs show the developmental stages of ticks throughout seasons in a densely human-populated area of south-eastern Canada, surrounded by woods (circled on the map). Values are already established for 2000 and are predicted for 2080.
\n[Source: reprinted from International Journal for Parasitology, 36(1), N.H. Ogden, A. Maarouf, I.K. Barker, M. Bigras-Poulin,
L.R. Lindsay, M.G. Morshed, C.J. O’Callaghan, F. Ramay, D. Waltner-Toews, D.F. Charron, Climate change
and the potential for range expansion of the Lyme disease vector Ixodes scapularis in Canada,
63–70, Copyright (2006), with permission from Elsevier]
White-footed mice (Peromyscus leucopus) in eastern North America’s wooded areas often host B. burgdorferi bacteria. To determine whether bacterial transmission from mice to tick nymphs could be prevented, mice were vaccinated with antigens from Lyme disease-causing B. burgdorferi. Scientists captured wild mice at two different sites in the woods once a month, over 4 months. Each time, they measured the levels of antibodies to B. burgdorferi present in the captured and re-captured mice, inoculated all of them, and released them into the woods. The control group was not vaccinated with B. burgdorferi antigen.
\n[Source: Copyright (2004) National Academy of Sciences, U.S.A. An ecological approach to preventing human infection:
Vaccinating wild mouse reservoirs intervenes in the Lyme disease cycle, Jean I. Tsao, J. Timothy Wootton, Jonas Bunikis,
Maria Gabriela Luna, Durland Fish, Alan G. Barbour, Proceedings of the National Academy of Sciences
Dec 2004, 101 (52) 18159–18164; DOI: 10.1073/pnas.0405763102]
The summer after vaccination, the prevalence of B. burgdorferi infection in tick nymphs collected on mice from the two sites was measured.
\n[Source: Copyright (2004) National Academy of Sciences, U.S.A. An ecological approach to preventing human infection:
Vaccinating wild mouse reservoirs intervenes in the Lyme disease cycle, Jean I. Tsao, J. Timothy Wootton, Jonas Bunikis,
Maria Gabriela Luna, Durland Fish, Alan G. Barbour, Proceedings of the National Academy of Sciences
Dec 2004, 101 (52) 18159–18164; DOI: 10.1073/pnas.0405763102]
State the domain into which ticks are classified.
\nUsing information from the text, identify one possible simple treatment for Lyme disease.
\nIdentify the month when small birds had the greatest chance of being infected by B. burgdorferi bacteria in the year 2000 and the month when they would be most likely to become infected according to the 2080 predictions.
\n2000:
\n2080:
\nUsing the life cycle diagram and the graph for the year 2000, analyse the distribution of adult ticks throughout the different seasons.
\nEvaluate the effect of the change in distribution of the different life stages of ticks on the spread of Lyme disease in south-eastern Canada.
\nState the reason for performing the experiment in the months of May to August.
\nSuggest possible reasons for the observed pattern of presence of antibodies in vaccinated mice.
\nAnalyse the data on the state of infection of tick nymphs with B. burgdorferi in control and vaccinated mice.
\nUsing all the data, discuss whether inoculating mice with the antigen to B. burgdorferi could be an effective method of controlling the spread of Lyme disease.
\neukaryote ✔
\nAccept eukaryotes.
\nantibiotics / named antibiotics ✔
\n«2000» August AND «2080» July ✔
\nBoth required.
\na. adults present through autumn and winter «according to the life cycle diagram»
OR
some adults «must» survive winter «despite graph suggesting zero» ✔
b. adults peak in October «& November»/in autumn/between September and December ✔
\nc. adults die after laying eggs in winter/beginning of spring ✔
\nd. smaller peak/10 % versus 55 % peak/smaller numbers of adults in April/spring ✔
\ne. adults absent from June to September/summer ✔
\nEach mark point, requires month or season.
Jan - Mar = winter
Apr - Jun = spring
Jul - Sep = summer
Oct - Dec = autumn = fall
Do not accept that there are the lowest number or no adults in winter.
\na. nymphs present through most of year/longer period/from March to November/through spring and summer «so more risk of infection» ✔
\nb. more adults in winter/in January/February so more risk of infection then ✔
\nc. infection will be possible through more/most months of/throughout the year ✔
\nd. Lyme disease likely to/will increase ✔
\nbecause nymphs are present/numbers of nymphs rise «in these months»
OR
build up immunity/antibodies in mice before nymphs «peak» ✔
Ignore references to larvae.
\na. low antibody level initially as mice not previously exposed to antigen/bacteria ✔
\nb. vaccination causes antibody production/development of immunity ✔
\nc. increased proportion of mice have been vaccinated in each successive month ✔
\nd. second vaccination/booster shot increases antibody level/speeds up antibody production ✔
\ne. memory cells produced so greater/faster antibody production ✔
\nf. many/rising numbers of nymphs which may spread the bacteria/antigens to mice ✔
\nIgnore any references to non-vaccinated/control mice – this means that no marks are awarded for them because the question is about vaccinated mice, but there is no penalty for including this information in an answer.
\na. at Site 1 there is little/no significant difference in the proportion of infected nymphs/numbers of infected and uninfected nymphs collected from both control and vaccinated mice ✔
\nb. at Site 2 the proportion of infected nymphs is lower in those collected from vaccinated than control mice
OR
at Site 2 «significantly» more nymphs are not infected from vaccinated than control mice ✔ For mpb and mpd, accept converse answers that give the proportions/percentages of uninfected nymphs rather than infected.
c. at both sites there are fewer infected than uninfected nymphs in those collected from both vaccinated and control mice ✔
\nd. proportion of infected nymphs is lower at Site 1 than Site 2 in nymphs collected from both control and vaccinated mice
OR
22 % of control mice and 23 % of vaccinated mice with infected nymphs at Site 1 AND 39 % of control mice and 29 % of vaccinated mice with infected nymphs at Site 2 ✔ Percentages are required for the second alternative of mpd.
Accept “ticks” instead of “tick nymphs” or “nymphs”
\nDo not accept quoting of untransformed numerical data.
\na. Site 2 suggests that vaccination could reduce «nymph» infection rate «so method might be effective» ✔
\nb. Site 1 suggests that vaccination does not reduce «nymph» infection rate «so method probably not effective» ✔
\nc. effective «to some extent» as vaccination increases antibodies/immunity in mice ✔
\nd. high antibody levels needed/ many mice need to be vaccinated «for the method to be effective» ✔
\ne. some nymphs are still infected / «absolute» numbers «rather than proportions» of infected nymphs are similar in those collected from control and vaccinated mice ✔
\nf. there are other hosts/mammals/birds ✔
\ng. difficult/expensive «to vaccinate many small mammals/mice»
OR
cheaper to use protective clothing/tick repellant/avoid wooded areas/other method ✔
The diagram shows one of Thomas Hunt Morgan’s crosses of Drosophila in the early 20th century.
\nState the type of inheritance shown.
\nIdentify the recombinants.
\nThe chi-squared value was calculated as shown. Deduce, with reasons, whether the observed ratio differed significantly from the expected Mendelian ratio.
\n«gene/autosomal» linkage ✔
\nReject sex linkage
\ngrey vestigial and black normal ✔
\nAccept Ggvv and ggVv or alternative acceptable upper/lower case genotypes.
\na. yes/observed ratio did differ significantly «from the expected Mendelian ratio»
OR
expected ratio is 1:1:1:1 / 575 of each type / 25 % of each type ✔ Correct ratio not needed in first alternative of mpa
b. 3 degrees of freedom ✔
\nc. critical value is 7.815 «at the 5 % level / 11.345 «at the 1 % level» ✔ Accept mpc if candidates indicate the critical value of chi squared by circling it.
\nd. chi-squared value «of 1002.6» exceeds the critical value ✔
\nAllow other levels of significance as long as the critical value is correctly stated for the chosen level.
\nDescribe what is shown in a cladogram.
\nOutline how variation in organisms of the same species could lead to natural selection.
\na. similarities/differences between organisms/species/clades ✔ mpa and mpd concern actual characteristics, not evolutionary relationships.
\nb. «probable» evolutionary relationships/closeness/common ancestry/phylogeny ✔ mpb concerns such relationships
\nc. divergence/splits/speciation/branches/nodes ✔ mpc concerns the structure branching of the cladogram
\nd. relative similarity/differences between base sequence/amino acid sequence ✔
\na. survival of the better adapted/fittest ✔
\nb. more reproduction of better adapted/fittest/individuals with favorable variations ✔
\nc. genes for favorable variations/adaptations passed on to offspring ✔ Accept answers in the converse.
\nd. competition for resources/more offspring produced than the environment can support/a struggle for existence ✔ Accept answers in the converse.
\nDraw the structure of a section of DNA showing all possible bases.
\nOutline the structural and genetic characteristics of eukaryotic chromosomes.
\nExplain how a polypeptide chain is synthesized in a eukaryotic cell.
\na. sugar, phosphate and base linked correctly to form at least one nucleotide ✔ For mpa, ignore labelling of the subunits of the nucleotide. Carbon atoms in deoxyribose do not have to be numbered but the phosphate should be linked to C5 and the base to C1. Shapes other than circles and rectangles could be used for the phosphate and base.
\nb. deoxyribose, phosphate and base/named base labelled at least once ✔
\nc. adenine paired with thymine and cytosine paired with guanine ✔ For mpc, full names of all four bases are required, but not relative sizes of the purine and pyrimidine bases.
\nd. two antiparallel chains of nucleotides linked by hydrogen bonds with all sugar-phosphate bonds in correct position ✔ For mpd, a bond should connect the C3 of deoxyribose on one nucleotide to the phosphate on the adjacent nucleotide. Two nucleotides in each strand is sufficient.
\na. linear/not circular DNA molecule
OR
one chromosome is one molecule of DNA/one chromosome is two DNA molecules «after replication» ✔
b. associated with histone proteins/nucleosomes ✔
\nc. centromere joins sister chromatids «after DNA replication» ✔
\nd. telomeres at the end «of the chromosome/chromatid» ✔
\ne. carries a sequence of genes / each gene occupies a specific locus ✔ Do not accept ‘sequence of bases’ for mpe.
\nf. alternative alleles of genes / homologous chromosomes carry same sequence of genes ✔
\ng. chromosomes in pairs / two «homologous chromosomes» of each type «in a diploid cell» ✔
\nh. non-coding sequences/example of a non-coding sequence ✔ Do not allow mph if the response states that chromosomes are always condensed.
\ni. supercoiled/condensed «during mitosis/meiosis» ✔
\na. translation occurs on ribosomes ✔
\nb. tRNA-activating enzymes attach amino acids to tRNAs ✔
\nc. small and large ribosome units assemble on mRNA
OR
translation/polypeptide synthesis starts at a start codon ✔
d. each tRNA arriving at the ribosome binds to the A site ✔
\ne. anticodon «on tRNA» binds to codon «on mRNA» ✔
\nf. according to complementary base pairing/A with U and G with C ✔
\ng. ribosome moves along the mRNA / mRNA moves over ribosome ✔
\nh. t-RNA shifts from the A site to P site/from the P to the E site ✔
\ni. peptide bond between amino acids «on tRNAs at A and P sites» ✔
\nj. tRNA released from ribosome at E site ✔
\nk. cycle repeats with other tRNAs / polypeptide grows as tRNAs bring more amino acids ✔
\nl. until stop codon on mRNA is reached ✔
\nm. components are disassembled / polypeptide leaves the ribosome ✔
\nAccept these points in an annotated diagram.
Do not award any marks for events in transcription.
Outline the reasons for the differences in blood concentrations between the renal artery and the renal vein.
\nOutline how two parents could have a child with any of the four ABO blood groups.
\nExplain the control mechanism of the heart rate.
\na. urea/waste products lower in vein due to excretion «in urine»/ultrafiltration but not reabsorption ✔
\nb. oxygen lower in vein due to use in cell respiration/in kidney tissue ✔
\nc. carbon dioxide higher in vein due to production by cell respiration/excretion by kidney cells ✔
\nd. glucose lower in vein due to use in cell respiration «by kidney tissue» ✔
\ne. sodium/chloride/ion concentrations changed due to production of hypertonic/hypotonic urine
OR
sodium/chloride/ion concentrations lower due to removal of excess ✔
f. ion/solute concentrations lower in vein than artery if ADH has been secreted
OR
ion/solute concentrations in vein vary depending on amount of water reabsorbed in the collecting duct ✔
g. drug/toxin concentrations lower in vein due to excretion in urine ✔
\nAccept any point given as the converse.
\nEach mark point includes a difference and reason for it.
\na. IA, i for one set of gametes/parental genotype ✔
\nb. IB, i for the other set of gametes/parental genotype ✔
\nc. «genotypes of offspring are» IAIB, IAi, IBi, ii ✔
\nd. «phenotypes of offspring are» AB, A, B, O ✔
\nAnswers can be given in a Punnett grid or in prose.
\nAccept the four possible blood groups of the offspring anywhere in the answer.
\na. sinoatrial node/SAN is a specialized group of muscle cells
OR
sinoatrial node/SAN is located in the right atrium ✔
b. acts as a pacemaker/controls the heart rate
OR
initiates/generates the heart beat/starts the cardiac cycle ✔
c. sends out electrical signal/impulses/depolarisations ✔
\nd. electrical signal stimulates contraction «of heart muscle» ✔
\ne. signal passes through walls of atria/passes to AV node ✔
\nf. then through walls of the ventricles ✔
\ng. medulla «oblongata of brain» can change/increase/decrease the rate ✔
\nh. through nerves/named example of nerve/autonomic/sympathetic/ parasympathetic nervous system ✔ In mph, only accept vagus nerve for slowing heart rate and sympathetic nerve for accelerating it.
\ni. one nerve increases the rate and the other decreases it ✔
\nj. epinephrine/adrenaline increases heart rate/force of contraction ✔
\nk. epinephrine/adrenaline prepares the body for vigorous activity/is part of fight or flight response ✔
\nCultivated rice, Oryza sativa, is one of the most important human foods. The two main sub-species of cultivated rice are O. sativa indica and O. sativa japonica. O. rufipogon is a wild species from which they probably evolved.
\nTo investigate whether the sub-species of cultivated rice evolved independently from the wild species, scientists analysed their chromosomes to find areas with similar DNA base sequences. Wild species tend to have many different alleles of genes present on each chromosome, but during the development of a crop plant by artificial selection from a wild species, this diversity drops considerably, increasing the base sequence similarity.
The number of different alleles of the genes on a chromosome can be represented by a diversity index. The following graph of part of chromosome 7 shows the diversity index of O. rufipogon and two varieties of cultivated rice, O. sativa indica and O. sativa japonica. The gene PROG1 allows the plant to stand upright, which is typical of cultivated rice. Its position is indicated by the vertical arrow on the graph.
[Source: He Z, Zhai W, Wen H, Tang T, Wang Y, Lu X, et al. (2011) Two Evolutionary Histories in the Genome of Rice:
the Roles of Domestication Genes. PLoS Genet 7(6): e1002100. https://doi.org/10.1371/journal.pgen.1002100]
In another experiment, scientists retrieved genome sequences of the wild rice O. rufipogon taken from a wide range of geographical sites (I, II and III) and those of the two sub-species of O. sativa from gene banks.
\nThe pie charts, presented along with a cladogram, show the proportion of alleles for three genes which confer specific characteristics to O. sativa. Mutations can produce derived alleles that are different from the original ancestral alleles. The control group represents wild rice species other than O. rufipogon.
\n[Source: reprinted by permission from Springer Nature: Nature, Huang, X., Kurata, N., Wei, X. et al. A map of rice
genome variation reveals the origin of cultivated rice. Nature 490, 497–501 (2012) doi:10.1038/nature11532]
State the reason that O. sativa and O. rufipogon are classified as different species.
\nDetermine which type of rice has the lowest overall diversity index.
\nCompare and contrast the trends for O. rufipogon and O. sativa indica.
\nState the proportion of the ancestral allele for the gene GS3 in the O. rufipogon-III population.
\nDistinguish between the proportion of ancestral and derived alleles for all three genes in O. sativa indica and O. sativa japonica.
\nState one reason for having a control group.
\nUsing all of the data, discuss whether there is evidence that the two sub-species of O. sativa might have evolved independently from O. rufipogon.
\ncannot interbreed to produce fertile offspring ✔
\nAccept converse or a good explanation.
\n«O. sativa» japonica ✔
\nSimilarities
both show diversity
OR
similar pattern/peaks and troughs in the first part of the chromosome / up to «approximately» 1.5 megabases
OR
similar diversity between 2.4 to 2.7 mb
OR
both highest at 0.7 mb ✔
Differences
there are «two» major drops in diversity for O. sativa indica whereas none for O. rufipogon/much wider fluctuations in O. s. indica
OR
O.s. indica much lower at PROG1
OR
O.rufipogon does not drop < 2.5 whereas O. s. indica approaches 0
OR
O.rufipogon generally higher than O. s. indica after 1.4-1.5 ✔
One answer from mpa and one from mpb required for 2 [max].
\nAccept a statement of where the drops occur.
\n/0.75/75% ✔
\nDo not accept 0.75% or 75 or ratios
\na. O.s. indica has more of the ancestral allele «for all three genes» ✔ Accept converse.
\nb. lower/higher values for ancestral/derived are not for the same genes ✔ Allow specific gene examples.
\nc. for O. s. indica the highest proportion is for DPL2 ancestral, but for O. s. japonica is GS3 derived allele ✔ Allow converse for smallest derived.
\nany reference to comparison ✔
\na. large difference in diversity index between O. s. indica and O. s. japonica «suggests independent evolution» ✔
\nb. «some of the» peaks/troughs for O. s. indica and O. s. japonica in different positions «suggests independent evolution» ✔
\nc. O.s. indica has a similar diversity index to O. rufipogon «which suggests closer relationship/recent divergence» ✔
Allow converse for japonica
d. O.s. japonica has very different proportions of ancestral and derived alleles compared to O. s. indica ✔
\ne. O.s. indica has similar large number of ancestral alleles to O. rufipogon I/II ✔ Allow converse for derived
\nf. O.s. japonica has a large number of derived alleles similar to O. rufipogon III
OR
«but» the number of derived alleles is greater in O. s. japonica than in O.rufipogon III ✔ Allow converse for ancestral
g. O.s. indica and O. s. japonica are in different clades ✔
OR
O.s. indica and O. rufipogon I are in the same clade ✔
h. evidence from one chromosome/3 genes/2 studies is not sufficient to form a conclusion ✔
\n\n Outline how these results indicate that blackberry distribution is limited by light intensity.\n
\n\n [2]\n
\n\n A pyramid of energy represents the amount of energy taken in by each trophic level per unit time and per unit area. Discuss the advantages and disadvantages of the use of pyramids of energy as models of energy flow in an ecosystem.\n
\n\n [4]\n
\n\n a. as light level changes so does the percentage cover «of blackberry» ✔\n
\n\n b. distribution is higher at intermediate/moderate light intensities ✔\n \n Accept appropriate numerical values\n \n
\n\n c. little growth at low light intensities\n
\n \n OR\n \n
\n little growth at high light intensities ✔\n \n Accept appropriate numerical values\n \n
\n d. distribution is bell shaped ✔\n
\n\n \n Advantages\n \n :\n
\n a. can show the energy loss/transfer between trophic levels ✔\n
\n b. permits a ‹quantitative› comparison between ecosystems ✔\n
\n\n c. shows change over time ✔\n
\n\n \n Disadvantages\n \n :\n
\n d. are difficult to produce accurately ✔\n
\n e. require destructive methods to obtain the data ✔\n
\n\n f. do not show all the interactions/food chains/feeding relationships «between different members of the community» ✔\n
\n\n g. cannot represent organisms that feed at different trophic levels ✔\n
\n\n \n Award\n \n [3 max]\n \n if only one side of the argument is given\n \n .\n
\n\n Explain why some biologists think protecting keystone species would help preserve biological diversity in an ecosystem.\n
\n\n [1]\n
\n\n Compare and contrast fundamental and realized niche of a species.\n
\n\n [2]\n
\n\n a large effect on the ecosystem/community structure/environment«relative to abundance»\n
\n \n \n OR\n \n \n
\n influence the balance of other populations in the ecosystem\n
\n \n \n OR\n \n \n
\n other species in the habitat would also disappear\n
\n \n \n OR\n \n \n
\n many other species dependent on them for survival ✔\n
\n a. both describe the habitat/role/relationship occupied by a species ✔\n
\n\n b. the fundamental niche is the potential role of a species in its ecosystem and realized niche is the actual role\n
\n \n \n OR\n \n \n
\n the fundamental niche depends on the adaptations of a species whereas the realized niche is limited by competition/predation\n
\n \n \n OR\n \n \n
\n realized niche is ‹usually› smaller than fundamental niche ✔\n
\n Models are used as representations of the real world. Evaluate the use of food webs to represent ecological communities.\n
\n\n [2]\n
\n\n Explain why some biologists think protecting keystone species would help preserve biological diversity in an ecosystem.\n
\n\n [1]\n
\n\n \n Advantages\n \n :\n
\n a. food webs summarize all possible food chains «in the community» ✔\n \n OWTTE\n \n
\n b. realistic representation ✔\n
\n\n \n Limitations\n \n :\n
\n c. some communities/ecosystems are too complex to represent ✔\n
\n d. only shows qualitative information/not quantitative data\n
\n \n \n OR\n \n \n
\n saprotrophs/abiotic factors not taken into account ✔\n
\n \n Needs one advantage and one limitation to obtain\n \n [2]\n \n .\n \n
\n \n Do not accept mentions of trophic levels\n \n .\n
\n a. «research demonstrates that keystone species» have a «disproportionately» large effect on their environment relative to abundance\n
\n \n \n OR\n \n \n
\n «keystone species» maintain a balance in the «stable» numbers of each species within a community\n
\n \n \n OR\n \n \n
\n «keystone species» have a large impact in preserving and stabilizing the biodiversity by preventing overpopulation/monopoly/out competition ✔\n
\n b. a keystone species’ disappearance would start a domino effect/other species in the habitat would also disappear\n
\n \n \n OR\n \n \n
\n their loss leads to an imbalance in the food web\n
\n \n \n OR\n \n \n
\n their loss leads to a decrease in biodiversity ✔\n
\n \n Do not accept protecting keystone species prevents disappearance of other species since it repeats the question\n \n .\n
\n\n Compare and contrast the information provided for baleen whales and sea turtles.\n
\n\n [2]\n
\n\n Outline how plastic ingestion may lead to biomagnification in these marine species.\n
\n\n [1]\n
\n\n State an example of an alien species.\n
\n\n [1]\n
\n\n Outline the impact of alien species on endemic species in ecosystems.\n
\n\n [2]\n
\n\n a. both show cases of entanglement/ingestion ✔\n
\n\n b. sea turtles similar number of species affected by entanglement/ ingestion while baleen whales more species affected by entanglement than ingestion\n
\n \n \n OR\n \n \n
\n higher percentage of sea turtles species affected by entanglement/ ingestion than baleen whales\n
\n \n \n OR\n \n \n
\n higher number of species of «baleen» whales than species of sea turtles ✔\n
\n \n Do not accept answers quoting numerical values only\n \n .\n
\n\n a. plastic broken down into microplastics to be able to enter the food chain ✔\n
\n\n b. plastic/microplastics become more concentrated at each trophic level ✔\n
\n\n \n Accept descriptions of biomagnification involving any of the species in the table\n \n .\n
\n\n name of species and where it is invasive\n \n e.g.\n \n introduction of cane toads in Australia ✔\n
\n\n \n Accept common name of species but not general groups e.g. toads\n \n .\n
\n\n a. ‹interspecific› competition with endemic/native species ✔\n \n May be outlined using a specific example\n \n
\n\n b. may cause the extinction/reduction of endemic/native species\n
\n \n \n OR\n \n \n
\n reduces biodiversity\n
\n \n \n OR\n \n \n
\n becomes invasive ✔\n
\n c. does not have endemic / natural predators so may increase in numbers ✔\n
\n\n d. upsets the balance between predators and prey\n
\n \n \n OR\n \n \n
\n disrupts food chains ✔\n
A short base sequence of mRNA and a table of the genetic code are shown below.
\nSequence of mRNA
\nTable of the genetic code
\nThe diagram shows a section of a polypeptide.
\nOutline the function of codons.
\nDetermine the sequence of amino acids that could be translated from the sequence of mRNA.
\nDetermine the DNA base sequence transcribed to form this sequence of mRNA.
\nSuggest a hypothesis that accounts for the slightly different meaning of some codons in a very limited number of organisms.
\nAnnotate the diagram to show a peptide bond between two amino acids.
\nState the type of reaction that removes water while linking amino acids together to form polypeptides.
\nOutline the function of Rubisco and of spider silk in relation to their three-dimensional conformation.
\n«three bases on mRNA» coding for one amino acid «in a polypeptide» ✔
\nmet-ser-arg-arg
OR
start-ser-arg-arg
OR
met-ser-arg-arg-stop
OR
start-ser-arg-arg-stop ✔
Do not accept peptides containing an amino acid/leu for the last codon.
\nTAC TCG GCT TCC ATC GAC ✔
\nthey occurred after the common origin of life OWTTE
OR
the genetic code is not «in fact» universal ✔
Look for alternatives.
any annotation between a C=O and the next NH ✔
\ne.g.
\ncondensation ✔
\nDo not accept anabolism alone.
\nAward [1] per correct row or correct column.
\nOutline the cell theory.
\nState two functions of life.
\nList three characteristics of eukaryotic homologous chromosomes.
\nUsing the following table, compare and contrast anaerobic cell respiration in yeasts and in humans. The first row has been completed as an example.
\na. cells can only arise from preexisting cells ✔
\nb. living organisms are composed of cells/smallest unit of life ✔
\nc. organisms consisting of only one cell carry out all functions of life in that cell/cells perform life functions «at some point in their existence» ✔
\nd. although most organisms conform to cell theory, there are exceptions ✔
\na. nutrition ✔
\nb. metabolism/respiration ✔
\nc. growth ✔
\nd. response/irritability ✔
\ne. excretion ✔
\nf. homeostasis ✔
\ng. reproduction ✔
\nDo not allow “feeding”, plants do not “feed”.
\nMark the first two answers only.
\na. linear DNA molecules
OR
DNA associated with histone «proteins» ✔
b. carry the same sequence of genes ✔
\nc. «but» not necessarily the same alleles «of those genes» ✔
\nd. both are present when nucleus is in diploid state ✔
OR
occur in pairs ✔
e. have same size/length/banding patterns ✔
\nf. centromeres are in the same position ✔
\nAward [1] per correct row.
\nState the immediate consequence of a species producing more offspring than the environment can support.
\nExplain the consequence of overpopulation on the survival and reproduction of better adapted individuals within a population.
\ncompetition/lack of resources/death/exceeding carrying capacity ✔
\nAllow a description of it.
Do not allow “overpopulation” or “natural selection”.
a. «better adapted» tend to survive more ✔
\nb. «better adapted» reproduce/produce more offspring ✔
\nc. pass on characteristics to their offspring «when they reproduce» ✔
\nd. their frequency increases «within the population» due to natural selection ✔
\ne. leading to evolution ✔
\nDraw a section of the Singer-Nicolson model of an animal cell membrane.
\nOutline the principles used by scientists to classify organisms.
\nExplain the movement of energy and inorganic nutrients in an ecosystem.
\na. bilayer of phospholipids with both “tails” towards the inside «of the bilayer» ✔ This can be taken unlabeled from diagram.
\nb. hydrophilic/polar and hydrophobic/non-polar annotation ✔
\nc. cholesterol between phospholipid tails ✔
\nd. glycoprotein ✔
\ne. integral proteins/channel proteins ✔
\nf. peripheral proteins ✔ Allow this if it does not extend across the membrane
\nElements should be clearly drawn, correctly positioned and annotated.
\na. use of the binomial system ✔
\nb. agreed/developed by scientists / OWTTE ✔
\nc. hierarchy of taxa used ✔ Names of the seven taxa not required.
\nd. three domains used/three domain names ✔ OWTTE
\ne. genome/DNA sequence similarities
OR
amino acid sequence of specific proteins ✔
f. species from a common ancestor are grouped together
OR
included in the same clade/branch in cladogram ✔
g. use evidence of evolutionary origin ✔ Allow example e.g. fossil record comparison
\nh. shared characteristics within a group
OR
similar embryonic development ✔
a. autotrophs/producers/plants obtain inorganic nutrients from the «abiotic» environment ✔
\nb. energy provided «mainly» by sunlight ✔
\nc. light energy converted «to chemical energy» through photosynthesis ✔
\nd. photosynthesis/producers/autotrophs convert inorganic carbon/carbon dioxide and water into carbon/organic compounds ✔
\ne. «these» carbon compounds/foods contain/are a source of «useable» energy «for life» ✔
\nf. carbon compounds/energy are transferred along food chains when eaten by consumers/heterotrophs ✔ Allow OWTTE for mpf for passed up trophic levels.
\ng. respiration returns carbon «dioxide» to the environment ✔
\nh. respiration releases stored/chemical energy as ATP/heat ✔
\ni. energy/ATP is used to carry out life functions/synthesis/growth/movement ✔
\nj. energy is lost/not recycled ✔
\nk. nutrients are recycled / example of recycled nutrient e.g. carbon ✔
\nl. decomposers recycle minerals/inorganic nutrients ✔
\nOutline how cuts in the skin are sealed to prevent blood loss.
\nOutline how two parents could have a child with any of the four ABO blood groups.
\nExplain how ventilation and lung structure contribute to passive gas exchange.
\na. platelets respond to/detect skin/blood vessel damage ✔
\nb. platelets release clotting factors ✔
\nc. clotting factors trigger a chain/cascade of reactions ✔
\nd. «leading to» formation of thrombin ✔
\ne. thrombin causes fibrinogen conversion into fibrin ✔
\nf. blood clot seals the wound due to fibrin network of fibres ✔
\nAccept answers presented as a flow chart.
\na. «first set of» gametes/parental genotype IA, i ✔
\nb. «other set of» gametes/parental genotype IB, i ✔
\nc. «genotypes of offspring are respectively» IAIB, IBi, IAi, ii ✔ All four correct required.
\nd. «phenotypes of offspring are respectively» AB, B, A, O ✔ All four correct required linked to genotypes
\nAward marks only for the first grid if more than one drawn;
e.g. of Punnett grid
Answers can be given in a Punnett grid or in prose.
\nAccept the four possible blood groups of the offspring anywhere in the answer.
\na. air carried through trachea AND bronchi/bronchioles AND alveoli ✔ All three required in correct order.
\nb. alveoli increase the surface area/thin walled for gas exchange ✔
\nc. gas exchange carried out through type I pneumocytes ✔
\nd. type II pneumocytes secrete surfactant to reduce surface tension ✔
\ne. moist surface/surfactant allows gases to diffuse in solution ✔
\nf. ventilation/moving blood maintains concentration gradients of oxygen and carbon dioxide ✔
\ng. between air in alveoli and blood in «adjacent» capillaries
OR
oxygen diffuses from alveoli to capillaries and carbon dioxide from capillaries to alveoli ✔ OWTTE
h. external intercostal muscles/diaphragm contract during inspiration ✔
\ni. lowering air pressure «in lungs»/increasing thorax volume ✔
\nj. relaxation of external intercostal muscles/diaphragm enable «passive» expiration ✔
\nk. internal intercostal «and abdominal muscles» contract «to force» expiration ✔
\nl. expiration due to increasing air pressure «in lungs»/decreasing thorax volume ✔
\nAccept correctly annotated diagram.
\n\n Outline how two parents could have a child with any of the four ABO blood groups.\n
\n\n [4]\n
\n\n Explain the control mechanism of the heart rate.\n
\n\n [7]\n
\n\n a. I\n \n A\n \n , i for one set of gametes/parental genotype ✔\n
\n\n b. I\n \n B\n \n , i for the other set of gametes/parental genotype ✔\n
\n\n c. «genotypes of offspring are» I\n \n A\n \n I\n \n B\n \n , I\n \n A\n \n i, I\n \n B\n \n i, ii ✔\n
\n\n d. «phenotypes of offspring are» AB, A, B, O ✔\n
\n\n \n Answers can be given in a Punnett grid or in prose.\n \n
\n\n \n Accept the four possible blood groups of the offspring anywhere in the answer.\n \n
\n\n a.\n \n sinoatrial node/SAN\n \n is a specialized group of muscle cells\n
\n \n \n OR\n \n \n
\n \n sinoatrial node/SAN\n \n is located in the right atrium ✔\n
\n b. acts as a pacemaker/controls the heart rate\n
\n \n \n OR\n \n \n
\n initiates/generates the heart beat/starts the cardiac cycle ✔\n
\n c. sends out electrical signal/impulses/depolarisations ✔\n
\n\n d. electrical signal stimulates contraction «of heart muscle» ✔\n
\n\n e. signal passes through walls of atria/passes to AV node ✔\n
\n\n f. then through walls of the ventricles ✔\n
\n\n g. medulla «oblongata of brain» can change/increase/decrease the rate ✔\n
\n\n h. through nerves/named example of nerve/autonomic/sympathetic/ parasympathetic nervous system ✔\n \n In mph, only accept vagus nerve for slowing heart rate and sympathetic nerve for accelerating it\n \n .\n
\n\n i. one nerve increases the rate and the other decreases it ✔\n
\n\n j. epinephrine/adrenaline increases heart rate/force of contraction ✔\n
\n\n k. epinephrine/adrenaline prepares the body for vigorous activity/is part of fight or flight response ✔\n
\n\n Draw a half-view of an animal-pollinated flower.\n
\n\n [4]\n
\n\n Explain the movement of energy and inorganic nutrients in an ecosystem.\n
\n\n [7]\n
\n\n a.\n \n sepals\n \n as outermost part of flower ✔\n
\n\n b.\n \n petals\n \n as largest part of flower ✔\n
\n\n c.\n \n stamen\n \n drawn with recognizable anther and filament\n
\n \n \n OR\n \n \n
\n \n anther\n \n and\n \n filament\n \n shown as parts of the stamen ✔\n
\n d.\n \n carpel/pistil\n \n drawn with recognizable stigma, style and ovary\n
\n \n \n OR\n \n \n
\n \n stigma\n \n ,\n \n style\n \n and\n \n ovary\n \n shown as parts of the carpel ✔\n
\n e.\n \n nectary\n \n at base of the ovary ✔\n
\n\n f.\n \n ovule\n \n inside the ovary ✔\n
\n\n \n As the question does not specify a labelled half-view, allow some marks for unlabeled structures: award one mark for any two of the six structures in the mark scheme (mpa to mpf). It must be clear what each unlabeled part is. The maximum mark is therefore 3 for an unlabeled half-view.\n \n
\n\n a. autotrophs/producers/plants obtain inorganic nutrients from the «abiotic» environment ✔\n
\n\n b. energy is provided «mainly» by sunlight ✔\n
\n\n c. light energy is converted «to chemical energy» through photosynthesis✔\n
\n\n d. photosynthesis/producers/autotrophs convert inorganic carbon/carbon dioxide and water into carbon/organic compounds ✔\n
\n\n e. carbon compounds/foods contain/are a source of «usable» energy «for life» ✔\n
\n\n f. carbon compounds/energy are transferred along food chains when eaten by consumers/heterotrophs ✔\n
\n\n g. respiration returns carbon «dioxide» to the environment ✔\n
\n\n h. respiration releases stored/chemical energy as heat/ATP ✔\n
\n\n i. energy/ATP is used to carry out life functions/synthesis/growth/movement ✔\n
\n\n j. energy is lost/is not recycled ✔\n \n Both related by “or” required\n \n .\n
\n\n k. nutrients are recycled / example of recycled nutrient e.g. carbon ✔\n
\n\n l. decomposers recycle minerals/inorganic nutrients ✔\n
\n\n \n Award\n \n [5 max]\n \n if only energy is mentioned\n \n .\n
\nA nomogram can be used to determine the body mass index (BMI) of an individual.
\nState the BMI of a person of mass 80 kg and 1.80 m in height.
\nExplain how measurement of BMI could help to assess whether a person has an increased risk of type II diabetes.
\n25
\nAccept 24.5 to 26.
\nUnits not required.
\na. BMI could indicate if a person is overweight/obese/too heavy for their height;
\nb. overweight/obesity increases the probability of developing type II diabetes;
\nDo not accept “High BMI increases the risk of diabetes.”
\nMost candidates correctly stated the BMI of the individual. Some did it by reading from the nomogram, and others by calculation.
\nIn (b) many candidates stated that a high BMI leads to obesity without explaining that a high BMI is only a possible indicator of obesity.
\nIn an investigation to compare the elasticity of arteries and veins, rings of the same diameter (20 mm) of artery and vein tissue were cut from blood vessels obtained from a mammal.
\n[Source: © International Baccalaureate Organization 2020.]
\nEach ring was attached to a clamp. Multiple masses of 10 g were added and removed. The vertical diameter of the artery and the vein was measured, both with the mass and once the mass had been removed.
\nThe results are shown in the table.
\nState the independent and dependent variables in this experiment.
\nIndependent:
\nDependent:
\nState one feature of the rings that has to be kept constant apart from their initial diameter.
\nExplain the differences between the results shown for vein and artery.
\na. Independent: mass;
\nb. Dependent: (vertical) diameter/length;
\nDo not accept elasticity
\na. width/section depth/slice of the ring;
\nb. same animal/age/freshness/temperature;
\nDon’t accept thickness or diameter.
\na. veins have thinner walls (than arteries);
\nb. veins sustain lower (blood) pressure (than arteries);
\nc. when stretched, veins become longer (than arteries);
\nd. veins have less muscle/elastic (fibre in their) walls (than arteries);
\ne. veins have lower elasticity/recover less/remain more stretched (than arteries after weights removed);
\nAccept inverse for arteries in all cases.
\nDo not accept a listing of numerical values without explanation.
\nThe experiment described an investigation to compare the elasticity of arteries and veins. In (a) the most common error was to reverse the correct responses of mass and diameter.
\nIn part (b) many candidates suggested that the samples should be of the same thickness.
\nIn part (b) many candidates suggested that the samples should be of the same thickness.
\nTo investigate whether carbon dioxide is required for photosynthesis, a plant was irrigated using water from which carbon dioxide had been removed and was then placed in the apparatus shown in the diagram. The apparatus was left in darkness for 24 hours to destarch the leaves. Then, after several hours in light, a leaf was removed from the plant and found to contain no starch when tested. A control was performed using a second plant. A leaf from this plant tested positive for starch.
\n[Source: © International Baccalaureate Organization 2020.]
\nDescribe the control for this experiment.
\nOutline how the carbon dioxide could be removed from the water used to irrigate the plant.
\nSuggest how a plastic bag placed around the plant pot prevents carbon dioxide from reaching the plant’s leaves.
\nA chromatograph was made of the photosynthetic pigments of a leaf of the plant.
\n[Source: Adapted from “Diversity of Photosynthetic Pigments” by Alexander F. Motten in Tested Studies for
Laboratory Teaching, Volume 16 of the Association for Biology Laboratory Education and used by permission of the
author.]
\n
Outline what measurements would be taken to identify pigment X.
\na. same apparatus with carbon dioxide present;
\nb. (control has) no sodium hydroxide/alkali;
\nc. control irrigated with untreated water/water with CO2;
\n“Same apparatus” alone does not get the mark.
\na. boiling (and cooling) the water;
\nb. expose the water to a vacuum;
\nAllow distillation of water.
\na. to prevent CO2 from (organisms in) the soil affecting the experiment;
\nb. the plastic bag is impermeable to gases;
\na. the distance travelled from the origin/O to the solvent front;
\nb. the distance travelled by the pigment (from the origin O to X);
\nAccept X to solvent front if the candidate indicates that this allows O to X to be calculated.
\nMany candidates had difficulty describing the control for this experiment. The question states that the experiment was to investigate whether carbon dioxide is required for photosynthesis therefore it seems straightforward that the plant would be in similar conditions with and without carbon dioxide. Some candidates assumed that the control plant would have similar conditions and no light.
\nThere were many suggestions on how carbon dioxide could be removed from water despite boiling the water being an assessment statement.
\nThe plastic bag caused considerable confusion with many candidates stating that it was to prevent carbon dioxide reaching the roots and being taken up the stem to the leaves for photosynthesis.
\nMost candidates managed to outline what measurements would be taken to identify the pigment on the chromatogram with many identifying the pigment although this was not required.
\nThe diagram shows part of two neurons.
\n[Source: © International Baccalaureate Organization 2020.]
\nOutline how the amphipathic properties of phospholipids play a role in membrane structure.
\nState the role of cholesterol in animal cell membranes.
\nDescribe what happens to the membranes of an animal cell during mitosis.
\nState the name of the structure shown.
\nX indicates the movement of a structure in the neuron. Explain what events trigger this movement and what happens next.
\na. part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar;
\nb. bilayer formed (formed naturally by phospholipids in water);
\nc. hydrophilic heads/parts face outwards and hydrophobic tails/parts face inwards.
\nDo not allow water loving/hating in mpa or mpc.
\na. controls/regulates/reduces fluidity of membrane / prevents crystallization of phospholipids;
\nb. reduces permeability to some substances.
\nDo not accept ‘stabilizes membrane’.
\na. nuclear membrane breaks down/disappears (in prophase/at start of mitosis);
\nb. nuclear membrane reforms around two new nuclei (in telophase/at end of mitosis);
\nc. plasma membrane pulled inwards at equator / cleavage furrow formed;
\nd. membrane pinches apart to form two cells / cytoplasm divided / cytokinesis.
\nsynapse/synaptic
\nAllow any answer including either of these terms unless out of context.
\na. depolarization of pre-synaptic membrane / action potential/nerve impulse arrives;
\nb. uptake of calcium / calcium ions diffuse in / calcium channels open;
\nc. structures containing neurotransmitter/vesicles move to/fuse with membrane;
\nd. neurotransmitter/acetylcholine released by exocytosis into cleft/binds to postsynaptic membrane/receptors;
\nMust see exocytosis.
\nKnowledge of the amphipathic properties of phospholipids was generally good.
\nHalf of candidates were able to state the role of cholesterol in animal cell membranes.
\nThe mark scheme allowed candidates to score marks either for describing changes to the nuclear membrane or the role that the plasma membrane plays in cytokinesis. The best answers referred to both. Many of the candidates who wrote about the nuclear membrane had rather sketchy understanding of how it breaks up into small vesicles at the end of prophase and then reforms around both daughter nuclei in telophase.
\nAs pointed out on some G2 forms, the question here was rather ambiguous as it wasn’t 100 % clear what structure was referred to, so any answer indicating that the candidate knew the diagram showed a synapse was accepted.
\nThis was generally well answered. Some suggested that the vesicle is released into the synaptic cleft, instead of neurotransmitter being released by exocytosis.
\nThe electron micrograph shows part of a cell including a mitochondrion.
\n[Source: Used with permission of McGraw-Hill Education, from Harrison’s Principles of Internal Medicine,
J L Jameson et al., 16th edition, 2004; permission conveyed through Copyright Clearance Center, Inc.]
Outline how the structures labelled X and Y are adapted to carry out the function of the mitochondrion.
\nX:
\n\n
Y:
\nExplain how ATP is generated in mitochondria by chemiosmosis.
\nX: large/increased SA area for ATP production/electron transport/oxidative phosphorylation/proton pumping
OR
X: small/narrow intermembrane space for generating proton gradient (rapidly/steeply);
Y: contains enzymes for Krebs cycle/link reaction;
\na. protons pumped across inner membrane of mitochondria/into intermembrane space;
\nb. using energy released by flow of electrons/by electron transport/by electron carriers;
\nc. proton gradient established/maintained / proton motive force generated;
\nd. protons pass/diffuse back through inner membrane/membrane of cristae/to matrix;
\ne. through ATP synthase;
\nf. ATP production coupled to flow of protons / ATP from ADP and Pi using energy from protons;
\nMarks can be awarded in an annotated diagram.
\nThe mean mark for this question was only 0.6 – less than half marks. Many students did not recognise they had to describe adaptations so, for example, the large surface area of the cristae or the small volume of the intermembrane space had to be included together with the function carried out. Weaker candidates had forgotten basic ideas of what happens in the mitochondrion.
\nThis was answered more successfully by most candidates and there were some impressively detailed and accurate answers.
\nProteins are an important group of chemicals found in all living organisms.
\nDraw a molecular diagram to show the formation of a peptide bond.
\nOutline how proteins are digested and the products of protein digestion absorbed in humans.
\nExplain how polypeptides are produced by the process of translation.
\na. two amino acids correctly drawn;
\nb. removal/production of H2O molecule shown;
\nc. peptide bond labelled between C of C=O and N of N-H;
\nAccept specific examples of amino acids correctly drawn for example glycine with H instead of R.
\nmpc can be awarded if the peptide bond is shown as in the dipeptide diagram but there are errors elsewhere.
\na. digested by peptidases/proteases;
\nb. pepsin/pepsinogen/endopeptidase secreted by stomach (lining)/digests proteins in stomach;
\nc. pancreas secretes/pancreatic juice contains endopeptidase/trypsin/peptidase;
\nd. endopeptidase digest proteins/polypeptides to shorter chains of amino acids/shorter peptides;
\ne. amino acids absorbed by active uptake/transport;
\nf. in small intestine/ileum;
\ng. villi increase the surface area for absorption;
\nh. absorbed into bloodstream/into capillaries;
\nPeptidase can be accepted instead of endopeptidase in mpc, but not in mpb or mpd.
\na. mRNA is translated;
\nb. mRNA binds with ribosome/with small subunit of ribosome;
\nc. tRNA-activating enzymes/aminoacyl tRNA synthetases attach specific amino acid to tRNA;
\nd. anticodon of 3 bases/nucleotides on tRNA;
\ne. start codon/AUG on mRNA;
\nf. tRNA carrying first amino acid/methionine binds to P/peptidyl site (when large subunit binds);
\ng. anticodon (on tRNA) binds to codon (on mRNA);
\nh. complementary base pairing (between codon and anticodon);
\ni. tRNA for next codon binds to A site/amino acyl site;
\nj. peptide bond forms between amino acids (on tRNAs) at P and A sites;
\nk. ribosome moves along mRNA to next codon/by three bases/in 5’ to 3’ direction;
\nl. tRNA released from E/exit site;
\nm. process/cycle repeats to elongate the polypeptide/until stop codon is reached;
\nn. release of polypeptide and mRNA/disassembly of ribosome complex at stop codon;
\nMarks can be awarded in an annotated diagram.
\nAccept UAA, UAG or UGA instead of stop codon in mpm or mpn but do not accept terminator sequence.
\nDo not award mpk for the ribosome moving to the start codon in a 5’ to 3’ direction.
\nMostly full marks were gained although some had the correct diagrams but failed to label the peptide bond.
\nNearly all candidates had some knowledge of protein digestion and many scored 3 or 4 of the 4-mark total. The relatively low mark allocation reflects that the program does not require detailed study of protein digestion or absorption. Few students referred to endopeptidases, which are stipulated in guidance for sub-topic 6.1 and also few stated that villi in the small intestine increase the surface area for absorption.
\nExplanations of translation were generally good, with many of the significant events included. Fewer candidates than in the past included unnecessary information about transcription. A common omission was the presence of a three-base (or nucleotide) anticodon on tRNA and a frequent small error was the idea that during initiation the first tRNA binds to the A site and then moves to the P site, rather than binding to the P site immediately.
\nWater is the medium of life.
\nOutline how hydrogen bonds form in water.
\nDescribe the processes that cause water to move from the roots of plants to their leaves.
\nExplain the role of the kidney in osmoregulation.
\na. water (molecules) are polar/dipolar/have partially positive and negative poles/have δ+ and δ-;
\nb. attraction/bonding between positive and negative (poles);
\nc. hydrogen bond formed between hydrogen and oxygen; Reject if H and O in same molecule.
\nd. bond/attraction between different water molecules/intermolecular;
\nMarks can be awarded in an annotated diagram.
\nReject answers stating or implying that there are whole positive or negative charges for mpa.
\na. water moved/transported in xylem vessels;
\nb. transported under tension/suction/pulled up (in xylem vessels);
\nc. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;
\nd. tension/pull generated when water evaporates from cell walls (in mesophyll);
\ne. transpiration is loss of water vapour from leaf (surface)/stomata;
\nf. cohesivity/cohesion in water due to hydrogen bonding/attractions between water molecules;
\ng. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;
\nh. transpiration stream is a column of/flow of water in xylem from roots to leaves;
\nDo not award marks for absorption of water by roots.
\na. osmoregulation is regulation of water and solute/salt balance/solute concentrations;
\nb. nephron (is the functional unit of the kidney/osmoregulates);
\nc. ultrafiltration in glomerulus / glomerular filtrate collected by Bowman’s capsule;
\nd. loop of Henle establishes/maintains hypertonic conditions in medulla;
\ne. osmosis/reabsorption of water (from filtrate) in the collecting duct;
\nf. brain/hypothalamus monitors blood solute concentration / pituitary secretes ADH;
\ng. ADH secreted when solute concentration of blood is too high/hypertonic/when dehydrated;
\nh. ADH increases permeability of collecting duct to water;
\ni. ADH causes more aquaporins (in membranes of collecting duct wall cells);
\nj. more water reabsorbed resulting in more concentrated/hypertonic urine/less volume of urine;
\nk. less/no ADH secreted when solute concentration (of blood) is too low/hypotonic;
\nl. less water reabsorbed resulting in dilute/hypotonic urine/large volume of urine;
\nReject ‘water balance’ and ‘water concentration’ for mpa.
\nAnswers were mostly good here, with many candidates scoring full marks. A very common misconception was that hydrogen bonds are strong – a single hydrogen bond is a weak interaction and it is only because there are many hydrogen bonds in water that they collectively exert large cohesive forces. A few candidates thought that hydrogen bonds were within rather than between water molecules.
\nThis was answered quite well. There were some traditional areas of confusion, with cohesion and adhesion either muddled up or treated as the same thing. Some candidates thought high pressure caused by roots and low pressure caused by leaves could exist at the same time in xylem. Few responses referred to water being transported in the vessels of xylem. In some answers water was said to evaporate from stomata, instead of the moist, blotting-paper-like cells walls of the mesophyll. Some answers included details of how water is absorbed into roots, which was outside the scope of the question. Despite these common faults, many candidates described clearly how tension is generated in xylem and how cohesive columns of water can be pulled up to leaves.
\nThere was some excellent knowledge of kidney function, frequently going way beyond the question’s requirements. In some cases, candidates just described all processes occurring in the kidney instead of actually answering the question. This generally led to the loss of the extra clarity mark.
\nA study was conducted to look at the short-term effects of a change in diet on the risk of disease in young adults. The table shows data on the habitual diet of the participants as well as the study diet followed for two weeks.
\n[Source: Horowitz, J.F., Ortega, J.F., Hinko, A., Li, M., Nelson, R.K. and Mora-Rodriguez, R.,
2018. Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in
overweight adults. PLoS ONE, 13(6), e0198372.]
Total blood plasma cholesterol levels were measured before the study began and once a week after starting the study diet. Mean results are shown in the bar chart, including the standard deviation.
\n[Source: Horowitz, J.F., Ortega, J.F., Hinko, A., Li, M., Nelson, R.K. and Mora-Rodriguez, R.,
2018. Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in
overweight adults. PLoS ONE, 13(6), e0198372.]
Control of blood glucose concentration was investigated using an oral glucose tolerance test. For this test, the person was given a concentrated glucose drink (at time zero) and then blood samples were taken every 15 minutes to determine the plasma insulin level. This test was done before the study diet and after two weeks on the study diet. Mean results are shown in the graph, including the standard deviation.
\n[Source: Horowitz, J.F., Ortega, J.F., Hinko, A., Li, M., Nelson, R.K. and Mora-Rodriguez, R.,
2018. Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in
overweight adults. PLoS ONE, 13(6), e0198372.]
Comment on the total energy content of the two diets.
\nDistinguish between the two diets.
\nCalculate, showing your working, the percentage change in mean cholesterol level after one week on the study diet.
\n\n
. . . . . . . . . . . . . . . . . . . .%
\nCompare the data for plasma insulin levels before and after the study diet.
\nState which cells secrete insulin.
\nOutline the reason for plasma insulin levels changing in the first 30 minutes of the test.
\nThe hypothesis made before the study was that saturated fats in the diet affected the risk of coronary artery blockage and diabetes. Using all the data in question 1, evaluate whether this hypothesis is supported by the study.
\na. energy is not changed (between the two diets);
\nb. study diet slightly lower in energy than habitual diet (but means/SD overlap);
\nc. spread of values show more variation for habitual diet / higher SD in habitual;
\na. they differ in percent of saturated and unsaturated fats (but not total fat);
\nb. percent of saturated fats is higher in study diet / lower in habitual diet;
\nc. (mono/poly) unsaturated fats decreased in study diet compared to habitual diet/more in habitual diet
OR
polyunsaturated fats in study diet only half of what they were in habitual diet;
d. (slightly) less carbohydrate in study;
\nAllow numerical points if they are a valid comparison using distinguishing terms.
\n((165–150) ÷ 150) × 100;
\n(=) 10 (%);
\nAllow up to 167 = 11.3 %
\n1 mark for correct working if above 167.
\na. both show same pattern of rise, level and then decrease / show same trend;
\nb. both show same/similar levels of insulin (at all times) due to overlapping error bars;
\nc. both rise for 30/45 minutes;
\nDo not give credit for contrasts.
\nβ cells of pancreas/islets (of Langerhans);
\nas blood glucose rises, insulin rises/increases to reduce the level/OWTTE;
\nBlood glucose must be mentioned as well as a rise in insulin.
\nHypothesis is partially supported
a. Increased saturated fats in study diet resulted in increase in cholesterol levels;
b. cholesterol level is risk for blockage of coronary arteries;
\nHypothesis is not supported
c. high insulin levels are sign of (Type II) diabetes;
d. insulin levels were the same in both diets so no increased risk;
\ne. study only 2 weeks long;
\nConsidering that all students must complete an Individual investigation as part of their Biology course, few understood the significance of the uncertainties shown in the first table or the overlap of the standard deviations in the graph in part d. Only the better candidates were able to say that the energy was extremely similar in both diets. A surprising number could not calculate a percentage change in part c.
\nIn part b, the command term was ‘distinguish’ which means that only differences were looked for. Similarly in d(i), ‘compare’ means that only similarities were needed. If both were required, the command term is ‘compare and contrast’.
\nIn d, few could state that there was no significant difference in the two curves due to the overlap of the error bars. Most knew that the beta cells of the pancreas secrete insulin, although a surprising number thought that they were produced by the red blood cells. In e, many gained two marks for linking saturated fats to cholesterol and its consequence.
\n\n
\n Distinguish between the distribution of\n \n D. tigrina\n \n and that of\n \n D. coronata\n \n .\n
\n\n [1]\n
\n\n Outline the principle of competitive exclusion.\n
\n\n [2]\n
\n\n Other than position in the tree, suggest\n \n two\n \n ways in which the niches of the warblers in the ecosystem may differ.\n
\n\n 1.\n
\n\n 2.\n
\n\n
\n\n [2]\n
\n\n The diagram shows the realized niches of the five species of warbler. Suggest how the fundamental niche of\n \n D. castanea\n \n might differ from its realized niche.\n
\n\n
\n\n [2]\n
\n\n a.\n \n D. tigrina\n \n lives higher in the tree than\n \n D. coronata\n \n ;\n
\n\n b.\n \n D. coronata\n \n occupies a greater area/height range than\n \n D. tigrina\n \n ;\n
\n\n c.\n \n D. tigrina\n \n found in the outer regions and\n \n D. coronata\n \n found on the inside;\n
\n\n \n Accept vice-versa\n \n .\n
\n\n a. no two species can occupy the same niche;\n
\n\n b. competition between them would cause one species to drive the other out\n
\n \n \n OR\n \n \n
\n one of the two species would need to adapt and evolve accordingly;\n
\n a. different food/prey;\n
\n\n b. different predators;\n
\n\n c. active at different times of the day;\n
\n\n d. present at different times of the year;\n
\n\n e. different nest sites;\n
\n\n f. different temperatures;\n
\n\n \n Do not accept “different habitats\n \n ”.\n
\n\n a. the realized is the actual niche and the fundamental is in the absence of competition;\n
\n\n b. with no competition the\n \n D. castanea\n \n would have a larger habitat/more food\n
\n \n \n OR\n \n \n
\n \n D.castanea\n \n could occupy the niches currently occupied by the other warbler species;\n
\n \n Accept named species\n \n .\n
\n\n Overall, well answered.\n
\n\n When describing competitive exclusion many candidates simply described competition. Some mentioned habitat rather than niche.\n
\n\n Weaker candidates gave incomplete or confused suggestions in differentiating between the realised and fundamental niche.\n
\n\n Calculate the number of species of grebe with problems due to entanglement.\n
\n\n [1]\n
\n\n Suggest how entanglement in plastics can lead to the death of marine birds.\n
\n\n [1]\n
\n\n Identify the group with the greatest number of species with problems due to ingestion of plastics.\n
\n\n [1]\n
\n\n Describe how ingested plastics can cause problems to marine birds.\n
\n\n [2]\n
\n\n 2;\n
\n\n a. birds are unable to fly/swim for food;\n
\n\n b. unable to escape predators;\n
\n\n c. birds drown;\n
\n\n d. birds suffocate / are strangled;\n
\n\n C / albatrosses, petrels and shearwaters;\n
\n\n a. fill up the stomachs (of young birds) so they feel full / starve to death;\n
\n\n b. damage the digestive system / cut the gut/stomach/oesophagus/intestines (leading to internal bleeding);\n
\n\n c. block passage of food (causing starvation);\n
\n\n d. cause choking (so cannot breathe);\n
\n\n e. contain/decompose to toxic chemicals (poisoning birds)\n
\n \n \n OR\n \n \n
\n toxins/microplastics in seawater build up/biomagnify (and poison wildlife);\n
\n This question outlining how plastic pollution affects marine birds was straightforward for all candidates.\n
\n\n Two components of biodiversity are richness and evenness. Deduce which of the two pools was higher in richness.\n
\n\n [1]\n
\n\n Two components of biodiversity are richness and evenness. Deduce which of the two pools was higher in evenness.\n
\n\n [1]\n
\n\n Calculation of Simpson’s reciprocal diversity index for both rockpools gives the following results.\n
\n\n \n
\n Outline what conclusion can be drawn from the results.\n
\n\n [1]\n
\n\n The graph shows how the number of ant species found on isolated islands near New Guinea depends on the size of the island.\n
\n\n \n
\n [Source: University of Windsor,\n \n The Theory of Island Biogeography\n \n . Available at:\n
\n http://web2.uwindsor.ca/courses/biology/macisaac/55-437/lecture9.htm.]\n
\n
\n\n Explain the relationship between island size and number of ant species.\n
\n\n [1]\n
\n\n A;\n
\n\n B;\n
\n\n B is more diverse/biodiverse than A;\n
\n\n \n Accept vice-versa. Do not accept greater Simpson’s reciprocal diversity index\n \n .\n
\n\n (the larger islands contain) more species as there are more habitats;\n
\n\n Candidates could deduce the richness and evenness of the pools but gave incomplete answers to the other two parts of this question.\n
\n\n The conclusion from the table showing values of Simpson’s reciprocal diversity index required mention of biodiversity, not simply that one pool is more diverse.\n
\n\n The graph showing the number of ant species according to island size required an explanation of the graph. Many candidates simply stated the relationship with no explanation.\n
\n\n Explain what information a pyramid of energy provides about an ecological community.\n
\n\n [4]\n
\n\n Outline a method that could have been used in this study to measure the numbers of sea urchins per 0.25 m\n \n 2\n \n .\n
\n\n [2]\n
\n\n Sea otters are considered keystone species in this environment. Suggest how the presence of sea otters could affect the algae population.\n
\n\n [3]\n
\n\n Sea urchins are destroying the giant kelp alga (\n \n Macrocystis pyrifera\n \n ) marine forests of South East Australia. Suggest a reason that sea urchins might be an invasive species in this environment.\n
\n\n [1]\n
\n\n a. shows the amount of energy at each trophic level (of a food chain/web);\n
\n\n b. (energy) measured over a period of time/year\n
\n \n \n OR\n \n \n
\n units are energy per area per time/kJ m\n \n –2\n \n year\n \n –1\n \n ;\n
\n c. the width/size of each bar represents the amount of energy;\n
\n\n d. the bottom level represent the producers;\n
\n\n e. subsequent levels represent consumers;\n
\n\n f. each level should be roughly one tenth of the size/10 % of the preceding level;\n
\n \n \n OR\n \n \n
\n organisms at the top of the food chain are shown to have much less energy available to them;\n
\n g. the energy that enters a community is ultimately lost as heat/in respiration\n
\n\n \n Some answers may be displayed in a diagram\n \n .\n
\n\n a. with\n \n quadrats\n \n (of 0.5 m side / 0.25m\n \n 2\n \n );\n
\n\n b. (quadrats) position determined at fixed distance by transects\n
\n \n \n OR\n \n \n
\n (quadrats) position determined at random;\n
\n c. random sampling / capture-recapture;\n
\n\n d. average number calculated;\n
\n\n a. sea otters (feeding on sea urchins) limit sea urchin population;\n
\n\n b. the largest sea urchins are eaten;\n
\n\n c. shown by low biomass/small size (of sea urchins);\n
\n\n d. fewer/smaller sea urchins allow for increase in algae population;\n
\n\n e. sea otters have a top down effect;\n
\n\n \n Allow converse reasoning\n \n .\n
\n\n the sea urchins’ limiting factors in their original habitat are missing\n
\n \n \n OR\n \n \n
\n lack of (natural) predators for sea urchins\n
\n The better candidates performed well in this question while weaker candidates are still confused by pyramids of energy. Many referred to numbers, or biomass at each trophic level. Very few candidates stated that the measurements were taken over a period of time.\n
\n\n Most candidates did well on this question, especially strong ones, although there was evidence that many candidates are not familiar with population evaluation skills. Candidates should be aware that writing \"quadrants\" (with a \"n\") is questionable, and that although it may not be assumed that they knew what echinoderms or sea urchins were, the capture-mark-release-recapture method was not really appropriate to evaluate their numbers, given the very limited areas of 0.25 m\n \n 2\n \n .\n
\n\n The majority of candidates could suggest that the sea algae population could be affected by sea otters feeding on sea urchins; most nevertheless did not use the graphs or mention that a top down effect was present.\n
\n\n Many suggested that sea urchins' predators were not present, but too many irrelevant or blank answers were seen.\n
\nThe photomicrograph below shows the protozoan Paramecium caudatum.
\n[Source: Deuterostome, CC BY-SA 3.0
https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons.]
State the genus of this organism.
\nState the domain in which it is classified.
\nOutline the method of nutrition carried out by P. caudatum.
\nOutline one aspect of how P. caudatum carries out homeostasis.
\nApart from the ribosomes, explain the evidence for the endosymbiotic theory of the origin of eukaryotic cells.
\nParamecium
\neukaryotes
\nheterotroph/consumer as it feeds on bacteria/algae/yeast/smaller single celled organisms
OR
heterotroph/consumer as it does not have chloroplasts
Heterotrophic must be qualified.
\na. lives in fresh water so water enters cell (by osmosis);
\nb. contractile vacuoles collect and expel water;
\nc. homeostasis is keeping internal conditions within limit/constant / involves osmoregulation/regulating water content/potential;
\na. mitochondria/chloroplasts show features in common with prokaryotes/similar size;
\nb. multiply by binary fission/in same manner;
\nc. have naked loop of DNA/circular DNA/own DNA;
\nd. surrounded by a double membrane;
\nIn a about half of the candidates named the genus correctly (Paramaecium), but significantly fewer could state the correct domain (eukaryota). The most common wrong answers were prokaryote and eubacteria. Only the better candidates were able to outline the method of nutrition (heterotrophy) in b and describe how homeostasis is carried out, in spite of being specifically stated in the syllabus. The endosymbiotic theory was quite well known.
\nThe diagram shows part of two neurons.
\n[Source: © International Baccalaureate Organization 2020.]
\nOutline how the amphipathic properties of phospholipids play a role in membrane structure.
\nState the name of the structure shown.
\nX indicates the movement of a structure in the neuron. Explain what events trigger this movement and what happens next.
\na. part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar; Both needed.
\nb. bilayer formed (formed naturally by phospholipids in water);
\nc. hydrophilic heads/parts face outwards AND hydrophobic tails/parts face inwards;
\nDo not allow water loving/hating in mpa or mpc.
\nsynapse/synaptic
\na. depolarization of pre-synaptic membrane / action potential/nerve impulse arrives;
\nb. uptake of calcium / calcium ions diffuse in / calcium channels open;
\nc. structures containing neurotransmitter/vesicles move to/fuse with membrane;
\nd. neurotransmitter/acetylcholine released by exocytosis into cleft/binds to postsynaptic membrane/receptors;
\nKnowledge of amphipathic properties in part a was good. In b, most could state that the diagram shows a synapse. A variety of spelling is usually allowed, but in this case ‘synapsis’ was not allowed in English as this refers to the pairing of homologous chromosomes. There were some G2 comments that the X could be taken as the structure. However X clearly points to an arrow, not a structure. Most were able to gain some marks by describing the events, but some lost marks by not clearly indicating that it is the vesicle containing the neurotransmitter that moves.
\nSketch the complementary strand to complete the section of a DNA diagram.
\nDefine mutation.
\nExplain how evolution by natural selection depends on mutations.
\na. correct base sequence: T, G, A;
\nb. strand drawn anti-parallel;
\nc. correct shapes used;
\nAward [2 max] if bonds are not from the correct carbon or if the nucleotides are not joined.
\nchange in genetic makeup/DNA/nucleotide/base sequence
\na. mutations cause variation among organisms of same species/population;
\nb. some variations/mutations make individual more suited to its environment/way of life;
\nc. individuals that are better adapted survive and produce offspring;
\nd. individuals pass on genetic characteristics/mutation/variation to offspring;
\ne. natural selection increases frequency of characteristics/alleles that make individuals better adapted;
\nIn part (a) most could gain marks for the correct shapes and sequence but did not gain the third mark as it was not shown as antiparallel. Some lost marks as a 2MAX was awarded if the bonds were not from the correct carbons. In part (b) the definition of mutation was quite well known, and most gained some marks in the explanation. Weaker students just wrote about evolution without relating it to mutations as asked. They also tended to give unnecessary examples, involving finches and moths etc.
\nLife is based on carbon compounds.
\nDraw a molecular diagram of alpha-D-glucose.
\nOutline how carbon compounds are produced in cells using light energy.
\nExplain the transformations of carbon compounds in the carbon cycle.
\na. hexagonal ring structure with O at one point (between C1 and C5);
\nb. correct orientation of OH groups (on carbons 1 to 4); Hydrogens not required
\nc. CH2OH group shown on fifth carbon with correct orientation;
OR
d. 6 carbon chain with oxygen on first C;
e. H and OH groups correctly orientated;
\nCarbons do not need to be numbered.
\nAllow boat or chair diagrams.
\nAllow [2 max] if linear structure drawn.
\na. occurs by the process of photosynthesis;
\nb. occurs in chloroplasts of plant cells/using chlorophyll;
\nc. chlorophyll absorbs red/blue light AND reflects green light; Both needed for marking point.
\nd. raw materials/starting products are carbon dioxide and water/shown in an equation;
\ne. water is split by photolysis;
\nf. oxygen is produced as waste/by-product/lost;
\ng. glucose formed/shown in an equation;
\nh. glucose molecules combine to form starch for storage;
\ni. light energy transformed to chemical;
\na. autotrophs/producers convert carbon dioxide into carbohydrates/carbon compounds in photosynthesis;
\nb. carbon dioxide diffuses/moves from the atmosphere /water into autotrophs/plants;
\nc. carbon compounds are transferred through food chains/OWTTE;
\nd. carbon dioxide produced by respiration diffuses out of organisms into water/atmosphere;
\ne. decomposers release carbon dioxide during decay/putrefaction;
\nf. methane is produced from organic matter in anaerobic conditions (by methanogens);
\ng. some methane diffuses into the atmosphere/accumulates in the ground;
\nh. methane is oxidized to carbon dioxide (and water) in the atmosphere;
\ni. peat forms when organic matter is not fully decomposed because of acidic/anaerobic conditions in waterlogged soils;
\nj. partially decomposed organic matter from past geological eras/fossils was converted into coal/oil/gas that accumulated in rocks;
\nk. carbon dioxide is produced by the combustion of biomass/fossilized organic matter/fuels;
\ni. hard parts of some animals/corals/molluscs are composed of calcium carbonate
\nm. can become fossilized in limestone;
\nAs this is an “explain” question, simply drawing a labelled diagram is not enough for [7]. Diagram would need sufficient annotations to meet the command term.
\nIf carbon compounds are referred to instead of carbon dioxide, penalise once then ecf.
\nThis was the less favourite option question, generally attempted by the better candidates. In part (a) the candidates tended to gain either 3 for a correct structure or zero. A G2 comment asked whether the different forms of glucose were needed; knowledge of the structure of alpha-glucose is stated in the syllabus.
\nMost gained some marks for what was really a description of photosynthesis in part (b), and the transformations of carbon compounds in the carbon cycle was quite well known. Some students lost marks by continuing to use the term ‘carbon compounds’ instead of carbon dioxide and the specific compounds. Many went off on a tangent to write about carbon dioxide and climate change etc. Many drew some quite detailed flow charts of the carbon cycle from which some marks could be gleaned or ambiguous statements clarified.
\nAccording to the cell theory, living organisms are composed of cells.
\nDraw the ultrastructure of a prokaryotic cell based on electron micrographs.
\nOutline what occurs in cells in the first division of meiosis.
\nExplain the role of cells in the defence against infectious disease.
\na. cell wall;
\nb. plasma membrane; Clearly shown as a separate line under the cell wall or the inner line
\nc. cytoplasm AND 70S ribosomes; Do not allow (small) circles
\nd. nucleoid/naked DNA;
\ne. plasmid
OR
pili
OR
flagella/flagellum;
Structures correctly drawn and labelled.
\nAward [2 max] if any exclusively eukaryotic structures are shown.
\nDo not allow cilia as they are not found in prokaryotes.
\na. halves the chromosome number/produces haploid cells;
\nb. at start of meiosis each chromosome consists of two sister chromatids attached by a centromere;
\nc. prophase (I): pairing of homologous chromosomes;
\nd. crossing over occurs;
\ne. chromosomes condense by supercoiling;
\nf. metaphase (I): pairs of homologous chromosomes/bivalents move to equator of spindle
OR
metaphase (I): orientation of pairs of homologous chromosomes (prior to separation) is random;
g. anaphase (I): centromeres do not divide
OR
anaphase (I): spindle fibre pulls chromosome/whole centromere with two sister chromatids to opposite poles;
h. telophase (I): arrival of centromere with sister chromatids at opposite poles;
\nSome of these can be awarded for correctly annotated diagrams.
\nNo credit for events in meiosis II.
\na. cells of skin provide a physical barrier/produce fatty acids/lactic acid/lysozyme which stops entry of microbes
OR
mucous membranes produce mucus to trap pathogens
OR
stomach cells produce hydrochloric acid which kills microbes;
b. platelets start the clotting process preventing access of pathogens;
\nc. (two types of) white blood cells fight infections in the body;
\nd. phagocytes ingest pathogens (by endocytosis/phagocytosis);
\ne. gives non-specific immunity to diseases / ingest any type of pathogen;
\nf. production of antibodies by lymphocytes/B cells;
\ng. in response to particular pathogens/antigens;
\nh. gives specific immunity;
\ni. lymphocyte/B cell makes only one type of antibody;
\nj. plasma cells produce large quantity of (one type of) antibody;
\nk. some lymphocytes act/remain as memory cells;
\nl. can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific antigen is re-encountered;
\nm. results in faster defence against second exposure to specific antigen/pathogen/disease;
\nIn part (a) many candidates were penalised for drawing clear eukaryotic structures such as mitochondria. The general level of the drawings was better than in the past. However a prokaryote should not be represented as a square. Ribosomes must be shown as dots, not circles.
\n\n
In part (b), those who had been well prepared easily gained the 5 marks, but there was a significant proportion who did not know the difference between mitosis and meiosis.
\nIn part (c), defence against infection was well known. Poor candidates lost marks by using loose, non-biological expressions such as the ‘white blood cells eat the infection/disease’. However, most had a fair idea of the cellular response to infection.
\n\n Outline how hydrogen bonds form in water.\n
\n\n [3]\n
\n\n Describe the processes that cause water to move from the roots of plants to their leaves.\n
\n\n [4]\n
\n\n a. water (molecules) are polar/dipolar/have partially positive and negative poles/have δ+ and δ-;\n
\n\n b. attraction/bonding between positive and negative (poles);\n
\n\n c. hydrogen bond formed between hydrogen and oxygen;\n \n Reject if H and O in same molecule.\n \n
\n\n d. bond/attraction between different water molecules/intermolecular;\n
\n\n \n Marks can be awarded in an annotated diagram.\n \n
\n\n \n Reject answers stating or implying that there are whole positive or negative charges for mpa\n \n .\n
\n\n a. water moved/transported in\n \n xylem\n \n \n vessels\n \n ;\n
\n\n b. transported under tension/suction/pulled up (in xylem vessels);\n
\n\n c. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;\n
\n\n d. tension/pull generated when water evaporates from cell walls (in mesophyll);\n
\n\n e. transpiration is loss of water\n \n vapour\n \n from leaf (surface)/stomata;\n
\n\n f.\n \n cohesivity\n \n /\n \n cohesion\n \n in water due to hydrogen bonding/attractions between water molecules;\n
\n\n g. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;\n
\n\n h. transpiration stream is a column of/flow of water in xylem from roots to leaves;\n
\n\n \n Do not award marks for absorption of water by roots\n \n .\n
\n\n Answers were mostly good here, with many candidates scoring full marks. A very common misconception was that hydrogen bonds are strong – a single hydrogen bond is a weak interaction and it is only because there are many hydrogen bonds in water that they collectively exert large cohesive forces. A few candidates thought that hydrogen bonds were within rather than between water molecules.\n
\n\n This was answered quite well. There were some traditional areas of confusion, with cohesion and adhesion either muddled up or treated as the same thing. Some candidates thought high pressure caused by roots and low pressure caused by leaves could exist at the same time in xylem. Few responses referred to water being transported in the vessels of xylem. In some answers water was said to evaporate from stomata, instead of the moist, blotting-paper-like cells walls of the mesophyll. Some answers included details of how water is absorbed into roots, which was outside the scope of the question. Despite these common faults, many candidates described clearly how tension is generated in xylem and how cohesive columns of water can be pulled up to leaves.\n
\nThe mechanism of action of vinblastine, an anticancer drug, was investigated over a range of concentrations. Vinblastine is an alkaloid isolated from the periwinkle plant (Catharansus roseus). The percentage of cells in mitosis and ratio of anaphase to metaphase in cells exposed to this drug in vitro for a fixed time were recorded. The data are displayed in two graphs.
\n[Source: Republished with permission of American Society for Pharmacology and Experimental Therapeutics, from Mechanism of Mitotic Block and Inhibition of Cell Proliferation by the Semisynthetic Vinca Alkaloids Vinorelbine and Its Newer Derivative Vinflunine, Molecular Pharmacology, Vivian K. Ngan, Krista Bellman, Bridget T. Hill, Leslie Wilson and Mary Ann Jordan, Volume 60 , Issue 1, 2001; permission conveyed through Copyright Clearance Center, Inc.]
\nBy referring to both graphs, evaluate the hypothesis that vinblastine targets cells in mitosis and prevents them from completing the process.
\nSome anticancer drugs inhibit mitosis by blocking the formation of the spindle. Suggest one other way in which vinblastine could block mitosis.
\nDiscuss one advantage and one disadvantage of using plant tissue to investigate drugs intended to treat cancer in humans.
\nAdvantage:
\n\n
Disadvantage:
\nEvaluation of evidence in graph on left
\na. increase in (percentage of) cells in mitosis (as vinblastine concentration rises) ✔
\nb. supports hypothesis that cells get stuck in/cannot complete mitosis ✔
\nEvaluation of evidence in graph on right
\nc. drop in anaphase-metaphase ratio due to fewer cells in anaphase/more cells in metaphase ✔
\nd. cells not progressing from metaphase to anaphase/get stuck in metaphase ✔
\nDo not allow mpa if the candidate is arguing that the hypothesis is not supported.
\na. causes microtubules/spindle fibres to break up / tubulin molecules to depolymerize ✔
\nb. prevents contraction of spindle microtubules/fibres ✔
\nc. disrupts/damages kinetochores/centromeres/microtubule motors/centrioles/centrosomes ✔
\nd. prevents separation/pulling apart of (sister) chromatids/chromosomes/centromeres ✔
\ne. prevents microtubules/spindle binding to chromatids/chromosomes/centromeres/DNA ✔
\nMark the first suggestion only in the answer.
\nDo not allow answers about DNA replication or other processes that precede mitosis.
\nAdvantage:
\navoids risks for humans/harm to humans / more ethical (than with human patients/volunteers) ✔
\nDisadvantage:
\ndifferences between plant and human cells so humans may not respond in same way
OR
plants have cell wall/no centrioles/other relevant difference between plant and human cells ✔
Not enough for mpa to say ‘not using humans’.
\nFor mpb there must be either a statement that differences between cells may cause a different response, or a specific example of a cell difference.
\nThis was another challenging data-analysis task with marks spread across the whole range, but full marks were rarely awarded. Many candidates thought that a rise in mitotic index had to mean that more cells were entering mitosis and they failed to deduce from the drop to zero of the anaphase-metaphase ratio that cells were getting stuck in metaphase. As in (d), it was necessary both to understand how cells progress through the cell cycle and be able to make deductions from trends in quantitative data.
\nA wide range of possible ways in which mitosis could be blocked was accepted and about a third of candidates suggested one of these. Methods of blocking DNA replication or cytokinesis were not accepted as the question specifically referred to mitosis.
\nMost candidates were able to suggest an advantage of using plant tissue and any reasonable answer was accepted. Fewer gave a disadvantage that was credited. Vague answers such as plants are different from humans were not rewarded with a mark.
\nThe image shows part of a plant cell with a chloroplast in close proximity to mitochondria.
\n[Source: Photo © E. Newcomb.]
\nState two structural similarities between mitochondria and chloroplasts.
\n1.
\n2.
\n\n
Compare and contrast mitochondria and chloroplasts in terms of the substrates they use and the products they produce.
\nOutline how the compounds produced by chloroplasts are distributed throughout the plant.
\na. (a loop of) DNA ✔
\nb. 70S ribosomes ✔
\nc. double membrane ✔
\nd. electron transport chains/enzyme complexes in (internal) membranes ✔
\ne. enzymes in a region of fluid/in stroma and matrix ✔
\nf. large area of (internal) membrane/cristae and thylakoids ✔
\nOnly two answers should be marked – the first on each line.
\nDo not award marks for functions rather than structures, for example ATP production.
\nAllow spaces inside cristae and thylakoids for mpf.
\na. ATP produced by both / ADP used by both ✔
\nb. oxygen produced by chloroplasts and used by mitochondria ✔
\nc. carbon dioxide produced by mitochondria and used by chloroplasts ✔
\nd. carbon/organic compounds built up in chloroplasts/anabolism and broken down in mitochondria/catabolism ✔
\nDo not award mpd for statements about carbohydrates or glucose (because the pyruvate used by mitochondria is not a carbohydrate).
\na. in phloem ✔
\nb. loading into sieve tubes/by active transport/by cotransport/by companion cells ✔
\nc. entry of water (to phloem) by osmosis/because of high solute concentration ✔
\nd. causes high/hydrostatic pressure ✔
\ne. flow from high pressure to lower pressure down pressure gradient ✔ from source to sink ✔
\nDo not award mpa if xylem included with phloem.
\nDo not award a mark solely for mentioning the term ‘translocation'.
\nSurprisingly few candidates achieved two marks for this question, with many thinking that these organelles were cells with cell membranes, cell walls or even nuclei.
\nTo answer this question, candidates had to know that chloroplasts photosynthesize and mitochondria respire aerobically. They then had to compare or contrast the substrates or products of these two processes. The mean mark was 0.6 out of 2, showing that many candidates failed to do this. Well-prepared candidates had no difficulty.
\nThis question also required a link to be made, after which marks were relatively easily earned. The phase 'distributed throughout the plant' indicated that it was carbon compounds, not oxygen, that were the issue, so phloem transport was the distribution method expected. Transport of oxygen out through stomata and transport within leaf cells were not relevant to the question as asked. The average score was only 0.6 out of 3.
\nPlasma cells secrete antibodies against specific antigens. Outline how plasma cells become activated.
\nA hybridoma is a cell produced by the fusion of a plasma cell with a tumour cell. Explain the advantages of using hybridoma cells in the production of monoclonal antibodies.
\nState one use of monoclonal antibodies.
\na. macrophages/phagocytes recognize/engulf pathogen and display antigens ✔
\nb. antigen binds to T cell/helper T cell / antigen causes activation of T cell ✔
\nc. antigen binds to antibodies in membrane of B cells ✔
\nd. (activated) T cells activate B cells (that have the antigen bound to them) ✔
\ne. activated B cells divide to produce a clone of cells ✔
\nf. active plasma cells develop from the clone of cells/from activated B cells ✔
\nAccept B-lymphocyte and Tlymphocyte instead of B cell and T cell throughout the answer.
\na. endless cell divisions/unregulated mitosis (in hybridoma cells) ✔
\nb. large clone/population of identical cells produced ✔
\nc. all cells (in clone) produce same type of antibody ✔
\nd. large amount of (chosen) antibody can be produced ✔
\nFor mpa it must be clear that it is the hybridoma cells not tumour cells that divide endlessly and that division is more than just rapid.
\na. pregnancy testing kits/detection of hCG (to diagnose pregnancy) ✔
\nb. produce antibodies for treating arthritis/C.difficile/anthrax/psoriasis/ulcerative colitis/asthma/ankylosing spondylitis/Crohn’s disease/multiple sclerosis/HIV/other named disease if verified / targeting tumor cells in treatment of cancer
OR
gives artificial/passive immunity (if injected) ✔
c. blood typing/testing urine for drugs/other verified specific use of monoclonal antibodies ✔
\nMark only the first answer.
\nSome explanations of the production of active plasma cells were excellent but weaker candidates were mostly very confused, with antigens and antibodies muddled up. Many candidates thought that plasma cells are already present and just need to be activated, rather than them being produced as a result of the activity of macrophages, T-cells and B-cells. There was too much focus on memory cells. The mean mark was low, 0.6 out of 3.
\nThis was another low-scoring question for most candidates. Many did not understand well enough how hybridoma cells are produced. There was also a lack of distinction between rapid proliferation of cells and proliferation that continues indefinitely.
\n\n Explain how cladistics can be used to investigate evolutionary relationships.\n
\n\n [2]\n
\n\n Cladistics and other evolutionary evidence suggest that mammals and birds have a more recent common ancestor than mammals and amphibians. Draw a cladogram to show the relationships between mammals, birds and amphibians.\n
\n\n [1]\n
\n\n a. (cladistics) shows evolutionary relationships through a common ancestor ✔\n
\n\n b. cladistics uses DNA/protein/derived/shared anatomical characteristics/traits ✔\n
\n\n c. time of divergence is related to the number of differences in DNA (base sequence) / protein (sequence of amino acids) ✔\n
\n\n d. homologous (versus analogous) traits are used to place an organism in a clade ✔\n
\n\n e. more shared characteristics mean a more recent common ancestor ✔\n
\n\n \n
\n \n What is important is that the proper relationships are shown not the style used\n \n .\n
\n\n Candidates found it hard to express themselves without contradiction.\n
\n\n Carrying examples into part (b) made for the most successful answers. Very few gained marks for providing answers that included molecular level cladistic evidence, e.g. DNA, protein sequencing, etc.\n
\n\n Candidates found it hard to express themselves without contradiction.\n
\n\n Examiners accepted a wide variety of diagrams and orientations as long as the relationships among amphibians, birds and mammals were appropriate. Nevertheless, there was poor performance on this question. It was the question most often left blank in Section A.\n
\nHormones are produced by one tissue in an organism and have their effect on a target tissue.
\nDescribe the genetic and hormonal control of male sexual characteristics in a human.
\nOutline how the hormone auxin controls phototropism in plant shoots.
\nCompare and contrast hormonal and nervous communication.
\na. male if (X and) Y chromosomes present ✔
\nb. gene on Y chromosome/SRY promotes development of testes (from embryonic gonads) ✔
\nc. testes secrete testosterone ✔
\nd. testosterone stimulates sperm production/spermatogenesis ✔
\ne. testosterone stimulates development (in fetus) of male genitals/primary sexual characteristics ✔
\nf. testosterone stimulates development of male secondary sexual characteristics
OR
testosterone causes changes to become adult male during puberty ✔
Allow first alternative for mpf if two secondary sexual characteristics are named instead of the general term ‘secondary sexual characteristics’.
\na. grows/bends towards (brightest) light/sun ✔
\nb. auxin moved from lighter to shadier side (of shoot/stem tip/apex) ✔
\nc. moved by auxin efflux pumps ✔
\nd. auxin promotes cell elongation/cell growth / auxin causes cell wall acidification/loosening ✔
\ne. more growth on shady side of stem (due to auxin concentration gradient) ✔
\nf. binds to auxin receptors (in target cells) ✔
\ng. auxin/auxin receptors promote expression of genes (for growth)/for H+ secretion into wall ✔
\nSimilarities
\na. both used for communication between cells/tissues/organs/parts of the body / WTTE ✔
\nb. both cause a response/change in specific/target cells
OR
both use chemicals that bind to receptors / hormones and neurotransmitters are both chemicals ✔
c. both can stimulate or inhibit (processes in target cells) / WTTE ✔
\nd. both can work over long distances/between widely separated parts of the body / WTTE ✔
\ne. both under (overall) control of the brain/CNS / brain (has role in) sending hormones and nerve impulses ✔
\nf. both use feedback mechanisms/negative feedback / both used in homeostasis ✔
\nDifferences
\nm. example of use of hormonal and use of nervous communication ✔
\nThere were some excellent answers that easily scored the three marks, but also many poorer ones, sometimes with no mention of genetic control or testosterone.
\nCandidates tended either to know the mechanism of phototropism very well, or not at all. It wasn't always clear that the terms shoot, stem and leaf were correctly distinguished in some candidates' minds.
\nWeaker candidates found this a real challenge as it was necessary to understand the nature of the nervous and endocrine systems well enough to be able to point out similarities and differences. Often there was no obvious comparing or contrasting in the answers.
\nCellular processes at the molecular level are regulated by enzymes.
\nOutline the process of DNA profiling.
\nOutline the role of DNA polymerase III in DNA replication.
\nExplain the factors that affect the rate of enzyme-controlled reactions in cells.
\na. sample of DNA obtained from person/hair/blood/mouth/crime scene ✔
\nb. PCR used to amplify/make copies of DNA (in sample) ✔
\nc. using Taq DNA polymerase / using DNA polymerase from thermophilic bacteria ✔
\nd. tandem repeats amplified/used ✔
\ne. gel electrophoresis used to separate DNA (into bands) ✔
\nf. separation according to length of fragments/number of repeats
OR
fragments of same length/number of repeats travel same distance ✔
g. pattern of bands/numbers of repeats is the profile/is unique to the individual ✔
\nh. example of application/forensics/crime investigation/paternity ✔
\nDo not accept ‘determine ancestry’ for mph. Other genes/chromosomes are more often used for that.
\nAccept STR for (short) tandem repeat in mpd.
\na. binds to template strand adjacent to a primer/at the primer ✔
\nb. adds nucleotides to template strand/to single stranded DNA ✔
\nc. using complementary base pairing ✔
\nd. links nucleotides with sugar-phosphate/phosphodiester bonds ✔
\ne. adds nucleotides/builds new strand in 5' 3' direction ✔
\nf. lagging strand is built in short segments/Okazaki fragments/synthesis is discontinuous ✔
\nAccept A to T and G to C instead of ‘complementary’ in mpc.
\nFor mpb it must be clear that nucleotides, not bases, are added to an existing strand of DNA. Do not accept ‘to replication fork’ for this.
\na. temperature increases rate up to optimum and higher temperatures decrease rate / graph ✔
\nb. faster molecular movement as temperature rises (so more substrate-active site collisions) ✔
\nc. high temperature/heat causes denaturation/irreversible change to active site (so rate reduces) ✔
\nd. rate decreased if pH is above and below optimum/if pH is too high or low / graph ✔
\ne. pH affects shape/structure of enzyme/active site /affects ionization (of amino acids) ✔
\nf. increases in substrate concentration cause rate to rise towards a plateau/WTTE / graph ✔
\ng. greater chance of substrate-active site collisions with higher substrate concentration
OR
active sites saturated/all full at high substrate concentrations ✔
h. higher enzyme concentration increases rate (as there are more active sites) ✔
\ni. enzyme inhibitors/competitive inhibitors/non-competitive inhibitors reduce the rate ✔
\nj. end-product inhibitors switch off metabolic pathway / act on enzyme at start of pathway rate ✔
OR
allosteric site used to control enzyme activity by binding of (non-competitive) inhibitor
Graphs can be used for mpa, mpd mpf and mph but x-axis must have the variable indicated.
\nFor mpa there must be exponential rise to optimum then faster drop.
\nFor mpd there must be a bellshaped curve but it need not be exactly symmetrical.
\nFor mpf and mph there must be decreasing increases in rate towards a plateau.
\nThere were some good outlines of DNA profiling. Most knew at least one application of the process. There was some confusion between profiling and sequencing and the use of tandem repeats was often missed.
\nFor the well-prepared candidate, this posed no problems and the four marks were easily earned. At the other end of the performance range almost all candidates mentioned Okazaki fragments — a benefit of giving something a distinctive name.
\nThis was expected to be high-scoring, because it is such a standard topic, but candidates may have been running out of steam by the time they reached it and many answers were too brief, with significant points omitted.
\n\n Identify in which continent the fewest types of neonicotinoid were detected in honey samples.\n
\n\n [1]\n
\n\n Using the data, outline the different use of thiamethoxam in North and South America.\n
\n\n [1]\n
\n\n Identify the total percentage of honey samples contaminated with neonicotinoid pesticides in the continent with the lowest overall levels of contamination.\n
\n\n
\n\n . . . . . . . . . . . . %\n
\n\n [1]\n
\n\n Deduce the conclusions that can be drawn from the data in the graph.\n
\n\n [2]\n
\n\n Suggest a reason for the effect of a diet reduced in acetylcholine on the larval survival rate.\n
\n\n [1]\n
\n\n Compare and contrast the effect of clothianidin and thiacloprid treatments on the concentration of acetylcholine in royal jelly.\n
\n\n [2]\n
\n\n Explain how neonicotinoids affect synaptic transmission in insects.\n
\n\n [3]\n
\n\n Oceania ✔\n
\n\n thiamethoxam is the most used in North America (but not in South America) / CONVERSE\n
\n \n \n OR\n \n \n
\n in South America they use different neonicotinoids to thiamethoxam ✔\n
\n \n OWTTE\n \n .\n
\n\n \n Allow numerical comparison\n \n .\n
\n\n 57 (%) ✔\n
\n\n \n Allow range 56 % to 58 %\n \n .\n
\n\n a. reduced acetylcholine does not affect larval survival in the first two days ✔\n
\n\n b. reduced acetylcholine causes increased mortality from day 3 onwards ✔\n
\n\n c. 90 % versus 40 % survival/other valid numerical comparison by day 5\n
\n \n \n OR\n \n \n
\n (much) greater decrease in survival occurs between days 4 and 5/by day 5 for the reduced group ✔\n
\n \n Do not accept answers without times/ days\n \n .\n
\n\n (larvae lacking acetylcholine/with reduced AcH cannot survive because) acetylcholine is a neurotransmitter taking message from one neuron to another in synapses of nervous tissues\n
\n \n \n OR\n \n \n
\n messages would not pass from one neuron to the other\n
\n \n \n OR\n \n \n
\n (larvae) unable to synthesise/produce AcH, so need it from their diet ✔\n
\n \n OWTTE\n \n .\n
\n\n a. both neonicotinoids reduce the concentration of acetylcholine in royal jelly (compared to control) ✔\n
\n\n b. clothianidin reduces the concentration of acetylcholine in royal jelly more than thiacloprid (but perhaps not statistically different) ✔\n
\n\n c. clothianidin is used in smaller concentrations (than thiacloprid) so no firm conclusion can be obtained ✔\n
\n\n \n Need one similarity and one difference.\n \n
\n\n a. neonicotinoids bind to the (acetylcholine) receptor (in insects) ✔\n
\n\n b. (binding happens) in (cholinergic) synapses/at motor end plate/between motor neuron and muscles ✔\n
\n\n c. neonicotinoids bind irreversibly (to receptors)\n
\n \n \n OR\n \n \n
\n (receptors are blocked so) acetylcholine is unable to bind ✔\n
\n d. acetylcholinesterase/enzymes cannot break down neonicotinoids ✔\n
\n\n e. (synaptic) transmission prevented ✔\n
\n\n f. (causing) insect paralysis/death ✔\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n Almost all were able to name Oceania.\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n Most identified that the proportion of the pesticide was greater in North than South America. This does not actually mean that a greater mass was used as these were relative frequency graphs.\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n Most read the graphs correctly, stating between 56 and 58 %.\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n An answer comparing the survival as the days progressed was expected. Answers tended to be too general.\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n At least half of the candidates did not know that acetylcholine is a neurotransmitter (section 6.5).\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n A compare and contrast question, that required one similarity and one difference. Most were able to state that both treatments reduce the acetylcholine concentration in the royal jelly and that clothianidin reduced it more than thiacloprid. The more astute also commented on the differences in concentrations of the insecticides.\n
\n\n In general, the data analysis question was well attempted by the majority of students.\n
\n\n Those students who had studied synaptic transmission scored well. However, many knew the role of acetylcholine here, but not in Q(1)(e).\n
\nCholinergic synapses use acetylcholine as their neurotransmitter. They are widespread in the body, passing on signals to muscle cells. These synapses are affected by neonicotinoid pesticides.
\nThe diagrams, which are not drawn to scale, show the synapse between two neurons and a detail of the synaptic cleft.
\nOn the diagrams, label with a letter H the hydrophilic end of a phospholipid.
\nOn the diagrams, label with a letter E a vesicle involved in exocytosis.
\nOn the diagrams, label with a letter P a location where a neonicotinoid pesticide could bind.
\nOutline how depolarization of the membrane of an axon occurs.
\nExplain how acetylcholine initiates an action potential in a postsynaptic membrane.
\nState the action of the enzyme acetylcholinesterase.
\nExplain what happens to an enzyme if there is a change of pH.
\nhydrophilic end of phospholipid shown ✔
\nFor 2a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
\nAward the mark if the full word is used to label the structure. Labels must be unambiguous.
\nan exocytic vesicle shown ✔
\nFor 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
\nAward the mark if the full word is used to label the structure. Labels must be unambiguous.
\nAccept any of the three intact circles or the exocytosis shown within the box.
\nany receptor protein on post synaptic membrane marked ✔
\nFor 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
\nAward the mark if the full word is used to label the structure.
\nMust unambiguously be pointed to a receptor on the synapse side. Accept labels of acetylcholinesterase.
\na. (local depolarization) causes ion / sodium / voltage gated channels to open ✔
\nb. altering membrane permeability to sodium ions/Na+/positive ions / Na+ diffuses into the cytoplasm ✔
\nc. membrane potential (of the axon) changes from negative to positive ✔
\nd. a threshold potential is reached and an action potential is generated ✔
\nDo not accept sodium pumps.
\nMp b requires evidence that the student recognizes sodium is an ion.
\nNumerical values indicating change from −70 to +30/+40/+55 can be accepted.
\na. acetylcholine binds to the receptor protein ✔
\nb. (causing a) change in tertiary structure / conformation of protein ✔
\nc. diffusion/entry of ions / Na+ through the receptor/channel protein allowed ✔
\nbreaks down acetylcholine (to acetyl and choline) ✔
\na. change in pH can cause a change in the tertiary structure of the enzyme
OR
enzyme denatured ✔
b. causing a change in the active site / substrate not able to bind ✔
\nc. the enzyme will not work as efficiently / decreased rate of reaction ✔
\nd. (may no longer be) the optimum/optimal pH for enzyme activity ✔
\nAllow answer in the form of a graph.
\nThere were numerous examples of ambiguous labels, for example, an H without an arrow placed beside the membrane making it unclear whether the student was indicating the hydrophobic or hydrophilic part of the membrane.
\nBoth receptor proteins and acetylcholinesterase were accepted as proteins to which acetylcholine can bind. The markscheme did not accept locations other than on the synaptic side.
\nIn many exams, candidates seemed to confuse the voltage gated ion channels with sodium-potassium pumps.
\nA broad bean is the seed of a species, Vicia faba, in the Fabaceae, a family of flowering plants. This family contains many species that are used as sources of food.
\nOn the diagram, label the testa and the radicle.
\nAn experiment was done to test the hypothesis that temperature affects the rate of germination of the broad bean. Outline two factors apart from temperature that should be controlled in this experiment.
\nState the genus of the broad bean.
\nBroad beans are rich in starch and cellulose. Compare and contrast the structure of starch and cellulose.
\nOnce the germinated bean grows above the ground, state the process used by the bean in the production of starch.
\na. testa labelled ✔
\nb. radicle labelled ✔ (must point to the bottom half of the embryo or the tip).
\na. same amount/type of soil/substrate / pH of soil ✔ e.g.: heater
\nb. same amount of water / humidity ✔
\nc. oxygen/aeration ✔
\nd. same measurement of germination / time ✔ e.g.: emergence of radicle
\ne. same number/source/age of seeds ✔
\nAccept discussion of light only in as much as it relates to temperature.
\nVicia ✔
\nNo mark if the species name is included.
\nMark can be awarded if the genus is not capitalized.
\na. both polymers of glucose molecules / polysaccharides
OR
both form 1,4 glycosidic bond ✔
b. starch is formed by alpha glucose while cellulose is formed by beta glucose
OR
in starch C1 hydroxyl groups are found in same plane while in cellulose on different planes
OR
in cellulose, alternatively the beta glucose needs to be placed upside-down in order to have C1 hydroxyl groups on the same plane
OR
two types of starch (amylose and amylopectin) but one type of cellulose ✔
One similarity and one difference.
\nComparison to cellulose needed.
\nphotosynthesis
\nDo not accept condensation or polymerization; if list of processes given, mark the first answer.
\nMany candidates did not correctly label the testa and in many cases the radicle was labelled too high up, pointing at the hypocotyl.
\nSeveral answers considered how light is needed for germination without any reference to its relationship to temperature.
\nThis question presented some confusion to Spanish speaking candidates since the word \"género\" is the same for \"genus\" and \"gender\", so some candidates write \"male/female\" in their answers.
\nCandidates seemed to find it difficult to find similarities and differences in structure between starch and cellulose, and many wrote about their functions, or simply mentioned that both were made of \"glucose\" without referencing that they are both polymers. There was limited recognition that there are two forms of starch: amylose and amylopectin that differ in their degree of branching.
\nAutosomal genes are located in chromosomes that are not sex chromosomes. The inheritance of autosomal genes is affected by whether the genes are linked or unlinked. Explain the two types of inheritance, using the example of parents that are heterozygous for two genes A and B.
\nOutline how sperm are produced from diploid cells in the testis and how this production can be sustained over many decades of adult life.
\nTestis cells are eukaryotic cells. Identify the structures seen under the electron microscope in testis cells that are not present in prokaryotic cells.
\na. unlinked genes are on different chromosomes / vice versa ✔
\nb. unlinked alleles migrate/segregate/are inherited independently (during meiosis) / vice versa ✔
\nc. (In unlinked inheritance) there is an equal chance for all 4 options to occur / AB, Ab, aB, ab / vice versa ✔
\nd. (dihybrid crosses involving) linked genes do not produce Mendelian ratios ✔
\ne. (excluding recombinants) there is a 1:1 chance of inheriting the different options/AB or ab ✔
\nf. in linked characteristics alleles might not migrate together if there is crossing over/ recombinants are formed ✔
\ng. crossing over occurs in prophase I of meiosis ✔
\nh. when the sister chromatids migrate in meiosis II the characteristics forming gametes are different/Ab, aB ✔
\ni. formation of recombinants causes changes in ratio/probability of inheritance/genetic variation ✔
\nj. correct named example of inheritance of linked/unlinked characteristics ✔
\nk. Punnett/paired diagrams of both unlinked and linked characteristics ✔
\nl. genes which are linked but are far apart on the chromosome can display independent assortment ✔
\nMp a could be awarded from an annotated diagram.
\nAllow annotated diagram of inheritance / could be shown in a Punnett square.
\nAllow annotated diagram of linked inheritance for mp f.
\nFor mp K, accept sex linked examples involving two genes.
\nIf the student interprets the question as sex-linked and autosomal inheritance, look for WTTE marks from the scheme.
\na. germinal epithelium divide endlessly (by mitosis giving rise to spermatagonia)
\nb. spermatogonia are diploid/2n ✔
\nc. spermatogonia divide by mitosis / provide a continuous supply throughout adult life ✔
\nd. (some) spermatogonia enlarge forming primary spermatocytes ✔
\ne. primary spermatocytes undergo the first division of meiosis/meiosis I ✔
\nf. secondary spermatocytes produced are haploid/n ✔
\ng. secondary spermatocytes undergo the second division of meiosis (to produce spermatids) ✔
\nh. spermatids develop tails
OR
spermatids differentiate into spermatozoa / spermatids associate with Sertoli cells ✔
Marks can be awarded to an annotated diagram.
\nDo not accept sperm or spermatozoa as equivalent to spermatagonia or spermatocytes.
\na. nucleus/nuclear membrane ✔
\nb. membrane bound organelles ✔
\nc. mitochondria ✔
\nd. rough ER/smooth ER/golgi apparatus ✔
\ne. lysosomes / centrioles ✔
\nf. large/80S ribosomes / ribosomes attached to a membrane ✔
\ng. linear chromosomes / histones ✔
\nQuestion 5 was not a popular question choice. Few candidates performed well on this question overall. Many students wrote vaguely about dominant and recessive alleles or confused gene linkage with \"sex-linkage\" or even multiple alleles and blood groups.
\nAlthough many candidates performed very well, many answers were vague and simply stated that sperm cells were made by meiosis without any further details. Commonly, the stages where meiosis occurs and where mitosis occurs were muddled as well as whether primary or secondary spermatocytes were diploid or haploid. The terms spermatozoa and spermatids were commonly used interchangeably.
\nExplain the role of hydrogen ions used in photosynthesis.
\nDescribe how the structure of the chloroplast is adapted to its function in photosynthesis.
\nThe enzyme Rubisco is used in carbon fixation during photosynthesis. Identify four other examples of proteins that illustrate the wide range of functions of this group of biochemicals in living organisms.
\na. hydrogen ions (and oxygen) are obtained from photolysis of water ✔
\nb. (excited) electrons (from Photosystem II) contribute to generate a hydrogen ion/proton gradient
OR
protons build up in the thylakoid space/inside the thylakoid ✔
c. ATP synthase generates ATP by chemiosmosis/using the proton gradient/by movement of protons from the thylakoid space to the stroma ✔
\nd. (two) high energy/excited electrons (from Photosystem II) are passed through an electron transport chain ✔
\ne. NADP accepts H+ (from the stroma) / electrons to produce NADPH+/reduced NADP ✔
\nf. ATP/reduced NADP/NADPH+ are used by light-independent reactions/Calvin cycle in the stroma ✔
\ng. (in Calvin cycle) carbon fixation to ribulose bisphosphate/RuBP produces G3P ✔
\nh. ATP is used to transform G3P to TP ✔
\ni. reduced NADP/NADPH+ is used to transform G3P to TP ✔
\nAccept answers in an annotated diagram.
\nAccept NADPH2/NADPH/NADPH + H+ in place of NADPH+.
\na. thylakoids have a small internal volume to maximize hydrogen gradient / quickly increase concentration gradient ✔
\nb. many grana / thylakoids give large surface area ✔
\nc. (grana) contain (photosynthetic) pigments/electron carriers/ATP synthase enzymes ✔
\nd. (photosynthetic) pigments/chlorophyll arranged into photosystems allowing maximum absorption of light energy ✔
\ne. stroma has suitable pH/ enzymes of Calvin/light independent cycle ✔
\nf. lamellae connect/separate grana maximizing photosynthetic efficiency ✔
\nAccept diagram showing the disposition in space.
\na. insulin receptors start the cellular signalling pathway/other receptor
\nb. leptin is a hormone / other protein hormone ✔
\nc. immunoglobulins/antibodies defend body from disease ✔
\nd. rhodopsin/photosystems convert light energy to electrical impulses ✔
\ne. keratin/collagen/other example is a structural protein ✔
\nf. tubulin/microtubules part of cytoskeleton/ involved in cell division
\ng. ATP synthase is an enzyme that catalyzes the formation of ATP / other enzyme and function ✔
\nh. fibrin/fibrinogen is a protein involved in clotting ✔
\ni. hemoglobin/sodium potassium pump/other example is a transport protein ✔
\nj. actin/myosin/other example is involved in muscle contraction ✔
\nk. any other named example and function ✔
\nl. a second other named example and function ✔
\nm. spider silk is used to form spider webs/capture prey ✔
\nAccept only one example of each protein type for example only one enzyme or only one hormone etc.
\nMp k can be awarded only once.
\nMp I can be awarded only once.
\nQuestion 6 was a popular question.
\nA relatively high scoring section, though many students failed to discuss the role of hydrogen in the light independent reactions.
\nQuestion 6 was a popular question.
\nWell answered.
\nQuestion 6 was a popular question.
\nWell answered by most except for when students gave multiple examples of proteins of the same functional type; for example, four examples of enzymes when the question asked for an illustration of the wide range of functions of proteins.
\nDescribe the structure of the DNA molecule.
\nOutline the role of three enzymes used in the replication of DNA.
\nInsulin is produced in β cells of the pancreas and not in other cells of the human body. Explain how differentiation of cells and regulation of gene expression allow proteins such as insulin to be produced in only certain types of body cell.
\na. two stranded/double helix ✔
\nb. antiparallel / strands running in opposite directions
OR
one strand organized 5’ to 3’ and the other 3’ to 5’ ✔
c. sugar-phosphate backbone ✔
\nd. each strand formed by chains of nucleotides ✔
\ne. each nucleotide is formed by a phosphate, a deoxyribose and a base / annotated diagram of a nucleotide clearly indicated as a nucleotide ✔
\nf. the bases are adenine, guanine, cytosine and thymine ✔
\ng. strands held together by hydrogen bonds (between complementary base pairs)
OR
A pairs with T and C pairs with G ✔
Both helix and two strands needed for mp a. Double helix is sufficient for the mark.
\nPoints can be awarded to annotated diagrams.
\nFor mp c, the explicit label sugar phosphate backbone is required.
\nTo award mp d from a diagram, at least three pairs of nucleotides should be shown.
\nFor mp e, the diagram would need to be labelled as a nucleotide.
\nFor mp e, expect deoxyribose not just sugar.
\nThe written names of the bases are required for mp f.
\nDo not penalize twice for mp f and g for using letters.
\nIf they only ever use the symbols A,T,C and G they are ineligible for mp f. If however, they say A pairs with T and C pairs with G, then they would get mp g. If they wrote adenine pairs with thymine and cytosine pairs with guanine, then they would obtain both mp f and mp g.
\na. helicase to separate/unwind DNA strands ✔
\nb. gyrase / toposiomerase to relax the tension as bacterial DNA is being uncoiled / prevent supercoiling ✔
\nc. primase to synthesise primers ✔
\nd. polymerase (I) removes primers and replaces with nucleotide ✔
\ne. polymerase (III) adds nucleotides (in a 5' to 3' direction) ✔
\nf. ligase joins (Okazaki) fragments together ✔
\nAccept the enzyme name without ‘DNA’ included; e.g. ‘DNA ligase’ or ‘ligase’ can both be accepted.
\na. insulin production is determined by a gene ✔
\nb. gene for insulin (is found in all cells), but only activated in (β cells of) pancreas ✔
\nc. stem cells differentiate into specialized cells/(into pancreatic β) ✔
\nd. during differentiation some genes are turned on and others off ✔
\ne. insulin is a hormone that regulates the amount of glucose/sugar in blood ✔
\nf. pancreatic β cells have sensors that detect glucose level in blood ✔
\ng. an increase in glucose will increase transcription of mRNA of insulin ✔
\nh. the site of transcription of insulin is in the pancreatic β cells ✔
\ni. gene transcription is regulated by proteins that bind to specific base sequence in DNA/ enhancers/silencers/promoter proximal elements ✔
\nj. regulatory sequences/proteins are specific to the gene they regulate / insulin regulator proteins are only found in in the pancreatic β cells ✔
\nk. (DNA) methylation (usually) inhibits gene expression / (histone) acetylation promotes gene expression / tightness of coiling of DNA around histones affects gene expression ✔
\nAccept sugar as equivalent to glucose.
\nA surprising number of students failed to discuss nucleotides explicitly.
\nWell answered.
\nIn general, scores for this question were quite low. Candidates were too vague in their answers. Many failed to discuss chemical modification of DNA and the role of non-coding sequences in the regulation of gene expression.
\nThe diagram shows some of the metabolic processes taking place in a plant cell.
\nIdentify the process Y and state the name of the organelle where it takes place in a plant cell.
\nProcess Y:
\nName of the organelle:
\nIdentify the molecule Z.
\nState the type of reaction which converts excess monosaccharides to polysaccharides.
\nThe process X uses oxygen and produces ATP. Identify the process X.
\nOutline the uses of ATP in plant cells.
\nWith reference to the diagram, identify one example of catabolism.
\nprocess Y: photosynthesis ✔
\norganelle: chloroplast ✔
\nBoth needed.
\nglycerol /glycerin ✔
\ncondensation/dehydration/synthesis/anabolic/anabolism ✔
\naerobic respiration ✔
\na. used as an energy source ✔
\nb. supplies/releases energy for biochemical reactions ✔
\nc. photosynthesis/active transport/other verifiable example of reaction or process ✔
\na. (letter X) breakdown of monosaccharides/respiration ✔
\nb. (letter Y) hydrolysis/photolysis in photosynthesis/oxidation of water ✔
\nAnswer must come from the diagram.
\nThe diagram was generally confusing to candidates.
\nThis was generally answered correctly.
\nThe diagram was generally confusing to candidates.
\nThis was generally missed.
\n\n
The diagram was generally confusing to candidates.
\nThis was often answered correctly.
\nThe diagram was generally confusing to candidates.
\nAlthough many answered cell respiration, aerobic respiration was expected since the question defined aerobic respiration.
\nThe diagram was generally confusing to candidates.
\nExpressing the nature of ATP in an outline of its use to plants was problematic for many students. Rather than a source of energy it was thought to be energy itself. Many processes use ATP in one part of the pathway or another, so a wide variety of seemingly contradictory answers received credit. For example, ATP is used in cell respiration during glycolysis. Credit was given for glycolysis as well as for respiration.
\nThe diagram was generally confusing to candidates.
\nToo often candidates did not make reference to the diagram when giving an example of catabolism. Some thought that ADP changing to ATP was catabolism. This probably comes from a failure to understand paired reactions.
\nSquid have been used for research on nerve impulses. The mechanism of nerve transmission in these animals is the same as that in humans. The image shows an oscilloscope trace from a nerve impulse in a squid’s giant axon.
\n[squid] Pixabay.
\nEstimate the resting potential for this axon.
\nOutline the role of the sodium–potassium pump in maintaining the resting potential.
\n−65 mV ✔
\nUnit required; minus sign required. Accept answers from −62mV to −65mV.
\na. sodium/potassium pump pumps sodium ions Na+ out of axon and pumps potassium ions/K+ in ✔
\nb. requires energy/ATP/against concentration gradients/active transport ✔
\nc. three sodium ions pumped out for every two potassium ions pumped in ✔
\nd. results in charge difference between inside and outside where outside is positive relative to inside ✔
\nStrong students answered this question showing clear understanding of the process. However, most students showed poor understanding with errors: correct direction of ions being pumped not known; the direction of ions during repolarization was mixed up with resting state. Few wrote about the charge difference between inside and outside of the axon during the resting potential.
\nThe image shows the wings of an insect, bird and bat.
\n[Source: [insect wing] Halvard Hatlen https://upload.wikimedia.org/wikipedia/commons/0/0f/Dip-trichoceridae-wing.png. [bird wing] University of Maryland, Department of Geology. [bat wing] University of Maryland, Department of Geology.]
\nBased on their structure, the insect and bat wings are analogous. Outline what is meant by an analogous trait.
\nThe bird and bat wings share homologous bone structures whereas the insect wing does not. Outline the conclusion that can be drawn about the evolution of these wings, based on homologous structures.
\nExplain how cladistics can be used to investigate evolutionary relationships.
\nCladistics and other evolutionary evidence suggest that mammals and birds have a more recent common ancestor than mammals and amphibians. Draw a cladogram to show the relationships between mammals, birds and amphibians.
\na. analogous traits have a different evolutionary history/ancestry ✔
\nb. different structures are adaptations for flight ✔
\nc. selective pressure leads to a similar solution to the problem of flying ✔
\na. bird and bat share a more recent common ancestor (than the insect) ✔
\nb. bird and bat are more closely related than insect and bat / insect and bird ✔
\nc. bird and bat wings evolved from a common ancestor (by natural selection) ✔
\na. (cladistics) shows evolutionary relationships through a common ancestor ✔
\nb. cladistics uses DNA/protein/derived/shared anatomical characteristics/traits ✔
\nc. time of divergence is related to the number of differences in DNA (base sequence) / protein (sequence of amino acids) ✔
\nd. homologous (versus analogous) traits are used to place an organism in a clade ✔
\ne. more shared characteristics mean a more recent common ancestor ✔
\nWhat is important is that the proper relationships are shown not the style used.
\nCandidates found it hard to express themselves without contradiction.
\nIn both parts the key to successful answers was in using the organisms given in the example. Contradictions were thus avoided.
\nCandidates found it hard to express themselves without contradiction.
\nIn both parts the key to successful answers was in using the organisms given in the example. Contradictions were thus avoided.
\nCandidates found it hard to express themselves without contradiction.
\nCarrying examples into part (b) made for the most successful answers. Very few gained marks for providing answers that included molecular level cladistic evidence, e.g. DNA, protein sequencing, etc.
\nCandidates found it hard to express themselves without contradiction.
\nExaminers accepted a wide variety of diagrams and orientations as long as the relationships among amphibians, birds and mammals were appropriate. Nevertheless, there was poor performance on this question. It was the question most often left blank in Section A.
\nOutline the structure of proteins.
\nCells produce a large variety of proteins with different sequences of amino acids. Explain how this is done.
\nOutline the range of functions of proteins in cells.
\na. protein formed from amino acids
OR
20 different amino acids ✔
b. linked together by peptide bonds ✔
\nc. may consist of one or more polypeptides linked together ✔
\nd. have a specific shape/conformation/folding ✔
\ne. shape determines function ✔
\na. protein is produced when a gene is expressed / switched on ✔
\nb. genetic code/codons consists of three nucleotides/bases/base triplet ✔
\nc. genetic code in DNA is transcribed/transcription (to mRNA) ✔
\nd. mRNA exits the nucleus ✔
\ne. mRNA (code) is translated/translation into a polypeptide/protein ✔
\nf. amino acid sequence/polypeptide formation occurs at a ribosome ✔
\ng. one codon translates to one amino acid ✔
\nh. tRNA carries code for specific amino acids ✔
\ni. tRNA anticodon matches with specific codon in mRNA ✔
\nj. amino acids joined (by peptide bonds) to form polypeptide ✔
\nk. sequence of amino acids determined by order of bases/nucleotides/codons in DNA/mRNA ✔
\nl. proteins vary based on which amino acids are used and their order
OR
protein variety increases by mutations to DNA ✔
a. enzymes that catalyse/speed up/control (the rate and direction of) metabolic reactions ✔
\nb. proteins can be hormones which are chemical messengers to cells ✔
\nc. proteins that transport through the membrane such as channel/carrier/pumps / that regulate what enters/leaves the cell ✔
\nd. hemoglobin in red blood cells that transports/ binds oxygen ✔
\ne. membrane proteins for cell/tissue recognition/cell adhesion/communication ✔
\nf. structural elements of muscle fibre/actin/myosin for movement
OR
spindle fibres move chromosomes ✔
g. histones condense DNA into chromosomes ✔
\nThe question requires answer that the function is in cells.
\nThis question was attempted by about 42 % of candidates.
\nMany candidates used the terms primary, secondary, tertiary and quaternary as their basic outline. These terms occur in AHL but not in the core. Therefore, without an outline of the meaning of these terms few marks could be earned.
\nThis question was attempted by about 42 % of candidates.
\nSome brilliant, clear answers were given earning the maximum marks. However, coherent essays explaining protein synthesis were rare. Some students dwelled on transcription without ever mentioning translation. Codons, anticodons and their locations were not clearly understood. Valid information did appear but it was frequently mixed up or incomplete.
\nThis question was attempted by about 42 % of candidates.
\nGenerally good answers were received for those who read the question correctly, i.e. protein function within cells.
\nOutline the process of inhalation.
\nExplain the process of gas exchange taking place in the alveoli.
\nDiscuss the relationship between atmospheric carbon dioxide concentration and global temperatures.
\na. diaphragm contracts / moves downwards/flattens ✔
\nb. external intercostal muscles contract ✔
\nc. (muscle contraction) moves the rib cage upwards and outwards ✔
\nd. increases volume of the thorax / lungs ✔
\ne. difference in pressure/decreasing pressure causes air to flow into lungs / lungs inflate ✔
\na. O2 diffuses into blood and CO2 diffuses out from blood ✔
\nb. blood entering the alveoli is high in CO2/low in O2 ✔
OR
air in alveolus is high in O2/low in CO2 ✔
c. diffusion (in either direction) take place due to concentration gradients ✔
\nd. concentration gradients maintained by ventilation/blood flow ✔
\ne. large surface area created by many alveoli/spherical shape of alveoli for more efficient diffusion ✔
\nf. rich supply of capillaries (around alveoli) allows efficient exchange ✔
\ng. type I pneumocytes are thin to allow easy diffusion/short distances ✔
\nh. gases must dissolve in liquid lining of alveolus in order to be exchanged ✔
\ni. type II pneumocytes secrete surfactants to reduce surface tension/prevent lungs sticking together ✔
\nj. type II pneumocytes create moist conditions in alveoli ✔
\na. greenhouse effect keeps Earth warm ✔
\nb. (over-time) earth’s temperature has fluctuated naturally ✔
\nc. changes in human activities have led to increases in CO2 concentration ✔
\nd. CO2 absorbs infrared/long wave radiation / trapping heat ✔
\ne. increase in atmospheric CO2 (concentration) correlates with/causes increased global average temperature ✔
\nf. use of fossil fuels increases atmospheric CO2 ✔
\ng. deforestation removes a carbon sink / less CO2 absorbed ✔
\nh. loss of polar ice causes less reflection of surface light/ more reradiation as heat contributing to (global) warming ✔
\ni. CO2 is not the only greenhouse gas/ there are other greenhouse gases ✔
\nThis question was attempted by approximately 56 % of candidates.
\nDone well by many. Diaphragm contraction and external intercostal muscle contraction was commonly known along with the idea that the rib cage moves upwards and outwards. Further understanding of the consequent pressure changes in the thorax was not clear.
\nThis question was attempted by approximately 56 % of candidates.
\nSome coherent, clear answers were given earning the maximum marks. However, commonly, valid ideas were given but were jumbled. Coherent writing was usually missing. The term diffusion was missing in many answers. The idea that alveoli are surrounded by capillaries to enable exchange of gases between the air in alveolus and the blood did not seem to be a mental picture for many. Concentration gradients as a reason for diffusion of oxygen or carbon dioxide between the alveoli and the blood was rarely explained. However, it seemed that everyone knew about surfactants from type II pneumocytes.
\nThis question was attempted by approximately 56 % of candidates.
\nThis question was actually quite narrow as reflected in the number of marks it could earn. Some candidates understood which part of the global warming topic was being asked of them and stuck to the topic. Good applications to the carbon cycle were given. Strong responses avoided deviations into consequences and how humans should behave. Several misunderstandings surfaced. For example that CO2 was responsible for the breakdown of the ozone layer. Many seemed to tie ozone layer destruction and global warming information together in their explanation. Atmospheric carbon dioxide and the ozone layer were confused. Some thought that the ozone layer traps heat.
\nThe diagram shows the stages in the cell cycle.
\nState processes occurring during interphase.
\nUsing the letter C, label the stage on the diagram where chromosome supercoiling occurs.
\nUsing the letter M, label the stage on the diagram where sister chromatids migrate to opposite poles.
\nDistinguish between the outcomes of a cell dividing either by mitosis or meiosis.
\nThe mitotic index is an important prognostic tool for predicting the response of cancer cells to chemotherapy. Outline how the mitotic index is calculated.
\na. DNA replication ✔
\nb. cell growth ✔
\nc. duplication of organelles/mitochondria / production of microtubules/protein synthesis ✔
\nAccept first two answers only.
\nprophase ✔
\nThe stage should be clearly labelled.
\nIf more than one stage is shown the candidate does not receive a mark.
\nanaphase ✔
\nThe stage should be clearly labelled.
\nIf more than one stage is shown the candidate does not receive a mark.
\na. mitosis produces two daughter cells while meiosis four ✔
\nb. mitosis produces cells with same number of chromosomes (2n) while in meiosis they are halved (n) ✔
\nc. mitosis produces body cells but meiosis produces gametes
OR
mitosis produces genetically identical cells but meiosis does not ✔
Allow answers in a table.
\nReference to both has to be present for the mark.
\nnumber of cells (seen under the microscope) undergoing mitosis divided by the total number of cells (observed in sample area) ✔
\nCan be given as a %.
\nThere were several G2 comments about Q2a, as it could be said that several processes were occurring. Perhaps it should have been more specific. Two out of DNA replication, cell growth and duplication of organelles were expected for the two marks. A surprising number of candidates in all languages, confused 'processes' with 'phases' and stated that interphase consisted of G1, S and G2. Most managed to label prophase and anaphase correctly on the diagram as required in part (b). The answers to part (b) were generally clear, but some lost marks for not comparing the same point. For example 'Mitosis produces diploid cells, but meiosis produces gametes' did not score a mark. The mitotic index was well known.
\nThere were several G2 comments about Q2a, as it could be said that several processes were occurring. Perhaps it should have been more specific. Two out of DNA replication, cell growth and duplication of organelles were expected for the two marks. A surprising number of candidates in all languages, confused 'processes' with 'phases' and stated that interphase consisted of G1, S and G2. Most managed to label prophase and anaphase correctly on the diagram as required in part (b). The answers to part (b) were generally clear, but some lost marks for not comparing the same point. For example 'Mitosis produces diploid cells, but meiosis produces gametes' did not score a mark. The mitotic index was well known.
\nThere were several G2 comments about Q2a, as it could be said that several processes were occurring. Perhaps it should have been more specific. Two out of DNA replication, cell growth and duplication of organelles were expected for the two marks. A surprising number of candidates in all languages, confused 'processes' with 'phases' and stated that interphase consisted of G1, S and G2. Most managed to label prophase and anaphase correctly on the diagram as required in part (b). The answers to part (b) were generally clear, but some lost marks for not comparing the same point. For example 'Mitosis produces diploid cells, but meiosis produces gametes' did not score a mark. The mitotic index was well known.
\nThere were several G2 comments about Q2a, as it could be said that several processes were occurring. Perhaps it should have been more specific. Two out of DNA replication, cell growth and duplication of organelles were expected for the two marks. A surprising number of candidates in all languages, confused 'processes' with 'phases' and stated that interphase consisted of G1, S and G2. Most managed to label prophase and anaphase correctly on the diagram as required in part (b). The answers to part (b) were generally clear, but some lost marks for not comparing the same point. For example 'Mitosis produces diploid cells, but meiosis produces gametes' did not score a mark. The mitotic index was well known.
\nThere were several G2 comments about Q2a, as it could be said that several processes were occurring. Perhaps it should have been more specific. Two out of DNA replication, cell growth and duplication of organelles were expected for the two marks. A surprising number of candidates in all languages, confused 'processes' with 'phases' and stated that interphase consisted of G1, S and G2. Most managed to label prophase and anaphase correctly on the diagram as required in part (b). The answers to part (b) were generally clear, but some lost marks for not comparing the same point. For example 'Mitosis produces diploid cells, but meiosis produces gametes' did not score a mark. The mitotic index was well known.
\nThe image represents the structure of the enzyme Rubisco from common pea (Pisum sativum).
\nState one function of Rubisco.
\nState a role of the active site of an enzyme.
\nState the genus of the plant where this Rubisco is found.
\nOutline one factor that could affect the activity of Rubisco.
\na. enzyme involved in photosynthesis/carbon fixation/Calvin cycle
OR
speeds up chemical reactions in photosynthesis ✔
b. carboxylation of RuBP ✔
\nc. production of carbohydrate in photosynthesis ✔
\nd. addition of carbon dioxide to form glucose (in Calvin cycle) ✔
\nEither photosynthesis or carbon fixation must be mentioned.
\nsite to which substrate binds
OR
catalytic site ✔
Give credit for the lock and key analogy.
\nPisum ✔
\na. name of factor ✔
\nb. how it affects rate of reaction ✔
\nExample answer:
\ntemperature ✔
\nas the temperature increases the rate of reaction increases until it reaches a maximum and then decreases rapidly ✔
\nAccept answers in a graph.
\nThere were some G2 comments about the function of Rubisco. It is mentioned in 2.4. and the stem did say that it was an enzyme. Simply stating that it was an enzyme involved in photosynthesis was sufficient for the mark. A detailed function was not required. Most could state the role of the active site and that the genus was Pisume. The word genus in this context confused some French candidates who answered 'male or female'. 3(d) was simply testing the factors controlling enzyme action. Weaker students just listed several factors. When it says one factor, only the first is considered. The difference between heat and temperature was occasionally confused. The examiners were looking for the effect of increasing and decreasing the factor. Many students seemed to be obsessed with denaturing. For example the answer 'temperature' (first mark): as the temperature is too high the enzyme denatures (no second mark as no mention of decreasing temperature). Few used the collision theory idea which was expected.
\nThere were some G2 comments about the function of Rubisco. It is mentioned in 2.4. and the stem did say that it was an enzyme. Simply stating that it was an enzyme involved in photosynthesis was sufficient for the mark. A detailed function was not required. Most could state the role of the active site and that the genus was Pisume. The word genus in this context confused some French candidates who answered 'male or female'. 3(d) was simply testing the factors controlling enzyme action. Weaker students just listed several factors. When it says one factor, only the first is considered. The difference between heat and temperature was occasionally confused. The examiners were looking for the effect of increasing and decreasing the factor. Many students seemed to be obsessed with denaturing. For example the answer 'temperature' (first mark): as the temperature is too high the enzyme denatures (no second mark as no mention of decreasing temperature). Few used the collision theory idea which was expected.
\nThere were some G2 comments about the function of Rubisco. It is mentioned in 2.4. and the stem did say that it was an enzyme. Simply stating that it was an enzyme involved in photosynthesis was sufficient for the mark. A detailed function was not required. Most could state the role of the active site and that the genus was Pisume. The word genus in this context confused some French candidates who answered 'male or female'. 3(d) was simply testing the factors controlling enzyme action. Weaker students just listed several factors. When it says one factor, only the first is considered. The difference between heat and temperature was occasionally confused. The examiners were looking for the effect of increasing and decreasing the factor. Many students seemed to be obsessed with denaturing. For example the answer 'temperature' (first mark): as the temperature is too high the enzyme denatures (no second mark as no mention of decreasing temperature). Few used the collision theory idea which was expected.
\nThere were some G2 comments about the function of Rubisco. It is mentioned in 2.4. and the stem did say that it was an enzyme. Simply stating that it was an enzyme involved in photosynthesis was sufficient for the mark. A detailed function was not required. Most could state the role of the active site and that the genus was Pisume. The word genus in this context confused some French candidates who answered 'male or female'. 3(d) was simply testing the factors controlling enzyme action. Weaker students just listed several factors. When it says one factor, only the first is considered. The difference between heat and temperature was occasionally confused. The examiners were looking for the effect of increasing and decreasing the factor. Many students seemed to be obsessed with denaturing. For example the answer 'temperature' (first mark): as the temperature is too high the enzyme denatures (no second mark as no mention of decreasing temperature). Few used the collision theory idea which was expected.
\nThe mass of an individual organism can affect its physiology and feeding ecology. The diagram shows the relative mass of carbon (black) and total wet mass (grey) of a marine crustacean, Calanus hyperboreus and a jellyfish, Bathocyroe fosteri.
\n[Source: Kristian McConville, Angus Atkinson, Elaine S. Fileman, John I. Spicer, Andrew G. Hirst. Disentangling the counteracting effects of water content and carbon mass on zooplankton growth. Journal of Plankton Research. 2017, Volume 39, Issue 2, Pages 246–256. https://doi.org/10.1093/plankt/fbw094. Adapted (and translated) by permission of Oxford University Press.]
\nState one process that results in the loss of carbon dioxide from a marine organism such as a crustacean or a jellyfish.
\nThe crustacean and the jellyfish obtain carbon compounds by feeding. State one source of carbon for marine organisms, other than feeding.
\nExplain how energy enters, flows through and is lost from marine food chains.
\nDeduce whether jellyfish or crustacea are a richer source of carbon in a food chain.
\nSuggest with a reason whether having a large body mass is an advantage or disadvantage for jellyfish.
\na. (aerobic/cellular) respiration ✔
\nb. gas exchange / diffusion ✔
\nDo not accept photosynthesis.
\nDo not accept breathing
\nOrganism is taken to be a living thing.
\na. photosynthesis ✔
\nb. absorption of (dissolved) carbon dioxide / (hydrogen)carbonate directly from the oceans ✔
\nAccept reference to carbonate or hydrogencarbonate ions.
\na. light energy is converted to chemical energy (in carbon compounds/sugars) by photosynthesis ✔
\nb. (chemical) energy (in carbon compounds) flows by means of feeding/through food chains/webs ✔
\nc. only (approximately) 10 % of energy is passed to the next trophic level ✔
\nd. energy released as heat (by respiration) ✔
\ne. energy is not recycled ✔
\nf. after death, energy may remain trapped as undigested detritus/fossils/fossil fuels ✔
\ncrustacean as they have more carbon per unit volume
OR
crustacean as jellyfish has little carbon per total body size ✔
OWTTE.
\nThe conclusion must be supported from the information given.
\na. advantage of large size is ability to eat /catch large prey ✔
\nb. (advantage as) lower rates of predation of large jellyfish ✔
\nc. (advantage as) can produce more reproductive cells ✔
\nd. (disadvantage as) can move slower to escape from predators/capture prey ✔
\ne. (disadvantage as) needs more energy/nutrients to maintain structure/move/grow ✔
\nf. (disadvantage as) low surface area to volume ratio and thus possibly difficulty with materials/gas/nutrient exchange ✔
\ng. (disadvantage as) more prone to mechanical damage during storms ✔
\nAccept other reasonable answer.
\nMust say advantage or disadvantage.
\nIn 4(a) most were able to state 'respiration' as the answer. 4(b) proved more difficult with fewer stating photosynthesis or absorption of dissolved CO2 or HCO3 from the seawater. Vague answers such as 'carbon in the water' were not credited. In 4(c) most knew that energy enters with sunlight, but did not state photosynthesis. They knew that it was transferred by feeding through the trophic levels and lost as heat due to respiration. A surprising number incorrectly stated that the energy was recycled and some drew diagrams of the energy flow showing this. In 4(d)(i) most were able to state that the crustacean was a richer source of carbon. An explanation using the data from the diagram was expected. In 4(d)(ii) most were able to present an argument for advantage or disadvantage.
\nIn 4(a) most were able to state 'respiration' as the answer. 4(b) proved more difficult with fewer stating photosynthesis or absorption of dissolved CO2 or HCO3 from the seawater. Vague answers such as 'carbon in the water' were not credited. In 4(c) most knew that energy enters with sunlight, but did not state photosynthesis. They knew that it was transferred by feeding through the trophic levels and lost as heat due to respiration. A surprising number incorrectly stated that the energy was recycled and some drew diagrams of the energy flow showing this. In 4(d)(i) most were able to state that the crustacean was a richer source of carbon. An explanation using the data from the diagram was expected. In 4(d)(ii) most were able to present an argument for advantage or disadvantage.
\nIn 4(a) most were able to state 'respiration' as the answer. 4(b) proved more difficult with fewer stating photosynthesis or absorption of dissolved CO2 or HCO3 from the seawater. Vague answers such as 'carbon in the water' were not credited. In 4(c) most knew that energy enters with sunlight, but did not state photosynthesis. They knew that it was transferred by feeding through the trophic levels and lost as heat due to respiration. A surprising number incorrectly stated that the energy was recycled and some drew diagrams of the energy flow showing this. In 4(d)(i) most were able to state that the crustacean was a richer source of carbon. An explanation using the data from the diagram was expected. In 4(d)(ii) most were able to present an argument for advantage or disadvantage.
\nIn 4(a) most were able to state 'respiration' as the answer. 4(b) proved more difficult with fewer stating photosynthesis or absorption of dissolved CO2 or HCO3 from the seawater. Vague answers such as 'carbon in the water' were not credited. In 4(c) most knew that energy enters with sunlight, but did not state photosynthesis. They knew that it was transferred by feeding through the trophic levels and lost as heat due to respiration. A surprising number incorrectly stated that the energy was recycled and some drew diagrams of the energy flow showing this. In 4(d)(i) most were able to state that the crustacean was a richer source of carbon. An explanation using the data from the diagram was expected. In 4(d)(ii) most were able to present an argument for advantage or disadvantage.
\nIn 4(a) most were able to state 'respiration' as the answer. 4(b) proved more difficult with fewer stating photosynthesis or absorption of dissolved CO2 or HCO3 from the seawater. Vague answers such as 'carbon in the water' were not credited. In 4(c) most knew that energy enters with sunlight, but did not state photosynthesis. They knew that it was transferred by feeding through the trophic levels and lost as heat due to respiration. A surprising number incorrectly stated that the energy was recycled and some drew diagrams of the energy flow showing this. In 4(d)(i) most were able to state that the crustacean was a richer source of carbon. An explanation using the data from the diagram was expected. In 4(d)(ii) most were able to present an argument for advantage or disadvantage.
\nPastry cream or confectioners’ custard is made with a combination of milk (rich in casein and lactose), egg yolks, sugar, starch and a flavouring such as vanilla.
\nDescribe the structure of starch.
\nExplain how amino acids in casein could reach the liver, starting from the moment when the person takes a bite of pastry cream pie.
\nCongenital lactase deficiency is a type of lactose intolerance that occurs in infants. It is inherited in an autosomal recessive pattern. Calculate the chance of congenital lactose intolerance in a child whose parents are both carriers for the disorder, showing fully how you reached your answer.
\na. starch is a carbohydrate ✔
\nb. starch is formed by carbon, hydrogen and oxygen ✔
\nc. it is a polymer/chain/polysaccharide ✔
\nd. formed from monosaccharides/simple sugars/glucose ✔
\ne. linked together by condensation/dehydration ✔
\nf. consists of amylose and amylopectin ✔
\ng. amylose is a long chain/unbranched ✔
\nh. amylopectin is branched ✔
\na. food is mechanically/physically digested in the mouth through mastication/chewing ✔
\nb. mixed with saliva (to form the bolus) in mouth ✔
\nc. moved through esophagus/peristalsis ✔
\nd. proteins digested in the stomach (pepsin) ✔
\ne. pancreas secretes enzymes into lumen of small intestine
OR
(endo)peptidases/trypsin) are secreted by pancreas ✔
f. enzymes digest macromolecules to monomers
OR
endopeptidases digest polypeptides to peptides/amino acids ✔
g. villi of small intestine absorb amino acids ✔ Allow pepsin.
\nh. amino acids carried to blood capillaries ✔
\ni. blood (capillaries) carry amino acids to (hepatic portal) vein/blood vessel going to liver ✔
\nj. amino acids absorbed by active transport/protein pumps in the villi ✔
\na. gametes of both parents shown as a capital and small letter (e.g. L and l) ✔
\nb. possible F1 genotypes ✔
\nc. 25 % lactose intolerant, 50 % carriers, 25 % lactose tolerant
OR
75 % tolerant and 25 % intolerant
OR
child has 25 %/1:4/ chances of inheritance of intolerance ✔
(attempted by about 40 % of the cohort)
\nThe question was put in the context of the composition of pastry cream, rather than just diving in with questions. In part (a), the structure of starch was quite well known. The better answers included descriptions of polymers and monomers, amylose and amylopectin, and glucose. Part (b) was challenging to many, requiring a knowledge of the digestive system, enzyme action and the blood supply to the liver. It was possible to gain all of the marks by omitting the references to the blood stream. This was an example of where a clear 'essay plan' would have helped. Weaker candidates wrote at length about carbohydrate and fat digestion which was not required. Surprisingly the position of the pancreas was often confused, with pancreatic enzymes appearing in the stomach. The fundamental idea of the digestion of large molecules (proteins) to small soluble ones (amino acids) which can be absorbed into the blood stream at the villi was missed by many. Part (c) was a straightforward genetics cross, put into the context of lactose intolerance. In general, it was well answered. Although the question clearly stated that it was an autosomal recessive condition, many weaker students were determined to make it sex linked.
\n(attempted by about 40 % of the cohort)
\nThe question was put in the context of the composition of pastry cream, rather than just diving in with questions. In part (a), the structure of starch was quite well known. The better answers included descriptions of polymers and monomers, amylose and amylopectin, and glucose. Part (b) was challenging to many, requiring a knowledge of the digestive system, enzyme action and the blood supply to the liver. It was possible to gain all of the marks by omitting the references to the blood stream. This was an example of where a clear 'essay plan' would have helped. Weaker candidates wrote at length about carbohydrate and fat digestion which was not required. Surprisingly the position of the pancreas was often confused, with pancreatic enzymes appearing in the stomach. The fundamental idea of the digestion of large molecules (proteins) to small soluble ones (amino acids) which can be absorbed into the blood stream at the villi was missed by many. Part (c) was a straightforward genetics cross, put into the context of lactose intolerance. In general, it was well answered. Although the question clearly stated that it was an autosomal recessive condition, many weaker students were determined to make it sex linked.
\n(attempted by about 40 % of the cohort)
\nThe question was put in the context of the composition of pastry cream, rather than just diving in with questions. In part (a), the structure of starch was quite well known. The better answers included descriptions of polymers and monomers, amylose and amylopectin, and glucose. Part (b) was challenging to many, requiring a knowledge of the digestive system, enzyme action and the blood supply to the liver. It was possible to gain all of the marks by omitting the references to the blood stream. This was an example of where a clear 'essay plan' would have helped. Weaker candidates wrote at length about carbohydrate and fat digestion which was not required. Surprisingly the position of the pancreas was often confused, with pancreatic enzymes appearing in the stomach. The fundamental idea of the digestion of large molecules (proteins) to small soluble ones (amino acids) which can be absorbed into the blood stream at the villi was missed by many. Part (c) was a straightforward genetics cross, put into the context of lactose intolerance. In general, it was well answered. Although the question clearly stated that it was an autosomal recessive condition, many weaker students were determined to make it sex linked.
\nTuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis.
\nOutline the structures in M. tuberculosis that are not present in a human cell.
\nExplain the production of antibodies when a patient is infected with the TB bacterium.
\nDescribe the risk to the human population of indiscriminate use of antibiotics.
\na. cell wall ✔
\nb. pili/flagella ✔
\nc. 70S ribosomes ✔
\nd. nucleoid / circular DNA
OR
naked DNA ✔
e. plasmids ✔
\nAs candidates do not need to know the structure of Mycobacterium tuberculosis, all prokaryotic structures are accepted.
\nIgnore references to membrane bound organelles.
\na. phagocytes/lymphocytes are white blood cells ✔
\nb. TB bacterium has a specific antigen ✔
\nc. this antigen is recognised by white blood cells ✔
\nd. a clone of lymphocytes/plasma cells/B cells are produced ✔
\ne. antibodies are produced by lymphocytes ✔
\nf. each lymphocyte produces just one type of antibody ✔
\ng. (this is) specific immunity ✔
\nh. (part of the) antibody/immunoglobulin binds to the antigen / specific antibody binds to the specific antigen ✔
\ni. antibodies are proteins/immunoglobulins ✔
\nj. (some) plasma cells become memory cells ✔
\nk. memory cells reproduce quickly ✔
\nl. memory cells prevent infection in the future ✔
\nAllow annotated diagrams to explain the process.
\na. antibiotics block bacterial processes ✔
\nb. example of bacterial process ✔ e.g. cell wall formation
\nc. variations exist naturally in a population / some are naturally resistant to the antibiotic ✔
\nd. bacteria that are not resistant to this antibiotic will die / only resistant will survive (when antibiotic given) ✔
\ne. this characteristic could be passed to next generation ✔
\nf. (natural selection) leads to changes in the proportions/frequency in the population ✔
\ng. human population will be exposed to antibiotic resistant bacteria and will not have antibiotic to kill them ✔
\nh. (antibiotic resistant bacteria) may pass resistance to other bacteria species/types by means of plasmids (so other bacteria species turn resistant too) ✔
\n(attempted by about 60 % of the cohort)
\nPart (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.
\n(attempted by about 60 % of the cohort)
\nPart (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.
\n(attempted by about 60 % of the cohort)
\nPart (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.
\nHemophilia is a sex-linked disease that affected some members of royal families in Europe during the 19th and 20th centuries.
\nThe diagram shows the structure of two chromosomes after the first division of meiosis, assuming that there was no crossing over or chiasmata formation.
\nDraw a Punnett square to show all the possible genotypes of Queen Victoria’s children.
\nDeduce the genotype of Queen Victoria’s daughter Alice.
\nDraw the same chromosomes to show their structure at the same stage of meiosis if there had been one chiasma between two gene loci.
\nState the stage of meiosis where chiasmata formation may occur.
\nExplain gene linkage and its effects on inheritance.
\nExplain the mechanism that prevents polyspermy during fertilization.
\na. parental alleles shown as XH and Xh (female) and XH and Y (male);
b. Punnett square with genotypes of offspring shown as XHXH and XHY and XHXh and XhY;
XH Xh;
\nall four upper arms with one A and both chromosomes with one B and one b on the lower arms;
\nThe chromatids can be shown as single lines rather than the wider versions in the question.
\nprophase I;
\na. located on the same chromosome;
b. genes/gene loci close together (on the same chromosome);
c. do not follow (the law of) independent assortment;
d. more chance of recombination if genes are further apart;
e. inherited together unless crossing over/recombination occurs;
f. ratios of offspring in dihybrid crosses are different from expected/non-Mendelian
OR
more offspring with parental phenotype combinations than expected;
a. cortical reaction (after first sperm nucleus enters the egg);
b. vesicles/cortical granules release their contents/enzymes (from the egg/zygote);
c. zona pellucida/glycoprotein coat/outer coat hardened / fertilization membrane formed;
d. enzymes of sperm/acrosome cannot digest (hardened coat)
OR
glycoproteins/ZP3 (in zona pellucida) altered so sperm cannot bind;
Generally this was well answered. Common errors were to omit X and or Y chromosomes, show alleles on both X and Y chromosomes, or show the male X chromosome with a recessive h allele.
\nAny symbols were accepted for the alleles, as long as they were shown superscript to X and the genotype was heterozygous. 70 % of candidates answered correctly.
\nThis was mostly well answered but some candidates showed one chromatid flopped over a non-sister chromatid, like crossed legs of a seated person, without any recombination having occurred. This showed a misunderstanding of the process of crossing over and chiasma formation. There were also a wide range of other answers showing confusion about events in meiosis.
\nMostly Prophase I was given as the answer but all other phases of Meiosis I were given by some candidates and others failed to specify 1st or 2nd division of meiosis.
\nThe was some confusion between autosomal and sex linkage. Candidates tended either to have a secure understanding of the mechanisms at work in autosomal gene linkage, or none. This question and (d) correlated closely with candidates’ overall score on the paper, so distinguished particularly effectively between stronger and weaker candidates.
\nAnswers were mixed. Most knew that there is a mechanism that prevents more sperm fusing with the egg after the first one and some gave accurate and detailed accounts of it.
\nThe image shows human red blood cells.
\n[Source: someoneice/123rf.com.]
\n\n
Outline what will happen to human red blood cells if transferred to distilled water.
\nStem cells can be used to treat Stargardt’s disease. State one other condition treated using stem cells.
\nExplain the propagation of nerve impulses along the membrane of a neuron.
\ncells absorb water by osmosis and swell/increase in volume
OR
cells burst/lyse;
leukemia/other diseases of the hematopoietic system / skin burns;
\na. depolarization of part of axon/membrane triggers/causes depolarization of next part;
b. local currents;
c. diffusion of sodium ions between depolarized part and next/polarized part (of axon);
d. resting potential reduced/polarization of membrane becomes less /change from -70 to -50mV;
e. sodium channels open when -50mV/threshold potential reached;
f. entry of sodium ions causes depolarization;
g. saltatory conduction in myelinated neurons/axons;
Allow answers in an annotated diagram
\nThis was mostly well answered with candidates realising that water entry due to osmosis would cause the cells to burst.
\nA range of conditions were suggested, most of which are not currently treatable using stem cells. Diabetes and neurological diseases were not accepted. 60 % of candidates gave an acceptable answer, with leukaemia much the commonest. The correlation between the mark on this question and overall scores was fairly low, which may suggest that inappropriate examples have been learned in some schools. It is important not to raise hopes of stem cell treatment for specific diseases when research is still continuing and regulatory approval is unlikely for many years.
\nThis was a more difficult question with a much higher correlation coefficient. Many candidates did not understand what is meant by propagation of an impulse along the membrane of a neuron and answers tended to include accounts of the whole sequence of events in an action potential or in synaptic transmission. The mark most frequently awarded was for the mechanism of saltatory conduction.
\nThe figure shows a tripeptide.
\nLabel one peptide bond in this molecule.
\nDescribe the secondary structure of proteins.
\nOutline the action taken by the diaphragm during inhalation.
\ncircle/bracket around peptide bond / arrow pointing to peptide bond / peptide bond labelled;
\nAllow either peptide bond
\nAllow if adjacent C=O and NH groups are included in the circle/bracket, but do not allow if other parts of the molecule are included.
\na. polypeptide wound into a helical structure / alpha/α helix
OR
polypeptide folded back on itself forming a pleated sheet / beta/β pleated sheet;
b. stabilized/held in shape by/due to hydrogen bonds (between C=O and N-H groups);
c. secondary structures are regular/unvarying (within polypeptides/proteins);
Allow annotated diagrams
\ncontracts/flattens/becomes less domed/increases volume of thorax;
\n80 % of candidates indicated one of the two peptide bonds in the diagram correctly.
\nThere were some good answers here describing secondary structures and the role of hydrogen bonding.
\n80 % of candidates knew that the diaphragm contracts than relaxes during inhalation, or stated that it flattens rather than moves upwards.
\nCompare and contrast the mode of nutrition of detritivores and saprotrophs.
\nExplain how some plant species are able to respond to changes in their abiotic environment and flower at a precise time of the year.
\nOutline the extension of the stem in plants.
\nAccept not autotrophic/not photosynthetic instead of heterotrophic.
\nDo not accept that both groups are decomposers or consumers for the similarity.
\na. genes for flowering are activated/gene activation/changes to gene expression;
b. shoot apex changes from producing leaves/stem to producing flowers;
c. daylength/duration of the day/night length/photoperiod measured/detected/responded to;
d. short day plants flower when they have a long night/period of darkness
OR
long day plants only flower when they have a short night/period of darkness;
e. so short day plants/SDPs flower in late summer/fall/autumn/winter
OR
so long day plants/LDPs flower in spring/(early) summer;
a. apical meristem (of shoot/stem) produces cells/elongates the stem
OR
cell division/mitosis in tip/apex of shoot/stem;
b. auxin stimulates cell/stem growth/extension/enlargement;
c. elongation of cells causes stem to grow (in length);
About half of candidates answered correctly and there were some well-informed answers, but also many that showed a lack of familiarity with nutrition in detritivores and saprotrophs.
\nThe only relevant changes in the abiotic environment were night length variation over the seasons of the year, which determines when flowering should occur. There were many complicated answers describing the interconversion of the forms of phytochrome, but according to the syllabus this level of detail is not expected and often the simpler ideas that plants can measure night length and respond by the timing of flowering in the year were omitted. Also mostly missing, were the idea of changes to gene expression in the shoot apex, so floral organs start to develop instead of leaves. The average score for this question was only slightly higher than one mark, but the correlation coefficient was high.
\nAgain, accounts were varied, with stronger ones clearly explaining how the shoot apical meristem generates cells by mitosis and how elongation of these cells, stimulated by auxin, causes stem elongation. Some candidates were side-tracked by phototropism but were able to score some marks from among irrelevant ideas.
\nRNA molecules play numerous roles in gene expression and so contribute to variation.
\nDraw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.
\nExplain transcription.
\nDistinguish between continuous and discrete variation, using examples.
\na. ribose drawn as pentagon and labelled sugar/ribose;
b. base drawn with correct link to (C1 of) ribose and labelled base/nitrogenous base;
c. phosphate drawn with correct link to (C5 of) ribose and labelled P/phosphate;
d. two (or more) ribonucleotides drawn with correct link (C3 to C5)
a. synthesis of RNA/mRNA / transcription of DNA to RNA;
b. RNA nucleotides linked together to form a strand/chain;
c. RNA strand assembled on DNA template/antisense strand / copy made of sense strand;
d. RNA polymerase carries out transcription/links RNA nucleotides;
e. uncoiling/separation of DNA strands;
f. 5’ end of nucleotides linked to 3’ end of (growing RNA) strand;
g. complementary base pairing (is the basis of copying the base sequence);
h. uracil instead of thymine in RNA;
i. starts at/RNA polymerase binds to a promoter;
j. regulated by transcription factors/DNA binding proteins/nucleosomes;
Annotated diagrams can be used.
\nMany candidates were able to draw the structure of an RNA nucleotide and link it correctly by a 5’ to 3’ bond to another nucleotide. The commonest error was to show two strands of nucleotides linked by base pairing, indicating confusion between RNA and DNA.
\nAnswers were mixed. Strong candidates had no difficulty in describing transcription in detail but weaker ones tended to get confused with replication and/or translation. Helicase was often stated as the enzyme that uncoils and splits the double helix, rather than RNA polymerase. Marks were not awarded merely for stating that transcription is 5’ to 3’ unless it was clear that the candidate understood that the 5’ terminal of a free nucleotide is linked to the 3’ terminal of the chain of nucleotides already linked up. Candidates are expected to show understanding in their answers, rather than just state memorized phrases.
\nDifferences between continuous and discrete variation were not well known and the average performance for this question was the lowest for any part of Section B. A general fault was to describe the two types of variation separately and in consequence forget to include both sides of a distinction. For example, some candidates stated that the environment can cause continuous variation, but most did not then also state that discrete variation is generally unaffected by environment. A mark was awarded for examples of the two types of variation, but in some cases an inappropriate example was chosen, such as hair colour for discrete variation or eye colour for continuous variation. The latter example was best avoided entirely as there are aspects of both continuous and discrete variation in the pigmentation of the iris.
\nPlants have developed efficient methods for transport and for synthesis of foods.
\nOutline how the properties of water make it an ideal transport medium in plants.
\nDistinguish between the xylem and phloem of plants.
\nExplain how the light-independent reactions of photosynthesis rely on the light-dependent reactions.
\na. polarity of water;
b. hydrogen bonds between water molecules;
c. cohesion between water molecules/water molecules stick together;
d. cohesion allows tensions/low pressures/transpiration pull/movement upward/against gravity;
e. adhesion to cellulose/cell walls generates tensions/pull (in xylem)
OR
adhesion to xylem walls/vessel walls causes capillary rise/upward movement;
f. solvent for many substances / many substances dissolve;
g. liquid at most temperatures experienced by plants / liquid so can flow;
Polarity of water and/or hydrogen bonding can be shown in an annotated diagram.
\na. light-dependent reactions produce ATP/reduced NADP;
b. ATP generated by chemiosmosis/by photophosphorylation/by ATP synthase;
c. reduced NADP produced by/using electrons from Photosystem I;
d. RuBP + CO2 to glycerate 3-phosphate (in light independent reactions);
e. glycerate 3-phosphate reduced to triose phosphate (in light independent reactions);
f. ATP/reduced NADP used in the light-independent reactions;
g. reduced NADP provides electrons/hydrogen / to reduce (glycerate 3-phosphate)
OR
reduced NADP used to convert glycerate 3-phosphate to triose phosphate;
h. ATP provides energy (for reduction of glycerate 3-phosphate);
i. ATP needed to regenerate RuBP
j. ATP/reduced NADP run out in darkness
k. Calvin cycle only possible with light/in the day/is indirectly dependent on light;
Most candidates knew at least some properties of water that make it useful as a medium for transport in plants. As in previous papers, cohesion and adhesion are often treated as the same process or were confused. Another common error is to refer to hydrogen bonds as strong – their effects are strong because so many hydrogen bonds are formed in water but, thinking of them individually, they are weak interactions.
\nThe average mark was 2/4 for the differences between xylem and phloem. As in 6(c) marks were often lost because only one side of a distinction was given in the answer. A common misconception is that transport in xylem is unidirectional whereas in phloem it is bidirectional. Simultaneous bidirectional transport in individual sieve tubes was hypothesized at one time but this has been falsified. Sap can move in either direction at different times in both phloem sieve tubes and xylem vessels and recent research shows that xylem sap drops back down to the roots as often as every night in some herbaceous plants, making the ascent of sap in air-filled vessels a daily task. Adhesion to cellulose (not lignin) in xylem walls and capillary action is therefore more important for water transport in plants than previously realized.
\nThis was a fair but challenging question and it yielded the highest correlation coefficient on the paper. Answers covered the whole gamut from the thoroughly confused to the masterly. Full names of intermediates in the Calvin cycle are preferred because abbreviations such as GP are often ambiguous. Weak points in some answers were the need for reduced NADP in the reduction of glycerate 3-phosphate to triose phosphate and the need also for energy from ATP both for this reduction reaction and for phosphorylation reactions in the regeneration of ribulose bisphosphate.
\nProteins carry out many important functions in cells and in organisms.
\nOutline the process of protein denaturation.
\nExplain the production of antibodies in humans.
\nDistinguish between competitive and non-competitive enzyme inhibition.
\na. change to conformation/shape/tertiary structure/3-D shape;
b. bonds within the protein/intramolecular bonds broken/changed;
c. pH and temperature (outside tolerated ranges) can cause denaturation;
d. vibrations/heat at high temperatures breaks bonds;
e. high pH/low pH/extreme pH alters ionization/charges (of amino acids and breaks ionic bonds);
f. protein cannot carry out its function
OR
active site of enzymes cannot bind substrates/catalyze reaction/no enzyme-substate complex;
g. permanent/irreversible change (usually)
OR
soluble proteins become insoluble/precipitate;
Allow any mark points if made clearly on an annotated graph or diagram.
\na. antigens stimulate antibody production;
b. antibodies produced by lymphocytes;
c. macrophages/phagocytes ingest/engulf pathogens and display antigens from them;
d. T-cells activated by binding antigen/by macrophage displaying antigen;
e. activated T-cells cause activation of B-cells;
f. mitosis/division of (activated) B-cells (to produce a clone of cells)
g. plasma cells formed from divided/activated/growing/differentiating B-cells;
h. plasma cells/plasma B-cells secrete antibodies;
i. clonal selection / plasma cells make same type of antibody/antibody specific to same antigen;
j. some activated B-cells become memory cells;
Accept mpd in a graph.
\nAccept mpa, mpb or mpc on an annotated diagram.
\n[Source: Enzyme inhibition curves,
ImranKhan1992, Available at: https://
commons.wikimedia.org/wiki/
File:Enzyme_kinetics_curve.png#/media/
File:Enzyme_kinetics_curve.png CC0 1.0
Universal (CC0 1.0) Public Domain Dedication
https://creativecommons.org/publicdomain/
zero/1.0/deed.en Source adapted.]
Most candidates knew that denaturation can be caused by excessive temperatures or extreme pH. Fewer candidates mentioned how bonds within a protein are broken by heat or kinetic energy. Even fewer explained how R-group ionization in acidic and basic amino acids can be altered by pH changes, breaking the ionic bonds within protein molecules that help to stabilize tertiary structure.
\nThis was another question giving scope to candidates to show the breadth of their biological understanding. The best answers were impressive but given the challenging nature of the topic it isn’t surprising that there were also some very muddled accounts. The weakest candidates struggled to remember the differences between antigens, antibodies and lymphocytes.
\nIn a parallel way to 6(c) and 7(b), there was a tendency for candidates to write about the two types of enzyme inhibition separately and as a result not to distinguish between them fully. For example. many candidates stated that competitive inhibitors show some chemical similarity to the substrate but did not then state that non-competitive inhibitors are typically dissimilar. An error which is repeatedly seen in enzyme questions is for candidates to state that the active site is on the substrate rather than the enzyme. Some candidates drew sketch graphs to show the difference between competitive and non-competitive inhibitors in the effect of substrate concentration, but either did not label the x-axis or labelled it ‘time’ rather than ‘substrate concentration’.
\nThe incidence of white syndrome, an infectious disease of coral, was investigated in a six-year study on Australia’s Great Barrier Reef. The map shows disease conditions on coral reefs at six study sites.
\n[Source: adapted from Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman,
H. and Melendy, A.M., 2007. PLOS Biology, [e-journal] 5(6), e124. https://doi.org/10.1371/journal.pbio.0050124.]
Satellites were used to record sea surface temperatures. The temperatures each week above a reef were compared with mean temperatures for that week between 1985 and 2004. If the sea surface temperature was 1 °C or more above the mean, this was recorded as a weekly sea surface temperature anomaly (WSSTA). The number of WSSTAs was calculated for the twelve months preceding the date on which a reef was surveyed for white syndrome.
\nOn each reef, the number of cases of white syndrome in a 1500 m2 sample area was surveyed once per year. The table shows these cases in relation to numbers of WSSTAs and coral cover on the reef. Low coral cover was 0–24 % and high coral cover was 50–75 %.
\n[Source: adapted from Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman,
H. and Melendy, A.M., 2007. PLOS Biology, [e-journal] 5(6), e124. https://doi.org/10.1371/journal.pbio.0050124.]
The graphs show the relationship between the weekly sea surface temperature anomalies (WSSTA) and coral cover during two twelve-month periods (1998–99 and 2002–03), which were the warmest in the six-year study. Each dot represents one studied reef.
\n[Source: adapted from Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman,
H. and Melendy, A.M., 2007. PLOS Biology, [e-journal] 5(6), e124. https://doi.org/10.1371/journal.pbio.0050124.]
Identify the site with the highest incidence of diseased colonies.
\nDeduce whether there is a trend in the incidence of white syndrome over the north-south range of latitude.
\nDescribe the evidence that is provided by the data in the table for the harmful effects of rising sea temperatures on corals.
\nThe researchers concluded that there was a threshold coral cover percentage, below which infection rates tended to remain fairly low. Using the data in the table, identify this threshold level.
\nSuggest a reason for a larger percentage of corals being infected with white syndrome on reefs with a higher cover of corals.
\nCompare and contrast the data for 1998–1999 and 2002–2003.
\nSuggest a reason for the correlation between coral cover and WSSTA in 1998–1999.
\nSome scientists predict that, if humans continue to produce carbon dioxide at the current rate, the pH of the oceans will become more acidic. Suggest possible effects on the coral reefs.
\nThis study was carried out over six years on the Australian Great Barrier Reef. State one advantage of field investigations compared with mesocosm experiments to study ecological processes.
\nCapricorn and Bunkers group;
\nthere is no clear trend;
\na. effects (generally) increase with temperature anomalies/WSSTA;
b. effects increase for all different coral covers
OR
in the 25-49 cover there is an anomaly/is not an increasing trend;
c. harmful effect is higher in coral covers between1 50-75 % / vice versa;
50 % / 25 % / 49 %
OR
25–49 % (coral cover);
a. the closer the corals are (to each other) the easier the transmission;
b. other organisms in the community may act as vectors/carriers / OWTTE;
Similarities
a. similar range of cover in both periods
OR
weak correlation/no correlation in either;
Differences
b. higher WSSTA range/more WSSTA in 1998–1999 than 2002–2003
OR
positive (2002–2003) versus negative correlation (98–99)
OR
more coral reefs with very low % of coral cover (0–20 %) in 2002–2003;
Must respond with one similarity and one difference for full marks.
\ncoral cover lower on reefs with higher temperature;
\na. corals would not be able to maintain their skeletons;
b. (if coral reefs are lost) habitat will be lost;
c. some organisms/coral can decline if the water becomes too acidified;
d. coral bleaching could occur/become worse
OR
coral could expel their mutualistic alga/zooxanthellae;
e. enzymes could be denatured;
real environmental conditions / larger scale investigations / more variables studied;
\nIn general, the data analysis question was well attempted by the majority of students. In 1a almost all were able to correctly name the Capricorn and Bunkers Group.
\nIn 1b many just described the incidence at various points, instead of stating that there was no clear trend. There were a few G2 comments that the map did not include compass points, which, perhaps, has some justification. However, by convention maps are usually shown with North at the top.
\nThe expression ‘coral cover’ did confuse some of the weaker students who imagined that the corals somehow has a sort of cover over them. Most students gained at least 1 mark in 1b (i), despite the criticism from the G2 forms that it was very complex.
\nMost were able to able to gain the mark in 1b (ii) for saying that the threshold level was 25 – 49 %.
\nIn 1b (iii) the better students realised that it was to do with higher density of corals, not just the more vague ‘higher cover’ given in the stem.
\nIn 1c contrasts were common, but few gained the comparison mark as well.
\nIn 1d weaker students incorrectly wrote about the sea becoming acidic as the pH was increasing (sic). Indeed, it is becoming more acidic, as the pH is decreasing slightly, leading to more bleaching and the decrease in population of some creatures.
\nIn 1e most were able to give a reason why the study was better than a mesocosm.
\nThe figure shows a tripeptide.
\nLabel one peptide bond in this molecule.
\nOutline the specific functions of three named proteins.
\nOutline the action taken by the diaphragm during inhalation.
\ncircle/bracket around peptide bond / arrow pointing to peptide bond / peptide bond labelled;
\nAllow either peptide bond
\nAllow if adjacent C=O and NH groups are included in the circle/bracket, but do not allow if other parts of the molecule are included
\na. Rubisco fixes CO2 from atmosphere during photosynthesis;
b. insulin controls blood glucose levels;
c. collagen forms connective tissue/ligaments;
d. spider silk forms the spider web;
e. rhodopsin involved in photoreceptor;
f. immunoglobulins/antibodies attach to antigens/pathogens;
g. actin/myosin performs muscle contraction;
h. hemoglobin carries oxygen in red blood cells;
Accept any other correct three named proteins
\nIf an enzyme is named, the correct substrate must be stated
\ncontracts/flattens/becomes less domed/increases volume of thorax;
\nHowever, in part 2b, many did not describe the functions of the named proteins correctly. Common mistakes were assuming that all hormones are proteins, with many incorrectly giving oestrogen, testosterone and thyroxin for example. Examples were anticipated from the list of proteins found in section 2.4 of the guide.
\nCompare and contrast the mode of nutrition of detritivores and saprotrophs.
\nThe image shows an example of a soil food web.
\n[Source: Anon., n.d. The Soil Food Web. [image online] Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/
photogallery/soils/health/biology/gallery/?cid=1788&position=Promo [Accessed 11 March 2020].]
\n
Draw a food chain from this food web, showing at least three organisms.
\nExplain the reasons for food chains rarely containing more than four or five trophic levels.
\nThe amount of food passing into food chains can be affected by the rate of photosynthesis. Explain the effect of one limiting factor on photosynthesis.
\nAll of the leaves in the image are from Solanum, a wild genus of tomato.
\n[Source: Courtesy: National Science Foundation, Credit Leonie Moyle.]
\n\n
State one cause of variation in a plant such as the tomato.
\nAccept not autotrophic/not photosynthetic instead of heterotrophic.
\nDo not accept that both groups are decomposers or consumers for the similarity.
\nfood chain of three or more organisms starting with plants;
\na. energy is lost between the trophic levels;
b. transfer between levels is only usually 10% efficient
OR
energy transformations take place in living organisms / the process is never 100% efficient;
c. energy is lost by the organism/used in respiration / released as heat/movement;
d. energy is lost as waste/feces/urine/undigested food/uneaten parts;
e. as energy is lost between trophic levels and so (higher ones) have less biomass / less biomass available for next level;
a. the rate of reaction will be limited by the limiting factor that is nearest to its minimum value;
\ntemperature:
b. enzymes that control photosynthesis are influenced by temperature;
c. as temperature increases, reaction rate will increase;
d. above a certain temperature, the rate of photosynthesis will decrease;
e. (where temperature is limiting) essential enzymes begin to denature/not working to optimum;
light intensity:
f. light is source of energy / converted into chemical energy;
g. as light intensity increases reaction rate will increase;
h. at a certain light intensity, rate of photosynthesis will plateau;
i. another factor becomes limiting;
CO2 concentration:
j. CO2 is fixed to form organic molecules;
k. as CO2 concentration increases, reaction rate will increase;
l. at a certain concentration of CO2, rate of photosynthesis will plateau;
m. another factor becomes limiting;
Accept answers using an annotated graph to explain
\nOnly accept the first factor described
\nDo not accept pH as a limiting factor
\na. mutations;
b. meiosis/crossing over/random assortment of homologous pairs;
c. sexual reproduction/recombination/random fertilisation;
Knowledge of detritivores and saprotrophs was good in 3a, with many of the poorer answers describing them as feeding on inorganic material.
\nMany took the word ‘draw’ literally in the food chain, and the drawings of the nematodes and fungi amused the examiners. A food chain must start with a plant. The stem did say “three organisms”. Detritus was not taken as an organism, so negated the mark.
\nMost were able to gain at least two marks for the explanation of energy loss through the trophic levels.
\nIn 3c, most knew that temperature/CO2 concentration/light affect photosynthesis but could not explain the idea of acting as a limiting factor. Only the better students gained all 3 marks.
\nIn 3d there was a great deal of variety in the leaf shape, so an answer in terms of sexual reproduction was looked for.
\nThis micrograph shows a transverse section of an artery.
\n[Source: OpenStax College, 2013. Comparison of artery and vein. [image online] Available at:
https://en.wikipedia.org/wiki/File:2102_Comparison_of_Artery_and_Vein.jpg Attribution 3.0 Unported
(CC BY 3.0) https:/creativecommons.org/licenses/by/3.0/deed.en [Accessed 11 March 2020].]
\n
Explain how the specialized structures of arteries help them to achieve their functions.
\nState one process that produces ATP.
\nOutline the use of ATP.
\nState one function of epinephrine in the human body.
\na. arteries carry blood at high pressure;
b. rupture of arteries is prevented by thick muscular/elastic walls;
c. narrow lumen to maintain a high blood pressure;
d. elastic tissue allows artery to stretch and recoil (to even out pressures);
e. arteries have muscle layers which contract to increase/control the blood flow;
f. folding in the endothelium allows stretching
OR
smooth endothelium reduces friction;
(cell) respiration/photosynthesis;
\nsource of energy (for use in the cell)
OR
example of use of ATP
OR
when ATP is converted to ADP + Pi (is hydrolyzed) the energy stored in the phosphate bond is released to be used by the cell;
increases heart rate
OR
prepares the body for action;
Most could state that the arteries carry blood at high pressure, but had difficulty getting the other two marks.
\nThe production of ATP and its use was well known. Although respiration was expected in b(i), photosynthesis was allowed, as ATP is produced in the light-dependent reactions.
\nIn part 4c, many students would have benefitted from the inclusion of (adrenaline) after epinephrine as they seemed to not know of the latter. Epinephrine is the term used in the syllabus (section 6.2), so it was expected that the students were familiar with it.
\nSubstances can move into and out of cells through the cell membrane.
\nOutline the significance of surface area to volume ratio in the limitation of cell size.
\nDescribe transport across cell membranes by osmosis.
\nExplain the adaptations of the small intestine to its function.
\na. surface area of the cell affects the rate of material exchange;
b. when the cell increases in size, so does its chemical activity/metabolism;
c. (when the cell increases in size/grows) more substances need to be taken in / more waste products need to be excreted;
d. as the volume of the cell increases, so does the surface area, however not to the same extent
OR
when the cell gets bigger, its surface area to volume ratio gets smaller;
e. substances will not be able enter the cell fast enough/cell volume will not be supplied
OR
metabolic rate will exceed the rate of exchange
OR
when the surface area: volume ratio is higher, the diffusion rate increases;
f. some cells have adaptations to increase their surface area/flatten/microvilli/shape of red blood cells;
g. cells in growth areas tend to divide and remain small
OR
cells divide when maximum size is reached;
a. form of diffusion;
b. osmosis is the movement of water molecules;
c. (movement) across a selectively/semi/partially permeable membrane/cell membrane;
d. from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
OR
movement of water molecules from a high concentration of water to a low concentration of water molecules;
e. it is a passive transport mechanism/does not use ATP;
f. channel proteins/aquaporins are used;
a. small intestine is where nutrients are absorbed into the bloodstream;
b. very long to maximize absorption;
c. (the small intestine) is lined with (smooth) muscle to allow for the mixing/ and moving of digested food;
d. muscles are circular and longitudinal;
e. that perform peristalsis;
f. the pancreas (and gall bladder) secretes substances into the small intestine to aid digestion;
g. contain villi, to increase surface area;
h. villi have microvilli to increase surface area even more;
i. villi absorb products of digestion/mineral ions/vitamins/glucose;
j. dense capillary network rapidly transports absorbed products;
k. lacteal absorbs lipids from the intestine (into the lymphatic system);
l. (most of the) chemical digestion (into monomers) occurs in small intestine/description of specific enzyme action;
Accept annotated diagrams as part of the explanation.
\n[Source: © 1999–2022, Rice University. Except
where otherwise noted, textbooks on this site are
licensed under a Creative Commons Attribution 4.0
International License https://creativecommons.org/
licenses/by/4.0/. Image available at https://
openstax.org/books/anatomy-and-physiology/
pages/23-5-the-small-and-large-intestines?
query=villus&target=%7B%22type%22%3A%
22search%22%2C%22index%22%3A0%7D#fsid1272744]
Most knew that as the cell increases in size, the SA:volume ratio decreases but had difficulty explaining its significance.
\nMost gained at least 2 marks describing osmosis. Common mistakes were not making it clear whether the concentration was referring to water or to the solute. A surprising number of students did not refer to water at all.
\nThe structure and function of the small intestine was quite well known. Many missed a mark for omitting to say that the products of digestion are absorbed into the blood. Some of the descriptions of microvilli were not clear, not making it evident that they were on the epithelial cells of the villi. The presence and functions of the muscle layers were well known.
\nOrganisms have evolved a great diversity of cell types.
\nDescribe the endosymbiotic theory.
\nExplain the need for halving the chromosome number during a sexual life cycle and how this is done.
\nOutline the binomial system of classification.
\na. theory that eukaryotic cells evolved from prokaryotes
OR
origin of eukaryotic organisms based on some organisms living inside/engulfed by other organisms
OR
prokaryotic cell engulfed another prokaryote including it in cytoplasm without digesting it;
b. mitochondria/chloroplasts have double membranes;
c. mitochondria/chloroplasts have their own DNA/loop of DNA/naked DNA;
d. mitochondria/chloroplasts have similar size to prokaryotes;
e. mitochondria/chloroplasts can reproduce by binary fission;
f. mitochondria/chloroplasts have 70S ribosomes (same as prokaryotes);
a. chromosome number is halved so the zygote/offspring has same number as the parent / so that chromosome number is not doubled;
b. process is meiosis;
c. DNA/chromosomes replicate (so each chromosome consists of two chromatids);
d. homologous chromosomes pair in prophase I;
e. (these) separate in anaphase I into two cells;
f. (after meiosis I) cells are haploid;
g. in meiosis II chromatids are separated;
h. result is four haploid cells/gametes;
i. each gamete is genetically unique;
j. (uniqueness) is due to crossing over/independent assortment/random alignment of chromosomes;
k. fertilization results in the formation of a diploid zygote;
l. (fertilization) results in variation in a population
a. the binomial system of names for species is universal among biologists
OR
named according to a globally recognized scheme;
b. allows to classify organisms into groups based on similar characteristics/common ancestry/DNA;
c. every species is given a binomial name;
d. members of the same species can mate and reproduce fertile offspring
e. genus is written first, followed by species;
f. genus is capitalized, (followed by) species is lower case
OR
an underlined correct example/stated that it must be underlined or italicized;
The theory of endosymbiosis was quite well known. Common errors included the original engulfing process being carried out by a eukaryote, rather than a prokaryote.
\nThis provided a departure from the usual questions requiring a straight recall of meiosis. In general, it was well attempted, with all but a few students failing to state that meiosis was at the heart of the question. Despite the question stating halving of the chromosome number, many insisted on writing about chromosomal errors such as Down syndrome, which was not pertinent to the question.
\nThis part was about the use of the binomial system for naming organisms. Weaker students insisted on describing the complete classification starting from kingdom, which was not required and put themselves in danger of losing the clarity mark for including superfluous material. Most were able to gain 2 marks for saying that the genus was written before species and about the use of capital/lower case or underlining or italicising.
\n\n Outline how the properties of water make it an ideal transport medium in plants.\n
\n\n [4]\n
\n\n Distinguish between the xylem and phloem of plants.\n
\n\n [4]\n
\n\n a. polarity of water;\n
\n b. hydrogen bonds between water molecules;\n
\n c. cohesion between water molecules/water molecules stick together;\n
\n d. cohesion allows tensions/low pressures/transpiration pull/movement upward/against gravity;\n
\n e. adhesion to cellulose/cell walls generates tensions/pull (in xylem)\n
\n \n OR\n \n
\n adhesion to xylem walls/vessel walls causes capillary rise/upward movement;\n
\n f. solvent for many substances / many substances dissolve;\n
\n g. liquid at most temperatures experienced by plants / liquid so can flow;\n
\n \n Polarity of water and/or hydrogen bonding can be shown in an annotated diagram\n \n .\n
\n\n \n
\n Most candidates knew at least some properties of water that make it useful as a medium for transport in plants. As in previous papers, cohesion and adhesion are often treated as the same process or were confused. Another common error is to refer to hydrogen bonds as strong – their effects are strong because so many hydrogen bonds are formed in water but, thinking of them individually, they are weak interactions.\n
\n\n The average mark was 2/4 for the differences between xylem and phloem. As in 6(c) marks were often lost because only one side of a distinction was given in the answer. A common misconception is that transport in xylem is unidirectional whereas in phloem it is bidirectional. Simultaneous bidirectional transport in individual sieve tubes was hypothesized at one time but this has been falsified. Sap can move in either direction at different times in both phloem sieve tubes and xylem vessels and recent research shows that xylem sap drops back down to the roots as often as every night in some herbaceous plants, making the ascent of sap in air-filled vessels a daily task. Adhesion to cellulose (not lignin) in xylem walls and capillary action is therefore more important for water transport in plants than previously realized.\n
\nHemoglobin is a conjugated protein consisting of four polypeptide chains with attached heme groups.
\n[Source: LAGUNA DESIGN/Science Photo Library via Getty Images.]
\nState the level of protein structure at which the polypeptide chains of hemoglobin are combined.
\nExplain the shape of the polypeptide chain at X.
\nOutline the role of tRNA in hemoglobin synthesis.
\nquaternary / fourth level;
\nMany candidates knew that association of polypeptide chains is quaternary structure. Any spelling of this term was accepted — only a minority got it correct.
\nThis was one of the most successfully answered of questions, with better prepared candidates recognizing the alpha helix and explaining it in terms of hydrogen bonding.
\nThere were varied answers to this question. The best made it clear that a tRNA molecule carries one specific amino acid corresponding to its anticodon and to the codon on mRNA to which it binds during translation.
\nThe diagram shows a section through part of the leg of an insect and antagonistic muscles X and Y. The tibia moves in the direction shown by the arrow when the muscle is flexing.
\n[Source: D G Mackean www.biology-resources.com.]
\nOutline a reason for the muscles being described as antagonistic.
\nDescribe the role of muscle Y.
\nOutline how the muscle attachment of insects differs from humans.
\nExplain the role of calcium ions in muscle contraction.
\ncause opposite movements / pull in opposite directions / when one contracts the other relaxes;
\ninsect muscle is attached inside (the skeleton) and human muscle outside (the skeleton);
insect muscle is attached to exoskeleton and human to endoskeleton/bones;
Most candidates were able to outline what antagonistic muscles are. The term 'extended' was sometimes used instead of 'relax' which is the preferred term as it indicates passivity.
\nThe mark scheme here was rather too demanding and very few candidates were awarded both marks. The biological distinctions between flexors and flexion were too fine and marks should have been awarded for other relevant ideas. A flexor is a muscle that causes flexion. Although we may speak colloquially of 'flexing our muscles' it is not a muscle that flexes but a limb at a joint.
\nThis question probed whether candidates understood the difference between the skeletons of mammals and insects and thus where muscles are attached. Although the diagram shows tendon-like attachment of insect muscles, many candidates claimed that the lack of tendons was the prime difference.
\nWell prepared candidates had no difficulty in describing the role of calcium in muscle contraction but they were in the minority. The discrimination index was very high.
\nOutline how the properties of water make it an effective coolant for the body.
\nDescribe how changes in weather conditions affect the transport and loss of water in plants.
\nExplain how water balance is restored in mammals when they are dehydrated.
\nThis question revealed widespread misunderstanding of the properties of water and changes of state. Most candidates though that water's high specific heat capacity explains its role as a coolant and that sweat cools the body by taking heat from it to raise its temperature. Few candidates referred to the requirement for heat to break hydrogen bonds as water evaporates. To be able to understand biological processes properly, a firm grounding in physics and chemistry is needed, but all too often it was lacking.
\nMore marks were scored here, but again there was widespread misunderstanding of the forces that cause water to evaporate and diffuse out of leaves, and of how conditions such as humidity, temperature and wind can influence the process. Few candidates mentioned concentration gradients between air spaces in the leaf and the atmosphere outside the leaf. There was a tendency to get trends the wrong way round, for example by suggesting that transpiration increases during rainfall because plants have plenty of water and therefore choose to open their stomata more widely.
\nThis was poorly answered by many candidates. The discrimination index was very high but the mean mark was only 1.5 out of 7. Often candidates did not get beyond the basic ideas of thirst and drinking water to rehydrate. The best candidates gave concise but detailed accounts of the roles of the hypothalamus, pituitary gland, collecting duct and aquaporins.
\nOutline reasons for the therapeutic use of stem cells.
\nDescribe how monoclonal antibodies are produced.
\nExplain the role of the electron transport chain in the generation of ATP by cell respiration.
\nMost candidates knew something of the therapeutic uses of stem cells, including differentiation for specific roles. There was a tendency for over-optimism over what can be fixed using stem cells. For example, stem cells are not a treatment for most cancers. The best answers stuck to well-established procedures such as the treatments for leukaemia using stem cells from bone marrow.
\nProduction of monoclonal antibodies was not widely understood and the mean mark was below 1 (out of 5). Many answers described the normal immune response by the body that results in production of antibodies, rather than the production of hybridoma cells that allow large-scale antibody manufacture.
\nFor well-prepared candidates this question posed no difficulty and there were some excellent detailed accounts of chemiosmosis. The discrimination index was the highest for any question on the paper, indicating that there was no room for lucky guesses about the biology here!
\n\n Describe the effect of neonicotinoid pesticides on the nervous system of insects.\n
\n\n [1]\n
\n\n State the cumulative increase in the mass of control colonies at 7 weeks.\n
\n\n [1]\n
\n\n Compare and contrast the cumulative increase in mass of the three groups of colonies once they were placed in the field.\n
\n\n [2]\n
\n\n Suggest a reason for the changes in mass in the colonies between weeks 6 and 8.\n
\n\n [1]\n
\n\n Using the data in the graph, predict how the use of neonicotinoid pesticides will affect bumblebee populations.\n
\n\n [3]\n
\n\n Identify the species whose eggs are most affected by a high dose of neonicotinoid.\n
\n\n [1]\n
\n\n Describe the overall effects of low and high doses of neonicotinoid on the egg lengths of all four species.\n
\n\n [2]\n
\n\n Deduce, based on the data presented, whether the levels of neonicotinoids used in agriculture cause direct harm to\n \n B. terrestris\n \n .\n
\n\n [1]\n
\n\n 230 g;\n
\n\n \n Accept range of 215 – 265. This takes into account the beginning and end of week 7.\n \n
\n \n Unit required.\n \n
\n \n Accept vice versa\n \n .\n
\n\n \n B. lucorum\n \n ;\n
\n\n \n Accept any other valid comparison\n \n .\n
\n\n \n OWTTE\n \n
\n\n A pleasing number were able to correctly describe the effect of the pesticides on the synapses and state the cumulative increase correctly in ai and aii. However, in aiii, many did not see the similarity in pattern for the compare mark. In part b stronger candidates commented on the fact that even at low doses the pesticide had a great effect and that the difference between high and low doses was negligible. Many examiners were bemused by the answers to c(i) where about half of the candidates failed to identify\n \n B. lucorum\n \n as the most affected species.\n \n B. pascuorum\n \n and\n \n B. terrestris\n \n were often incorrectly given. In cii. Better candidates described the overall effects as asked, with weaker candidates just describing each species in turn. In 1d the top candidates looked at all of the data, but those who just stated that the effect was not noticeable as the egg length was not significantly smaller gained the mark.\n
\nThe onion (Allium cepa) root cells shown in the micrograph are in different stages of mitosis.
\n[Source: Reischig, J., 2014. Mitosis (261 13) [Pressed; root meristem of onion]. [image online] Available at https://commons.wikimedia.org/wiki/File:Mitosis_(261_13)_Pressed;_root_meristem_of_onion_(cells_in_prophase,_metaphase,_anaphase,_telophase).jpg This file is licensed under the Creative Commons Attribution -ShareAlike 3.0 Unported (CC BY-SA 3.0) https://creativecommons.org/licenses/by-sa/3.0/deed.en [Accessed 3 December 2019].]
\nIdentify, with a reason, the stage shown at X.
\nCalculate the length of the entire cell labelled Y, showing your working.
\nState the role of cyclins in the cell cycle.
\nDistinguish between the structure of chromosomes in prokaryotes and eukaryotes.
\nExplain Cairns’s technique to measure the length of the DNA molecule.
\nOWTTE
\n50 μm = 27/28/29 mm, Y = 8/9/10 mm
OR
50 x 9 /27
OR
16.7 μm (accept answers in the range of 14.8 μm to 17.2 μm)
Award [1] for correct ratios not precise measurements in the work or [1] for correct answer with correct unit.
\nAccept only differences.
\nDifferentiating terms expected;
\nThis question was generally well done; a few identified the incorrect phase of mitosis, but the observations about cellular events being observed was often correct.
\nMost answers were correct, but some calculations were difficult to follow; some errors in unit conversions led to incorrect decimal places.
\nThis question led to a large number of correct answers. Some answers focused on the difference between prokaryotes and eukaryotes instead of their chromosomes.
\nAsking students to outline Cairn’s technique led to a number of blank answers. That there was a role for autoradiography was better known than how this was used to obtain the length estimate.
\nA group of students used quadrat sampling and the chi-squared test to find out whether the distributions of two plant species were associated with each other or not. These two species were found in the ground vegetation in a woodland ecosystem.
\n[Source: (left) Pixabay.
(right) Topic, J., n.d. Veronica montana 2. [image online] Available at: http://www.freenatureimages.eu/Plants/
Flora%20S-Z/Veronica%20montana/#Veronica%2520montana%25202%252C%2520Bosereprijs%252C%2520Saxifr
aga-Jasenka%2520Topic.jpg [Accessed 3 December 2019].
The numbers of quadrats with one, both or neither species present were counted and recorded. The observed frequencies from 150 quadrats are shown in the following contingency table.
\nState the alternative hypothesis for this study.
\nTo calculate chi-squared, expected values must first be calculated. Assuming that there is no association between the two species, calculate the expected number of quadrats in which both species would be present, showing your working.
\n\n
\n
\n
\n
\n
State the number of degrees of freedom for this test to determine the critical value of chi-squared.
\nWhen the data in the table were used to calculate chi-squared, the calculated value was 0.056. The critical value is 3.84. Explain the conclusion that can be drawn from the calculated and critical values for chi-squared.
\nthere is a positive / negative association between the two species;
they tend to grow together / they tend to grow apart;
OWTTE
\nAward [1] for proper values chosen/equation or [1] for answer.
\n1 (df)
OR
(r-1) (c-1);
There were many correct answers, but a number confused the alternate and null hypotheses.
\nCalculating the degrees of freedom proved problematic for many.
\nDNA forms chromosomes, but much of eukaryote DNA is non-coding.
\n[Source: Pixabay.]
\nPeople can be analysed genetically for risks of specific diseases before they actually develop. One of these is type I diabetes.
\nDescribe the structure of nucleosomes.
\nExplain how the two strands of the DNA double helix are held together.
\nThere are multiple tandem repeats of nucleotide segments of DNA found in the non-coding DNA between genes. Outline how tandem repeats are used for DNA profiling.
\nExplain what happens in a person when they develop type I diabetes.
\nState how type I diabetes should be treated to avoid harmful health consequences of the condition.
\na unit of DNA wound/coiled around 8 histone proteins / octamer;
\nOWTTE
\nFull names required for c and d though use ecf.
\nOWTTE
\n“sugar” is NOT accepted in place of glucose. However, this should only be penalized once; i.e., utiltize ECF.
\nThe structure of a nucleosome was correctly described for many, but some did not mention the number of histones.
\nThere was a noticeable number of blank answers with respect to this question about using tandem repeats as the foundation of DNA profiling. Others had a good idea about PCR and gel electrophoresis, but many didn't mention restriction enzymes and did not clearly understand that patterns are compared.
\nThis question about blood sugar regulation was well done by most. This was a question where the use of general terms rather than specific terms led to lost marks. For example, sugars instead of glucose and ‘the body’ instead of ‘the pancreas’
\nThree-toed sloths (Bradypus variegatus) are placental mammals that live in trees in Central and South America. They eat leaves and fruit and get almost all their water from succulent plants.
\n[Source: Adapted from Laube, S., 2003. Three-toed-sloth (Bradypus variegatus), Lake Gatun, Republic of Panama. [image online] Available at: https://meta.wikimedia.org/wiki/User:Bradipus#/media/File:Bradypus.jpg]
\nThree-toed sloths change their body posture in response to the temperature of their environment (ambient temperature). Researchers assessed posture on a scale from 1 to 6, with 1 being when the sloth was curled into a tight ball and 6 when it had all limbs spread. The percentage of time the sloths were observed in each position was recorded at ambient temperatures from 22 °C to 34 °C. The researchers also measured the body temperature of the sloths over the same range of ambient temperatures.
\n[Source: Adapted from Cliffe, R.N., Scantlebury, D.M., Kennedy, S.J., Avey-Arroyo, J., Mindich, D. and Wilson, R.P., 2018. The metabolic response of the Bradypus sloth to temperature. PeerJ, [e-journal] 6: e5600. http://dx.doi.org/10.7717/peerj.5600. Licensed under a Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.]
\nThe daily food intake of three-toed sloths and daily ambient temperatures were monitored over a 160-day period from February to early July. The graphs show the mean results.
\n[Source: Cliffe et al. (2015), Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus variegatus). PeerJ 3:e875; DOI 10.7717/peerj.875 Licensed under a Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.]
\nState the relationship between sloth body temperature and ambient temperature.
\nExplain how this relationship differs from that in humans.
\nDescribe the trend in body posture as ambient temperature rises from 22 to 34 °C.
\nSuggest reasons for this trend.
\nThe mean daily food intake fluctuated from day to day. State the month that contains the day on which the mean intake of food was highest.
\nOutline the relationship between ambient daily temperature and food intake in March.
\nSuggest, with a reason, how the activity of the sloth varies with ambient temperature.
\nState one feature of the sloth that would indicate it is a mammal.
\nbody temperature increases with ambient temperature / positive correlation;
\nSince direct can be either –/+, no credit for direct correlation alone.
\nhumans maintain/regulate a constant body temperature at different ambient temperatures/maintained by homeostasis;
\nAs ambient temperature increases, the sloth spends more time with limbs spread
OR
as ambient temperature increases the sloth spends less time curled in a ball
OR
as ambient temperature rises, the posture changes from 1 to 6;
May;
\nReason required.
\nCharacteristic must be exclusive to mammals.
\nIf more than one answer, use the first one given.
\nOften steady or constant body temperatures for humans was given rather than homeostasis. Some candidates erroneously described humans as warming up in response to cold ambient temperatures.
\nThere were a few beautiful responses where surface area was specifically incorporated into the answer achieving maximum marks.
\nGood reasoning was not seen often, best answers needed some of the elements in this linkage: higher temperature led to uncurling which led to movement which led to eating because of energy needs; \"sloth prefers staying curled up so cold does not affect them\" or \"sloth enjoys staying curled up\" were not acceptable
\nHave hair/fur or feed young with milk were the best answers. Many students wrote \"give birth to live young;\" While this is true of most mammals, monotremes lay eggs. Furthermore, some other animals give birth to live young (ovoviviparous) such as some reptiles and some groups of fish. We credited only characteristics exclusive to all mammals.
\nThe diagrams represent the structure of a protein before and after it has become denatured.
\n[Source: Dean Williams, L., 2019. Molecular Interactions. [online] Available at: https://ww2.chemistry.gatech.edu/~lw26/
structure/molecular_interactions/mol_int.html [Accessed 20 August 2019].]
State how many different types of amino acid there are, which can become part of a polypeptide when mRNA is translated.
\nOutline one cause of denaturation in proteins.
\nExplain how denaturation affects the activity of an enzyme.
\n20;
\nAmazingly, most candidates missed gaining the mark for this question: a huge variety of incorrect numbers appeared, \"4\" was most frequent. Many candidates seemed to confuse the 4 nucleotide bases with the 20 amino acids.
\nMost candidates answered these questions about enzymes either really well or very poorly. Some erroneous answers mistook the active site as being separate to the enzyme or part of the substrate. Another problem had to do with temperature as a denaturant. Lowering the temperature does not denature the enzyme, though it may slow the reaction down. The candidate needed to say increased or hot temperature to gain a mark.
\nThe graph shows the mean annual changes in global temperatures between 1880 and 2014. The mean temperature from 1951 to 1980 was used as the value of zero change in temperature.
\n[Source: National Aeronautics and Space Administration, n.d. GISS Surface Temperature Analysis (v3). [online]
Available at: https://data.giss.nasa.gov/gistemp/graphs_v3/ [Accessed 20 August 2019].]
Calculate the increase in mean global temperature between 1880 and 2010.
\n.......... °C
\n\n
\n
Outline how changes in temperature over short time periods could give a misleading impression of changes to the Earth’s climate.
\nExplain how increased carbon dioxide in the air leads to the greenhouse effect.
\n0.87; (accept values between 0.8 and 0.9)
\nshort-term reading could show global temperatures falling while the trend is rising
OR
fluctuations from year to year may not show long-term trend;
Do not accept “reflected” for mpb.
\nMany candidates got the calculations right.
\nAnswers to this question suffered due to poor expression of an answer. Other candidates answers were succinct and to the point as they used the data to make the point clear.
\nThere were elegant answers to this question demonstrating a high level of understanding. Many other candidates mixed up all sorts of ideas here, including the idea of ozone being a cause of global warming and stating as a \"fact\" that heat travels from the sun to the earth. Among the confused ideas was the notion that carbon dioxide is a toxic pollutant.
\nThe pedigree chart shows the incidence of hemophilia in some of the descendants of Queen Victoria.
\nState the probability that Edward had hemophilia.
\nExplain the reasons for none of the females in the pedigree chart having hemophilia.
\n0.5/50 %;
\nApproximately 50 % of the candidates were correct with their answer of 50 %. It was expected that candidates would recognize that Edward is male because he has a box rather than circle in the pedigree. Then from there, they would calculate the 50 %.
\nThere were many good answers for this question showing understanding of haemophilia being sex linked and recessive Quite a number then failed to get the third mark by being too vague in their explanations. Some candidates thought that hemophilia is carried on Y chromosome; many understand that females have two X chromosomes; talk about females as carriers was often seen, but without any details to expand the answer; or that males are most likely to have hemophilia but, again, no details; for the most part answers showed shallow understanding.
\nThe graph shows the change in the membrane potential of an axon during an action potential.
\nState the approximate value of the membrane potential at X.
\nY is the threshold potential. State what happens when the threshold potential is reached.
\nDescribe the movements in ions that occur during time t.
\nExplain how a nerve impulse is passed on to other neurons.
\n0 mV; (accept answers in the range of – 10 mV to +10 mV) (Units required)
\nsodium channels (start to) open
OR
depolarization/axon begins to depolarize
OR
action potential occurs;
Reject pumping of ions.
\nNa+/sodium ions diffuse into the axon (in the first part/half of t);
K+ /potassium ions diffuse out of the axon (in the second half/part of t)
Do not accept the name of the element without indication that it is an ion.
\nThere were some truly brilliant answers to Question 5. However, the weakest answers in the examination were also found here. Of the questions, these were the most commonly left blank.
\nCandidates were often unable to approximate X. Missing units resulted in no marks awarded.
\nFundamental terms such as action potential or depolarization were not used in Q5(b) and (c). Some candidates failed to distinguish between elements and ions.
\nSome candidates talked about ion movement but with no reference to a specific ion; some described the Na+/K+ pump here and received no marks.
\nUse of term \"synapse\" was often missing; involvement of the brain was sometimes built into the passage of a nerve impulse to another neuron.
\nOutline reasons for the therapeutic use of stem cells.
\nDescribe how leaf cells make use of light energy.
\nExplain how cells and cell components in the blood defend the body against infectious disease.
\nWavelengths accepted for mpg.
\nPlatelets: [3 max]
\nECF may be applied when candidates use white blood cells in place of specific terms.
\nSome excellent answers; stem cells was usually well answered, leaf cells was often more complex than necessary but marks were usually able to be gained, though many answers were missing the obvious, for example leaf cells contain chloroplasts. Defence against disease; far too often over-complicated and confused answers. It is important that students are aware of the level they need to know. There is no need to over-complicate and risk loss of understanding, which was evident across many answers and which was so confused across the roles of macrophages, B cells and T cells. Equally, for cell components, more often than not this was ignored or mistakenly discussed as skin and mucus over platelets.
\nMostly well done. Appropriate examples provided for stem cell therapies. It seems that many candidates think that all stem cells can differentiate into \"any\" type of cells, which is not correct. Other candidates showed understanding of the discrimination between the source and the potential of the cells. Occasionally candidate showed confusion about the term \"stem\" in that \"stem cells show the way plants grow\"; sometimes therapeutic use of stem cells was confused with gene editing.
\nGenerally speaking, students know how leaf cells make use of light energy; at least three marks were usually gained. Some candidates made the answer too complex and involved, when a simple straight-forward answer would have been better.
\nAmong all candidates, there was knowledge of how each of the blood components (platelets, phagocytes, lymphocytes) can defend against infectious disease; each of the marking points in the mark scheme was eventually awarded; popular ideas were that fibrin is formed, phagocytes engulf pathogens and antibodies provide specific immunity.
\nCorrect terminology was confused or was not employed in too many answers.
\nDraw a labelled diagram to show the structure of a single nucleotide of RNA.
\nDistinguish between the processes of meiosis and mitosis.
\nExplain the development of antibiotic resistance in terms of natural selection.
\nAward [2 max] if more than one nucleotide drawn.
\n“Sugar” alone is insufficient.
\nAward [6 max] if pathogen is used instead of bacteria throughout the answer with no mention of bacteria.
\nOccasionally, deoxyribose was seen rather ribose; sometimes more than one nucleotide was shown limiting maximum marks.
\nThe best answers had clear comparative statements or were in a comparison table. The worst answers were continuous prose descriptions of the processes, as these all too often failed to be able to be pieced together to make full marking points.
\nAbout half of the candidates did not mention cell divisions, but simply mentioned \"two cycles\" which was too vague and instead of writing haploid/diploid, many said 23/46, which only applies to human or certain organisms.
\nThis natural selection question had better responses than in previous years. There seems to be a widespread misconception that bacteria can mutate after exposure to an antibiotic rather than surviving because of a genetic variation. Some confusion was demonstrated concerning the use of antibiotics: some candidates did not understand that antibiotics are used against bacteria (not viruses). There was lots of confusion in the use of different but similar words — antibiotics / antibody / antigen
\n\n Draw a labelled diagram to show the structure of a single nucleotide of RNA.\n
\n\n [3]\n
\n\n Describe how DNA profiling can be used to establish paternity.\n
\n\n [5]\n
\n\n Explain the reasons for variation in human height.\n
\n\n [7]\n
\n\n \n
\n Many candidates were able to draw a nucleotide and label the subunits correctly. Weaker candidates did not know what a nucleotide was, so often drew a diagram of either a DNA or RNA polynucleotide.\n
\n\n Answers to this question were very variable, with some very well-informed accounts including the use of genes containing of tandem repeats and details of the collection of DNA samples from both parents and the child. There were also many accounts with errors of understanding. A common misconception is that paternity is established by finding the male whose profile has most similarities to that of the child. This does not prove that a man is the father of the child and instead the father's profile must contain all bands in the child's profile that do not occur in that of the mother.\n
\n\n This was a relatively high scoring question, with a mean mark of 2.4 (out of 7). Teachers expressed surprise in G2 forms that their students were being expected to make 7 valid points in their answer, but the wide-ranging mark scheme ensured that this was possible. There was some confusion between polygenic inheritance and multiple alleles. Weaker candidates tended to think that dominant alleles make us taller and recessive alleles cause shortness. Another fault in many answers was to focus on natural selection and evolution of height — if anything natural selection will reduce variation in human height rather than cause it.\n
\nAn oval-shaped stem cell cluster is shown in the micrograph.
\n[Source: Sontag, S., Förster, M., Seré, K. and Zenke, M., 2017. [online] Available at: https://bio-protocol.org/e2419 [Accessed
6 December 2019]. Source adapted.]
State the main characteristic of stem cells.
\nCalculate the maximum diameter of the stem cell cluster on the micrograph, showing your working and giving the units.
\n\n
\n
\n
..........
\nState one therapeutic role of stem cells.
\nDiscuss how the use of stem cells to treat hereditary diseases could affect the person who received the treatment and their progeny.
\nundifferentiated/pluripotent/ability to divide/differentiate into any types of cells/differentiate along different pathways;
\nAllow answer in range of 1150 μm to 1350 μm.
treatment of Stargardt’s disease/leukemia/diabetes/heart disease/Parkinson’s disease;
\nAny other verifiable condition.
\nIn a. most were able to say the stem cells were undifferentiated. Many put ‘unspecialised’ instead, which was not credited. There were several, justified G2 comments about the size of the micrograph, which made it difficult to measure the diameter accurately. However, this was generally very poorly answered, with over half failing to get the mark for either the correct answer or the correct calculation. In 1d the better candidates stated that the person could now live a normal life, but as it was not in the genome the progeny could still be affected. The word ‘progeny’ was commented on by several teachers, who pointed out that ‘children’, or ‘offspring’ would have been clearer for those working in an additional language.
\nMethane can be the product of anaerobic respiration in some organisms.
\nDescribe anaerobic respiration in humans and in yeast.
\nDistinguish between the thermal properties of water and methane.
\nExplain the role of methane in climate change.
\nDo not accept mpd if CO2 also included.
\nOther verifiable sources.
\nIn 3a. the top candidates scored all three marks with ease, but many, especially in Spanish scored zero. A common mistake was to say that humans produce lactic acid and CO2. In bi, many were confused by the term ‘thermal properties’, but most were able to gain at least one point for comparing their boiling/freezing temperatures, specific heat capacities and latent heat of fusion. The best candidates also explained the differences in terms of hydrogen bonds between the water molecules. In bii, few got past stating that methane is a greenhouse gas/causes an increase in temperature of the atmosphere. The better candidates stated that it was a powerful greenhouse gas, but it had a relatively short life span compared to CO2. A surprising number thought that methane’s main danger was that it destroyed the ozone layer.
\nEnzyme activity is affected by temperature and pH.
\nExplain the decrease in activity of the enzyme on either side of the optimum temperature.
\nIn biotechnology, enzymes are used to transfer genes to bacteria. Outline how two specific enzymes are used for the transfer.
\nOWTTE
\nAccept correct mention of reverse transcriptase.
\nIn a. many incorrectly stated that the enzyme was denatured either side of the optimum. Denaturation was reasonably well known, but the explanation for increased activity with rising temperature was not. Surprisingly some candidates missed the obvious references to temperature and wrote about pH instead. Knowledge of the enzymes involved in gene transfer was very patchy, with over a quarter of candidates leaving it blank. This was the worst scoring question on the paper.
\nPeople with the inherited disease hemophilia have a deficiency in one of the proteins that act as clotting factors in blood.
\nOutline how a protein is made in a cell by the process of translation.
\nA family has a history of hemophilia that is caused by a sex-linked recessive allele. A woman from this family is a carrier and marries a man who does not have the allele. Showing your working, determine the probability of their children having the disease.
\nExplain how blood clotting occurs and the consequence for a person who has hemophilia.
\nSex needs to be mentioned for mpc but info can be taken from the Punnett square.
\nClotting process:
\ne.g. surgery, contact sports
\nWell prepared candidates were able to clearly explain the production of a protein by translation. Weak candidates usually scored a few points. Many started with a very detailed description of transcription, which was not necessary and put them in danger of losing the quality mark.
\nA standard sex -linked genetics question. It was expected that an indication that half of the boys would suffer from haemophilia, not just a ‘25 % possibility’.
\nThe formation of a clot by the final formation of fibrin was reasonably well known, although many implied that the thrombin directly converted the fibrinogen and did not mention the cascade of reactions. In the consequences for haemophiliacs the fact that the clot would not form and they risked bleeding to death and were more susceptible to infections were well known. There were some G2 comments that 7 marks seemed difficult to attain, this did not seem to be the case with an average of 4.1 marks for the question.
\nThe growing human population has an increasing demand for energy derived from crop plants. At the same time, increasing droughts that are part of climate change make it difficult to grow crops in some parts of the world.
\nOutline energy flow through a community in a natural ecosystem.
\nExplain how natural selection can cause traits such as drought resistance to develop in wild plants.
\nSuggest possible benefits and risks of using genetic modification to develop varieties of crop plant with traits such as drought resistance.
\nBenefits:
\nMust include at least one benefit and one risk for [3 max].
\nThis was well attempted with an average score of 3. The question was about energy, but weaker students also tended to include biomass. Fortunately, very few students failed to grasp the idea that the energy is lost from the ecosystem and not recycled.
\nThe inclusion of ‘drought resistance’ in the stem of the question should have been a suggestion to include it, but many answers were far to general with a vague attempt to explain Natural Selection in general without explaining how the initial variation in the population came about. There were quite a few ‘Lamarckian’ answers with claims of individuals adapting to cope with the change. The average for this question was only 2.8, putting it as one of the most poorly answered.
\nAgain, drought resistance was in the stem, but not always used. There were many general ‘rants’ about GMO crops without setting out the facts.
\n\n Outline\n \n four\n \n different processes,\n \n with examples\n \n , that allow substances to pass through the plasma membrane.\n
\n\n [4]\n
\n\n Humans need to balance water and solute concentrations and also excrete nitrogenous wastes. Explain how the different parts of the kidney carry out these processes.\n
\n\n [7]\n
\n\n Describe adaptations in mammals living in desert ecosystems to maintain osmolarity in their bodies.\n
\n\n [4]\n
\n\n \n Marks can be awarded to clearly annotated diagrams\n \n .\n
\n\n This question in section B was the second most commonly selected, but in performance, tended to do more poorly than the other questions.\n
\n\n Most candidates could list the processes that allow passage across the plasma membrane, but many answers lacked some of the necessary elements, most commonly leaving out examples (e.g. an example for gradient).\n
\n\n Generally, there were good accounts of the functioning of the nephron, but some elements were missing such as which substances are reabsorbed in the proximal tubule and which are absorbed in the loop of Henle, although permeability was mentioned. Discussion about the role of ADH was well done. The discussion of the management of nitrogenous wastes was least well done in this question.\n
\n\n Long loops of Henle was the most common desert adaptation discussed. There were common misconceptions about camels’ humps being water storage organs rather than the production of metabolic water.\n
\n\n State the relationship between BCH and brain mass of shrews.\n
\n\n [1]\n
\n\n Outline how the shrew labelled P differs from the normal relationship between BCH and brain mass.\n
\n\n [1]\n
\n\n Suggest a reason that researchers use BCH rather than brain mass to indicate brain size.\n
\n\n
\n\n [1]\n
\n\n State the season when shrew brain mass is greatest.\n
\n\n [1]\n
\n\n Compare and contrast the results for winter and spring.\n
\n\n [2]\n
\n\n State the activity and season that occupied the greatest mean percentage of observation time.\n
\n\n [1]\n
\n\n Suggest a reason for the difference in the time observed eating and drinking.\n
\n\n [2]\n
\n\n Calculate the percentage of containers that contained food.\n
\n\n [1]\n
\n\n Outline a reason that the path length was standardized.\n
\n\n [1]\n
\n\n Compare and contrast the results for trials 2 and 9.\n
\n\n [2]\n
\n\n With reference to all the data, suggest a reason for the difference in standardized mean path length for summer and winter.\n
\n\n [2]\n
\n\n positive correlation/the greater the BCH the greater the brain mass;\n
\n\n \n No mark for ‘positive relationship’ or for directly proportional\n \n
\n\n positive correlation/the greater the BCH the greater the brain mass;\n
\n\n \n No mark for ‘positive relationship’ or for directly proportional\n \n
\n\n easier to measure/doesn’t require dissection/non-invasive / shrew not harmed/killed/more ethical;\n
\n\n \n The mark can be awarded for one of these reasons even if it is not the first reason given in the answer\n \n .\n
\n\n Summer;\n
\n\n \n Compare part of answer = similarity\n \n :\n
\n\n \n For the second alternative in mpb, the answer must not state simply that BCH is higher in spring as there is much overlap. Do not accept quoted figures without the similarity or difference being stated.\n \n
\n\n \n rest\n \n ing in\n \n spring\n \n ;\n
\n\n 4(%);\n
\n\n compensates for the different distances between entrances and food/OWTTE;\n
\n \n \n OR\n \n \n
\n to enable (fair/valid) comparison/OWTTE;\n
\n \n Do not accept unspecific answers such as ‘to be consistent’.\n \n
\n\n \n Similarity between 2 and 9\n \n :\n
\n\n Most candidate described the relationship shown on the graph as a positive correlation. Some referred to it incorrectly as directly proportional, but that would be revealed by data points along a straight line on the graph. The term 'positive relationship' is vague and should be discouraged.\n
\n\n Most candidate described the relationship shown on the graph as a positive correlation. Some referred to it incorrectly as directly proportional, but that would be revealed by data points along a straight line on the graph. The term 'positive relationship' is vague and should be discouraged.\n
\n\n Almost all candidates understood what made P an outlier, but not all could express lucidly how it deviated from the trend.\n
\n\n Many candidates came up with unlikely reasons for using BCH as a measure, rather than that it was easier to measure and less harmful to the shrew. A common misconception was that brain mass was an inaccurate measure of brain size because brain density might be variable. It was surprising that so many candidates thought the height of the brain case would give a better measure of brain size, especially as the scatter graph showed that BCH is not directly proportional to brain mass.\n
\n\n 90\n \n % of candidates correctly identified summer as the season when brain mass was greatest.\n
\n\n This was a 'compare and contrast' question, so similarities and differences were expected in the answer. There were plenty of choices for differences between winter and spring but only one acceptable similarity — that BCH was lower in both winter and spring than in summer. Some answers failed to discriminate between BCH and brain mass — the trends were not the same so these terms were not interchangeable. Some candidates gave a comparison and then repeated it as the converse statement — for example 'sample mass is higher in spring than winter and is lower in winter than spring'. Clearly this is not necessary.\n
\n\n 80\n \n % of candidates identified the season and activity correctly.\n
\n\n This was generally well answered, with candidates mostly focussing on food being harder to find in winter and more energy being needed to maintain body heat.\n
\n\n Percentage calculations often cause widespread difficulties but most candidates found this one easy. The only common mistake was to divide 4 by a hundred, and then forget to multiply by a hundred to turn the proportion into a percentage.\n
\n\n This was another question that tested candidates' ability to express a relatively simple idea clearly, rather than having to understand a more complicated idea. The key part of the answer was that standardizing path lengths allowed results to be compared to find the effect of the intended independent variable, by cancelling out another variable.\n
\n\n As in part (e), a similarity and a difference were required. When analysing data, candidates should try to keep in mind whether aspects are of interest and lead to conclusions significant conclusions or not. Here the two interesting findings were that the path length was highest in winter in both trials and that the shrews' performance between trial 2 and 90 had improved in all seasons, so they found the food with a shorter path length.\n
\n\n The instruction to refer to all the data does not indicate that conclusions from each graph should be restated, but rather than the research findings should be combined to produce a hypothesis. The expected reasoning here was that the lower BCH of shrews in winter showed brain mass was on average lower, so cognitive ability was reduced and shrews were slower to find food.\n
\n\n Identify the protein labelled in the diagram.\n
\n\n [1]\n
\n\n Outline how nucleosomes affect the transcription of DNA.\n
\n\n [1]\n
\n\n Identify X, the enzyme which copies a DNA sequence.\n
\n\n [1]\n
\n\n Identify Y, non-coding DNA at the start of a gene.\n
\n\n [1]\n
\n\n Explain the role of lactose in the expression of the gene for lactase production.\n
\n\n [3]\n
\n\n State\n \n one\n \n reason that identical twins may show different methylation patterns as they grow older.\n
\n\n [1]\n
\n\n histone;\n
\n\n \n ‘Affects transcription’ is in the question — no mark.\n \n
\n\n \n RNA polymerase\n \n ;\n
\n\n promoter;\n
\n\n \n Accept the converse of mpa to mpd with lactose absent.\n \n
\n\n (different) environment/illness/disease/diet;\n
\n\n \n Not mutation\n \n
\n\n Nearly all candidates know that the proteins in nucleosomes are histones.\n
\n\n Many candidates knew something of the role of nucleosomes in regulating DNA transcription. A wide range of answers was accepted for the single mark.\n
\n\n About half of candidates knew that X must be RNA polymerase. Some candidates thought that it was DNA polymerase, perhaps because the question referred to copying a DNA sequence. This answer was not accepted as the diagram did not show replication.\n
\n\n Fewer candidates knew that Y must be the promoter. This could be deduced from the diagram which showed that Y is the site to which RNA polymerase first binds. Other incorrect suggestions were primer, intron, start codon and telomere.\n
\n\n Many candidates realised from the diagram that lactose binds to the repressor. Fewer could explain convincingly that this removes the repressor from the promoter, so RNA polymerase can bind and transcribe the gene for lactase. Many candidates earned a mark by giving the last idea on the mark scheme, which is that presence of lactose results in lactase production. It should be emphasized that the lac operon does not need to be taught to future students — the expectation here was that answers would be based on general understanding of transcription and on the information provided in the question. These was no expectation that candidates would already be familiar with the lac operon.\n
\n\n Most candidates knew that differences in environmental factors can cause differences in methylation pattern.\n
\n\n Identify structure X.\n
\n\n [1]\n
\n\n Identify structure Y.\n
\n\n [1]\n
\n\n State the region of the kidney in which the loop of Henle is situated.\n
\n\n [1]\n
\n\n Explain the role of the hormone ADH in osmoregulation.\n
\n\n [2]\n
\n\n Outline\n \n two\n \n adaptations for water conservation in leaves of desert plants.\n
\n\n [2]\n
\n\n \n proximal\n \n convoluted tubule/PCT;\n
\n\n \n proximal\n \n convoluted tubule/PCT;\n
\n\n glomerulus/Bowman’s capsule;\n
\n\n medulla;\n
\n\n \n Mark the first two answers only\n \n
\n (thick) wax layer/cuticle;\n
\n hairs on leaves/rolled leaves;\n
\n sunken stomata/stomata in pits/stomata opening at night/CAM physiology;\n
\n (leaves reduced to) spines/needles/no/few/small leaves/low surface area (to volume ratio) of leaves;\n
\n thick stems/water storage tissue/vertical stems (to avoid most intense sunlight);\n
\n 60null% of candidates identified the proximal convoluted tubule.\n
\n\n 60null% of candidates identified the proximal convoluted tubule.\n
\n\n 80% of candidates identified the structure as the glomerulus or Bowman's capsule.\n
\n\n 70\n \n % of candidates knew that the loop of Henle is in the medulla.\n
\n\n This was generally well answered. A few candidates thought that ADH could both increase and decrease water reabsorption in the nephron. The commonest omission was hypertonic conditions being the signal that results in ADH secretion.\n
\n\n A wide range of leaf adaptations were suggested and most candidates gave one or two that were accepted. Root adaptations were not accepted as the question asked for features of leaves.\n
\n\n Outline the processes occurring during interphase in the cell cycle.\n
\n\n [4]\n
\n\n Describe what occurs in a neuron when an action potential is propagated along the axon.\n
\n\n [4]\n
\n\n Explain how cells in the bloodstream cause a specific immune response.\n
\n\n [7]\n
\n\n \n Allow organelles ‘replicated’ for mpb.\n \n
\n\n \n Allow organelles ‘replicated’ for mpb.\n \n
\n\n \n Do not award mpa for sodium being pumped in (rather than diffusing).\n \n
\n \n Allow mpc with ECF if direction of both sodium and potassium is wrong, or if both movements are described as pumping.\n \n
\n \n Accept B and T cells instead of B and T lymphocytes.\n \n
\n\n Many candidates included growth and DNA replication in their answers. Other processes such as protein synthesis and cell respiration were mentioned less often.\n
\n\n Many candidates included growth and DNA replication in their answers. Other processes such as protein synthesis and cell respiration were mentioned less often.\n
\n\n Answers were very varied and some were excellent. A few missed the point and described synaptic transmission. In some weaker answers there was confusion about the role of the sodium-potassium pump and what makes sodium and potassium ions move across the membrane during depolarisation and repolarisation. The mechanism of propagation by the formation of local currents was sometime attempted but rarely expressed clearly.\n
\n\n Here again there was a wide range of answers. The best focussed throughout on how specificity in immune responses is achieved. The weakest were very muddled, with confusion between antigens and pathogens and between antigens and antibodies. Nearly all candidates mentioned memory cells and their role in long term immunity.\n
\n\n Describe how ATP is produced by Photosystem II in the light-dependent stage of photosynthesis.\n
\n\n [5]\n
\n\n Explain how carbohydrates are transported from plant leaves.\n
\n\n [7]\n
\n\n There was evidence that many candidates had prepared carefully for this topic and were fully familiar with the sequence of events in the light-dependent reactions that result in ATP production. In some of the weaker answers the protons were moving in the wrong direction and some candidates were confused about the differences in structure between chloroplasts and mitochondria and therefore the nature of the proton gradient.\n
\n\n This was answered well by many candidates, with correct use of terminology. Some answers gave considerable detail about methods used to load assimilate into phloem. The specific structures used for transport (sieve tubes) were not always named and their adaptations were rarely included. Many candidates stated that phloem transport is bidirectional and that xylem transport is not — a hypothesis that has been falsified! Transport in a single phloem sieve tube can only be in one direction at one time, though the direction can be reversed if the hydrostatic pressure gradient switches, for example when a growing leaf changes from being a sink to a source. Xylem transport can also be bidirectional over time, as xylem sap sinks down to the roots of deciduous trees when leaves are lost in the fall and some plants allow xylem sap to sink to the roots every night when transpiration stops.\n
\n\n Outline how adaptive radiation provides evidence for evolution.\n
\n\n [3]\n
\n\n Describe polyploidy and how it can lead to speciation.\n
\n\n [5]\n
\n\n Explain how a newly discovered plant species would be classified and named.\n
\n\n [7]\n
\n\n \n Allow any of the marking points with reference to a named example e.g. Darwin’s finches or pentadactyl limb.\n \n
\n\n \n Allow any of the marking points with reference to a named example e.g. Darwin’s finches or pentadactyl limb.\n \n
\n\n \n Reject non-disjunction as a cause of polyploidy as it usually applies to a single bivalent.\n \n
\n\n Naming:\n
\n\n \n For mpi, common names such as ‘mosses’ are acceptable\n \n .\n
\n\n Some candidates misread the question and wrote about radiation causing mutations. Other answers described how a species changes over time due to natural selection. A few answers gave a convincing account of how structures such as the pentadactyl limb give evidence for evolution because similarities of structure despite differences of function indicate common ancestry.\n
\n\n Some candidates misread the question and wrote about radiation causing mutations. Other answers described how a species changes over time due to natural selection. A few answers gave a convincing account of how structures such as the pentadactyl limb give evidence for evolution because similarities of structure despite differences of function indicate common ancestry.\n
\n\n A minority of answers defined polyploidy correctly and then explained how it results in speciation through reproductive isolation. Weaker responses muddled up aneuploidy with polyploidy so claimed that non-disjunction of a pair of chromosomes causes polyploidy. The average score for this question was low, because it tended to be weaker candidates who were answering it, and also because it is a relatively difficult part of the program.\n
\n\n A very wide range of ideas was credited with marks in this question. Most candidates knew that plants are either classified according to their observable traits or the base sequences of their DNA. Many answers also included the hierarchy of taxa used in traditional classification and traits of plant phyla. At least some features of binomial nomenclature were included in most answers. The best answers included changes to plant classification that have resulted from sequence analysis and cladistics.\n
\n\n State the mean annual decline in insect biomass.\n
\n\n [1]\n
\n\n Compare and contrast the results for Hymenoptera and Lepidoptera.\n
\n\n [2]\n
\n\n Calculate the number of Coleoptera species that would be expected to exist after one year from a starting number of 400\n \n 000 species, assuming the mean rate of decline.\n
\n\n [1]\n
\n\n Identify in how many of the diets sucrose was the greatest mass of food consumed.\n
\n\n [1]\n
\n\n Compare and contrast the results for the 1:1 and the 1:10 P:L diets.\n
\n\n [2]\n
\n\n Calculate the mass of lipid eaten when the bumblebees were presented with the 5:1 diet.\n
\n\n [1]\n
\n\n Suggest a reason that the mass of protein and lipid mixture eaten at 25:1 is lower than at 50:1.\n
\n\n [1]\n
\n\n State the relationship between high lipid content and survivability on day 7.\n
\n\n [1]\n
\n\n Suggest with a reason which P:L diet is closest to the normal diet of these bumblebees.\n
\n\n [1]\n
\n\n Discuss whether these studies show that habitat destruction can affect global bumblebee numbers.\n
\n\n [2]\n
\n\n 2.5\n \n %;\n
\n\n \n % required\n \n
\n\n 2.5\n \n %;\n
\n\n \n % required\n \n
\n\n 392\n \n 000 (species);\n
\n\n 5;\n
\n\n \n Accept vice versa for all answers.\n \n
\n\n 0.025g;\n
\n\n \n Unit needed\n \n
\n\n the higher the lipid content, the fewer bees survived/negative correlation;\n
\n\n 10:1 diet as this has the highest survival rate (after 7 days);\n
\n\n \n Reason must be given.\n \n
\n\n \n Accept other reasonable discussion using the data.\n \n
\n\n In general, the Data based question was well answered. In (a) all but a handful of candidates failed to state 2.5, but unfortunately many failed to also include the unit. In (b) Most gave a difference, but similarities were rarer. The statement that both show a decline would have earned the mark. A surprising number were confused by the arithmetic involved in (c), with most calculating that the decrease was 8000, but either forgetting to subtract this from the 400,000, or doing it incorrectly. In (d) almost everyone counted 5 correctly and in (e) a large proportion gained both marks for a comparison and a contrast. The calculation in (f) involved the use of ratios. There was a G2 comment that this was not common knowledge. However, as all IB students continue to study mathematics at some level, it was not considered beyond their capabilities. 40\n \n % of the students gained the mark. A similar number gained the mark in (g), usually for an correct answer, but many answered in terms of the actual ratios, which did not gain the mark. In (h) 80\n \n % were able to spot that there was an inverse relationship between high lipid content and survivability, but in (i) fewer were able to cite the 10:1 diet as closest to the normal diet, some failing to state the reason. The answers t (j) were very varied, with most gaining marks for the possible loss of natural food sources or\n
\n the bees having to look for others. Better candidates noted that there was no control where the bees were fed their normal diet as it was a simulation, not using pollen. There was a G2 comment that the graph was somewhat crowded and should have been bigger. This is perhaps true.\n
\n In general, the Data based question was well answered. In (a) all but a handful of candidates failed to state 2.5, but unfortunately many failed to also include the unit. In (b) Most gave a difference, but similarities were rarer. The statement that both show a decline would have earned the mark. A surprising number were confused by the arithmetic involved in (c), with most calculating that the decrease was 8000, but either forgetting to subtract this from the 400,000, or doing it incorrectly. In (d) almost everyone counted 5 correctly and in (e) a large proportion gained both marks for a comparison and a contrast. The calculation in (f) involved the use of ratios. There was a G2 comment that this was not common knowledge. However, as all IB students continue to study mathematics at some level, it was not considered beyond their capabilities. 40\n \n % of the students gained the mark. A similar number gained the mark in (g), usually for an correct answer, but many answered in terms of the actual ratios, which did not gain the mark. In (h) 80\n \n % were able to spot that there was an inverse relationship between high lipid content and survivability, but in (i) fewer were able to cite the 10:1 diet as closest to the normal diet, some failing to state the reason. The answers t (j) were very varied, with most gaining marks for the possible loss of natural food sources or\n
\n the bees having to look for others. Better candidates noted that there was no control where the bees were fed their normal diet as it was a simulation, not using pollen. There was a G2 comment that the graph was somewhat crowded and should have been bigger. This is perhaps true.\n
\n Annotate the diagram to illustrate the amphipathic nature of phospholipids.\n
\n\n [2]\n
\n\n Outline a function of cholesterol in cell membranes.\n
\n\n [1]\n
\n\n Describe\n \n two\n \n pieces of evidence that show that eukaryotic cells originated by endosymbiosis.\n
\n\n [2]\n
\n\n reduces fluidity of membrane / reduces permeability of membrane (to some molecules);\n
\n\n \n Accept ‘controls’ or ‘maintains ‘ as a BOD.\n \n
\n\n Part (a) was a variation on a classic Section B question on the phospholipid bilayer. Approximately 1 in 10 students left it blank, suggesting that many did not think that they had to reply to it as there was no answer box. Cholesterol's function in reducing fluidity and permeability was not well known in (b). In (c) poorer candidates described endosymbiosis, rather than answering the question. Many knew about double membranes, own DNA and 70s ribosomes, but did not score any marks as they failed to say that they were in mitochondria and/or chloroplasts. This is perhaps a case where planning the answer was lacking, rather than rushing in.\n
\n\n Part (a) was a variation on a classic Section B question on the phospholipid bilayer. Approximately 1 in 10 students left it blank, suggesting that many did not think that they had to reply to it as there was no answer box. Cholesterol's function in reducing fluidity and permeability was not well known in (b). In (c) poorer candidates described endosymbiosis, rather than answering the question. Many knew about double membranes, own DNA and 70s ribosomes, but did not score any marks as they failed to say that they were in mitochondria and/or chloroplasts. This is perhaps a case where planning the answer was lacking, rather than rushing in.\n
\n\n It has been argued that variation in the global mean surface temperature has been caused by variation in energy from the Sun. Analyse whether evidence from the graphs supports this argument.\n
\n\n [2]\n
\n\n Explain how increased levels of atmospheric carbon dioxide contribute to global warming.\n
\n\n [3]\n
\n\n State\n \n one\n \n other gas that contributes to global warming.\n
\n\n [1]\n
\n\n (\n \n Evidence does not support this argument because\n \n :)\n
\n\n \n Do not award marks for claims that the evidence supports the argument, for example, claims that the fluctuations coincide.\n \n
\n\n \n Do not accept answers relating to ozone in the atmosphere because the question refers to carbon dioxide.\n \n
\n \n For mpa do not accept heat instead of LW/IR radiation.\n \n
\n methane/nitrous oxide/water vapour/ozone/CFCs/other halogenated gases;\n
\n\n Most candidates earned one of the two marks for the idea that the long-term trend in global mean surface temperature is an increase whereas there has if anything been an overall decrease in energy from the Sun between 1980 and 2020. Few answers pointed out that ten-year cycles in Sun's energy that are seen do not correspond to fluctuations in surface mean temperature. Weaker answers tended to pick out single instances of a rise in Sun's energy corresponding with a rise in surface temperature. Very few answers included any comments on the small variation in Sun's energy — only ranging from 1360.2 to 1362.2 W m-2 which is less than 0.15\n \n % fluctuation, with no overall increase, whereas surface temperature has increased by more than a degree Celsius in the 40 years from 1978 to 2018.\n
\n\n Understanding of the greenhouse effect is very patchy and in some cases is lamentably weak. Some candidates still think that 'increased CO\n \n 2\n \n burns a hole through the ozone layer' or that the greenhouse effect is due to carbon dioxide being reflected back to the Earth's atmosphere. Nearly all answers included errors of understanding. Global warming will have a major impacts on all IB students' future lives so we owe it to them to make sure they understand how it is happening. Perhaps they should memorise these elements:\n
\n\n Nearly all candidates could name another greenhouse gas that is contributing to global warming. Methane was the commonest answer. Water vapour was also allowed as it is increasing in the atmosphere.\n
\n\n Draw the symbol for individual X on the diagram.\n
\n\n [1]\n
\n\n Calculate the probability of male Y having an allele for the disorder.\n
\n\n [1]\n
\n\n Explain how the information in the box labelled B indicates that the gene is\n \n not\n \n sex-linked.\n
\n\n [2]\n
\n\n Explain how a single base substitution mutation in DNA can cause a change to a protein.\n
\n\n [2]\n
\n\n \n
\n \n
\n 50\n \n %/ 0.5/ 1/2;\n
\n\n The majority of candidates managed to draw the correct symbol for a heterozygous female in the correct place in (a). However, there were a few who thought that X was male. Just over a half correctly gave 50\n \n % for (b). There were a large number of very pleasing answers in (c) to explain why the gene is not sex linked including that if it were sex linked, then you could not have a heterozygous male, and if it were sex linked, the daughter would have been heterozygous. The explanations in (d ) were not of such a high quality, with a large proportion failing to note that it hinges on the coding for a different amino acid, thus producing a different protein. Students should be careful about restating the stem as the 'protein changes' did not score as that was in the stem.\n
\n\n The majority of candidates managed to draw the correct symbol for a heterozygous female in the correct place in (a). However, there were a few who thought that X was male. Just over a half correctly gave 50\n \n % for (b). There were a large number of very pleasing answers in (c) to explain why the gene is not sex linked including that if it were sex linked, then you could not have a heterozygous male, and if it were sex linked, the daughter would have been heterozygous. The explanations in (d ) were not of such a high quality, with a large proportion failing to note that it hinges on the coding for a different amino acid, thus producing a different protein. Students should be careful about restating the stem as the 'protein changes' did not score as that was in the stem.\n
\n\n Identify the structure labelled X.\n
\n\n [1]\n
\n\n Outline how the neuron is stimulated to release the neurotransmitter.\n
\n\n [2]\n
\n\n Explain the action of neonicotinoid pesticides in insects.\n
\n\n [3]\n
\n\n (Neurotransmitter) vesicle;\n
\n\n \n Do not accept Vacuole.\n \n
\n\n (Neurotransmitter) vesicle;\n
\n\n \n Do not accept Vacuole.\n \n
\n\n The majority managed to label X as a vesicle in (a). In (b) weaker candidates tried to explain the propagation of the action potential, rather than the release of the neurotransmitter. Better candidates were able to give almost textbook answers to explain the actions of the pesticide. Many got tangled up trying to explain, with those who had never seen this part of the syllabus, just saying that pesticides kill insects.\n
\n\n The majority managed to label X as a vesicle in (a). In (b) weaker candidates tried to explain the propagation of the action potential, rather than the release of the neurotransmitter. Better candidates were able to give almost textbook answers to explain the actions of the pesticide. Many got tangled up trying to explain, with those who had never seen this part of the syllabus, just saying that pesticides kill insects.\n
\n\n Outline how energy flows in an ecosystem.\n
\n\n [4]\n
\n\n Describe how plants affect the amount of carbon dioxide in the atmosphere.\n
\n\n [4]\n
\n\n Explain how a newly discovered plant species would be classified and named.\n
\n\n [7]\n
\n\n \n \n OR\n \n Shows correct equation\n \n
\n\n \n Naming\n \n :\n
\n\n \n Mpi accept common names, mosses, ferns etc\n \n .\n
\n\n Explaining the energy flow in part (a), was one of the best answered questions on the paper. However, the\n
\n conversion of light energy to chemical energy was missed by many. Some weaker candidates are still confusing energy with biomass, and lost marks due to contradictory statements such as saying that in the end the energy goes into the soil and is taken up by the plants again.\n
\n In both (a) and (b) many students drew diagrams, Carbon cycles etc. Some of these aided the explanation, but the majority contributed nothing.\n
\n\n Explaining the energy flow in part (a), was one of the best answered questions on the paper. However, the\n
\n conversion of light energy to chemical energy was missed by many. Some weaker candidates are still confusing energy with biomass, and lost marks due to contradictory statements such as saying that in the end the energy goes into the soil and is taken up by the plants again.\n
\n In both (a) and (b) many students drew diagrams, Carbon cycles etc. Some of these aided the explanation, but the majority contributed nothing.\n
\n\n In (b) many saw the words 'Carbon Dioxide' and immediately treated it as a long answer on greenhouse gases, thus putting their quality mark in jeopardy for superfluous content. Most were aware of photosynthesis removing CO\n \n 2\n \n from the atmosphere by fixing it into organic compounds and many mentioned plant respiration as increasing it. However, there were some disturbing answers where students mixed up photosynthesis and respiration. The question was about plants, but many answered in terms of animals.\n
\n\n There were a few G2 comments that part (c) on classification was above the level expected at Standard Level. This part was in common with Higher Level, but did only contain material from the Core (cladistics appears in section 5.4). There were a large number of marking points to score on. The naming was well known, as was classification into taxa and the different plant phyla. Some weaker students tried to go all the way back to eukaryotes and prokaryotes and animalia and plantae, giving far too much superfluous information. Good explanations of the use of clades were seen by better candidates.\n
\n\n In the grass plant\n \n Halopyrum mucronatum\n \n , the enzyme amylase breaks bonds in polysaccharides during germination. The graph shows how the activity of the enzyme varies with the concentration of polysaccharide.\n
\n\n \n
\n \n [Source: Material from: Siddiqui, Z.S. and Khan, M.A., The role of enzyme amylase in two germinating seed morphs of\n \n
\n \n \n Halopyrum mucronatum\n \n (L.) Stapf. in saline and non-saline environment, published 2011,\n \n Acta Physiologiae\n \n \n
\n \n \n Plantarum\n \n , reproduced with permission of SNCSC.]\n \n
\n
\n\n What is the reason for the curve levelling off?\n
\n
\n
\n A. There is insufficient substrate for the enzyme to act on.\n
\n\n B. The product acts as an enzyme inhibitor.\n
\n\n C. The enzymes have all been consumed in the reaction.\n
\n\n D. All the enzyme active sites are occupied by substrate.\n
\n\n [1]\n
\n\n Identify the relationship between migration distance and plumage colouration.\n
\n\n [1]\n
\n\n Suggest\n \n two\n \n reasons, other than mate selection, for variation in plumage colouration in red knots.\n
\n\n [2]\n
\n\n D\n
\n\n State the purpose of lime water in flask B.\n
\n\n [1]\n
\n\n Suggest a reason that the pot was covered with a plastic bag.\n
\n\n [1]\n
\n\n Suggest a suitable control for this experiment.\n
\n\n [1]\n
\n\n Using the information in the graph, describe how the experiment could be extended to determine the optimum temperature for immobilized HRP.\n
\n\n [2]\n
\n\n to check there is no carbon dioxide (left in the air)/show any carbon dioxide present;\n
\n\n \n Do not accept absorb carbon dioxide\n \n
\n\n soil releases CO\n \n 2\n \n from microorganisms/decomposers/bacteria/fungi\n
\n \n \n OR\n \n \n
\n respiration by microorganisms may affect the result;\n
\n using the same apparatus without a plant\n
\n \n \n OR\n \n \n
\n cover the whole plant with a plastic bag;\n
\n \n Do not accept a controlled variable\n \n
\n\n \n c. Accept numbers such as 1 or 5 degrees.\n \n
\n\n State\n
\n\n
\n\n where the DNA of each individual could be taken from.\n
\n\n [1]\n
\n\n how the DNA is amplified.\n
\n\n [1]\n
\n\n Deduce with a reason the identity of the father.\n
\n\n [2]\n
\n\n Describe how the rose shoot could be treated to show what part of the plant loses water.\n
\n\n [1]\n
\n\n A standard potometer only measures water uptake. Explain how this apparatus measures the amount of water lost by the shoot as well as uptake.\n
\n\n [2]\n
\n\n from blood sample/hair/cheek/saliva/semen;\n
\n\n \n Do not accept red blood cells\n \n
\n\n from blood sample/hair/cheek/saliva/semen;\n
\n\n \n Do not accept red blood cells\n \n
\n\n PCR/polymerase chain reaction\n
\n \n \n OR\n \n \n
\n using Taq polymerase;\n
\n
\n\n \n Must mention both the mother and father\n \n
\n\n parts of the shoot/leaves are removed/covered in petroleum jelly/Vaseline;\n
\n\n \n Accept part wrapped in plastic\n \n
\n\n \n OWTTE\n \n
\n\n State a variable that should be kept constant.\n
\n\n [1]\n
\n\n Identify the independent variable.\n
\n\n [1]\n
\n\n Outline the steps to produce a 10 % egg white solution.\n
\n\n [2]\n
\n\n Each measurement was repeated several times. Explain the need for replicates of each treatment.\n
\n\n [2]\n
\n\n Using the data, deduce whether pretreatment with ultrasound is effective for hydrolysing proteins.\n
\n\n [1]\n
\n\n enzyme/substrate/egg white concentration/volume/pH/temperature;\n
\n\n \n Accept another reasonable variable.\n \n
\n\n (with/without) ultrasound pretreatment / time;\n
\n\n \n Accept use of volume instead of mass.\n \n
\n\n \n Mp a: do not accept “accurate”.\n \n
\n\n it is not, as hydrolysis is higher without ultrasound pretreatment\n
\n \n \n OR\n \n \n
\n it is not, as hydrolysis is lower with ultrasound pretreatment;\n
\n \n Do not accept numerical values only with no attempt to evaluate their importance.\n
\n \n
\n \n Do not accept increase/decrease instead of lower/higher.\n \n
\n\n State\n \n two\n \n variables measured by the probes.\n
\n\n 1:\n
\n\n 2:\n
\n\n [2]\n
\n\n Suggest one advantage of using a mesocosm in this type of research.\n
\n\n [1]\n
\n\n Outline the requirements for sustainability within a sealed mesocosm.\n
\n\n [2]\n
\n\n environmentally safe/cheaper/control of variables easier;\n
\n\n \n The question is “outline”, so do not accept only a list of variables without a brief account.\n \n
\n\n Draw\n \n one\n \n phospholipid molecule on the diagram to show a possible position in the membrane.\n
\n\n [2]\n
\n\n The receptor contains seven alpha helices and one other secondary structure. Deduce what this other secondary structure is.\n
\n\n [1]\n
\n\n Discuss briefly whether amino acids on the surface of the protein are likely to be polar or non-polar.\n
\n\n [2]\n
\n\n Outline the role of melatonin in humans.\n
\n\n [2]\n
\n\n phospholipid shown with circular head and 2 tails;\n
\n e.g.\n
\n \n
\n \n [Source: Material from: Stauch, B., Johansson, L.C., McCorvy, J.D.,\n \n
\n \n et al., Structural basis of ligand recognition at the human MT1\n \n
\n \n melatonin receptor, published 2019,\n \n Nature\n \n , reproduced with\n \n
\n \n permission of SNCSC.]\n \n
\n phospholipid shown in either the upper or lower half of the membrane only with its head at the margin of the membrane and tails towards centre;\n
\n\n \n Accept answers with two or more phospholipids shown, as long as all are correct.\n \n
\n\n phospholipid shown with circular head and 2 tails;\n
\n e.g.\n
\n \n
\n \n [Source: Material from: Stauch, B., Johansson, L.C., McCorvy, J.D.,\n \n
\n \n et al., Structural basis of ligand recognition at the human MT1\n \n
\n \n melatonin receptor, published 2019,\n \n Nature\n \n , reproduced with\n \n
\n \n permission of SNCSC.]\n \n
\n phospholipid shown in either the upper or lower half of the membrane only with its head at the margin of the membrane and tails towards centre;\n
\n\n \n Accept answers with two or more phospholipids shown, as long as all are correct.\n \n
\n\n beta pleated sheet/beta-loop/beta strands;\n
\n\n \n Reject ‘beta helix’\n \n
\n\n polar/hydrophilic where exposed to the cytoplasm/to fluid outside cell/to polar phospholipid heads;\n
\n non-polar/hydrophobic where exposed to the (core of the) membrane/hydrophobic tails (of phospholipids);\n
\n \n Do not award mpb unless the answer is referring to secretion, not just correlations.\n \n
\n\n Calculate how long all the heart chambers are in diastole at the same time.\n
\n\n [1]\n
\n\n State the letter on an ECG corresponding with the events from 0.0 to 0.1 s.\n
\n\n [1]\n
\n\n Describe the state of the heart valves at 0.3 s.\n
\n\n [2]\n
\n\n Explain how cardiac muscle is adapted to its function.\n
\n\n [3]\n
\n\n 0.4 s;\n
\n\n P;\n
\n\n State where in the body the blood would be flowing at point X on the graph.\n
\n\n [1]\n
\n\n Outline the reason that the curve for fetal hemoglobin is to the left of normal adult hemoglobin.\n
\n\n [1]\n
\n\n lungs/alveoli;\n
\n\n Explain how natural selection could increase the prevalence of an antibiotic resistance gene in a species of soil bacterium.\n
\n\n [3]\n
\n\n There are viruses in soils that are pathogens of animals. Outline a reason for antibiotics in soil not eliminating these viruses.\n
\n\n [1]\n
\n\n Explain the reasons for antibiotics secreted into soil not harming insects or other animals in the soil.\n
\n\n [2]\n
\n\n \n Do not award marks for general statements about natural selection – answers must refer to antibiotic resistance.\n \n
\n\n \n Do not award marks for general statements about natural selection – answers must refer to antibiotic resistance.\n \n
\n\n
\n\n
\n\n Outline the role of ADH in osmoregulation.\n
\n\n [4]\n
\n\n Explain the regulation of metabolic pathways by end-product inhibition.\n
\n\n [4]\n
\n\n Describe the hormone feedback mechanisms that help to prepare a woman’s body for pregnancy, sustain the pregnancy and then give birth.\n
\n\n [7]\n
\n\n \n Do not accept answers suggesting that ADH has a different effect when body fluids are hypotonic.\n \n
\n\n \n Do not accept answers suggesting that ADH has a different effect when body fluids are hypotonic.\n \n
\n\n \n For mpe accept other examples of end-product inhibition if verified. Please add a note in such cases.\n \n
\n\n \n Preparing the woman’s body for pregnancy\n \n
\n\n a. FSH stimulates estrogen secretion (by the developing follicle);\n
\n b. estrogen increases FSH receptors so boosting estrogen production/so causing positive feedback;\n
\n c. estrogen stimulates repair/thickening of the endometrium/uterus lining;\n
\n d.\n \n high levels of estrogen\n \n stimulate LH production/inhibit FSH secretion (negative feedback);\n
\n e. LH (surge/peak) stimulates ovulation;\n
\n \n Sustaining pregnancy\n \n
\n\n f. LH stimulates the development of corpus luteum / corpus luteum secretes progesterone;\n
\n g. progesterone inhibits FSH/LH secretion (negative feedback);\n
\n h. progesterone maintains lining of uterus/endometrium (for pregnancy/implant of embryo);\n
\n i. progesterone inhibits uterine contractions;\n
\n j. HCG (secreted by embryo) stimulates maintenance of corpus luteum;\n
\n \n Childbirth\n \n
\n\n k. oxytocin stimulates uterine/myometrial contractions which stimulate oxytocin secretion;\n
\n l. positive feedback (mechanism used to stimulate childbirth);\n
\n Outline the changes to chromosomes that occur during prophase in the first division of meiosis.\n
\n\n [4]\n
\n\n Describe how plants are named and classified, including recognition features of plant phyla.\n
\n\n [7]\n
\n\n Describe the processes that are carried out by enzymes that bind to DNA.\n
\n\n [7]\n
\n\n Explain the effects that the environment can have on DNA in living organisms.\n
\n\n [4]\n
\n\n
\n\n \n Each phylum of plants requires at least 2 features for the mark.\n \n
\n\n \n Mp-e does not require all 4 phyla to be named.\n \n
\n\n \n replication\n \n
\n\n a. helicase unwinds the double helix/DNA;\n
\n b. helicase breaks hydrogen bonds between/separates/unzips DNA strands;\n
\n c. (DNA) gyrase/topoisomerase releases tensions in DNA as it unwinds;\n
\n d. (DNA) primase adds\n \n RNA\n \n primers (where DNA polymerase can bind);\n
\n e. DNA polymerase (III) replicates DNA/adds nucleotides (to make new strand);\n
\n f. DNA polymerase I replaces RNA (primers) with DNA;\n
\n g. DNA ligase seals nicks/joins sugar-phosphate backbones/joins (Okazaki)fragments;\n
\n \n transcription\n \n
\n\n h. RNA polymerase used for transcription;\n
\n i. RNA polymerase unwinds / separates DNA strands / binds to the promoter;\n
\n j. RNA polymerase copies DNA base sequence of a gene/makes mRNA;\n
\n k. restriction enzymes/endonucleases cut DNA at specific base sequences;\n
\n l. telomerase adds nucleotides to the ends of chromosomes/makes telomeres;\n
\n \n Mutation\n \n
\n\n a. (environment can cause)\n \n mutation\n \n ;\n
\n b. mutations are base sequence changes;\n
\n c. radiation/UV/gamma rays can cause mutations/changes to base sequences;\n
\n d. mutagenic/carcinogenic chemicals can cause mutations / mustard gas/another example;\n
\n \n Epigenetics\n \n
\n\n e. (environment) can cause changes to\n \n gene expression\n \n ;\n
\n f. methylation (patterns) in DNA changed (in response to environmental factors);\n
\n g. methylation inhibits (gene transcription) / acetylation promotes (gene transcription);\n
\n h. body temperature/stress/diet (can affect gene expression);\n
\n \n For mpd do not allow ‘mutagen’ instead of ‘mutagenic chemical’ as it includes forms of radiation as well as chemicals.\n \n
\n\n \n Allow smoking and asbestos as examples of mutagens /carcinogens in mpd.\n \n
\n\n Explain how vesicles are used by cells to move materials.\n
\n\n [5]\n
\n\n Describe the transport of carbon compounds such as sucrose and amino acids in phloem.\n
\n\n [7]\n
\n\n Arginine and ornithine are in the same group of biochemicals. Identify this group.\n
\n\n [1]\n
\n\n The concentration of urea in blood plasma is typically about 30 mg per 100 ml. In urine it can be as high as 1800 mg per 100 ml. Explain how this increase in concentration is achieved.\n
\n\n [3]\n
\n\n amino acids;\n
\n\n
\n\n State the name and function of structures I and II.\n
\n\n Structure I name and function: ..............................................................................................................\n
\n\n ..........................................................................................................................................................\n
\n\n Structure II name and function: .............................................................................................................\n
\n\n ..........................................................................................................................................................\n
\n\n
\n\n [2]\n
\n\n State one cause of lung cancer.\n
\n\n [1]\n
\n\n Suggest one difference between tissue taken from a lung cancer tumour and normal lung tissue that might be seen in micrographs.\n
\n\n [1]\n
\n\n The lung tumour in the light micrograph was slow-growing. Predict with a reason what would have been visible in the micrograph if the tumour was growing rapidly.\n
\n\n [2]\n
\n\n \n Both name and function must be included in each marking point.\n \n
\n\n \n The role in pollination should be stated. Do not accept “pollination” on its own.\n \n
\n\n
\n\n
\n\n \n See\n \n
\n\n a. more mitosis\n
\n \n \n OR\n \n \n
\n cells in prophase/metaphase/anaphase/telophase;\n
\n \n Why\n \n
\n\n b. more dividing cells/tumour cell divide uncontrollably\n
\n \n \n OR\n \n \n
\n a higher mitotic index;\n
\n
\n\n State with a reason whether pangolins are autotrophic or heterotrophic.\n
\n\n [1]\n
\n\n Pangolins are unique among mammals in having evolved scales, which are a recognition feature of reptiles. Explain which features you expect pangolins to have, which would show that they are mammals, not reptiles.\n
\n\n [2]\n
\n\n heterotrophic because it feeds on/eats food/other organisms /eats ants/termites/ doesn’t photosynthesise/does not produce its own food;\n
\n\n \n Do not accept “it is not autotrophic” as it is part of the stem question.\n \n
\n\n \n Mpa: pangolin underside may be covered with sparse fur.\n \n
\n\n \n Do not accept double circulation. Mpe: accept “do not lay eggs” as WTTE Do not accept internal fertilization.\n \n
\n\n Explain how changes to the cell cycle can result in tumour formation.\n
\n\n [4]\n
\n\n Outline the role of the right atrium in the cardiac cycle.\n
\n\n [4]\n
\n\n
\n\n
\n\n
\n\n State the\n \n mode\n \n for the number of steps per day in Japan and USA, rounding your answers up or down to the nearest 1000 steps.\n
\n
\n
\n Japan: ..................................................................................................\n
\n\n
\n USA: ....................................................................................................\n
\n [1]\n
\n\n Suggest reasons for the differences in LAI between the boreal and equatorial zones.\n
\n\n [3]\n
\n\n Analyse the data shown in the graph for evidence of a relationship between LAI and El Niño events.\n
\n\n [2]\n
\n\n The data in the graph show a long-term trend in global LAI.\n
\n\n
\n\n State the trend.\n
\n\n [1]\n
\n\n Global ecosystem modelling suggests that most of the change in LAI is due to increases in atmospheric carbon dioxide. Explain how rising atmospheric carbon dioxide (CO\n \n 2\n \n ) concentration could cause the observed change in LAI.\n
\n\n
\n\n Compare and contrast the data in the chart for males and females.\n
\n\n [2]\n
\n\n Suggest\n \n two\n \n hypotheses to account for the relationship between the mean number of steps per day and the proportion of people who are obese.\n
\n\n
\n\n Hypothesis 1: ...........................................................................................................\n
\n\n ...............................................................................................................................\n
\n\n
\n\n Hypothesis 2: ...........................................................................................................\n
\n\n ...............................................................................................................................\n
\n\n [2]\n
\n\n State the relationship between activity inequality and obesity shown in the scattergraph.\n
\n\n [1]\n
\n\n Using only evidence from the data in Question 1, suggest\n \n two\n \n strategies for reducing obesity in countries where this health problem is most prevalent.\n
\n\n [2]\n
\n\n Japan: 6000\n \n and\n \n USA: 4000;\n \n (both needed)\n \n
\n\n Japan: 6000\n \n and\n \n USA: 4000;\n \n (both needed)\n \n
\n\n \n Mpa refers to changes in LAI, not whether levels were high or low.\n \n
\n\n \n The example given for mpb must correspond with the trend given in mpa. The graph does not show the years clearly so we must show some lenience in mpb – award this mark if it is clear which period the candidate was referring to.\n \n
\n\n \n For mpa, do not accept answers implying that decreases in LAI cause El Niño or increases in LAI prevent El Niño.\n \n
\n\n Increase/increasing/upwards/rising (trend);\n
\n\n \n Reject ‘positive’, ‘positive trend’ and ‘positive correlation’\n
\n \n \n Accept linear increase.\n \n
\n Increase/increasing/upwards/rising (trend);\n
\n\n \n Reject ‘positive’, ‘positive trend’ and ‘positive correlation’\n
\n \n \n Accept linear increase.\n \n
\n \n If the answer focuses on greenhouse effect or global warming, do not award mpa, but mpb can be awarded if one of the alternatives is included in the answer.\n \n
\n\n Similarities (Compare)\n
\n\n a. lower percentage of obesity with more steps per day in both males and females\n
\n \n \n OR\n \n \n
\n percentage obesity is most similar at 1000 steps\n
\n \n \n OR\n \n \n
\n correlation of steps to percent obesity plateaus after 8000 steps for both males and females;\n
\n
\n Differences (Contrast)\n
\n b. the range difference of obesity percentage among different steps is bigger in females (9% - 31% versus 18% to 30%)\n
\n \n \n OR\n \n \n
\n walking has a greater impact on lowering obesity rates in females than males\n
\n \n \n OR\n \n \n
\n men show a greater percentage of obesity\n
\n \n \n OR\n \n \n
\n at 1000 steps per day there are more obese women than men;\n
\n
\n\n \n One similarity and one difference required for two marks.\n \n
\n\n \n (any order)\n \n
\n\n
\n\n \n Accept hypothesis and a null hypothesis for the two.\n \n \n
\n \n
\n as activity inequality rises percentage obesity rises\n
\n \n OR\n \n
\n Positive/direct correlation/relationship;\n
\n
\n\n \n Do not accept answers that involve diet as that is beyond the scope of the data given.\n \n
\n\n Identify the trophic level of sea lions. [\n
\n\n [1]\n
\n\n Outline the additional information that would be required to convert the food web into a pyramid of energy.\n
\n\n [2]\n
\n\n Outline a reason that the orca whale would be difficult to represent in a pyramid of energy.\n
\n\n [1]\n
\n\n secondary consumer / third trophic level/ trophic level 3 / 3;\n
\n\n orca is in different trophic levels depending on its food source;\n
\n\n Outline how the prickly pear cactus may become an ecological problem in Kruger National Park.\n
\n\n [2]\n
\n\n Explain the change in numbers of the cactus moth throughout the study period.\n
\n\n [2]\n
\n\n Suggest a reason for the loss of bird habitats.\n
\n\n [1]\n
\n\n Outline how\n \n in situ\n \n conservation may help preserve populations of amphibians.\n
\n\n [2]\n
\n\n Distinguish between species richness and evenness as components of biodiversity.\n
\n\n [1]\n
\n\n richness is how many species are in an area, evenness considers the relative number of each species;\n
\n\n Label with a P on the\n \n -axis the level of commercial sea fishing activity that would result in maximum sustainability.\n
\n\n [1]\n
\n\n Outline the reason for the change in yield in region III of the diagram.\n
\n\n [1]\n
\n\n Suggest a reason that it is difficult to keep global commercial sea fishing activity at a sustainable level.\n
\n\n [1]\n
\n\n Explain how the population of fish in a pond could be estimated using the capture-mark-release-recapture method.\n
\n\n [3]\n
\n\n label P on\n \n x\n \n -axis below the peak yield;\n
\n \n Dotted line for guidance, not required in answer.\n \n
\n \n
\n \n Do not accept if P is\n \n only\n \n on the curve\n \n
\n\n fish are being caught faster than they can reproduce;\n
\n\n Compare and contrast the effect on men and women of an increase in serum cholesterol.\n
\n\n [2]\n
\n\n Suggest a reason for high total serum cholesterol not always being an indicator of CHD risk.\n
\n\n [1]\n
\n\n \n Accept vice versa\n \n
\n\n Annotate the diagram by adding arrows to show how deoxygenated blood enters the heart.\n
\n\n [1]\n
\n\n Explain the function of the left ventricle.\n
\n\n [2]\n
\n\n arrows added to the diagram to show how deoxygenated blood enters the heart;\n
\n\n \n
\n \n [Source: CC BY-SA 3.0, https://commons.wikimedia.org/\n \n \n w/index.php?curid=606903. Open access.]\n \n
\n\n arrows added to the diagram to show how deoxygenated blood enters the heart;\n
\n\n \n
\n \n [Source: CC BY-SA 3.0, https://commons.wikimedia.org/\n \n \n w/index.php?curid=606903. Open access.]\n \n
\n\n State how many water molecules are shown in the diagram.\n
\n\n [1]\n
\n\n State how many water molecules are shown in the diagram.\n
\n\n [1]\n
\n\n Identify the interactions that are shown between the water molecules.\n
\n\n [1]\n
\n\n With reference to the diagram, explain how water in sweat evaporates.\n
\n\n [2]\n
\n\n Outline the reasons for secretion of sweat in humans.\n
\n\n [2]\n
\n\n 10;\n
\n\n 10;\n
\n\n hydrogen bonds/H bond;\n
\n\n
\n\n
\n\n Explain the conclusion that can be drawn from Cross 1.\n
\n\n [2]\n
\n\n Deduce reasons for the difference between the results of Cross 2 and Cross 3.\n
\n\n [2]\n
\n\n There were 79 progeny in Cross 4. Predict the expected results by completing the table.\n
\n\n [1]\n
\n\n The proportion of banded and unbanded individuals in a population of\n \n D. plantarius\n \n can change. Suggest how such a change could occur.\n
\n\n [1]\n
\n\n \n Accept answers given in the form of Punnet squares.\n \n
\n\n
\n\n
\n\n
\n| \n 0\n | \n\n 79\n | \n\n 0.000/0\n | \n
\n \n All three parts of the answer must be correct for the mark to be awarded.\n \n
\n\n
\n\n \n Do not accept a list. Natural selection or sexual selections alone are not adequate.\n \n
\n\n Outline the sequence of events that occurs during mitosis.\n
\n\n [5]\n
\n\n Explain how the presence of a Y chromosome in the cells of a human embryo causes it to develop as a male.\n
\n\n [3]\n
\n\n
\n\n \n If events are not in the correct order, then award a maximum of 4 marks.\n \n
\n\n \n Names of phases of mitosis not required.\n \n
\n\n \n Award a max of 1 mark for listing the phases in the correct order when the list constitutes the entire response.\n \n
\n\n
\n\n \n If events are not in the correct order, then award a maximum of 4 marks.\n \n
\n\n \n Names of phases of mitosis not required.\n \n
\n\n \n Award a max of 1 mark for listing the phases in the correct order when the list constitutes the entire response.\n \n
\n\n
\n\n \n Accept SRY gene in mp-a\n \n
\n\n Describe how plants are named and classified, including recognition features of plant phyla.\n
\n\n [7]\n
\n\n Outline the changes to chromosomes that occur during prophase in the first division of meiosis.\n
\n\n [4]\n
\n\n Outline the stages of the carbon cycle that involve plants or materials made by plants.\n
\n\n [5]\n
\n\n
\n\n \n Each phylum of plants requires at least 2 features for the mark.\n \n
\n\n \n Mp-e does not require all 4 phyla to be named.\n \n
\n\n
\n\n \n Carbon may be used in place CO\n \n 2\n \n an carbohydrates\n \n
\n\n Using the data in the stacked column graph, describe the features that characterize the B2 enterotype.\n
\n\n [2]\n
\n\n Estimate the median number of bacterial cells per gram of feces in the R enterotype.\n
\n\n [1]\n
\n\n Using the graph, identify the percentage who had received the tuberculosis vaccine in 2012.\n
\n\n [1]\n
\n\n Calculate the difference in time between 40 % of children receiving the hepatitis B vaccine and 40 % receiving the pneumococcal vaccine.\n
\n\n [1]\n
\n\n Outline the conclusions that can be drawn from the graph showing data for PCV and\n \n S. pneumoniae\n \n .\n
\n\n [2]\n
\n\n Suggest reasons for the difference between the data for\n \n S. pneumoniae\n \n and\n \n H. influenzae\n \n .\n
\n\n [2]\n
\n\n Using the data in the violin plots, deduce the age range at which children are most likely to develop ARI due to\n \n S. pneumoniae\n \n .\n
\n\n [2]\n
\n\n Suggest reasons for the difference between the fraction attributed to\n \n S. pneumoniae\n \n in all cases of ARI and in cases where antibiotics were used to treat the infection.\n
\n\n [2]\n
\n\n When there is no vaccination, the estimated number of cases for lower-middle income countries is larger than in either low income or upper-middle income countries. Suggest\n \n one\n \n reason for this.\n
\n\n [1]\n
\n\n Using the data, predict the effect of universal PCV vaccination in LMICs.\n
\n\n [1]\n
\n\n Outline likely health benefits of universal PCV vaccination of children, other than reducing the incidence of ARI.\n
\n\n [2]\n
\n\n
\n\n
\n\n 1.9 × 10\n \n 11\n \n / 190000 million / 190 billion (cells per gram);\n
\n\n \n Cells per gram not needed as in stem. Accept 1.80 × 10\n \n 11\n \n to 1.95 × 10\n \n 11\n \n .\n \n
\n\n 89%;\n
\n\n \n Allow 88 to 91 %.\n \n
\n\n 13\n \n years\n \n ;\n
\n\n \n Allow any value between 13 and 14 years inclusive.\n \n
\n\n \n Comparative terms expected (do not award mp for just stating values).\n \n
\n\n \n Mpb: do not allow “directly or inversely proportional”.\n \n
\n\n \n Mpb: Do not just accept “positive correlation” or “negative correlation” without a clear indication of the variables being correlated\n \n
\n\n \n Mp a to c: accept vice versa.\n \n
\n\n \n Comparative terms expected (do not award mp for just stating values).\n \n
\n\n \n Mpa and mpb: there must be a clear reference to time.\n \n
\n\n \n Mpc: there must be clear indication to vaccination.\n \n
\n\n \n Allow no points for bacterial virulence / disease severity, etc., as bacterial pathogenicity is not addressed on graphs.\n \n
\n\n
\n\n \n Mpa: Comparative terms expected (do not award mp for just stating values).\n \n
\n\n \n Lower-middle income subgroup is not the same as LMIC (includes all 3 subgroups).\n \n LMIC should not be used as equivalent.\n \n \n
\n\n \n Mpb: accept vice versa.\n \n
\n\n (worldwide) drastic/huge/important/significant decrease/decrease of 42 million ARI cases (treated with antibiotics);\n
\n\n \n There must be an indication of magnitude of decrease in the answer for the mark to be awarded.\n \n
\n\n \n Accept 30 to 50 million for the decrease.\n \n
\n\n \n Since the question already states “other than reducing the incidence of ARI”, this reason should not be accepted.\n \n
\n\n State the relationship between DDE concentration and eggshell thickness.\n
\n\n [1]\n
\n\n Suggest\n \n one\n \n reason for a decline in the brown pelican population with the most eggshell thinning.\n
\n\n [1]\n
\n\n Deduce, giving reasons, which brown pelican population decreased the most in the years following the study.\n
\n\n [2]\n
\n\n Outline the biomagnification of DDE in brown pelicans.\n
\n\n [3]\n
\n\n the higher the DDE concentration, the lower the thickness/higher the thinning\n
\n \n \n OR\n \n \n
\n negative correlation/inverse relationship;\n
\n \n Accept vice versa.\n \n
\n\n more risk of predation/ eggs break more easily/fewer chicks hatch;\n
\n\n \n \n OWTTE\n \n \n
\n\n State how many chromosomes there would be in male or female gametes of\n \n L. purpureus\n \n .\n
\n\n [1]\n
\n\n Arginine and ornithine are in the same group of biochemicals. Identify this group.\n
\n\n [1]\n
\n\n Using a Punnett grid, explain the results of this cross.\n
\n\n [3]\n
\n\n Suggest a reason for the F\n \n 2\n \n generation not corresponding exactly to the expected ratio of purple-flowered and white-flowered plants.\n
\n\n [1]\n
\n\n 12;\n
\n\n \n No alternative.\n \n
\n\n amino acids;\n
\n\n
\n\n \n Accept other upper-case and lower-case letters for the alleles.\n \n
\n \n \n \n
\n \n No punnet grid = 2 MAX.\n \n
\n \n Allow ECF if alleles incorrect.\n \n
\n\n chance /\n \n \n OWWTE\n \n \n ;\n
\n\n \n (any of the usual mechanisms causing recombination of alleles) NOT mutation.\n \n
\n\n \n Any wording that implies the bigger the sample size, the closer to 3:1.\n \n
\n\n Outline the effect of nitrogen compounds from agricultural land leaching into a lake.\n
\n\n [3]\n
\n\n State the relationship between elevation and nitrogen concentration in leaves of pitcher plants.\n
\n\n [1]\n
\n\n Suggest\n \n two\n \n reasons, other than elevation, for the differences in nitrogen concentration in leaves of pitcher plants.\n
\n\n [2]\n
\n\n the higher the elevation, the greater the nitrogen content\n
\n \n \n OR\n \n \n
\n positive relationship;\n
\n State one cause of lung cancer.\n
\n\n [1]\n
\n\n State the name and function of structures I and II.\n
\n\n Structure I name and function: ..............................................................................................................\n
\n\n ..........................................................................................................................................................\n
\n\n Structure II name and function: .............................................................................................................\n
\n\n ..........................................................................................................................................................\n
\n\n
\n\n [2]\n
\n\n Structure III is the ovary. Outline the processes that occur in the ovary.\n
\n\n [2]\n
\n\n Structure IV contains a gland that secretes a sugary liquid. Suggest a benefit to the plant of secreting this liquid.\n
\n\n [1]\n
\n\n
\n\n \n Both name and function must be included in each marking point.\n \n
\n\n \n The role in pollination should be stated. Do not accept “pollination” on its own.\n \n
\n\n \n Mpb: Do not accept union of pollen and ovule (Too vague).\n \n
\n\n \n Do not accept carpel/pistil as equivalent of ovary\n \n
\n\n \n Mpa: Both parts of the sentence are needed to award the point.\n \n
\n\n State with a reason whether pangolins are autotrophic or heterotrophic.\n
\n\n [1]\n
\n\n State with a reason whether pangolins are autotrophic or heterotrophic.\n
\n\n [1]\n
\n\n Explain what information is needed to find the trophic level of pangolins.\n
\n\n [2]\n
\n\n Outline evidence for evolution from the limbs of mammals such as pangolins.\n
\n\n [2]\n
\n\n Sex is determined in the same way in pangolins as in humans. State how many autosomes there are in somatic cells of\n \n M. pentadactyla\n \n .\n
\n\n [1]\n
\n\n heterotrophic because it feeds on/eats food/other organisms /eats ants/termites/ doesn’t photosynthesise/does not produce its own food;\n
\n\n \n Do not accept “it is not autotrophic” as it is part of the stem question.\n \n
\n\n heterotrophic because it feeds on/eats food/other organisms /eats ants/termites/ doesn’t photosynthesise/does not produce its own food;\n
\n\n \n Do not accept “it is not autotrophic” as it is part of the stem question.\n \n
\n\n \n Do not award points for indicating that predator information is needed.\n \n
\n\n
\n\n 38\n
\n \n \n OR\n \n \n
\n 19\n \n pairs\n \n ;\n
\n Explain how temperature affects enzymes.\n
\n\n [4]\n
\n\n Describe how human activities have caused average surface air temperatures on Earth to increase.\n
\n\n [7]\n
\n\n \n Graphs would need to be well annotated.\n \n
\n\n \n Must not be bell shaped.\n \n
\n\n \n Graphs would need to be well annotated.\n \n
\n\n \n Must not be bell shaped.\n \n
\n\n
\n\n Describe the organelles and other structures in animal cells that are visible in electron micrographs.\n
\n\n [7]\n
\n\n Explain how animal cells produce the ATP that they need.\n
\n\n [5]\n
\n\n \n Allow any point made on an annotated diagram.\n \n
\n\n \n Do not award any marks for just labels. Descriptions are needed.\n \n
\n\n \n Allow any point made on an annotated diagram.\n \n
\n\n \n Do not award any marks for just labels. Descriptions are needed.\n \n
\n\n
\n\n Draw a plan diagram in the right-hand box with labels to show the distribution of tissues in this transverse section of a leaf.\n
\n\n [3]\n
\n\n Calculate the actual thickness of the leaf, from upper to lower surface along the line a–b, showing your working.\n
\n\n [2]\n
\n\n Suggest a reason for using a lower power objective lens when first focusing on a slide under the microscope.\n
\n\n [1]\n
\n\n Identify\n \n one\n \n adaptation of the leaf for the absorption of light visible in this micrograph.\n
\n\n [1]\n
\n\n \n 1 mark for each correctly labelled tissue shown as a layer in proportion.\n \n
\n\n \n 1 mark for each correctly labelled tissue shown as a layer in proportion.\n \n
\n\n \n Allow ECF – ruler measurements\n \n /\n \n 1mm\n \n
\n\n Outline the role of UV radiation as a mutagen.\n
\n\n [4]\n
\n\n Explain how the function of DNA is linked to its molecular structure.\n
\n\n [7]\n
\n\n Describe the use of gene knockout technology in research.\n
\n\n [4]\n
\n\n \n Accept points in clearly labelled diagram\n \n
\n\n Identify the recombinant phenotypes amongst the offspring.\n
\n\n [1]\n
\n\n Explain whether these results fit the predicted Mendelian ratios for this cross.\n
\n\n [2]\n
\n\n brown body vestigial wings\n \n \n AND\n \n \n black body, normal wings\n
\n\n brown body vestigial wings\n \n \n AND\n \n \n black body, normal wings\n
\n\n Explain how vaccination can result in specific immunity to a viral disease.\n
\n\n [7]\n
\n\n Outline the role of membrane proteins in the movement of specific ions at specific times in the transmission of nerve impulses.\n
\n\n [4]\n
\n\n Describe the barriers that exist to hybridization between species.\n
\n\n [4]\n
\n\n Describe how toxins such as DDT might concentrate in the bodies of birds.\n
\n\n [4]\n
\n\n Toxins often act as inhibitors. Compare and contrast competitive and non-competitive enzyme inhibition.\n
\n\n [7]\n
\n\n Rotenone is a naturally occurring toxin that blocks the electron transport chain in insects and fish. Outline the consequences of exposure to a toxin like rotenone for cell respiration.\n
\n\n [4]\n
\n\n \n
\n a valid example of each;\n
\n\n \n Answers must have at least one similarity for 7 max\n \n .\n
\n\n Outline how sex is determined in humans.\n
\n\n [2]\n
\n\n Explain how sexual reproduction gives rise to genetic variation.\n
\n\n [2]\n
\n\n Outline how mutation can lead to cancer.\n
\n\n [4]\n
\n\n Outline\n \n one\n \n example of a human disease caused by an autosomal recessive allele.\n
\n\n [4]\n
\n\n Explain the role of mutations in evolution.\n
\n\n [7]\n
\n\n Explain the mechanisms involved in thermoregulation in humans.\n
\n\n [7]\n
\n\n Explain the relationship between temperature and the activity of enzymes.\n
\n\n [4]\n
\n\n Distinguish between the thermal properties of air and water as they relate to the habitat of animals.\n
\n\n [4]\n
\n![https://](\"https://%20commons.wikimedia.org/wiki/%20File:Enzyme_kinetics_curve.png#/media/%20File:Enzyme_kinetics_curve.png\"){kind=link}
![https://en.wikipedia.org/wiki/File:2102_Comparison_of_Artery_and_Vein.jpg](\"https://en.wikipedia.org/wiki/File:2102_Comparison_of_Artery_and_Vein.jpg\"){kind=link}
![https://commons.wikimedia.org/wiki/File:Mitosis_(261_13)_Pressed;_root_meristem_of_onion_(cells_in_prophase,_metaphase,_anaphase,_telophase).jpg](\"https://commons.wikimedia.org/wiki/File:Mitosis_(261_13)_Pressed;_root_meristem_of_onion_(cells_in_prophase,_metaphase,_anaphase,_telophase).jpg\"){kind=link}