HL Paper 2
Distinguish between the transfers of energy and inorganic nutrients in ecosystems.
Outline the role of methanogenic archaeans in the movement of carbon in ecosystems.
Describe how autotrophs absorb light energy
Markscheme
energy is lost (between trophic levels) / not all passed on / not reused / must be supplied;
nutrients are recycled/reused;
a. methane produced from organic matter;
b. in anaerobic conditions;
c. methane diffuses into atmosphere/accumulates in ground/soil;
d. oxidized/converted to carbon dioxide (in atmosphere);
a. light absorbed by (photosynthetic) pigments;
b. chlorophyll absorbs blue and red / drawing of absorption spectrum for chlorophyll;
c. photosystems are groups of pigment molecules/are light harvesting complexes;
d. photosystems are located in thylakoid membranes;
e. electrons excited/raised to higher energy level;
Examiners report
A sizeable number of candidates seemed to miss the point here, in what was intended to be an easy question. The command term ‘distinguish’ is an instruction to give the key differences. In this case, the distinction was that energy flows through an ecosystem and is ultimately lost whereas inorganic nutrients can be recycled.
Answers were rather polarised, with candidates either understanding the role of methanogens or stumbling round in a mire of uncertainty. Although this may seem an obscure part of biology, methanogenesis is of great significance in terms of climate change.
As this question was near the end of Section A, the mark scheme was constructed so as to reward stronger answers, rather than simple ideas such as ‘light is absorbed by chlorophyll’. Answers were very varied. Few candidates including any ideas about photosystems.
Coral reefs are among the most spectacular ecosystems on Earth. They support a rich diversity of life and provide economic benefits to the people who use them. In Papua New Guinea in the Pacific Ocean north of Australia the following data were collected. Coral cover is the percentage of the reef surface covered by live hard coral.
In order to test the effect of temperature, live samples of a species of coral, Pocillopora damicornis, were placed in an experimental chamber at a constant pH, water depth and low light. All the coral samples were started at 26°C and half of them were rapidly increased to 30°C.
The pie charts show the percentage of live and dead tissues at the end of the experiment.
Acidification of the world’s oceans is an increasing threat to the health of oceanic life including coral reefs. Corals perform calcification to create their calcium carbonate exteriors. An experiment was conducted on Heron Island, Southern Great Barrier Reef, Australia. For the experiment the pH was altered by dissolving carbon dioxide in the water. Three different coral species were used, with each test group at two different temperature ranges and three different pH values. The white line in each photograph represents 5 cm.
Calculate the difference in coral cover in 1996 and 2002. No working required.
Describe the evidence that the ocean temperature has an effect on coral cover.
Suggest causes for the changes in ocean temperature.
Identify one advantage of conducting this experiment in the laboratory rather than in the ocean.
Comment on whether the experimental data supports the observed data from the ocean.
(i) Describe the trend in calcification when the pH is decreased at 25 –26°C.
(ii) In environmental studies, a critical value is the level at which a population declines or shows signs of poor health. Suggest a critical pH for P. onkodes.
(iii) Using all of the data, comment on the hypothesis that ocean acidification in warming seas will have the same effect on all species of coral.
Suggest another marine animal that has parts made of calcium carbonate and may therefore be damaged due to ocean acidification.
Outline causes of ocean acidification.
Discuss the need for international cooperation to solve the problems of declining coral populations.
Markscheme
60 (%)
a. coral cover decreases as temperature rises (between 1996 and 1998/2000 and 2002) / negative correlation between temperature and coral cover / coral cover highest when temperature is lowest/vice versa
b. coral cover remains constant when temperature drops (between 1998/1999 and 2000)/remains (nearly) constant when temperature stops rising (between 2002 and 2003)
c. no proof of causation / only a correlation / other factors could be affecting the coral
Do not award this mark for “inversely proportional”, but the mark can still be awarded if other parts of the answer give one of the alternative parts of the mark point.
a. increased carbon dioxide/methane in the atmosphere / carbon dioxide emissions from burning of fossil fuels / other specific source of a named greenhouse gas
b. increased greenhouse effect / more heat/long wave radiation trapped in the atmosphere
c. heat transfer from atmosphere to ocean / ocean absorbs heat from atmosphere
No marks for increased CO2 in the oceans, global warming or climate change.
The idea of an increase must be included, not just greenhouse effect or heat trapping.
control of variables/pH/light/temperature / no predators of coral
a. supports because there is more dead coral/less % cover at the higher temperature
b. (experimental data) does not support (observed data) because experimental temperatures were (all) higher/rose much faster
The answer must make it clear whether or not the data provides support.
(i) less calcification in all three/each species (as pH decreased)
(ii) 7.6 / 7.7 / 7.8
Accept any pH that is 7.6 or higher, but lower than 7.9.
(iii)
a. greater reduction in calcification as pH drops at the higher temperature in P. onkodes than on the other two species (so hypothesis not supported)
This answer is based on the larger drop in calcification between 8.2 and 7.6 at both temperatures in onkodes than the other two species.
b. net loss in calcification at lowest pH and highest temperature in P. onkodes whereas there is still calcification in the other two species (so hypothesis not supported)
This answer is based only on whether there are positive values for calcification or negative.
c. warming reduces calcification at all pH levels in A. intermedia but not in the other two species (so hypothesis not supported)
This answer is based on the drop in calcification at each pH when the temperature rises in intermedia, whereas in the other species there is a rise at one or more of the pHs.
d. combined effect of acidification and warming is a larger reduction inncalcification in A. intermedia than in the other two species (so hypothesis not supported)
This answer is based on the larger overall drop in calcification between pH 8.2 at 25/26°C and 7.6 at 28/29°C.
e. more calcification as temperature rises at lower pH/pH 7.9 and 7.6 in P. lobata whereas there is less in the other two species (so hypothesis not supported)
The answer must either state pHs 7.9 and 7.6 or specify lower pH or greater acidification.
f. more calcification as pH drops from 8.2 to 7.9 at higher temperature in P. lobata whereas there is a drop/no rise in the other two species (so hypothesis not supported)
The answer must state the two pH values and state higher temperature or 28-29°C.
Mollusca/named marine mollusc with a shell/crustacean/named marine crustacean/Porifera/sponges/named calcareous marine sponge
Reject terrestrial examples. Reject sea shells, shellfish. Specific named examples must be verified if it is uncertain whether they have calcified parts.
a. carbon dioxide makes an acid/carbonic acid in water
b. (carbon dioxide from) burning fossil fuels/forest fires
c. carbon dioxide forms solution with/dissolves into water/oceans/rain
Do not award a mark for stating only that carbon dioxide causes ocean acidification.
Do not award marks for methane sources or sources of unspecified greenhouse gases or statements about increased carbon dioxide in the atmosphere.
a. international cooperation needed to reduce carbon dioxide emission/concentrations
b. carbon dioxide produced anywhere increases the greenhouse effect/global warming/ocean acidification/health of coral everywhere
c. ocean currents/tides/wind move carbon dioxide/acid/heat around the world / oceans of the world are interconnected/part of one overall system
d. (some) coral reefs are in international waters (or words to that effect) / coral reefs cannot be protected by single national governments alone
e. the more groups of people/nations/corporations that reduce their carbon emissions, the lower the impact on coral will become / not enough for one country/group/corporation to reduce carbon dioxide emissions
f. sharing of technology/research/information/resources
g. aid to poorer/developing countries (to help with coral conservation)
h. reference to an economic/ecological benefit of conserving coral reefs
Examiners report
White-nose syndrome (WNS) is a disease caused by the fungus Geomyces destructans. This kills bats from many species during their winter hibernation Scientists used echolocation to record the number of bat flights over a station in 10 minute sample periods during the summers of 2007 to 2009. The graph shows the mean number of flights and number of recording samples for little brown bats (Myotis lucifugus), a species affected by the fungus, along with those of hoary bats (Lasiurus cinereus), which are not affected.
Scientists have hypothesised that bats affected by WNS could be more likely than unaffected bats to emerge from hibernation during the winter. Bats occasionally undergo short interruptions in hibernation accompanied by an increase in body temperature. Scientists used sensors attached to M. lucifugus to monitor the temperature of a group infected with the fungus over the winter months. A second uninfected group was also monitored. The graphs show the results for two typical individuals.
The graph shows data from a small group of infected bats that died during hibernation. The average time interval between hibernation emergence periods and the date of death were recorded using temperature sensors for these bats.
Calculate the total number of M. lucifugus flights that were recorded in the summer of 2007
Suggest one limitation of this recording method in determining the accurate mean number of individual bats flying.
Calculate the percentage decline in the mean number of M. lucifugus flights for 2009 when compared to 2008.
Evaluate the conclusion that the decline in the population of M. lucifugus is due to infection by the fungus.
Distinguish between the patterns of hibernation of the uninfected and infected bats.
Based on the data and your biological knowledge, suggest how the infection could lead to premature death in a bat.
Outline the relationship between date of death and the mean interval between hibernation emergence periods.
Discuss whether the data in the graph show that there is a causal link between the date of death and the interval between hibernation emergence periods.
Suggest one reason, other than the interval between hibernation emergence periods, for some infected bats surviving longer than others.
Using all of the data, predict the effect of WNS on bat populations.
Markscheme
(32 × 79 =) 2528
a. same bat may be recorded more than once
b. some bats may not fly over [the recording station]
OR
only bats flying over the station are recorded
c. two bats flying close/together might be recorded as one
82 / 82.1 / 82.14 (% decline)
Conclusion supported
(2008 to 2009) M. lucifugus declines more (than L. cinereus)
OR
(2007 to 2009) M. lucifugus declines whereas L. cinereus increases/fluctuates/did not decline
OR
more affected than unaffected bats in 2007 and 2008 but more unaffected in 2009
Conclusion not supported
other factors could be causing the difference between the species/the decrease in M. lucifugus
OR
there will be differences between the two bat species apart from WNS infection
OR
both species decreased from 2008 to 2009
Award one mark maximum for an argument supporting the conclusion and one mark maximum for an argument against the conclusion. Marks should only be awarded for statements that make an explicit or clearly implied comparison between the species.
a. more (frequent) interruptions/emergences from hibernation/shorter periods of hibernation/more spikes in temperature (indicating emergence) in infected bats
b. more fluctuation in (body) temperature (during hibernation) in infected (than uninfected bats)
c. emergences/interruptions become more frequent during the hibernation period in infected bats versus (about) about the same frequency in uninfected
a. energy needed to raise body temperature / lost during temperature spikes
b. energy/heat released by/comes from (cell) respiration/metabolism
c. food/fat (stores/reserves) used in (cell) respiration/in generating energy/heat/raising body temperature
d. bats die/starve if fat/energy/food stores used up
e. hibernation conserves food stores/reduces use of energy
f. no/little food available/food harder to find (in winter/during hibernation period)
g. (more) energy/food used when flying/hunting
h. (more) heat loss/hypothermia (in winter/cold weather)
i. higher chance of being killed by predators when flying/emerged from hibernation
later date of death with longer/bigger intervals (between hibernation emergence)/with less frequent interruptions (to hibernation)
The correlation must be described.
Arguments for a causal link
a. there is a trend/correlation/relationship (shown by the data in the graph)
b. explanations of how more frequent emergence from hibernation could cause earlier death (are plausible)/example of an explanation
Arguments against a causal link
c. there is a correlation but this does not show a causal link / correlation does not prove causation
d. more data/further research is needed to show the causes
e. there is (much) variation/spread in the data
f. other factors can affect the date of death
a. differences in body mass
OR
differences in reserves/stores of food/energy/fat
b. bats may be predated during a flight / chance events might affect the date of death
c. more effective/stronger immune system/immunity (in some bats)
d. more resistance to cold (in some bats)
e. larger bats lose heat less rapidly
f. infected at a different/later date
a. higher mortality/more deaths
b. shorter life expectancy/premature death/death before reproduction
c. extinction/reduction in (size of) of bat populations
d. L. cinereus/species of bats not affected by WNS may increase
OR
L.cinereus/species of bats not affected by WNS may experience less competition
e. infection may affect birth rates/fertility
f. bats will emerge more from hibernation/in winter
g. bats will use up food/energy reserves faster in winter/faster due to (more) interruptions
h. bat (populations) develop/evolve greater resistance to WNS
Examiners report
The image shows a food web.
Using the food web, identify a detritivore.
Using the food web, identify a saprotroph.
State the name of the domain to which birds, such as the Elf owl, belong.
Outline the energy flow through this food web.
Markscheme
earthworm/woodlouse
bacteria/fungi
Do not accept protozoans or nematodes as they are consumers.
eukaryote/eukaryota/eukarya
a. light energy of Sun is converted by plant/autotroph to chemical energy «in carbon compounds through photosynthesis»
b. detritivores/saprotrophs decay plant material «that accumulates in the soil» to obtain energy OWTTE
c. consumers release energy from the carbon compounds by cell respiration energy lost as heat
d. energy is used by organisms for metabolism
e. energy is transferred between organisms/trophic levels through the food chains/web
For mp e, accept specific example such as energy is transferred from primary to secondary consumer etc.
f. energy is lost at each trophic level «so lengths of food chains/web are restricted»
OR
approximately 80/90 % of energy is lost «between trophic levels»
Vice versa
Award mark points that refer to the specific organisms from this food web.
Examiners report
The Chinese soft-shelled turtle, Pelodiscus sinensis, lives in salt water marshes. The turtle can live under water and out of water.
These turtles have fully developed lungs and kidneys, however, many microvilli have been discovered in the mouth of P. sinensis. A study was undertaken to test the hypothesis that oxygen uptake and urea excretion can simultaneously occur in the mouth.
Initial experiments involved collecting nitrogen excretion data from P. sinensis. The turtle urinates both in water and out of water. When in water it allows waste products to be washed out of its mouth. When out of water it regularly dips its head into shallow water to wash its mouth. The table shows the mean rates of ammonia and urea excretion from the mouth and kidney over six days.
It was noted that during long periods out of water, turtles rhythmically moved their mouths to take in water from a shallow source and then discharge it. Changes in the dissolved oxygen and the quantity of accumulated urea in the rinse water discharged by the turtles were monitored over time as shown in this graph.
In order to test whether a urea transporter was present in the mouth tissues of the turtles, phloretin (a known inhibitor of membrane proteins that transport urea) was added to the water in which a further set of turtles submerged their heads. The results of that treatment are shown.
Further research was conducted to determine where mRNA expression of a urea transporter gene might be occurring in P. sinensis. Gel electrophoresis was used to analyse different tissue samples for mRNA activity.
Expression of the urea transporter gene by cells in the turtle’s mouth was assessed by measuring mRNA activity. Turtles were kept out of water for 24 hours and then injected with either a salt solution that matched the salt concentration of the turtle, dissolved ammonia or urea, followed by another 24 hours out of water.
Deduce whether the excretion of ammonia or urea changes more when a turtle emerges from water.
Compare and contrast the changes in urea excretion in the mouth with the changes in urea excretion in the kidney when a turtle emerges from the water.
Describe the trends shown by the graph for dissolved oxygen in water discharged from the mouth.
Suggest reasons for these trends in dissolved oxygen.
Deduce with a reason whether a urea transporter is present in the mouth of P. sinensis.
Outline the additional evidence provided by the gel electrophoresis results shown above.
Identify which of these turtle groups represent the control, giving a reason for your answer.
Suggest a reason for the greater expression of the gene for the urea transporter after an injection with dissolved ammonia than an injection of urea.
The salt marshes where these turtles live periodically dry up to small pools. Discuss the problems that this will cause for nitrogen excretion in the turtles and how their behaviour might overcome the problems.
Markscheme
a. urea
b. for both mouth and kidney
c. percentage change/change in μmol day−1 g−1 greater with urea/other acceptable numerical comparison
a. both higher/increased on emergence from/with turtle out of water
b. both increased by 0.66 «μmol−1 g−1 when turtle emerges from water»
c. % increase is higher in kidney / kidney 940% versus mouth 73/75% / increase is higher proportionately higher in kidney / kidney x10 versus mouth nearly double/x1.73
d. urea excretion by mouth greater than kidney out of water «despite larger % increase in kidney excretion»
decrease «when head is submerged» and increase when head is out of water
a. oxygen absorbed from water/exchanged for urea when head dipped in water«so oxygen concentration decreases»
b. lungs cannot be used with head in water / can «only» be used with head out of water
c. oxygen from water «in mouth» used in «aerobic cell» respiration
d. oxygen from air dissolves in water when head out of water «so oxygen concentration increases»
a. urea transporter is present
b. less urea «excreted»/ lower rate «of urea excretion» / excretion almost zero when phloretin/inhibitor was present
a. mRNA only in mouth and tongue/in mouth and tongue but not esophagus intestine kidney or bladder
b. bands / lines indicate mRNA for/expression of urea transporter gene
c. urea transporter gene expressed / urea transporters in mouth/tongue / not expressed/made in esophagus/intestine/kidneys/bladder
d. mRNA/transcription/gene expression/urea transporters higher in tongue/more in tongue «than mouth»
salt solution is control because it does not contain a nitrogenous/excretory waste product / it matches the salt concentration of the turtle / the turtle’s body already contains salt / because the turtle lives in salt water/salt marshes / because nothing has been altered
a. ammonia is «highly» toxic/harmful
b. ammonia is more toxic than urea/converse
c. ammonia converted to urea
d. urea concentration raised «by injecting ammonia»
e. difference between ammonia and urea «possibly» not «statistically» significant
Problems:
a. urea becomes more concentrated «in small pools» / lower concentration gradient «between tongue/mouth and water»
b. less water available for urine production/excretion by kidney
OR
less water in ponds for mouth rinsing/more competition for pools (to use for mouth rinsing)
Behaviour to overcome problems:
c. «still able to» dip mouth into/mouth rinse in water/pools
d. «still able to» excrete urea «though the mouth» in the small pools
e. more conversion of ammonia to urea/urea excretion rather than ammonia
f. more urea transporters/expression of urea transporter gene
g. urea excreted «in mouth/via microvilli» by active transport/using ATP
h. excretion with little/no loss of water
Examiners report
Compare and contrast the mode of nutrition of detritivores and saprotrophs.
Explain how some plant species are able to respond to changes in their abiotic environment and flower at a precise time of the year.
Outline the extension of the stem in plants.
Markscheme
Accept not autotrophic/not photosynthetic instead of heterotrophic.
Do not accept that both groups are decomposers or consumers for the similarity.
a. 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);
Examiners report
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.
The 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.
Again, 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.
Draw a half-view of an animal-pollinated flower.
Outline the growth of plant shoot apex.
Explain the movement of energy and inorganic nutrients in an ecosystem.
Markscheme
a. sepals as outermost part of flower ✔
b. petals as largest part of flower ✔
c. stamen drawn with recognizable anther and filament
OR
anther and filament shown as parts of the stamen ✔
d. carpel/pistil drawn with recognizable stigma, style and ovary
OR
stigma, style and ovary shown as parts of the carpel ✔
e. nectary at base of the ovary ✔
f. ovule inside the ovary ✔
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.
a. growth in shoots is indeterminate/unlimited ✔
b. produces stem and leaves ✔ For mpb, both stem and leaves are needed and buds or branches should not be accepted as alternatives.
c. growth/cell growth/cell elongation controlled/affected by hormones/auxin/IAA ✔
d. new/extra cells produced by mitosis/cell division / apex is a meristem ✔
e. tropism/phototropism / grows towards the sun/light ✔
f. auxin moved away from sunny side/to shady side of shoot «apex»
OR
auxin efflux pumps set up concentration gradients ✔
a. autotrophs/producers/plants obtain inorganic nutrients from the «abiotic» environment ✔
b. energy is provided «mainly» by sunlight ✔
c. light energy is converted «to chemical energy» through photosynthesis✔
d. photosynthesis/producers/autotrophs convert inorganic carbon/carbon dioxide and water into carbon/organic compounds ✔
e. carbon compounds/foods contain/are a source of «usable» energy «for life» ✔
f. carbon compounds/energy are transferred along food chains when eaten by consumers/heterotrophs ✔
g. respiration returns carbon «dioxide» to the environment ✔
h. respiration releases stored/chemical energy as heat/ATP ✔
i. energy/ATP is used to carry out life functions/synthesis/growth/movement ✔
j. energy is lost/is not recycled ✔ Both related by “or” required.
k. nutrients are recycled / example of recycled nutrient e.g. carbon ✔
l. decomposers recycle minerals/inorganic nutrients ✔
Award [5 max] if only energy is mentioned.
Examiners report
Outline the roles of helicase and ligase in DNA replication.
Explain how natural selection can lead to speciation.
Outline the features of ecosystems that make them sustainable.
Markscheme
helicase:
a. unwinds/uncoils the DNA «double helix» ✔
b. breaks hydrogen bonds «between bases» ✔
c. separates the «two» strands/unzips the DNA/creates replication fork ✔
ligase:
d. seals nicks/forms a continuous «sugar-phosphate» backbone/strand ✔
e. makes sugar-phosphate bonds/covalent bonds between adjacent nucleotides ✔
f. after «RNA» primers are removed/where an «RNA» primer was replaced by DNA ✔
g. «helps to» join Okazaki fragments ✔
a. variation is required for natural selection/evolution/variation in species/populations ✔
b. mutation/meiosis/sexual reproduction is a source of variation ✔
c. competition/more offspring than the environment can support ✔
d. adaptations make individuals suited to their environment/way of life ✔
e. survival of better adapted «individuals)/survival of fittest/converse ✔
f. inheritance of traits/passing on genes of better adapted «individuals»
OR
reproduction/more reproduction of better adapted/fittest «individuals» ✔
g. speciation is formation of a new species/splitting of a species/one population becoming a separate species ✔
h. reproductive isolation of separated populations ✔
i. geographic isolation «of populations can lead to speciation» ✔
j. temporal/behavioral isolation «of populations can lead to speciation» ✔
k. disruptive selection/differences in selection «between populations can lead to speciation» ✔
l. gradual divergence of populations due to natural selection/due to differences in environment ✔
m. changes in the gene pools «of separated populations»/separation of gene pools ✔
n. interbreeding becomes impossible/no fertile offspring «so speciation has happened» ✔
a. recycling of nutrients/elements/components/materials ✔
b. carbon/nitrogen/another example of recycled nutrient/element ✔
c. decomposers/saprotrophs break down organic matter/release «inorganic» nutrients ✔
d. energy supplied by the sun
OR
energy cannot be recycled «so ongoing supply is needed»
OR
energy is lost from ecosystems as heat ✔
e. energy flow along food chains/through food web/through trophic levels ✔
f. photosynthesis/autotrophs make foods/trap energy
OR
autotrophs supply the food that supports primary consumers ✔
g. oxygen «for aerobic respiration» released by autotrophs/photosynthesis/plants ✔
h. carbon dioxide «for photosynthesis» released by respiration ✔
i. populations limited by food supply/predator-prey/interactions/competition
OR
populations regulated by negative feedback
OR
fewer/less of each successive trophic level «along the food chain»/OWTTE ✔
j. supplies of water from rainfall/precipitation/rivers/water cycle ✔
Examiners report
This was generally well answered, with most candidates knowing at least something of the roles of these two enzymes. Most candidates knew that ligase connects Okazaki fragments but some claimed that it creates hydrogen bonds between nucleotides on template and the new strand. Many candidates did not distinguish between unwinding of DNA and separating the strands. Two details that should be more widely known are that helicase separates the two strands of a DNA molecule by encouraging the breakage of hydrogen bonds between bases and that ligase seals nicks by making sugar phosphate bonds.
Most candidates think they understand evolution by natural selection but many do not. Here the focus was on speciation - the splitting of a species into two or more species. Often answers described the evolution of one species over time, rather than speciation itself. An idea central to natural selection that was frequently missing from an answer is adaptation or fitness. Often traits were referred to as ‘favourable’ and therefore likely to lead to survival and reproduction but there is a circularity of argument there. Survival depends on traits fitting the environment, hence being an adaptation to it. The mostly common ideas seen in answers were differential survival and reproduction, due to differences in traits. A common fault was to confuse individuals and species and to refer to a whole species surviving and reproducing more successfully than another species.
There were some vague answers to this question but also some impressive ones that explained ecological processes including nutrient recycling, energy flow and regulation of population sizes.
Outline energy flow through a food chain.
Draw a fully labelled graph of the action spectrum for photosynthesis.
Explain Calvin’s experiment and what was discovered about photosynthesis through his work.
Markscheme
a. energy from the sun/light energy is converted to chemical energy by photosynthesis ✔
b. «chemical» energy flows through the food chains by feeding ✔
c. 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 ✔
f. 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.
b. 400 and 700 nm as limits ✔
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 ✔
d. peaks of activity at 430 nm AND at 660 nm ✔
e. peaks indicated as «violet» blue light AND peak indicated as «orange» red light ✔
a. Calvin cycle is light-independent ✔
b. carbon fixation
OR
carboxylation of ribulose bisphosphate/RuBP occurs ✔
c. algae placed in thin glass container/“lollipop” apparatus ✔
d. given plenty of light and bicarbonate/ CO2 ✔
e. at start of experiment algae supplied radioactive carbon/HCO3-/14C ✔
f. samples taken at intervals / heat/alcohol killed samples ✔
g. C-compounds separated by chromatography ✔
h. 14C/radioactive-compounds identified by autoradiography ✔
i. showed that RuBP was phosphorylated ✔
j. after five seconds/immediately more glycerate-3-phosphate/3-PGA labelled than any other compound ✔
k. shows glycerate-3-phosphate/3-PGA first «carboxylated» compound/the first stable product ✔
l. next compound to be detected containing radioactive carbon was triose phosphate/G3P/glyceraldehyde 3 phosphate ✔
m. showed that a wide range of carbon compounds was quickly made in sequence ✔
n. showed that a cycle of reactions was used to regenerate RuBP ✔
Examiners report
Was well answered with most students being knowledgeable about ecology.
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.
This question was commonly answered poorly with students showing a lack of knowledge of both the Calvin cycle as well as the Calving experiment.
White clover (Trifolium repens) is native to Eurasia but is now a common plant found worldwide in lawns, next to roads, in pastures and similar habitats.
[Source: © International Baccalaureate Organization 2020.]
Some T. repens plants are able to produce the toxin hydrogen cyanide (HCN) by cyanogenesis. A study at 128 sites (2509 plants) in Toronto (Canada) looked at the proportion of T. repens plants producing HCN. The sites were at regular intervals from the city centre towards rural areas.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
To determine whether similar patterns in cyanogenesis were seen in other cities, studies were also carried out around New York City and Boston (USA) and Montreal (Canada).
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
The researchers considered two possible ecological causes for the observed gradients in cyanogenesis. First, the researchers assessed the extent of herbivory of T. repens plants in Toronto by measuring the percentage of leaf area that was eaten. The graph shows the results for both cyanogenic and non-cyanogenic plants.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
The researchers then investigated a possible correlation between cyanogenesis and exposure to freezing conditions. It had been proposed that when a cyanogenic plant freezes, its cells burst, releasing HCN which is toxic to the plant. Snow can insulate the ground and plants from freezing temperatures. However, snow is more likely to melt in cities, which then exposes plants to freezing temperatures.
All four of the cities studied receive below freezing temperatures and winter snowfall. Researchers looked at the number of days below freezing (0 °C) that did not have snow cover in these cities.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
State the distance from the city centre at which the highest proportion of plants sampled contained HCN.
Outline the relationship shown in the graph.
Deduce whether the pattern of cyanogenesis was the same in all of the areas around all four cities.
Discuss whether the data supports the hypothesis that the gradient in cyanogenesis is due to its benefits against herbivory in rural areas.
Identify with a reason the city where the plants were more insulated from freezing temperatures.
Using all of the data so far, suggest whether exposure to freezing temperatures in the four cities is supported as a reason for the differences in HCN production in T. repens.
Markscheme
32 (km);
Accept answers in the range of 31 to 33 (km).
positive correlation / (proportion with) HCN increases as distance increases;
a. Toronto, NYC and Boston show same pattern/all show positive correlation/relationship/WTTE;
b. Montreal shows negative correlation/negative relationship/WTTE so is different;
a. (hypothesis) not supported;
b. large overlap/little difference between cyanogenic and non-cyanogenic (in herbivory);
c. smaller difference between cyanogenic and non-cyanogenic in rural areas;
d. both show negative correlation between herbivory and distance from city center/same trend;
e. some support/hypothesis partly supported by lower herbivory in cyanogenic (at all distances).
Do not accept ‘No’ unqualified as an answer.
Do not award mpe if the answer states that the hypothesis is supported without doubt/fully.
Montreal because it has the lowest number of days below 0 °C without snow cover;
Do not award the mark for Montreal if the reason is not given.
a. cities with more days without snow cover have positive correlation between distance from city center and HCN / vice versa for Montreal;
b. fewer plants with HCN within cities that have more days without snow cover/have more exposure to freezing temperatures / converse for Montreal which has fewer days without snow cover;
c. HCN is 0.2 (or less) HCN in cities that have more days without snow cover proportion whereas city with fewer days/Montreal it is 0.5/more than 0.4;
d. in Toronto cyanogenic and non-cyanogenic plants show little difference in herbivory;
e. support for hypothesis/exposure to freezing temperatures as reason.
Do not accept ‘Yes’ unqualified as an answer, but accept it if supported by reasoning.
Examiners report
This was intended to be an easy first mark but only 70 % answered it correctly, with others mostly giving the maximum distance rather than the distance with the highest proportion of plants containing cyanide.
This was answered more successfully with 90 % of candidates gaining the mark. The answer ‘positive relationship’ was not enough and either ‘positive correlation’ or description of it were required.
Here candidates were expected to state that the four cities did not all show the same pattern, as three have a positive correlation, but the fourth city (Montreal) has a weak negative correlation. A few failed to include Toronto among the cities with a positive correlation and instead singled it out for showing a stronger correlation than the others.
This was possibly the hardest part of the data-based question. Candidates were spread approximately equally between 0, 1 and 2 marks. Many candidates did not realise that the differences between herbivory in cyanogenic and non-cyanogenic plants were unlikely to be significant, especially in the rural areas that were furthest from the city. The hypothesis stated in the question was therefore not supported. Candidates should be encouraged to consider both the closeness of means and also the spread, when analysing data.
90 % of candidates identified Montreal as the city where plants were more insulated because it was where there were fewer freezing days without snow cover.
Many candidates found this to be another difficult question. There was a lot of data to consider and as in (c) it was necessary to take into account whether differences were likely to have been significant. In the histogram showing the amount of cold experienced, the large error bars show that only differences between Montreal and the other three cities are great enough to be considered reliable. When combined with data for the proportion of plants with HCN in each city, it was clear that there was support for exposure to cold as a reason.
More than 8 million different species are alive today but over the course of evolution, more than 4 billion may have existed.
Outline the criteria that should be used to assess whether a group of organisms is a species.
Describe the changes that occur in gene pools during speciation.
Discuss the process, including potential risks and benefits, of using bacteria to genetically modify plant crop species.
Markscheme
a. organisms can potentially interbreed;
b. to produce fertile offspring;
c. same sequence of genes (on chromosomes) / same types of chromosomes;
d. similar traits/phenotype/WTTE;
e. same chromosome number/karyotype;
a. gene pool is all genes/alleles in an (interbreeding) population;
b. gene pool splits/divides/separated during speciation;
c. due to reproductive isolation (of groups within a species);
d. temporal/behavioral/geographic isolation (can cause reproductive isolation);
e. divergence of gene pools;
f. allele frequencies change;
g. natural selection different (in the isolated groups so there is divergence);
h. different (random) mutations occur (in the isolated populations so there is divergence);
i. speciation has occurred when differences between populations prevent interbreeding;
Do not award both mpc and mpi for the same idea (reproductive isolation separating populations vs speciation due to interbreeding not being possible).
Process:
a. genetic modification by gene transfer between species;
b. gene/Bt gene/DNA segment transferred from bacterium to plant/crop;
c. gene/DNA codes for/responsible for desired protein/gene product;
d. bacteria have/produce plasmids / gene/DNA inserted into plasmid;
e. using restriction enzymes/endonucleases to cut DNA;
f. using DNA ligase to join DNA;
g. bacterium transfers (modified) plasmid to plant cell;
Benefits:
h. increase crop yields / more food produced / less land needed to grow food;
i. increase pest/disease resistance / use less pesticides/insecticides/fungicides;
j. improves crops to be more nutritious/increased vitamin content;
k. increased tolerance to saline soils/drought/high temperatures/low temperatures;
Risks:
l. GM organisms could spread to sites (where they will cause harm);
m. transferred gene could spread to other species / spread of herbicide resistance to weeds;
n. GM crops that produce pesticide could kill non-pest insects/monarch butterflies / insect pests could develop resistance to pesticides/insecticides/Bt toxin;
Examiners report
This was the most successfully answered part of Question 8. Many knew that members of a species can interbreed and produce fertile offspring. Surprisingly few mentioned that similarities in characteristics or phenotype are found in species.
Answers to this part of the question were mostly poor. Gene pools and speciation are the subject of sub-topic 10.3, but many candidates struggled to link these two concepts. Various misunderstandings were seen. As in previous exams, some candidates thought that speciation is evolutionary change over time in a species, rather than the splitting of a species into two or more separate species. Mutations were sometimes described as though they happen in response to a need for new traits, rather than them happening spontaneously and occasionally being selected for.
This part of the question was also poorly answered by many candidates. Only a simple account of the procedures used to genetically modify plants was expected and not the details of the use of Agrobacterium that are part of Option B. There was confusion among many candidates about how bacteria might be involved in genetic modification of plants. Two possibilities that were rewarded were the transfer of genes from bacteria or of plasmids derived from bacteria. Where marks were scored, they were mostly for risks and benefits, but many accounts of these were unconvincing. Some of the risks that were suggested were not evidence-based. As with vaccination, it is important that myths about the dangers of procedures such as genetic modification are not propagated.
All living organisms depend on a continuous supply of energy.
Explain the stages of aerobic respiration that occur in the mitochondria of eukaryotes.
Outline how ventilation in humans ensures a supply of oxygen.
Describe the reasons for the shape of a pyramid of energy.
Markscheme
a. «cell» respiration is the «controlled» release of energy from organic compounds to produce ATP
b. «cell respiration» involves the oxidation and reduction of electron carriers
c. in link reaction pyruvate is converted into acetyl coenzyme A, CO2 is released and NAD is reduced
d. in the Krebs cycle, a 4 C molecule combines with acetyl CoA
e. decarboxylation releases 2 CO2 molecules for each pyruvate / conversion of 6C to 5C/5C to 4C releases CO2
f. «3» reduced NAD and «1» reduced FAD are produced
g. ATP generated in the Krebs cycle
h. reduced molecules/FAD/NAD are carried to the cristae/inner membrane of the mitochondria
i. transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping
j. protons accumulate in intermembrane space/ between cristae/inner membrane and outer membrane
OR
proton / electrochemical gradient between intermembrane space and matrix is established
k. protons diffuse through ATP synthase to generate ATP
l. chemiosmosis is the use of a proton/electrochemical gradient to generate ATP
m. oxygen is the final electron acceptor
Accept any of the points in a correctly annotated diagram.
a. ventilation is exchange of gases between lungs and air.
b. during inhalation diaphragm contracts AND lowers.
Both needed.
c. external intercostal muscles contract, raising ribs upwards and outwards
d. increase in volume AND decrease in pressure within thoracic cavity
e. air drawn into alveoli bringing fresh supply of oxygen
f. oxygen concentration in alveolar sacs is higher than in blood capillaries
g. «oxygen concentration gradient» causes oxygen to diffuse out of alveoli into red blood cells in capillaries
a. pyramid of energy has stepped shape with largest bottom step being producers, then first consumer, second consumer, etc
b. light energy «from sun» converted to chemical energy in carbon compounds by photosynthesis
c. energy released by respiration is used in living organisms AND converted to heat
d. heat «energy» is lost from ecosystems
e. approximately 10 % of energy in trophic level converted into new material for next level
f. energy also lost as undigested material/uneaten material/feces/excretion
Examiners report
The map shows the widespread distribution of coral reef ecosystems (indicated by black dots) in the world’s oceans. Death of coral reefs is related to increasing atmospheric carbon dioxide concentrations.
[Source: National Oceanic and Atmospheric Administration, 2021. Where Reef Building Corals Found. [map online] Available at: https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral05a.html [Accessed 20 May 2021].]
Explain how increased atmospheric carbon dioxide concentrations can lead to coral death.
Markscheme
a. carbon dioxide dissolves in oceans/seawater ✔
b. carbonic acid formed/equation/lowers pH/makes water acidic ✔
c. prevents deposition of calcium carbonate/causes calcium carbonate to dissolve ✔
d. skeleton of (hard) corals degraded ✔
e. carbon dioxide is a greenhouse gas/causes warming/increases temperatures ✔
f. warmer oceans cause corals to expel zooxanthellae ✔
g. bleaching due to death/expulsion of mutualistic organisms/algae ✔
Allow zooxanthellae instead of algae in mpf.
Reject reacts and diffuses instead of dissolves in mpa.
Examiners report
Answers relating to global warming and therefore rising sea temperatures and also answers relating to ocean acidification were accepted. Many candidates knew that carbon dioxide causes acidification but fewer knew about the resulting problems with mineral deposition to form the skeleton in a hard coral.
A community living in the water of an estuary was used to investigate how climate change may affect ecological systems. The food web in this community included phytoplankton (producers), zooplankton (consumers) and saprotrophic bacteria. Small plastic mesocosms were set up with water from the estuary containing only these three groups of organisms. The mesocosms were subjected to four different temperatures and two nutrient levels (control and nutrients added) to replicate local variations of the conditions in the estuary during springtime warming.
The graph shows the biomass of the community for each of the eight mesocosms at the end of the experimental period. Biomass was measured in terms of the amount of carbon present. The horizontal line indicates the initial biomass.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
The graph shows the mean biomass of autotrophs and heterotrophs in the eight mesocosms. The horizontal lines indicate the initial biomasses.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
Chlorophyll concentration was used as an estimate of the photosynthetic capacity of the community. The rate of photosynthesis and mass of chlorophyll per unit volume were measured in a mesocosm at three different temperatures.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
In a larger study, California grassland was exposed to elevated temperature and nitrate concentration for five years. The graph shows the total biomass production in individual and in combined treatments. Error bars denote one standard error.
[Source: adapted from Dukes, J.S., Chiariello, N.R., Cleland, E.E., Moore, L.A., Shaw, M.R., Thayer,S., Tobeck, T.,
Mooney, H.A. and Field, C.B., 2005. PLOS Biology, 3(10), e319. https://doi.org/10.1371/journal.pbio.0030319.]
Describe the effect of temperature on the total biomass.
Compare and contrast the effects of temperature on the biomass of autotrophs and heterotrophs with added nutrients.
Explain the effect of temperature on the rate of photosynthesis in this mesocosm.
Suggest reasons for the decreases in biomass of autotrophs as temperature rises, despite the increases in photosynthesis.
Describe the effects of temperature and nitrate concentration on biomass.
Suggest two abiotic factors, other than temperature and nutrient supply, that may affect the production of biomass of the grasslands.
The first study used mesocosms and the second study was carried out in natural grassland. Discuss the use of mesocosms as opposed to a study in a natural environment.
Markscheme
a. negative correlation/decrease (in biomass) as temperature rises in added-nutrients (mesocosms);
b. little/no (significant) change in biomass as temperature increases in control (mesocosms);
a. autotroph biomass decreases and heterotroph biomass increases with higher temperatures;
b. decrease in autotrophs is greater/larger/more than increase in heterotrophs
OR
little difference in biomass (between auto and heterotrophs) at highest temperature/27 °C;
c. autotrophs show smaller and smaller gains in biomass from initial as temperature rises/WTTE;
d. heterotrophs no gain in biomass at 21 °C then larger and larger gains as temperature rises;
rate of photosynthesis increases as temperature rises because:
a. temperature is the limiting factor for photosynthesis;
b. higher temperatures increase enzyme activity;
c. faster molecular motion/more molecular kinetic energy/more frequent enzyme-substrate collisions;
d. Calvin cycle/light independent reactions (of photosynthesis) speed up;
biomass of autotrophs decreases as temperature rises because of:
a. more herbivory/grazing/feeding by (zooplankton/heterotrophs);
b. higher populations/numbers/biomass of zooplankton/heterotrophs;
c. more mortality/more decomposition/decay of autotrophs/phytoplankton;
d. respiration (rate higher than photosynthesis rate in autotrophs/phytoplankton);
a. increased temperature raises biomass;
b. increased nitrate raises biomass more than increased temperature;
c. increased nitrate and temperature raises biomass by same amount as nitrate alone;
a. water availability/rainfall/humidity;
b. light/sunlight (intensity) / daylength;
c. salinity of soil / high/low soil pH;
d. chemical pollution/herbicides/allelopathy/parasitic weeds;
Mark the first two answers only.
Do not accept carbon dioxide or weather conditions.
advantages of mesocosms/converse problems with studies in natural environments
a. easier to manipulate/control variables/conditions / less susceptible to outside influences
OR
easier to replicate
OR
take up less space;
disadvantages of mesocosms/converse opportunities with studies in natural environments
b. some trophic levels missing/incomplete food chains in mesocosms
OR
large animals cannot be included / ethical concerns about enclosing animals in mesocosms
OR
some variables lacking in mesocosms / doesn’t show what happens in natural ecosystems;
Allow only one mark for an advantage and one mark for a disadvantage as this is a discuss question.
Examiners report
The word ‘total’ confused some candidates who tried to write about both the control and added-nutrients without distinguishing between them. Of those candidates who realised that ‘total’ must mean the total biomass within a mesocosm, almost all got the decrease in ‘with nutrients’ mesocosms as temperature increased, but many were not discerning enough with the control mesocosms. The differences were less than the error bars, so were clearly insignificant and should have been ignored Question 2.
Most candidates stated that autotroph biomass decreased and heterotroph biomass increased as temperature increased, but this contrast was only given one mark. Far fewer candidates gave a second worthwhile comparison or contrast. Very few candidates realised that at all temperatures the autotrophs had gained biomass from the initial level, but the gains were less as temperature rose. Similarly, few candidates stated that the heterotrophs had not gained mass at 21°C but gained increasing amounts of biomass as the temperature rose.
About half candidates merely stated the relationship shown in the graph, rather than actually explaining it. An explanation based on enzyme activity were expected. Performance in this question and in (d) correlated well with the overall performance of each candidate on the paper, probably because biological understanding was required. Other parts of question 1 were less well correlated, as is typical for data analysis questions.
Enzymes do not denature at the temperatures used in this experiment. Also, the autotrophs are phytoplankton living in seawater so transpiration cannot be the cause of biomass reductions at higher temperatures. The data in previous graphs showed higher biomasses of heterotrophs at higher temperatures and thus greater rates of herbivory were the obvious explanation for reduced autotroph biomass.
This was another question where many candidates’ answers lacked discernment. The increase in biomass with nitrate was clearly greater than that with temperature alone, but the increase with nitrate and temperature combined was not significantly different from nitrate alone. A useful way of thinking about answering questions such as this is ‘If I read out my answer to someone over the phone, would they correctly sketch the relative size of the bars without seeing the actual bar chart?’ With many candidates the answer to this would have been no.
About half of answers given were accepted. Carbon dioxide concentration was unlikely to vary enough to affect grassland biomass production. Vague answers such as ‘pH’ were not accepted but soil pH was.
Because this was a ‘discuss’ question, one mark was awarded for arguments in favour or mesocosms and one mark for counter arguments. The best answers weighed up the relative advantage of these two approaches, rather than just singing the praises of one of them.
Extensive areas of the rainforest in Cambodia are being cleared for large-scale rubber plantations. Distinguish between the sustainability of natural ecosystems such as rainforests and the sustainability of areas used for agriculture.
Describe the roles of the shoot apex in the growth of plants.
Research suggests that many living plant species are polyploid. Explain how polyploidy occurs and, using a named example, how polyploidy can lead to speciation.
Markscheme
a. sustainable communities/ecosystems allow continued survival of organisms/OWTTE ✔
b. natural ecosystems can be sustainable over long periods of time/OWTTE ✔
c. natural ecosystems/rainforest more sustainable than agricultural areas/plantations ✔
d. diverse community/high biodiversity/higher biodiversity in natural ecosystems/rainforest
OR
less/low biodiversity in agricultural areas/agricultural soils ✔
e. agricultural areas/monocultures more affected by pests/diseases ✔
f. nutrient recycling «efficient» in natural ecosystems/rainforest ✔
g. nutrients removed with crops/nutrients removed when crops are harvested
OR
less formation of humus/less organic matter in agricultural soils ✔
h. more water recycling/more rainfall/more transpiration in natural ecosystems/rainforest ✔
i. larger biomass/more carbon stored «in biomass» in natural ecosystems/rainforest ✔
j. shallower soils/less soil erosion/degraded soils/infertile soils in agricultural areas ✔
a. shoot apex is an «apical» meristem/has undifferentiated cells ✔
b. mitosis «in shoot apex» ✔
c. cell division/cytokinesis/cells produced «in shoot apex» ✔
d. cell elongation «in shoot apex» ✔
e. stem/shoot growth «due to the cell division and elongation in the shoot apex» ✔
f. produces auxin ✔
g. auxin stimulates growth/cell elongation ✔
h. growth towards light ✔
i. differentiation of cells «produced by the shoot apex» ✔
j. leaf initiation/leaf development begins/leaf «primordia» formation «at shoot apex» ✔
k. flowers produced «by shoot apex» ✔
a. polyploidy is having more than two sets of «homologous» chromosomes ✔
b. triploid has three sets/is 3n ✔
c. tetraploid has four sets/is 4n ✔
d. Allium/vizcacha rats/other named example» ✔
e. details of chromosome numbers in diploid and polyploid species in the example ✔
f. non-disjunction/failure of chromosome pairs to separate during meiosis ✔
g. diploid gamete «can lead to polyploidy» ✔
h. fusion of diploid and haploid gamete produces triploid cells ✔
i. DNA replication but no subsequent mitosis doubles the chromosome number/produces tetraploid «from diploid»
OR
fusion of two diploid gametes produces tetraploid/4n ✔
j. polyploid/tetraploid «crossed» with diploid/non-polyploid produces infertile offspring ✔
k. meiosis fails in triploids because «homologous» chromosomes cannot pair up ✔
l. polyploid individuals are reproductively isolated
OR
polyploidy causes instant/immediate speciation
OR
tetraploids can form a new species because they can cross with each other
OR
polyploids cannot cross/produce fertile offspring with diploids ✔
m. speciation by polyploidy is common in plants/commoner in plants than animals ✔
n. polyploid individuals tend to be larger ✔
Examiners report
Outline how greenhouse gases interact with radiation and contribute to global warming.
Outline how plants make use of the different wavelengths of light.
Explain how organic compounds are transported within plants.
Markscheme
a. carbon dioxide is a greenhouse gas
b. methane/nitrogen oxide/water vapour is a greenhouse gas
c. sunlight/light/(solar) radiation passes through the atmosphere (to reach the Earth’s surface)
d. CO2 in atmosphere/greenhouse gases absorb/trap/reflect back some radiation/heat (emitted by the Earth’s surface)
e. CO2 in atmosphere/greenhouse gases allow short wave radiation to pass (through atmosphere) but absorb long wave/infra-red
f. solar radiation/sunlight is (mostly) short wave
g. radiation/heat emitted by the Earth is long wave/infra-red
Allow answers presented in a clearly annotated diagram.
a. light used in photosynthesis/light-dependent reactions/photolysis/photosystems/photophosphorylation/excitation of electrons/switch to flowering
b. chlorophyll absorbs red AND blue light (more)
c. chlorophyll/leaf/plant reflects/does not absorb/does not use green light
d. absorption spectrum of chlorophyll has peaks in the red and blue/sketch graph to show this
e. action spectrum shows which wavelengths plants use in photosynthesis/sketch graph of action spectrum showing peaks in the blue and red
f. accessory/other (named) photosynthetic pigments absorb different wavelengths/colours
g. violet is the shortest wavelength and red the longest
h. red light and far red/infra-red absorbed to measure length of light/dark periods
a. transported in/translocated in/loaded into phloem
b. in sieve tubes
c. by mass flow
d. from sources to sinks
e. from leaves/other example of source to roots/other example of sink
f. loading (of sugars/organic compounds) by active transport
g. cause high concentration of solutes (in phloem/sieve tubes)
h. water uptake (in phloem/sieve tubes) by osmosis/water diffuses into phloem
i. rise in (hydrostatic) pressure at source (in phloem)
j. creates a (hydrostatic) pressure gradient/higher pressure in source than sink
k. flow can be in either direction/bidirectional
Examiners report
Most of the surface of the Earth is covered with a wide diversity of ecosystems. Outline two general characteristics of all ecosystems.
Vascular plants can be found in a wide variety of ecosystems.
Outline active transport in phloem tissue.
Vascular plants can be found in a wide variety of ecosystems.
Explain how a plant replaces the water it loses in transpiration.
Markscheme
a. organisms/community plus the environment / biotic and abiotic «components»
b. interactions
c. ecosystems show sustainability
d. nutrients are recycled in ecosystems
e. energy flows through ecosystems
f. producers «are part of all ecosystems»
g. decomposers/saprotrophs «are part of all ecosystems»
a. active transport/pumps used to load sugars/sucrose into phloem/companion cells/sieve tubes
b. loading in sources/unloading in sinks
OR
sucrose/sugars moved from source to sink
c. active transport moves H+ out of phloem/sieve tubes «to make H+ gradient in the leaf/source»
d. H+ gradient used for co-transport of sucrose into phloem/sieve tubes/companion cells
Accept protons or hydrogen ions instead of H+ ions.
Accept the equivalent of mpc and mpd for unloading in the sink.
a. transpiration/evaporation of water causes suction/tension
b. water sucked/drawn out of xylem «in leaf»
c. water moves up in xylem
d. due to suction/tension/pulling forces
e. cohesion of water/hydrogen bonds between water molecules
f. movement from roots to leaves
g. water enters root by osmosis/due to higher solute concentration inside root
Examiners report
In ecosystems, energy is used to convert inorganic compounds into organic matter. Energy enters ecosystems through producers.
Explain the processes by which light energy is converted into chemical energy.
Describe how energy flows through and is used by organisms in ecosystems.
Markscheme
a. plants/producers/autotrophs convert light to chemical energy by photosynthesis
b. chlorophyll/photosynthetic pigments absorb light
c. electrons are excited/raised to higher energy level
d. excited electrons pass along chain of electron carriers
e. energy from electrons used to pump protons across thylakoid membrane/into thylakoid space
f. chemiosmosis/proton gradient used to make ATP
g. ATP synthase generates ATP
h. pigments arranged in photosystems
i. electrons from Photosystem II flow via the electron chain to Photosystem I
j. electrons from Photosystem I are used to reduce NADP
k. ATP and reduced NADP used in the light independent reactions/Calvin cycle
l. carbohydrate/glucose/carbon compounds produced containing energy
Award marking points for any point made on a clearly annotated diagram.
a. producers/plants/autotrophs obtain energy from light/sun/inorganic sources
b. food contains energy / energy passed in the form of food/carbon compounds (along food chains/between trophic levels)
c. consumers obtain energy from other organisms/from previous trophic level
This mark point distinguishes consumers from producers.
d. energy released (in organisms) by (cell) respiration
Reject energy used in respiration.
e. ATP produced
f. energy/ATP used for biosynthesis/movement/active transport/other valid use of ATP
g. less energy available / energy lost at each trophic level
Examiners report
Boreal forests stretch across Canada, Russia and Scandinavia. This northern ecosystem accounts for 29 % of the world’s forest areas. The long, cold winters favour tall evergreen trees with either needles or scale-like leaves. These trees are wind-pollinated and their seeds are not enclosed in a fruit. The photograph shows a typical boreal forest in winter.
Identify the dominant plant phylum in the boreal forest.
In some areas there are gaps in the boreal forest where trees fail to grow and peat tends to accumulate. Suggest reasons for this.
An increase in global temperatures poses a critical threat to boreal forests. Explain the consequences of climate change to this northern ecosystem.
Suggest one advantage for the evergreen trees of the boreal forest being pollinated by wind.
Discuss the advantages of the production of seeds enclosed in fruit.
The boreal forests are situated close to the north pole and even in summer the intensity of sunlight is lower than at the equator. Sketch a graph showing the effect of light on the rate of photosynthesis, labelling the axes.
In some boreal species, Rubisco is down-regulated during the winter months. Describe the role of Rubisco in photosynthesis.
Markscheme
coniferophyta/conifer/coniferous/gymnosperms/pinophyta ✔
a. waterlogged soils/poor drainage
OR
acidic soil
OR
anaerobic conditions/soil ✔
b. organic matter not «fully» decomposed «leading to peat formation»
OR
decomposers/saprotrophs less active/fewer in cold «temperatures» ✔
a. higher temperatures so more transpiration/droughts/dehydration/water shortage ✔
b. more forest fires ✔
c. more/new pests/diseases because of the changed conditions ✔
d. competition from trees/plants «that colonize/spread to boreal forests» ✔
e. trees/«named» organisms «of boreal forests» not adapted to warmer conditions
OR
trees/«named» organisms migrate/change their distribution due to warmer conditions ✔
f. trees die so loss of habitat for animals ✔
g. faster decomposition/nutrient cycling «so conditions in the ecosystem change» ✔
h. standing water/floods due to more snow/permafrost melting ✔
animals/insects/mutualistic «relationships» not needed «for pollination»
OR
pollen not eaten by animals/insects ✔
a. seeds are protected «inside the fruit» ✔
b. seed dispersal by fruits ✔
c. example of a strategy for seed dispersal by a fruit ✔
d. dispersal reduces competition/spreads seeds away from parent plant/to colonize new areas ✔
For mpc suitable strategies are dispersal by wind, by animals ingesting/carrying away succulent fruits, by animals being attracted to colourful/sweet/tasty fruits, by animals burying nuts, by burrs or other hooked fruits sticking to animals and by self-explosion.
a. x-axis labelled as light intensity/amount of light AND y-axis labelled as rate of photosynthesis/rate of oxygen release/rate of carbon dioxide uptake ✔
b. curve starting at/slightly to the right of the x-axis origin and rising rapidly and then more slowly and plateauing but never dropping ✔
a. carbon fixation/fixes carbon dioxide/carboxylation
OR
rubisco is used in the Calvin cycle/light independent stage ✔
b. carbon dioxide linked to RuBP/ribulose bisphosphate «by rubisco» ✔
c. glycerate 3-phosphate/glycerate phosphate produced «by rubisco» ✔