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.

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 ✔

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.

Identify the following processes as either anabolism or catabolism by placing a tick (√) in the correct box.

Outline the importance of enzymes to metabolic processes.

a. increase rate of reaction/speed up reaction

b. lower activation energy

c. a specific enzyme for each reaction/substrate

d. metabolic process/pathway blocked if an enzyme is inhibited/absent

e. end-product inhibition can control metabolic pathways

f. differences in metabolism as cells produce different enzymes during differentiation

State the highest concentration of cotinine in the urine, giving the units.

Deduce, with a reason, whether the concentrations of cotinine and NNAL-Gluc would be higher in the urine or in the blood plasma of a smoker.

Suggest one advantage of using the urine concentration of cotinine rather than NNAL-Gluc to give a measure of the amount of tobacco smoke inhaled by a person.

42 nmol ml^–1 ✔

higher in urine due to concentration of waste products «during the process of urine production»
OR
higher in urine because water is reabsorbed «from glomerular filtrate/in the collecting duct»/ because «waste products» are not reabsorbed ✔

higher «concentrations» so easier to measure/identify/find
OR
wider spread/greater range «of values/concentrations»
OR
units are larger/nmol rather than pmol ✔

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.

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» ✔

Identify the structure labelled X.

State the stage of mitosis of this cell. 

Compare and contrast the location of ATP synthase and the movement of protons during aerobic cell respiration and photosynthesis.

Using the table, distinguish between the production of ATP, use of oxygen and release of CO₂ in aerobic cell respiration between the cytoplasm and the mitochondrion.

The graph shows energy levels throughout an uncatalysed reaction. Draw a curve to show how the action of an enzyme would affect this reaction.

cell wall

metaphase

location of ATP synthase

a. cristae/inner mitochondrial membrane versus thylakoid membranes 

movement of protons

b. protons moved/pumped as a result of electron flow/electron transport in both 

c. (pumped by the electron transport chain) from the matrix to the intermembrane space versus from the stroma to the thylakoid space 

d. through ATP synthase/synthetase in both (respiration and photosynthesis) 

e. protons move (through ATP synthase/synthetase) down the concentration gradient in both 

f. move (down concentration gradient) from the intermembrane space to the matrix versus from the thylakoid space to the stroma

curve starting and ending at the same energy level but rising to a lower peak 

The sketch shows the relationship between the reaction rate and substrate concentration in the presence and the absence of a competitive inhibitor.

Explain the effect of the competitive inhibitor on the reaction rate.

Describe its location.

Describe its function.

a. competitive inhibitor «slows the reaction rate as it» competes for the active site
OR
competitor has similar shape/structure/composition to substrate «and slows the reaction rate» 

b. binding of competitor is reversible 

c. «as the substrate concentration increases» more substrate binds to the active site than the competitor «and reaction rate increases» 

d. «as the substrate concentration increases» the reaction rate reaches the maximum plateau «same as with no inhibitor»

the inner mitochondrial membrane critstae/thylakoid membrane

a. protons build up in the intermembrane space due to electron transport chain OWTTE

b. protons move through ATP synthase down the concentration gradient
Accept H⁺ ions in place of protons 

c. catalyses formation of ATP OWTTE

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.

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

Calcium is absorbed from food in the human gut by both active and passive processes. Outline active transport, including the benefits of the process.

Describe the role of oxygen in aerobic cell respiration.

Adult humans may absorb more than five hundred litres of oxygen per day. Explain how gas exchange is maintained in the human respiratory system.

a. moved against a concentration gradient/lower to higher concentration ✔

b. energy/ATP required/used ✔

c. pump/carrier «protein» «carries out active transport» ✔

d. absorption «by active transport» into a cell is possible even if exterior concentrations are «very» low
OR
allows all/nearly all of/more of the substance/calcium to be absorbed «whereas diffusion can only even out concentrations»
OR
unidirectional/allows the direction of movement to be controlled
OR
allows a concentration gradient to be built up/potential energy to be stored/membrane potential to be generated/maintained
OR
allows a specific concentration to be maintained «in a cell» ✔

a. terminal/final electron acceptor ✔

b. at the end of electron transport chain ✔

c. oxygen also accepts protons/hydrogen ions ✔

d. water produced/ $\frac{1}{2}$O₂ + 2 electrons + 2H⁺ → H₂O ✔

e. helps to maintain proton gradient «across inner mitochondrial membrane by removal of protons from the stroma» ✔

f. oxygen is highly electronegative/electrons strongly attracted to oxygen ✔

g. avoids anaerobic respiration/buildup of lactic acid ✔

h. allows more electrons to be delivered to the electron transport chain
OR
allows NADFAD to be regenerated/reduced NAD/FAD converted back to NAD/FAD ✔

i. oxygen allows maximum yield of energy «from glucose» allows complete oxidation of glucose/allows fats to be used in respiration ✔

a. ventilation/inhaling brings fresh air/air with high oxygen concentration to the lungs
OR
ventilation/exhaling gets rid of stale air/air with high concentration of carbon dioxide ✔

b. ventilation due to muscle contractions causing pressure/volume changes in the thorax ✔

c. contraction of external intercostal muscles AND diaphragm occurs during inspiration
OR
contraction of internal intercostal muscles/abdomen wall muscles during «forced» expiration ✔

d. alveoli surrounded by «many» capillaries ✔

e. blood flow/pumping of heart «brings blood to/takes blood away from alveoli/lungs» ✔

f. concentration gradients «of oxygen/ CO₂» maintained «by ventilation/blood flow» ✔

g. O₂ AND CO₂ diffuse ✔

h. CO₂ from capillaries/blood/vessel to alveolus/air AND O₂ from alveoli into capillaries/blood/vessel ✔

i. large numbers of alveoli increase surface area ✔

j. short distance so rapid diffusion/gas exchange ✔

k. type I pneumocytes/alveolus wall/capillary walls are one cell thick/very thin ✔

l. alveoli «lining» moist for dissolving of gases/rapid diffusion
OR
type II pneumocytes keep the «lining of» the alveolus moist ✔

m. type II pneumocytes secrete surfactant to reduce surface tension/prevents alveoli from collapsing ✔

Outline reasons for the therapeutic use of stem cells.

Describe how monoclonal antibodies are produced.

Explain the role of the electron transport chain in the generation of ATP by cell respiration.

1. unspecialized/undifferentiated stem cells can divide/differentiate along different pathways;
2. (stem cells are accessible as they) come from embryos/bone marrow/umbilical cord blood/adult tissue;
3. (stem cells) can regenerate/repair/regrow diseased/damaged tissues in people;
4. valid specific example;
5. drugs can be tested on stem cells (in laboratories to see if they are harmful);

1. mice/rabbit/small mammal injected with one type of antigen;
2. cells from the spleen/antibody-producing cells are removed;
3. plasma cells that produce antibodies (are used);
4. myeloma/tumor cells that divide endlessly (are used);
5. fusion of plasma cells with tumor/myeloma cells / fusion produces hybridoma cells;
6. selection of hybridoma cells / medium used that only allows growth of hybridoma cells;
7. fused cells/hybridoma cells are cultured/grown in tissue culture/grown in a fermenter;
8. (hybridoma) cells divide endlessly and produce the desired antibodies;

1. electron transport chain performs chemiosmosis / chemiosmosis generates ATP;
2. receives energy/electrons from oxidation reactions/from Krebs cycle/glycolysis;
3. receives electrons from reduced NAD/NADH/reduced FAD/FADH;
4. energy released as electrons pass from carrier to carrier (in the chain);
5. release of energy (from electron flow) coupled to proton pumping;
6. protons pumped into intermembrane space;
7. creates proton gradient;
8. protons diffuse back/move down the concentration gradient (across membrane);
9. protons pass through ATP synthase;
10. protons return to the matrix;
11. flow of protons provides energy for generating ATP;
12. electrons transferred to oxygen at end of electron transport chain;

Most 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.

Production 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.

For 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!

Outline the process of DNA profiling.

Outline the role of DNA polymerase III in DNA replication.

Explain the factors that affect the rate of enzyme-controlled reactions in cells.

a. sample of DNA obtained from person/hair/blood/mouth/crime scene ✔

b. PCR used to amplify/make copies of DNA (in sample) ✔

c. using Taq DNA polymerase / using DNA polymerase from thermophilic bacteria ✔

d. tandem repeats amplified/used ✔

e. gel electrophoresis used to separate DNA (into bands) ✔

f. 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 ✔

h. example of application/forensics/crime investigation/paternity ✔

Do not accept ‘determine ancestry’ for mph. Other genes/chromosomes are more often used for that.

Accept STR for (short) tandem repeat in mpd.

a. binds to template strand adjacent to a primer/at the primer ✔

b. adds nucleotides to template strand/to single stranded DNA ✔

c. using complementary base pairing ✔

d. links nucleotides with sugar-phosphate/phosphodiester bonds ✔

e. adds nucleotides/builds new strand in 5' $\to$ 3' direction ✔

f. lagging strand is built in short segments/Okazaki fragments/synthesis is discontinuous ✔

Accept A to T and G to C instead of ‘complementary’ in mpc.

For 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.

a. temperature increases rate up to optimum and higher temperatures decrease rate / graph ✔

b. faster molecular movement as temperature rises (so more substrate-active site collisions) ✔

c. high temperature/heat causes denaturation/irreversible change to active site (so rate reduces) ✔

d. rate decreased if pH is above and below optimum/if pH is too high or low / graph ✔

e. pH affects shape/structure of enzyme/active site /affects ionization (of amino acids) ✔

f. increases in substrate concentration cause rate to rise towards a plateau/WTTE / graph ✔

g. 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) ✔

i. enzyme inhibitors/competitive inhibitors/non-competitive inhibitors reduce the rate ✔

j. 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.

For mpa there must be exponential rise to optimum then faster drop.

For mpd there must be a bellshaped curve but it need not be exactly symmetrical.

For mpf and mph there must be decreasing increases in rate towards a plateau.

There 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.

For 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.

This 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.

Outline how the properties of water make it an ideal transport medium in plants.

Distinguish between the xylem and phloem of plants.

Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions.

a. 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.

a. 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.

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.

This 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.

State two structural similarities between mitochondria and chloroplasts.

1.

2.

Compare and contrast mitochondria and chloroplasts in terms of the substrates they use and the products they produce. 

Outline how the compounds produced by chloroplasts are distributed throughout the plant.

a. (a loop of) DNA ✔

b. 70S ribosomes ✔

c. double membrane ✔

d. electron transport chains/enzyme complexes in (internal) membranes ✔

e. enzymes in a region of fluid/in stroma and matrix ✔

f. large area of (internal) membrane/cristae and thylakoids ✔

Only two answers should be marked – the first on each line.

Do not award marks for functions rather than structures, for example ATP production.

Allow spaces inside cristae and thylakoids for mpf.

a. ATP produced by both / ADP used by both ✔

b. oxygen produced by chloroplasts and used by mitochondria ✔

c. carbon dioxide produced by mitochondria and used by chloroplasts ✔

d. carbon/organic compounds built up in chloroplasts/anabolism and broken down in mitochondria/catabolism ✔

Do not award mpd for statements about carbohydrates or glucose (because the pyruvate used by mitochondria is not a carbohydrate).

a. in phloem ✔

b. loading into sieve tubes/by active transport/by cotransport/by companion cells ✔

c. entry of water (to phloem) by osmosis/because of high solute concentration ✔

d. causes high/hydrostatic pressure ✔

e. flow from high pressure to lower pressure down pressure gradient ✔ from source to sink ✔

Do not award mpa if xylem included with phloem.

Do not award a mark solely for mentioning the term ‘translocation'.

Surprisingly few candidates achieved two marks for this question, with many thinking that these organelles were cells with cell membranes, cell walls or even nuclei.

To 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.

This 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.

Outline the process of protein denaturation.

Explain the production of antibodies in humans.

Distinguish between competitive and non-competitive enzyme inhibition.

a. 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.

a. 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.

Accept mpa, mpb or mpc on an annotated diagram.

[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.

This 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.

In 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’.

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.

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 

Explain the role of hydrogen ions used in photosynthesis.

Describe how the structure of the chloroplast is adapted to its function in photosynthesis.

The 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.

a. hydrogen ions (and oxygen) are obtained from photolysis of water ✔

b. (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 ✔

d. (two) high energy/excited electrons (from Photosystem II) are passed through an electron transport chain ✔

e. NADP accepts H⁺ (from the stroma) / electrons to produce NADPH⁺/reduced NADP ✔

f. ATP/reduced NADP/NADPH⁺ are used by light-independent reactions/Calvin cycle in the stroma ✔

g. (in Calvin cycle) carbon fixation to ribulose bisphosphate/RuBP produces G3P ✔

h. ATP is used to transform G3P to TP ✔

i. reduced NADP/NADPH⁺ is used to transform G3P to TP ✔

Accept answers in an annotated diagram.

Accept NADPH₂/NADPH/NADPH + H⁺ in place of NADPH⁺.

a. thylakoids have a small internal volume to maximize hydrogen gradient / quickly increase concentration gradient ✔

b. many grana / thylakoids give large surface area ✔

c. (grana) contain (photosynthetic) pigments/electron carriers/ATP synthase enzymes ✔

d. (photosynthetic) pigments/chlorophyll arranged into photosystems allowing maximum absorption of light energy ✔

e. stroma has suitable pH/ enzymes of Calvin/light independent cycle ✔

f. lamellae connect/separate grana maximizing photosynthetic efficiency ✔

Accept diagram showing the disposition in space.

a. insulin receptors start the cellular signalling pathway/other receptor

b. leptin is a hormone / other protein hormone ✔

c. immunoglobulins/antibodies defend body from disease ✔

d. rhodopsin/photosystems convert light energy to electrical impulses ✔

e. keratin/collagen/other example is a structural protein ✔

f. tubulin/microtubules part of cytoskeleton/ involved in cell division

g. ATP synthase is an enzyme that catalyzes the formation of ATP / other enzyme and function ✔

h. fibrin/fibrinogen is a protein involved in clotting ✔

i. hemoglobin/sodium potassium pump/other example is a transport protein ✔

j. actin/myosin/other example is involved in muscle contraction ✔

k. any other named example and function ✔

l. a second other named example and function ✔

m. spider silk is used to form spider webs/capture prey ✔

Accept only one example of each protein type for example only one enzyme or only one hormone etc. 

Mp k can be awarded only once.

Mp I can be awarded only once.

Question 6 was a popular question.

A relatively high scoring section, though many students failed to discuss the role of hydrogen in the light independent reactions.

Question 6 was a popular question.

Well answered.

Question 6 was a popular question.

Well 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.

Explain how chemical energy for use in the cell is generated by electron transport and chemiosmosis.

Outline four different functions of membrane proteins.

Distinguish between anabolism, catabolism and metabolism.

a. NAD/FAD carries/is reduced by gaining «two» H «atoms»/«two» electrons 

b. reduced NAD produced in glycolysis/link reaction/Krebs cycle 

c. reduced NAD/FAD delivers electrons/hydrogen «atoms» to ETC 

d. ETC is in mitochondrial inner membrane/cristae 

e. electrons release energy as they flow along the chain/from carrier to carrier 

f. electrons from ETC accepted by oxygen/oxygen is the final electron acceptor 

g. proteins in the inner mitochondrial membrane/electron carriers act as proton pumps 

h. protons pumped into intermembrane space/proton gradient across inner mitochondrial membrane/proton concentration higher in intermembrane space than in matrix 

i. energy «from electrons» used to pump protons into intermembrane space/generate a proton gradient / high H⁺ concentration is a store of «potential» energy 

j. ATP synthase in inner mitochondrial membrane/cristae 

k. energy released as protons pass down the gradient/through ATP synthase 

l. ATP synthase converts ADP to ATP/phosphorylates ADP 

m. oxidative phosphorylation «is ATP production using energy from oxidizing foods»

Accept H+ but not H/hydrogen in place of protons in any part of the answer.

Accept NADH or FADH in place of reduced NAD or FAD.

a. receptor/binding site for hormone/neurotransmitter 

b. cell-to-cell communication / cell recognition 

c. channels «for passive transport» / facilitated diffusion 

d. pumps / active transport 

e. cell adhesion 

f. «immobilized» enzymes/enzymes embedded in the membrane 

g. electron transport / electron carriers

a. metabolism is all enzyme-catalyzed reactions in a cell/organism/is anabolism plus catabolism 

b. anabolism is synthesis of polymers/complex/larger molecules/larger substances «from smaller molecules/monomers» 

c. catabolism is breaking down «complex» molecules/substances «into simpler/smaller ones/into monomers»

Outline the functions of rough endoplasmic reticulum and Golgi apparatus.

Outline the control of metabolism by end-product inhibition.

Explain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane.

a. ribosomes on RER synthesize/produce polypeptides/proteins ✔

b. proteins from RER for secretion/export/use outside cell/for lysosomes ✔

c. Golgi alters/modifies proteins/example of modification ✔

d. 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.

a. 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 ✔

c. inhibits/binds to/blocks the first enzyme in chain/pathway ✔

d. non-competitive inhibition ✔

e. end-product/inhibitor binds to an allosteric site/site away from the active site ✔

f. changes the shape of the active site/affinity of the active site «for the substrate» ✔

g. 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 ✔

i. 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 ✔

Allow mark points shown in clearly annotated diagrams.

To gain mpd, mpe and mpf the answer must be in the context of end-product inhibition, not enzyme inhibition generally.

a. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔

b. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔

c. phospholipid bilayer in water/in membranes ✔

d. hydrophilic heads «of phospholipids» face outwards/are on surface ✔

e. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔

f. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔

g. some amino acids are hydrophilic and some are hydrophobic ✔

h. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔

i. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔

j. 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 ✔

Allow mark points shown in clearly annotated diagram.

In any part of the answer, accept polar instead of hydrophilic and non-polar or apolar instead of hydrophobic.

Most 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.

Answers 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.

Candidates 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.

Explain the processes by which light energy is converted into chemical energy.

Describe how energy flows through and is used by organisms in ecosystems.

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

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.

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 ✔

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.

Describe the process of photolysis in photosynthesis.

a. water is split/breaks

b. using energy from light

c. electrons «from photolysis» pass to photosystem II

d. oxygen is a «waste» product

e. hydrogen ions/protons are produced

Allow answer given as an equation

[Max 3 Marks]

Outline how the structures labelled X and Y are adapted to carry out the function of the mitochondrion.

X:

Y:

Explain how ATP is generated in mitochondria by chemiosmosis.

X: 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;

a. protons pumped across inner membrane of mitochondria/into intermembrane space;

b. using energy released by flow of electrons/by electron transport/by electron carriers;

c. proton gradient established/maintained / proton motive force generated;

d. protons pass/diffuse back through inner membrane/membrane of cristae/to matrix;

e. through ATP synthase;

f. ATP production coupled to flow of protons / ATP from ADP and Pi using energy from protons;

Marks can be awarded in an annotated diagram.

The 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.

This was answered more successfully by most candidates and there were some impressively detailed and accurate answers.