Outline the properties of water molecules that permit them to move upwards in plants.

Define osmolarity. 

Deduce, with a reason, which red blood cell has been placed in a hypertonic solution.

State what change there has been in the cell surface area to volume ratio in red blood cell 1.

a. water molecules are polar
OR
can form hydrogen bonds 

b. cohesion between water molecules allows continuous water columns
OR
cohesion between water molecules allows transpiration stream «to form in xylem» 

c. adhesion of water to the walls of xylem vessel «helps water rise» 

d. water evaporates at environmental temperatures allowing transpiration pull OWTTE

«measurement of» solute concentration of a solution OWTTE

cell 2 because it has plasmolized/lost water/volume has decreased

decreased

Draw a labelled diagram to show the fluid mosaic model of the plasma membrane.

Outline how neurons generate a resting potential.

Hydrogen bonds can exist both within and between molecules in living organisms and have an impact on their structure and function. Explain the importance of hydrogen bonding for living organisms.

a. two correctly orientated layers of phospholipids/phospholipid bilayer shown withheads facing in opposite directions 

b. phospholipids shown with two parts labelled hydrophilic/phosphate head AND hydrophobic/hydrocarbon tail

c. protein (any) shown as a globular structure embedded in one/both layers of phospholipid 

d. peripheral protein shown as globular structures at the surface of the membrane AND integral protein shown as embedded globular structures 

e. glycoprotein shown as embedded globular structure with antenna-like carbohydrateprotruding 

OR 

carbohydrate shown as branched/antenna-like structure attached either to a protein or to a phospholipid 

OR 

channel protein(s) shown with a pore passing through it 

OR 

pump protein shown as a transmembrane globular structure 

f. cholesterol shown in between adjacent phospholipids 

Do not award the mark unless the structure is labelled with the underlined name. 

a. sodium-potassium pump 

b. sodium /Na⁺ out and potassium /K⁺ in 
OR
sodium/Na⁺ concentration higher outside and potassium/K⁺ higher inside 

c. three Na⁺ pumped for every two K⁺ (hence negative inside) 
OR
inside of axon holds negative ions/Cl^- ions/negatively charged proteins/organic anions (hence negative inside) 

d. by active transport / using ATP 

e. inside (of axon/neuron) is negative in comparison to outside 
OR
electrochemical concentration/charge difference (across the membrane) is the resting potential 

f. resting potential is –70 mV

a. cohesion in water/water molecules stick together (due to hydrogen bonds) 

b. cohesion helps transport under tension of water/sap in xylem / transpiration stream

c. adhesion between water and cell walls/cellulose/polar molecules 

d. adhesion/capillary action helps water to rise in plants/stems/xylem / helps keep leaf walls moist 

e. solvent properties (due to hydrogen bonds) with polar/hydrophilic molecules 

f. solvent properties exemplified by glucose/other example of a polar solute 

g. high latent heat of evaporation / (much) energy required for evaporation so water useful as coolant/for sweating 

h. high (specific) heat capacity so water temperature changes less 

i. base pairing between bases/nucleotides/strands in DNA by hydrogen bonding 

j. base pairing between bases in RNA and DNA for transcription/between codon and anticodon for translation 

k. proteins have hydrogen bonding in secondary structure/α helix/β pleated sheet 

l. proteins have hydrogen bonding between R groups/in tertiary structure/to maintain conformation 

m. habitats because water is liquid due to high boiling point/due to water freezing on the surface 

n. habitats on water surface due to surface tension

The image shows human red blood cells.

[Source: someoneice/123rf.com.]

Outline what will happen to human red blood cells if transferred to distilled water.

Stem cells can be used to treat Stargardt’s disease. State one other condition treated using stem cells.

Explain the propagation of nerve impulses along the membrane of a neuron.

cells absorb water by osmosis and swell/increase in volume
OR
cells burst/lyse;

leukemia/other diseases of the hematopoietic system / skin burns;

a. depolarization of part of axon/membrane triggers/causes depolarization of next part;
b. local currents;
c. diffusion of sodium ions between depolarized part and next/polarized part (of axon);
d. resting potential reduced/polarization of membrane becomes less /change from -70 to -50mV;
e. sodium channels open when -50mV/threshold potential reached;
f. entry of sodium ions causes depolarization;
g. saltatory conduction in myelinated neurons/axons;

Allow answers in an annotated diagram

This was mostly well answered with candidates realising that water entry due to osmosis would cause the cells to burst.

A range of conditions were suggested, most of which are not currently treatable using stem cells. Diabetes and neurological diseases were not accepted. 60 % of candidates gave an acceptable answer, with leukaemia much the commonest. The correlation between the mark on this question and overall scores was fairly low, which may suggest that inappropriate examples have been learned in some schools. It is important not to raise hopes of stem cell treatment for specific diseases when research is still continuing and regulatory approval is unlikely for many years.

This was a more difficult question with a much higher correlation coefficient. Many candidates did not understand what is meant by propagation of an impulse along the membrane of a neuron and answers tended to include accounts of the whole sequence of events in an action potential or in synaptic transmission. The mark most frequently awarded was for the mechanism of saltatory conduction.

Cell biologists play an important role in research into disease, fertility, evolution and many other areas of science.

Describe the origin of eukaryotic cells according to the endosymbiotic theory.

Cell biologists play an important role in research into disease, fertility, evolution and many
other areas of science.

Compare and contrast the processes of spermatogenesis and oogenesis.

Cell biologists play an important role in research into disease, fertility, evolution and many
other areas of science.

Outline the evidence for evolution provided by selective breeding.

a. mitochondria and chloroplasts are similar to prokaryotes

b. «host» cell took in another cell by endocytosis/by engulfing «in a vesicle»

    Allow “taking in” in place of “engulfing”

c. but did not digest the cell/kept the «ingested» cell alive

    OR

    symbiotic/mutualistic relationship «between engulfed and host cell»

d. chloroplasts and mitochondria were once independent/free-living «organisms»

e. DNA «loop» in chloroplast/mitochondrion

f. division/binary fission of chloroplast/mitochondrion

g. double membrane around chloroplast/mitochondrion

h. 70s ribosomes «in chloroplast/mitochondrion»

Award up to [2] for evidence from mpe to mph

[Max 4 Marks]

a. both result in haploid cells/gametes

b. both involve mitosis at the start/in the «germinal» epithelium

c. both have cell growth «before meiosis»

d. both involve «two divisions of» meiosis

e. both involve differentiation to produce a gamete

f. both are stimulated by hormones

    OR

    spermatogenesis stimulated by testosterone and oogenesis stimulated by FSH

A table is not required but both statements in one row of the table must either be explicitly stated or clearly implied to award the mark

[Max 8 Marks]

a. crop plants/domesticated animals/livestock produced by selective breeding

b. specific example of a domesticated animal/crop plant and the wild species from which it
was developed

   OR  

   specific example of a domesticated animal/crop plant and the features in it which have been improved «compared with the wild species»

   For example dogs have been developed from wolves

c. artificial selection/crossing selected varieties/eliminating undesirable varieties

d. «selective breeding/artificial selection can cause» significant/rapid change over time/from the original wild species

e. «changes due to selective breeding/artificial selection» shows natural selection can cause change/evolution «in a species»

[Max 3 Marks]

Outline the effect of hypoxia on body mass and erythrocyte percentage.

Using the data in the graph, deduce whether hypoxia increases or decreases the endurance of the rats’ diaphragm muscle.

Using the data presented in this question, explain the effect of hypoxia on the body.

Analyse the graph to obtain two conclusions about the concentration of sodium–potassium pumps.

Muscle fibres are stimulated to contract by the binding of acetylcholine to receptors in their membranes and the subsequent depolarization.

Suggest a reason for increasing the concentration of sodium–potassium pumps in the membranes of diaphragm muscle fibres.

Outline the effect of hypoxia on the force of contraction of the diaphragm.

Hypoxia caused a 13 % increase in the surface area to volume ratio of the diaphragm. Suggest a reason for this change.

Using all relevant data in the question, evaluate the effectiveness of the rats’ adaptation to hypoxia.

Discuss the advantages and disadvantages of using rats as models in this investigation.

Erythrocyte percentage increased AND body mass reduced/smaller increase in mass

a. increases endurance «in relation to the control»

b. higher force/endurance at every testing time/throughout

    OR

    smaller decreases in force «over time»

c. the magnitude of the difference is similar throughout the five minutes experiment/testing

d. differences are «statistically» significant

e. endurance of control is «approximately» 35 % versus endurance of hypoxia «approximately» 55 % «after 5 minutes»

Accept ±5 % for both percentages

[Max 2 Marks]

a. diaphragm more endurance/stronger/generates more force for more ventilation/inspiration

b. right ventricle mass increases to pump more blood

c. erythrocyte percentage increases to transport oxygen

d. less growth/body mass which reduces oxygen demand

Reject “loss of body mass”

The physiological reason is required for each mark

[Max 2 Marks]

a. hypoxia increases the concentration of sodium–potassium pumps

b. nitric oxide needed for/stimulates «production of» sodium-potassium pumps

c. nitric oxide synthase inhibitor reduces the concentration of pumps
   OR
   concentration of pumps reduced by inhibiting nitric oxide production

Award up to [1] for a conclusion on lines labelled 1 and up to [1] for a conclusion on the lines labelled 2

[Max 2 Marks]

a. resting potential restored faster

b. increases the «maximum» frequency/rate of contractions

    OR

    can contract again sooner

Accept shorter refractory period for mpa

Do not accept faster contraction/depolarization/ repolarization

[Max 1 Mark]

reduces «force of» twitch AND peak tetanic contraction

a. decrease in volume/atrophy/loss of cells/less muscle fibres/less tissue in the diaphragm

b. SA to volume ratio increased to make oxygen uptake into muscle/cells faster

Do not accept reduction in area of diaphragm

[Max 1 Mark]

a. not effective because body mass lost

b. effective because body mass still increases/rats still grow

c. not effective because contractions/force exerted by diaphragm decreases

d. effective because more sodium-potassium pumps so more/faster rate of diaphragm/muscle contractions

e. effective because endurance of diaphragm increases

f. effective because mass of right ventricle increases

g. effective because erythrocyte percentage increases

For each marking point the candidate must make it clear whether they are arguing for adaptation being effective or not. This can be done by giving the physiological benefit of a change, for example greater mass of right ventricle so more blood pumped.

[Max 3 Marks]

Advantages:

a. small size

   OR

    easy to look after in research labs

b. short lifespan

    OR

    study can extend over several generations

c. can be killed «to get experimental results» if benefits of research justify it

d. «mammalian» so similarities with humans

e. fewer ethical objections than if humans are used/not ethical to subject humans to hypoxia/does not cause harm to humans

Accept any one of the advantages

Disadvantages:

f. ethical objections

   OR

   wrong to cause suffering to animals/rats

g. rat physiology/anatomy not same as human

Accept any one of the disadvantages

[Max 2 Marks]

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 ✔

State the property of amphipathic phospholipids that enables them to form a bilayer.

State the reason cis and trans fatty acids are said to be unsaturated.

State the name of this process.

Explain how water is used in photosynthesis.

have both a hydrophilic and a hydrophobic region
OR
have both a polar and a non-polar region ✔

they have a double bond between carbon/C «atoms»
OR
they could hold more hydrogen ✔

Accept clearly annotated diagrams to that effect.
Do not accept double bonds between C and any other atom

photolysis / light-dependent «reactions/stages» / photophosphorylation ✔

a. water is split/broken «up»/lysed/undergoes photolysis ✔ For mpa, reject “water is cut in half”. For mpa, accept photolysis only if the context shows that water is being split.

b. producing/providing electrons ✔

c. replaces electrons lost by Photosystem II / PSII / P680 / chlorophyll a ✔ For mpc, do not accept just chlorophyll.

d. allows electrons «to continue» to pass along the electron transport chain ✔

e. provides protons/H+ «inside thylakoid» to help generate a «proton» gradient/maintain high concentration inside thylakoid ✔ For mpe, reject pumping of protons into the thylakoid as photolysis produces them inside the thylakoid.

Outline how the amphipathic properties of phospholipids play a role in membrane structure.

State the role of cholesterol in animal cell membranes.

Describe what happens to the membranes of an animal cell during mitosis.

State the name of the structure shown.

X indicates the movement of a structure in the neuron. Explain what events trigger this movement and what happens next.

a. part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar;

b. bilayer formed (formed naturally by phospholipids in water);

c. hydrophilic heads/parts face outwards and hydrophobic tails/parts face inwards.

Do not allow water loving/hating in mpa or mpc.

a. controls/regulates/reduces fluidity of membrane / prevents crystallization of phospholipids;

b. reduces permeability to some substances.

Do not accept ‘stabilizes membrane’.

a. nuclear membrane breaks down/disappears (in prophase/at start of mitosis);

b. nuclear membrane reforms around two new nuclei (in telophase/at end of mitosis);

c. plasma membrane pulled inwards at equator / cleavage furrow formed;

d. membrane pinches apart to form two cells / cytoplasm divided / cytokinesis.

synapse/synaptic

Allow any answer including either of these terms unless out of context.

a. depolarization of pre-synaptic membrane / action potential/nerve impulse arrives;

b. uptake of calcium / calcium ions diffuse in / calcium channels open;

c. structures containing neurotransmitter/vesicles move to/fuse with membrane;

d. neurotransmitter/acetylcholine released by exocytosis into cleft/binds to postsynaptic membrane/receptors;

Must see exocytosis.

Knowledge of the amphipathic properties of phospholipids was generally good.

Half of candidates were able to state the role of cholesterol in animal cell membranes.

The mark scheme allowed candidates to score marks either for describing changes to the nuclear membrane or the role that the plasma membrane plays in cytokinesis. The best answers referred to both. Many of the candidates who wrote about the nuclear membrane had rather sketchy understanding of how it breaks up into small vesicles at the end of prophase and then reforms around both daughter nuclei in telophase.

As pointed out on some G2 forms, the question here was rather ambiguous as it wasn’t 100 % clear what structure was referred to, so any answer indicating that the candidate knew the diagram showed a synapse was accepted.

This was generally well answered. Some suggested that the vesicle is released into the synaptic cleft, instead of neurotransmitter being released by exocytosis.

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»

Discuss alternative models of membrane structure including evidence for or against each model.

Outline the process used to load organic compounds into phloem sieve tubes.

a. early evidence showed membranes are partially permeable AND organic solvents penetrate faster than water 

b. suggests they have non-polar regions 

c. chemical analysis showed membranes consist mainly of proteins and lipids 

d. layer of phospholipids spread over water, orientate themselves into monolayer with nonpolar/hydrophobic tails out of water and polar/hydrophilic heads in water surface 

e. when shaken with water form micelles/particles with tails inwards away from water 

f. Davson–Danielli model proposed phospholipid bilayer coated with protein molecules on both surfaces 

g. evidence from electron microscopy «supported Davson–Danielli model» 

h. three-layered structure/ sandwich/railway tracks/two dark bands with a light band between 

i. model could not account for hydrophobic proteins / artifacts due to low resolution 

j. fluorescent labelling / freeze fracturing later used to investigate membrane structure 

k. led to Singer-Nicholson / fluid mosaic model of protein molecules floating in fluid lipid bilayer 

l. shows particles/proteins project partially and sometimes right through lipid bilayer 

m. indicates peripheral and integral proteins present

Accept any of the points clearly explained in an annotated diagram.

a. active transport/loading of sucrose/amino acids/organic metabolites 

b. sucrose moves by apoplastic / symplastic routes 

c. «loading» at source into companion cells «of sieve tubes» 

d. movement «of sucrose» through plasmodesmata 

e. high concentration of solutes in phloem leads to water movement by osmosis

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!

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 

Outline four types of membrane transport, including their use of energy.

Draw the structure of a dipeptide.

ADH (antidiuretic hormone) is a peptide hormone that is produced in the hypothalamus. Explain its action in the human body.

a. simple diffusion is passive movement of molecules/ions along a concentration gradient ✔

b. facilitated diffusion is passive movement of molecules/ions along a concentration gradient through a protein channel «without use of energy» ✔

c. osmosis is the passage of water through a membrane from lower solute concentration to higher ✔ OWTTE

d. active transport is movement of molecules/ions against the concentration gradient «through membrane pumps» with the use of ATP/energy ✔

e. endocytosis is the infolding of membrane/formation of vesicles to bring molecules into cell with use of energy
OR
exocytosis is the infolding of membrane/formation of vesicles to release molecules from cell with use of energy ✔

f. chemiosmosis occurs when protons diffuse through ATP synthase «in membrane» to produce ATP ✔

mpa, mpb and mpc require reference to concentration.  

Active transport requires mention of the use of energy.

a. two amino acids, one with NH₂/NH₃+ end and one with COOH/COO– end ✔

b. peptide bond between C=0 and N—H correctly drawn ✔

c. «chiral» C with H and R group on each amino acid ✔

d. peptide bond labelled/clearly indicated between C terminal of one amino acid and N terminal of the second amino acid ✔

Labels not required for amino group and carboxyl group.

a. ADH plays a role in osmoregulation/regulating blood solute concentration ✔

b. acts on the collecting ducts of the kidney ✔

c. acts in «late» distal convoluted tubule ✔

d. hypothalamus detects plasma/blood osmolarity/solute concentration ✔

e. if plasma/blood is too concentrated/hypertonic, «posterior» pituitary releases ADH ✔

f. ADH stimulates insertion of aquaporins/water channels / increases permeability of collecting duct ✔

g. water moves «through aquaporins» by osmosis into the medulla/blood ✔

h. urine becomes more concentrated/smaller volume ✔

i. negative feedback occurs ✔ OWTTE for negative feedback acceptable.

j. if blood is hypotonic no ADH is released ✔

k. water is not reabsorbed from the collecting ducts/permeability of the collecting duct decreases ✔

l. urine becomes more dilute/less concentrated / higher volume ✔

OWTTE for all mp.

Question 6 was the most popular extended response question. It was less common for students to correctly describe endocytosis and exocytosis. A common misconception was for students to describe both types of bulk transport as passive processes when they both require energy. Students also commonly failed to make reference to concentration gradients in their discussions of active and passive transport.

Students were successful at indicating the presence of two chiral carbons. Commonly the carboxyl group was represented as an aldehyde group. Nitrogen atoms were commonly shown as having just two bonds.

This question was commonly very well answered. Common errors included muddling the roles of the hypothalamus and the pituitary gland in the production and release of ADH. It was uncommon for students to include a discussion of negative feedback in their answers.

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.

Compare and contrast the structure of a typical prokaryotic cell with that of a mitochondrion.

Explain how mitochondria could have been formed from free living prokaryotes.

differences

a. prokaryote has cell wall but mitochondrion does not ✔

b. mitochondrion has double membrane whereas prokaryote has single membrane
OR
«Gram negative» bacteria have cell wall between two membranes whereas mitochondria has intermembrane space between two membranes ✔

c. mitochondrion has cristae/invaginations of inner membrane but prokaryote does not
OR
prokaryote «may have» flagella/pili/«slime» capsule which mitochondria do not have ✔

similarities

d. 70S ribosomes in both ✔

e. DNA in both / loop of DNA in both / naked DNA in both ✔

f. shape similar/both rod shaped/OWTTE
OR
size of both is similar/both about 3 μm long ✔

g. both are membrane-bound/OWTTE ✔

a. endocytosis/engulfing of prokaryote by a larger/another/anaerobic prokaryote/cell ✔

b. double membrane of the mitochondrion is the result of endocytosis
OR
inner membrane of mitochondrion from engulfed cell and outer from food vacuole ✔

c. «engulfed prokaryotic cell» was aerobic/respired aerobically/consumed oxygen
OR
«engulfed prokaryotic cell» provided energy/ATP ✔

d. «engulfed prokaryotic cell» not destroyed/not digested
OR
«endo»symbiotic/mutualistic relationship developed ✔

e. «engulfed prokaryotic cell» had its own DNA/own «70S» ribosomes ✔

Do not award mpc for “mitochondrion makes ATP”.

Answers were spread over the whole of the four-mark range. The scale bars allowed the sizes of both to be calculated – they were close, but even so a substantial number of candidates claimed that one or other was significantly larger.

This was generally well answered with candidates explaining how endocytosis would have got a free-living prokaryote into a larger cell and because it made itself useful, there was a selective advantage in developing a mutualistic relationship rather than digesting the engulfed cell. Some candidates gave valid evidence for endosymbiosis – presence of DNA, 70S ribosomes or the double membrane. 

Outline the process of inspiration in humans.

Describe the functions of valves in the mammalian heart.

Explain how blood solute concentrations are kept within narrow limits in the human body.

a. diaphragm and external intercostal muscles contract ✔

b. diaphragm moves down/becomes flatter
OR
external intercostals raise the ribcage/move the ribcage up/out ✔

c. muscles/diaphragm/intercostals increase volume of thorax/expand the thorax
OR
muscles/diaphragm/intercostals decrease pressure in the thorax ✔

d. as volume «of thorax/lungs» increases the pressure decreases ✔

e. air enters «lungs» due to decreased pressure/higher pressure outside body ✔

f. air flows to lungs through trachea and bronchi/bronchioles ✔

Accept thoracic cavity or chest cavity in place of thorax in any part of the answer.

Do not allow “oxygen” instead of air in mpe or mpf.

a. prevents backflow/ensures one-way flow/controls direction of flow ✔

b. open valves allow blood to flow through
OR
opening and closing of valves controls timing of blood flow «during cardiac cycle» ✔

c. closed «semilunar» valves allow ventricles/chambers to fill with blood
OR
closed «semilunar» valves allow pressure in ventricles to rise «rapidly» ✔

d. valves open when pressure is higher upstream/OWTTE/converse for closed valves ✔

e. AV/bicuspid/tricuspid/mitral valves prevent backflow from ventricle to atrium
OR
AV/bicuspid/tricuspid/mitral valves open when pressure in atrium is higher «than in the ventricle»/when atrium is pumping/contracting ✔

f. semilunar/aortic/pulmonary valves prevent backflow from artery to ventricle
OR
semilunar/aortic/pulmonary valves open when pressure in ventricle is higher «than in the artery»/when ventricle is pumping/contracting ✔

Allow mpa, mpb, mpc or mpd if the point is made through the example of one specific valve.

a. solute concentration of blood monitored by the brain/hypothalamus ✔

b. pituitary gland secretes ADH ✔

c. ADH secreted when solute concentration/osmolarity is too high/a person is dehydrated/OWTTE ✔

d. collecting duct more permeable to water ✔

e. «more» aquaporins/opens aquaporins «in the plasma membrane of collecting duct cells» ✔

f. «more» water reabsorbed «into the medulla» ✔

g. medulla is hypertonic/hyperosmotic «so water can be reabsorbed from filtrate» ✔

h. small volume of urine/concentrated urine produced «with ADH» ✔

i. no/little/less ADH secreted if «blood» solute concentration is too low ✔

j. collecting duct less permeable to water/less water reabsorbed/large volume of urine produced/dilute urine produced «with low/no ADH» ✔

k. insulin causes blood glucose «concentration» to be reduced ✔

l. glucose stored as glycogen in the liver ✔

m. glucagon causes blood glucose «concentration» to be increased ✔

n. negative feedback ✔

Accept hypertonic for solute concentration too high and hypotonic for too low.

The mechanisms used to cause ventilation of the lungs are poorly understood by many candidates, despite them being used throughout our lives. Cause and effect are often confused – air rushes in to cause a volume increase in the lungs for example.

Most candidates got the general idea of valves in the heart preventing backflow, but many did not make any other valid points. A clear statement of what is achieved by valves opening and by valves closing was expected and then comments on the specific roles of the atrio-ventricular and the semi-lunar valves.

This was another question where many candidates struggled. Sub-topic 11.3 of the programme is titled ‘The kidney and osmoregulation’ but perhaps the connection was not made. A significant minority wrote only about regulation of blood sugar levels Others gave an exhaustive account of kidney function, but mostly did not have time left to write enough about osmoregulation. A small proportion of candidates gave a convincing account of the negative feedback mechanisms that keep blood solute concentrations within narrow limits.

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.

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

By referring to both graphs, evaluate the hypothesis that vinblastine targets cells in mitosis and prevents them from completing the process.

Some anticancer drugs inhibit mitosis by blocking the formation of the spindle. Suggest one other way in which vinblastine could block mitosis.

Discuss one advantage and one disadvantage of using plant tissue to investigate drugs intended to treat cancer in humans.

Advantage:

Disadvantage:

Evaluation of evidence in graph on left

a. increase in (percentage of) cells in mitosis (as vinblastine concentration rises) ✔

b. supports hypothesis that cells get stuck in/cannot complete mitosis ✔

Evaluation of evidence in graph on right

c. drop in anaphase-metaphase ratio due to fewer cells in anaphase/more cells in metaphase ✔

d. cells not progressing from metaphase to anaphase/get stuck in metaphase ✔

Do not allow mpa if the candidate is arguing that the hypothesis is not supported.

a. causes microtubules/spindle fibres to break up / tubulin molecules to depolymerize ✔

b. prevents contraction of spindle microtubules/fibres ✔

c. disrupts/damages kinetochores/centromeres/microtubule motors/centrioles/centrosomes ✔

d. prevents separation/pulling apart of (sister) chromatids/chromosomes/centromeres ✔

e. prevents microtubules/spindle binding to chromatids/chromosomes/centromeres/DNA ✔

Mark the first suggestion only in the answer.

Do not allow answers about DNA replication or other processes that precede mitosis.

Advantage:

avoids risks for humans/harm to humans / more ethical (than with human patients/volunteers) ✔

Disadvantage:

differences between plant and human cells so humans may not respond in same way
OR
plants have cell wall/no centrioles/other relevant difference between plant and human cells ✔

Not enough for mpa to say ‘not using humans’.

For mpb there must be either a statement that differences between cells may cause a different response, or a specific example of a cell difference.

This was another challenging data-analysis task with marks spread across the whole range, but full marks were rarely awarded. Many candidates thought that a rise in mitotic index had to mean that more cells were entering mitosis and they failed to deduce from the drop to zero of the anaphase-metaphase ratio that cells were getting stuck in metaphase. As in (d), it was necessary both to understand how cells progress through the cell cycle and be able to make deductions from trends in quantitative data.

A wide range of possible ways in which mitosis could be blocked was accepted and about a third of candidates suggested one of these. Methods of blocking DNA replication or cytokinesis were not accepted as the question specifically referred to mitosis.

Most candidates were able to suggest an advantage of using plant tissue and any reasonable answer was accepted. Fewer gave a disadvantage that was credited. Vague answers such as plants are different from humans were not rewarded with a mark.

Identify, with a reason, the stage shown at X.

Calculate the length of the entire cell labelled Y, showing your working.

State the role of cyclins in the cell cycle.

Distinguish between the structure of chromosomes in prokaryotes and eukaryotes.

Explain Cairns’s technique to measure the length of the DNA molecule.

1. anaphase;
2. the (replicated) chromosomes/chromatids are separating/moving to opposite poles of the cell;

OWTTE

50 μm = 27/28/29 mm, Y = 8/9/10 mm
OR
50 x 9 /27
OR
16.7 μm (accept answers in the range of 14.8 μm to 17.2 μm)

Award [1] for correct ratios not precise measurements in the work or [1] for correct answer with correct unit.

1. (group of regulatory proteins that) control/regulate the cell cycle;
2. activate cyclin-dependent kinases (which control cell cycle processes);

1. prokaryotes (usually) have one chromosome while eukaryotes have numerous chromosomes;
2. prokaryotes have a circular chromosome while eukaryotes have linear ones;
3. eukaryotes’ chromosomes are associated with histones/proteins but prokaryotes/Eubacteria have naked DNA vs eukaryote DNA associated to proteins/histones;

Accept only differences.

Differentiating terms expected;

1. Cairns grew prokaryotes/E. coli in radioactive thymidine/thymine/thymine containing tritium;
2. contents of cell put on photographic film/surface (for several weeks) / used autoradiography and electron microscopes;
3. measured the length of the DNA molecule and photographed it / produced image of DNA;
4. could show the new strands were all labelled with thymidine/thymine;

This question was generally well done; a few identified the incorrect phase of mitosis, but the observations about cellular events being observed was often correct.

Most answers were correct, but some calculations were difficult to follow; some errors in unit conversions led to incorrect decimal places.

This question led to a large number of correct answers. Some answers focused on the difference between prokaryotes and eukaryotes instead of their chromosomes.

Asking students to outline Cairn’s technique led to a number of blank answers. That there was a role for autoradiography was better known than how this was used to obtain the length estimate.

Identify the stage of mitosis labelled X in the image, giving a reason.

[Source: Copyright 2002, The Trustees of Indiana University]

Outline what is indicated by the mitotic index of tissue taken from a tumour.

DNA has regions that do not code for proteins. State two functions of these regions.

1.

2.

Outline the difference in methylation pattern between tumorous and normal tissue samples.

Suggest a way methylation may affect tumour cell genes.

telophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔

OWTTE

a. mitotic index is an indication of the ratio/percentage of cells undergoing mitosis/cell division ✔

b. cancer cells «generally» divide much more than normal «somatic» cells ✔

c. a high/elevated mitotic index in tumours / possible diagnosis of cancer /measure of how aggressive/fast growing the tumour is ✔

a. promoters / operators / regulation of gene expression/transcription ✔

b. telomeres/give protection to the end of chromosomes «during cell division» ✔

c. genes for tRNA/rRNA production ✔

d. other valid function for non-coding sequence ✔

Do not accept stop codon, accept centromeres (connecting sister chromatids).

a. «overall» much more methylation in the colon tumour samples than normal ✔

b. tumour and normal samples the markers 258 and 269 similar degree of methylation/fewer differences ✔

c. degree of methylation on certain markers may correlate with the presence of cancer / correct example of a marker only methylated in tumour cells eg marker 32 ✔

a. «DNA» methylation may inhibit transcription of genes that would prevent cancer/tumor formation ✔

b. «DNA» methylation may increase mitosis/cell division leading to tumor formation ✔

Do not accept discussion of histone methylation.

Both anaphase and telophase were accepted as answers, but students had difficulty providing a reason that was an appropriate justification; for example, students might say telophase and then support this with the statement because chromatids are seen moving to opposite poles. Students sometimes referred to chromatids as homologous chromosomes. As this is an image of plant cells, students would be mistaken by referring to a cleavage furrow. The mechanism of cytokinesis involves the formation of a cell plate.

Many students could accurately define the term mitotic index. An area of misunderstanding was to categorize the mitotic index as a rate rather than a ratio.

This question was well answered. The occasional answer made reference to processes associated with translation.

Many students detected that greater degrees of methylation in tumour samples. Fewer could accurately summarize the specific differences between methylation in the two samples.

Students lacked the understanding of epigenetics necessary to make reasonable suggestions.

Outline four different processes, with examples, that allow substances to pass through the plasma membrane.

Humans need to balance water and solute concentrations and also excrete nitrogenous wastes. Explain how the different parts of the kidney carry out these processes.

Describe adaptations in mammals living in desert ecosystems to maintain osmolarity in their bodies.

1. simple/passive diffusion down a concentration gradient / from high concentration to low concentration (without the use of channels/proteins); (e.g., CO₂ / O₂ / H₂O / steroid hormones)
2. osmosis is the diffusion of water from an area of high water potential / low solute concentration to low water potential / high solute concentration;
3. facilitated diffusion is passive transport/diffusion through a protein channel; (e.g., glucose)
4. active transport requires energy/ATP to move the molecules through a protein channel (e.g., Na-K pump / sodium potassium pump) against a concentration gradient/from low solute concentration to high concentration;
5. endocytosis is the infolding of membranes to form a vesicle and take in a large molecule; (e.g., macrophages engulfing pathogens)
6. exocytosis is the fusion of vesicles with membranes to release a large molecule; (e.g. neurotransmitters)

1. humans are osmoregulators/maintain the internal concentrations of the blood/osmolarity within specific/ limited range / OWTTE;
2. glomerulus / Bowman’s capsule (in the nephron) carry out ultrafiltration;
3. proximal convoluted tubule selectively reabsorbs glucose/solute/salts/amino acids;
4. loop of Henle maintains hypertonic conditions in the medulla/absorbs salts (by active transport);
5. loop of Henle reabsorbs water (by osmosis);
6. (osmoreceptors in the hypothalamus) cause production of ADH if the blood is too concentrated / person is dehydrated / OWTTE;
7. ADH causes more uptake of water/increases permeability in the collecting duct;
8. resulting in a more concentrated urine / lower volume of urine;
9. excess amino acids are broken down producing nitrogenous waste / ammonia / urea as a result;
10. ammonia is toxic and is converted into non-toxic urea;
11. urea is eliminated in the urine;

Marks can be awarded to clearly annotated diagrams.

1. behavioural adaptations to avoid over-heating / hiding in burrows/out of sun during hot period of day / active at cooler times of the day/nocturnal animals / panting;
2. adaptations for heat exchange such as large ears;
3. may have longer loop of Henle (to reabsorb more water);
4. may produce more ADH (according to osmotic concentrations of the blood) / produce concentrated urine / lower volume of urine;
5. camel humps that store fat that releases (metabolic) water when broken down;
6. reduced sweat;
7. any other valid adaptation; (e.g., light coloured coats)

This question in section B was the second most commonly selected, but in performance, tended to do more poorly than the other questions.

Most candidates could list the processes that allow passage across the plasma membrane, but many answers lacked some of the necessary elements, most commonly leaving out examples (e.g. an example for gradient).

Generally, there were good accounts of the functioning of the nephron, but some elements were missing such as which substances are reabsorbed in the proximal tubule and which are absorbed in the loop of Henle, although permeability was mentioned. Discussion about the role of ADH was well done. The discussion of the management of nitrogenous wastes was least well done in this question.

Long loops of Henle was the most common desert adaptation discussed. There were common misconceptions about camels’ humps being water storage organs rather than the production of metabolic water.

Cells go through a repeating cycle of events in growth regions such as plant root tips and animal embryos. Outline this cell cycle.

Draw a labelled diagram of the formation of a chiasma by crossing over.

Explain the control of gene expression in eukaryotes.

a. mitosis is the division of a nucleus to produce two genetically identical daughter nuclei 

b. consists of four phases: prophase, metaphase, anaphase, telophase 

c. cytokinesis occurs after mitosis 

d. interphase is the metabolically active phase between cell divisions  OWTTE

e. the interphase consists of the S phase, G1 and G2 

f. DNA replicates in the S phase 

g. cell growth
OR
preparation for mitosis
OR
duplication of organelles in G1 and G2

a. «crossing over/chiasmata shown between» homologous chromosomes  

b. centromere drawn and labelled  

c. single strand break «SSB»/DNA cut between homologous chromosomes 

d. non-sister chromatids labelled
OR
sister chromatids labelled 

e. chiasma between homologous chromosomes labelled «shown forming after SSB»

Homologous chromosomes must be labelled and correctly drawn.

It is likely that more than one diagram will need to be included to demonstrate the stages.

a. mRNA conveys genetic information from DNA to the ribosomes «where it guides polypeptide production» 

b. gene expression requires the production of specific mRNA «through transcription» 

c. most genes are turned off/not being transcribed at any one time/regulated
OR
some genes are only expressed at certain times 

d. some genes are only expressed in certain cells/tissues
OR
«cell» differentiation involves changes in gene expression 

e. transcription factors/proteins can increase/decrease transcription 

f. hormones/chemical environment of cell can affect gene expression 

g. example of cell environment
eg: auxin/insulin/cytoplasmic gradient in embryo

h. transcription factors/proteins may prevent or enhance the binding of RNA polymerase 

i. nucleosomes limit access of transcription factors to DNA/regulate gene expression/transcription
OR
activate or silence genes 

j. DNA methylation/acetylation appears to control gene expression «as epigenetic factor»
OR
methylated genes are silenced 

k. «some» DNA methylation patterns are inherited 

l. introns may contain positive or negative gene regulators
OR
gene expression can be regulated by post-transcriptional modification/splicing/mRNA processing

The image is an electron micrograph of the lining of the small intestine.

(i) Label the microvilli using the letter M and a nucleus using the letter N.

(ii) State the function of the goblet cell.

(iii) Deduce, with a reason, whether or not the goblet cell is likely to divide.

Explain how the cell cycle is controlled.

(i)

Award [1] for one microvillus labelled M and one nucleus labelled N.

Both are essential for the mark.

Do not award the mark if any structure is labelled incorrectly.

(ii)

secretion/exocytosis / produce mucous

Candidates are not required to have studied goblet cells, so are just expected to deduce from the vesicles that the function is secretion; allow enzyme secretion but reject answers suggesting secretion of something that is clearly incorrect such as secretion of bile.

(iii)

not likely to divide as specialized/differentiated
OR
not likely to divide (as nucleus) is in interphase/not in mitosis

Do not award a mark for stating that the goblet cell lacks a nucleus.

a. cell cycle is a sequence of stages / cell cycle is G₁, S, G₂ and mitosis

b. (control of the cell cycle) by cyclins/cyclin

c. levels of cyclins rise (and fall)/fluctuate during the cell cycle/surge at different times/have to reach a certain concentration

d. conditions inside as well as outside the cell affect regulation

e. four cyclins/different cyclins to enter different stages of/events in the cell cycle / cyclins regulate the sequence/timing of the cell cycle / cyclins trigger the next stages

The idea of different cyclins acting at different phases must be clear.

f. cyclin-dependent kinases / cyclins bind to kinases and activate them

g. kinases phosphorylate other proteins

h. phosphorylated proteins perform specific functions in the cell cycle 

Identify the dark structure indicated by I.

Identify the protein producing the thick filament in the dark band indicated by II.

Identify the structure indicated by III.

Discuss whether the tissue shown in the micrograph consists of cells or not.

Explain how calcium is involved in muscle contraction.

nucleus ✔

myosin ✔

muscle fibre/muscle cell ✔

Reject myofibril because it would be much narrower – diameter 

a. «muscle fibres are» multinucleate/contain many nuclei «whereas cells are expected to have only one/so muscle fibers are an exception to the cell theory» ✔

b. one cell membrane/sarcolemma enclosing a whole muscle fibre «as expected for cells» ✔

c. very large/much larger/longer/than most cells ✔

d. muscle fibres formed by fusion of cells/are syncytia ✔

a. action potential/nerve impulse causes release of calcium ✔

b. from sarcoplasmic reticulum/specialized endoplasmic reticulum ✔

c. binds to troponin ✔

d. causes tropomyosin to move/be removed «from binding sites» ✔

e. exposes myosin-binding sites on actin/allows myosin «heads» to bind to actin ✔

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.

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.

Autosomal genes are located in chromosomes that are not sex chromosomes. The inheritance of autosomal genes is affected by whether the genes are linked or unlinked. Explain the two types of inheritance, using the example of parents that are heterozygous for two genes A and B.

Outline how sperm are produced from diploid cells in the testis and how this production can be sustained over many decades of adult life.

Testis cells are eukaryotic cells. Identify the structures seen under the electron microscope in testis cells that are not present in prokaryotic cells.

a. unlinked genes are on different chromosomes / vice versa ✔

b. unlinked alleles migrate/segregate/are inherited independently (during meiosis) / vice versa ✔

c. (In unlinked inheritance) there is an equal chance for all 4 options to occur / AB, Ab, aB, ab / vice versa ✔

d. (dihybrid crosses involving) linked genes do not produce Mendelian ratios ✔

e. (excluding recombinants) there is a 1:1 chance of inheriting the different options/AB or ab ✔

f. in linked characteristics alleles might not migrate together if there is crossing over/ recombinants are formed ✔

g. crossing over occurs in prophase I of meiosis ✔

h. when the sister chromatids migrate in meiosis II the characteristics forming gametes are different/Ab, aB ✔

i. formation of recombinants causes changes in ratio/probability of inheritance/genetic variation ✔

j. correct named example of inheritance of linked/unlinked characteristics ✔

k. Punnett/paired diagrams of both unlinked and linked characteristics ✔

l. genes which are linked but are far apart on the chromosome can display independent assortment ✔

Mp a could be awarded from an annotated diagram.

Allow annotated diagram of inheritance / could be shown in a Punnett square.

Allow annotated diagram of linked inheritance for mp f.

For mp K, accept sex linked examples involving two genes.

If the student interprets the question as sex-linked and autosomal inheritance, look for WTTE marks from the scheme.

a. germinal epithelium divide endlessly (by mitosis giving rise to spermatagonia)

b. spermatogonia are diploid/2n ✔

c. spermatogonia divide by mitosis / provide a continuous supply throughout adult life ✔

d. (some) spermatogonia enlarge forming primary spermatocytes ✔

e. primary spermatocytes undergo the first division of meiosis/meiosis I ✔

f. secondary spermatocytes produced are haploid/n ✔

g. secondary spermatocytes undergo the second division of meiosis (to produce spermatids) ✔

h. spermatids develop tails
OR
spermatids differentiate into spermatozoa / spermatids associate with Sertoli cells ✔

Marks can be awarded to an annotated diagram.

Do not accept sperm or spermatozoa as equivalent to spermatagonia or spermatocytes.

a. nucleus/nuclear membrane ✔

b. membrane bound organelles ✔

c. mitochondria ✔

d. rough ER/smooth ER/golgi apparatus ✔

e. lysosomes / centrioles ✔

f. large/80S ribosomes / ribosomes attached to a membrane ✔

g. linear chromosomes / histones ✔

Question 5 was not a popular question choice. Few candidates performed well on this question overall. Many students wrote vaguely about dominant and recessive alleles or confused gene linkage with "sex-linkage" or even multiple alleles and blood groups. 

Although many candidates performed very well, many answers were vague and simply stated that sperm cells were made by meiosis without any further details. Commonly, the stages where meiosis occurs and where mitosis occurs were muddled as well as whether primary or secondary spermatocytes were diploid or haploid. The terms spermatozoa and spermatids were commonly used interchangeably.