IB Biology extended response questions. With markschemes, by theme.

• cohesion: water molecules stick to each other (because of hydrogen bonding);
• cohesion allows water columns (in xylem) to be pulled up plants under tension (without breaking);
• cohesion produces surface tension, allowing small animals (e.g. pond skaters) to live on the water surface;
• adhesion: water sticks to (polar/hydrophilic) surfaces such as cellulose;
• adhesion draws water through/along cell walls / capillary action (e.g. keeping leaf cell walls moist);
• water rises in narrow spaces (against gravity) by capillary action, OWTTE;

• water is a good solvent for polar/ionic substances, so glucose, amino acids, ions and mineral salts dissolve and are carried dissolved in plasma/xylem sap;
• cohesion (hydrogen bonding) keeps the water column in the xylem continuous / unbroken under tension, so water can be pulled up to the leaves (transpiration stream);
• adhesion of water to (hydrophilic) cell walls helps water move through walls and up narrow xylem vessels (capillary action);
• high specific heat capacity means the blood/sap resists rapid temperature change, helping distribute heat and stabilize the organism's temperature;
• limitation: hydrophobic substances (e.g. lipids/O₂) dissolve poorly, so they need carriers (lipoproteins, haemoglobin), a constraint imposed by water's polarity;
• (on balance) water's polarity and hydrogen bonding suit it well as a transport medium, with the main limitation being the transport of non-polar substances, OWTTE;

A balanced answer should include at least one limitation to reach the top band.

• the base sequence stores/encodes (genetic) information;
• (any) sequence and length are possible, giving unlimited capacity/diversity;
• complementary base pairing (A–T, G–C) allows each strand to act as a template;
• (so) DNA can be replicated accurately (and information passed on);
• hydrogen bonds between strands are (individually) weak, so the strands can be separated (for replication/transcription);
• the covalent sugar–phosphate backbone makes the molecule stable, protecting the information;

• complementary bases pair by hydrogen bonding: adenine with thymine (uracil in RNA) and guanine with cytosine;
• hydrogen bonds are individually weak, so the two DNA strands can be separated to expose the bases;
• in replication, each separated strand acts as a template;
• free DNA nucleotides pair with the exposed bases, so each new strand is complementary to its template and two identical molecules are formed;
• in expression, one DNA strand acts as a template for making RNA;
• RNA nucleotides pair with the exposed DNA bases (uracil with adenine), so the base sequence of the gene is copied into RNA, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D1.2 codons of mRNA pair with complementary anticodons of tRNA, so the base sequence determines the amino acid sequence;

Award [3 max] if only replication or only expression is addressed.

• the two DNA strands are antiparallel, one running 5′→3′ and the other 3′→5′;
• DNA polymerase can only extend a strand by adding nucleotides to a free 3′ end, so new DNA is always synthesized 5′→3′;
• eukaryotic DNA is wrapped around cores of eight histone proteins to form nucleosomes, packaging the very long DNA molecule so it fits within the nucleus;
• nucleosome packaging must be (partially) undone locally to allow replication enzymes access to the DNA, and re-formed on the new DNA behind the replication fork;
• (so) both directionality and nucleosome structure constrain how replication machinery must operate, despite the underlying accuracy provided by complementary base pairing, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D1.1 (as a result) one new strand (the leading strand) is made continuously, while the other (the lagging strand) is made discontinuously as Okazaki fragments, later joined by DNA ligase;

• (under present conditions) all observed cells arise by division of pre-existing cells;
• Pasteur's experiments falsified spontaneous generation (broth in swan-necked flasks stayed sterile);
• however, the first cells must (ultimately) have arisen from non-living matter;
• conditions on the early Earth differed from today (little/no oxygen, abundant energy sources), OWTTE;
• a stepwise origin is proposed: organic molecules → polymers → self-replication → protocells;
• (so) the principle holds now but cannot have held at the origin of life;

• for: RNA can store genetic information in its base sequence;
• for: RNA can act as a catalyst / ribozymes exist;
• for: the catalytic core of the ribosome is RNA / RNA still catalyses peptide-bond formation;
• against: RNA is (chemically) unstable / hard to synthesize its nucleotides without enzymes (pre-biotically);
• against: no self-replicating RNA has been demonstrated in nature / the hypothesis remains untested directly, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• A2.3, D1.1, D1.2 for: (some) viruses and cellular processes still use RNA centrally (e.g. primers, mRNA), consistent with an earlier RNA role;

• strength: it showed that organic building blocks (amino acids and other carbon compounds) can form from simple inorganic gases with only an energy source, without any living organisms;
• strength: it was a controlled, repeatable laboratory test of a specific hypothesis (a strongly reducing atmosphere), and the results have been reproduced;
• limitation: it only produced monomers, not polymers, self-replicating molecules or cells, so it does not demonstrate the origin of life itself;
• limitation: the assumed atmosphere (methane/ammonia-rich) may not match the real early atmosphere (possibly CO₂/N₂-dominated), which would give much lower yields;
• limitation: it cannot be directly tested against the actual past (the early Earth cannot be re-run), so it demonstrates plausibility rather than what actually happened;
• judgement: it provides strong evidence that the first step (abiotic synthesis of organics) was possible, but weak/indirect evidence for the later steps, so it is suggestive rather than conclusive, OWTTE;

A judgement weighing strengths against limitations is required for the top band.

• (cell theory:) organisms are made of cells, the smallest units of life, which come from pre-existing cells;
• all cells share a plasma membrane, cytoplasm, ribosomes and DNA;
• (however) cells are highly diverse in size, shape and internal structure;
• atypical examples challenge the general pattern: red blood cells (no nucleus) / skeletal muscle fibres (very large, multinucleate) / aseptate fungal hyphae (continuous cytoplasm);
• (overall) the theory holds as a generalization, with recognized exceptions, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• A2.1 viruses are non-cellular, but they are not considered living / cannot reproduce independently;

• evidence for: mitochondria have their own circular DNA (like a prokaryote) and 70S ribosomes (as in bacteria, not the 80S of the eukaryotic cytoplasm);
• evidence for: mitochondria are surrounded by a double membrane, the inner one resembling a prokaryotic membrane, consistent with engulfment by a vesicle;
• evidence for: mitochondria replicate by a fission-like process independently of the cell and cannot be made from scratch by the cell;
• evidence for: mitochondrial gene/rRNA sequences are more similar to certain bacteria than to the host nucleus;
• limitation: mitochondria can no longer live independently (many genes have moved to the nucleus), so the case is inferred, not directly observed / the event cannot be repeated;
• judgement: the multiple independent lines of molecular and structural evidence make endosymbiosis a strongly supported theory, though indirect, OWTTE;

Requires a judgement supported by weighing evidence; award the top band only with an explicit evaluation.

• shared features: small size, nucleic acid genome, protein capsid, no cytoplasm/metabolism, reliance on a host cell;
• these shared features reflect the shared lifestyle of obligate intracellular parasitism (constraints of infecting cells);
• diversity: DNA or RNA, single- or double-stranded genomes, enveloped or naked, varied shapes and hosts;
• diversity suggests viruses (may) have several separate origins / do not form a single clade;
• hypotheses of origin: progressive (escaped genetic elements) / regressive (reduced cells) / virus-first, OWTTE;
• (so) similar selection pressures could produce convergent shared features in unrelated lineages;

• their genomes are RNA, and the enzymes copying RNA (RNA polymerase/reverse transcriptase) lack proofreading;
• (so) mutation rates are very high, generating many variants;
• enormous numbers of virions are produced per infection / generation times are very short, so selection acts quickly;
• influenza can also exchange genome segments when two strains infect one cell (reassortment/antigenic shift);
• strong selection from host immunity favours new surface-antigen variants (antigenic drift);
• consequence: vaccines must be reformulated (annually) / drug resistance evolves / no effective HIV vaccine yet, OWTTE;

• for: viruses have genetic material (DNA or RNA) and can evolve/adapt by mutation and natural selection;
• for: they reproduce (make many copies) and show heredity, so pass on characteristics;
• against: they have no cytoplasm, ribosomes or metabolism of their own;
• against: they cannot reproduce independently. They are obligate intracellular parasites that require a host cell's machinery;
• against: outside a host they are inert particles (can even be crystallized), carrying out no functions of life;
• judgement: because they lack independent metabolism and reproduction, most biologists place them outside the usual definition of life / 'at the edge of life', though this depends on how life is defined, OWTTE;

A reasoned judgement referring to the criteria for life is required for the top band.

• members of one species share (most of) their characteristics but are not identical / vary genetically;
• between species, differences are (usually) larger and include (base) sequences of DNA/genomes;
• the more distantly related two species are, the more their (base/amino-acid) sequences differ;
• every individual (except identical twins/clones) has a unique genome, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D1.3, D2.1 sources of variation within a species: mutation / sexual reproduction (meiosis and fertilization);

• by the biological species concept, a species is a group of organisms that can interbreed to produce fertile offspring;
• the hybrids are fertile, so by this concept polar and brown bears could be regarded as one species;
• but hybrids are rare in the wild / the two usually live and breed apart, so few genes pass between them;
• they differ in many traits (fur colour, skull shape, diet), so on appearance / by the morphological concept they are separate species;
• competing species concepts give different answers here, so the classification depends on which definition is used;
• the two may be part-way through speciation / have diverged but not completely, so where one species ends is an arbitrary decision;
• a conclusion supported by the evidence, e.g. they remain separate species because they differ in many traits and rarely interbreed, OWTTE;

Accept either conclusion if it is supported by evidence from the stem. Award [3 max] if only one side is argued.

• cladograms are built from (shared) base/amino-acid sequence differences;
• the pattern of branching shows which species share the most recent common ancestors;
• if a traditional group is found not to be a clade (paraphyletic/polyphyletic), it is reclassified;
• example: plants placed in the figwort family (Scrophulariaceae) were moved to other families when sequences showed closer kinship elsewhere;
• (or) "reptiles" are not a clade unless birds are included, since crocodiles are closer to birds than to lizards;
• (reclassification restores) groups whose members all descend from one ancestor, OWTTE;

• before this, all prokaryotes were grouped together (in one kingdom) on the basis of their similar appearance / lack of a nucleus;
• rRNA (and its gene) is present in all cells with the same function, so it can be compared across all living organisms;
• rRNA sequences change (very) slowly / are highly conserved, so differences can still be measured between very distantly related organisms;
• sequences give many objective, quantifiable characters (base positions) instead of judgements about appearance;
• archaea were found to differ from bacteria as much as from eukaryotes, so the prokaryotes were split into two domains, Bacteria and Archaea;
• a new taxonomic level, the domain, was added above kingdom: Bacteria, Archaea and Eukarya;
• archaea were found to share a more recent common ancestor with eukaryotes than with bacteria;
• the shared prokaryotic features are ancestral characters, so they are not evidence of close relationship;
• this was a revolutionary change / paradigm shift, but like any classification it remains a hypothesis that other genes could modify, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• A2.2.12 mitochondria and chloroplasts descend from engulfed bacteria, so some genes in eukaryotic cells show bacterial ancestry;

Discuss: credit evidence, its interpretation and some evaluation (e.g. the point on the paradigm shift and the provisional nature of the classification).

• an ancestral population was split by the Congo River into two populations on opposite sides;
• neither ape can swim, so the river acts as a geographical barrier and gene flow between the two populations stopped;
• conditions differed on the two sides of the river (e.g. gorillas compete for ground vegetation north of the river but are absent south of it), so the populations faced different selection pressures;
• differential selection favoured different heritable characteristics in each population, e.g. less aggressive, female-led groups in bonobos but more aggressive, male-dominated groups in chimpanzees;
• (so) genetic differences accumulated and the populations diverged over many generations;
• eventually the populations became different enough to be separate species, OWTTE;

Accept other correct differences between the two species or their habitats for mp 3 and 4. Award any 4.

• adaptive radiation is the (rapid) diversification of a single lineage/ancestral species into many species, each occupying a different (ecological) niche;
• it occurs where there is ecological opportunity, e.g. colonization of new islands/lakes or niches left vacant after a mass extinction;
• with few competitors, (separate) populations can exploit a range of different food sources/habitats;
• different niches impose different selection pressures / diversifying selection;
• (so) different adaptations are favoured in different populations, e.g. beak size/shape matching diet;
• populations become reproductively isolated (e.g. on separate islands / by using different resources or breeding sites);
• isolation plus differential selection leads to speciation, repeated many times (in rapid succession);
• the resulting species are closely related and show homologous structures adapted to different functions / divergent evolution;
• e.g. one colonizing bird species radiating into seed-eating, insect-eating and nectar-feeding species on an archipelago, OWTTE;

• strength: it can protect a species from immediate threats (hunting, habitat loss, disease) and boost numbers under controlled, intensive care;
• strength: it preserves genetic material (e.g. seed/gene banks) for the future and can supply individuals for reintroduction;
• limitation: captive populations are small, so genetic diversity is easily lost through inbreeding/genetic drift;
• limitation: animals may lose natural behaviours / become adapted to captivity, making reintroduction difficult, and the natural habitat may no longer exist;
• limitation: it is expensive and can cover only a few species, whereas in situ methods protect whole ecosystems;
• judgement: ex situ breeding is a valuable last resort and complement to in situ conservation, but not a substitute for protecting habitats, OWTTE;

A judgement weighing strengths against limitations is required for the top band.

• citizen scientists are members of the public (volunteers, not professional scientists) who record organisms and submit the data;
• evidence of a crisis needs surveys repeated over many years to show changes in species richness, evenness or population size, and volunteers make long-term repeated surveys affordable;
• large numbers of observers can cover many more sites, a wider area and a wider range of habitats than professional scientists alone;
• the large number of records gives large samples, so trends can be detected with more confidence / rare species are more likely to be recorded;
• concern: volunteers may misidentify species, especially similar-looking or small ones, so the records contain errors;
• concern: effort varies between observers and years (time spent, skill, weather), so an apparent decline may reflect less effort rather than fewer organisms;
• concern: records are biased towards places near roads and towns, towards easily recognized groups such as birds and butterflies, and towards times when people are free, so some habitats and groups are under-sampled;
• concern: data are only verifiable if the methods are recorded and the data are checked, e.g. by publication in a peer-reviewed source;
• these problems are reduced by standardized methods (fixed routes, times and counting rules), training, photographs checked by experts and statistical correction for effort;
• citizen-science data analysed by scientists and published in peer-reviewed journals contribute to international assessments such as IPBES reports;
• judgement: citizen science greatly extends the scale of monitoring and, with standardized methods and expert checking, gives reliable evidence, but it works best alongside professional surveys, OWTTE;

• both are polysaccharides / polymers of glucose (joined by glycosidic bonds by condensation);
• starch is made of α-glucose, whereas cellulose is made of β-glucose;
• starch chains are coiled/branched (amylose/amylopectin), whereas cellulose chains are straight (with alternating glucose orientation);
• starch is an energy store (in seeds/tubers/chloroplasts), whereas cellulose is structural (cell walls);
• starch is (relatively) easy to hydrolyse for glucose release, whereas cellulose fibres/microfibrils resist digestion and give tensile strength;

Credit only explicitly comparative points for differences; award converse.

• triglycerides release about twice as much energy per gram as carbohydrates when oxidised, so a large energy store adds relatively little mass;
• they are non-polar/hydrophobic, so they are stored as droplets without associated water, whereas glycogen is stored with water, adding further to the advantage per gram of tissue;
• they are insoluble in water, so even large stores have no osmotic effect / do not change the solute concentration of adipose cells;
• they are chemically stable, so the store can be kept for long periods (months) until needed;
• triglycerides/adipose tissue conduct heat poorly, so a layer beneath the skin slows heat loss from the warm body to colder surroundings;
• insulation matters most for endotherms in cold habitats, which have thicker layers (e.g. blubber of marine mammals in polar seas), OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• C1.2 the high energy yield per gram is due to fatty-acid chains having less oxygen and more oxidizable hydrogen and carbon than carbohydrates;

The two roles are long-term energy storage and thermal insulation. Justify requires each property to be linked to the role it suits. Award at most 3 marks for either role alone.

• strength: there is a positive correlation between saturated-fat intake and CHD across the populations studied;
• limitation: a correlation does not by itself establish causation;
• limitation: this is population-level (ecological) observational data with many confounding variables that differ between countries (total calories, sugar intake, exercise, smoking, wealth, healthcare);
• limitation: no controlled manipulation of the variable, so other factors could explain the pattern;
• judgement: the evidence is suggestive but insufficient on its own; controlled intervention or large cohort studies would be needed to support a causal claim, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• B3.2 strength: there is a plausible mechanism, saturated fat can raise blood (LDL) cholesterol, contributing to atherosclerosis;

Must weigh strengths AND limitations and reach a judgement for full marks; correlation-vs-causation point is essential.

• named fibrous example, e.g. collagen: three polypeptides wound into a (rope-like) triple helix;
• elongated, insoluble strands (cross-linked into fibres) give high tensile strength, suits its structural role in skin/tendons/ligaments;
• named globular example, e.g. myoglobin (or haemoglobin/insulin/lysozyme): compact folded (spherical) shape, hydrophilic surface, soluble;
• a specific binding site/pocket (e.g. haem holding oxygen / active site) allows its chemical function;
• (in general) fibrous proteins serve structure, whereas globular proteins serve chemical/functional roles, OWTTE;
• any correct additional structural detail linked to function credited;

Two named proteins required for full marks; accept other valid examples.

• primary structure (the sequence and position of each amino acid) determines where each R group lies and so how the chain folds / its three-dimensional conformation;
• in HbS only one of 146 amino acids is changed, so the primary structure differs at a single position;
• a charged, hydrophilic R group on the surface is replaced by a non-polar/hydrophobic one, creating a hydrophobic patch on the surface of the molecule;
• hydrophobic patches on different haemoglobin molecules can cluster together (away from water), so HbS molecules stick to each other (when deoxygenated) and the protein becomes less soluble / forms fibres, OWTTE;
• in HbS the folded shape of each molecule is (almost) unchanged, yet its behaviour changes, because the change is on the surface where molecules meet, OWTTE;
• a substitution in the hydrophobic core or in a binding site (e.g. the haem pocket) can disrupt folding or binding, so function is lost;
• but a substitution by an amino acid with a similar R group (e.g. one non-polar for another) may leave the conformation (almost) unchanged;
• a substitution at a position that is not critical to folding or binding may have little or no effect on function;
• (so) the effect depends on where the change is and how different the new R group is, not on the number of amino acids changed, OWTTE;

Discuss: at least one point showing a large effect and one showing little or no effect are needed for full marks. Points about the disease or about natural selection are not asked for here and earn no marks.

• crystallins are globular proteins: each polypeptide is folded into a compact, rounded (tertiary) structure;
• non-polar/hydrophobic R groups are clustered in the core of each molecule, away from water;
• polar/charged/hydrophilic R groups on the surface form hydrogen bonds / attractions with water, so the molecules stay dissolved even at a very high concentration;
• neighbouring molecules with like charges on their surfaces repel each other, so they stay spread out rather than sticking together, OWTTE;
• dissolved, evenly spread molecules do not form particles large enough to scatter light, so light passes through the fibre cells / the lens is transparent, OWTTE;
• a mutation changes the amino acid sequence (primary structure), and so can change how the chain folds / its conformation;
• replacing a polar or charged surface R group with a non-polar one creates a hydrophobic patch on the surface, so neighbouring molecules stick together;
• a change to an amino acid in the core, or one that forms a bond holding the fold, can stop the chain folding correctly / make it unfold more easily;
• over a lifetime, damage breaks bonds holding the tertiary structure, so molecules gradually unfold / are denatured;
• unfolding exposes hydrophobic R groups that were buried in the core, which cluster with those of other molecules (hydrophobic interactions), so the crystallins aggregate / come out of solution;
• denaturation is (usually) irreversible / aggregated chains cannot refold;
• the fibre cells have no nucleus or ribosomes, so damaged crystallins cannot be replaced and the damage accumulates with age / cataracts are commoner in older people;
• the aggregates are large enough to scatter light, so the lens becomes cloudy and less light passes through it / vision is blurred, OWTTE;

The answer must connect the molecule (R groups and folding) to the fibre cells or lens and to vision: award [5 max] if no point links a change in the molecules to the cloudiness of the lens. Do not accept 'the proteins die' or 'the peptide bonds break'.

• membrane is a phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward;
• phospholipids (and some proteins) move laterally, making the membrane fluid;
• proteins are scattered through the bilayer like a mosaic;
• integral proteins span/embed in the bilayer, whereas peripheral proteins sit on a surface;
• glycoproteins/glycolipids occur on the outer surface / cholesterol among the phospholipids (in animal cells);

• finding 1: the particles are (integral) proteins embedded in / spanning the bilayer, not only lying on its surfaces;
• (so) proteins are scattered through the bilayer as a mosaic rather than forming continuous layers;
• finding 2: the proteins moved sideways/laterally within the membrane, so the membrane is fluid;
• fixed/rigid continuous layers of protein (as in the earlier model) could not allow the labelled proteins to mix, OWTTE;
• comment: models are provisional and are revised or replaced when new evidence conflicts with them (falsification);
• comment: new techniques (electron microscopy, fluorescent labelling) produced the evidence that drove the change, OWTTE;

NOS: at least one comment point is required for full marks. Both findings are given in the stem; do not expect recall of the techniques.

• the nuclear envelope separates transcription (nucleus) from translation (cytoplasm);
• (so) mRNA can be processed/spliced before translation / translation of unfinished mRNA is prevented;
• the envelope also protects DNA (from cytoplasmic processes/damage);
• mitochondria concentrate the enzymes and substrates of aerobic respiration together;
• cristae give a large surface area for electron transport chains;
• the inner membrane allows a proton gradient to be maintained (in a small intermembrane space) for ATP synthesis, OWTTE;

• benefit: the enzymes and substrates of the Krebs cycle are concentrated together in the small volume of the matrix, so the reactions are faster;
• benefit: the inner membrane is folded into cristae, giving a large area for electron transport chains and ATP synthase;
• benefit: the intermembrane space is small, so a proton gradient builds up quickly across the inner membrane;
• benefit: these reactions are kept separate from incompatible processes in the cytoplasm, OWTTE;
• cost: substrates and ADP must be moved into the mitochondrion and ATP moved out across its membranes, which needs transport proteins / energy;
• cost: the cell must make and maintain two extra membranes for every mitochondrion, OWTTE;
• judgement: the much faster / greater production of ATP outweighs the costs of transport and of making the membranes, OWTTE;

A balanced answer needs at least one cost and a judgement for [5]; benefits alone score a maximum of [4].

• both carry out transcription and translation and both use membranes at the cell surface;
• in eukaryotes a nuclear envelope separates transcription (nucleus) from translation (cytoplasm), whereas in prokaryotes both occur together in the cytoplasm;
• (so) in eukaryotes the mRNA can be processed/modified (e.g. splicing) before translation, whereas in prokaryotes translation can begin before transcription finishes;
• eukaryotes have membrane-bound organelles that compartmentalize metabolism (e.g. respiration in mitochondria, digestion in lysosomes), whereas prokaryotes lack such organelles;
• (so) eukaryotic compartments can maintain different internal conditions (e.g. pH) and concentrate enzymes with substrates, whereas the prokaryote relies on the whole cytoplasm;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D2.2 both, however, can regulate which genes are expressed, OWTTE;

Points must be explicitly comparative; award converse statements.

• all body cells (of an organism) carry the same genome/genes;
• only some genes are expressed in any given cell / gene expression is differential;
• which genes are expressed is controlled by (chemical) signals / gradients of signalling molecules in the embryo;
• a cell's position (in the gradient) determines its developmental pathway;
• expressed genes produce the proteins that give the cell its specialized structure and function, OWTTE;

• cell structure reflects function because differentiation switches on the genes appropriate to each role;
• red blood cells: small and biconcave for a high surface area-to-volume ratio / no nucleus, maximizing haemoglobin (for oxygen transport);
• intestinal epithelial cells: microvilli increase absorptive surface area, with many mitochondria powering active transport;
• sperm: flagellum for movement / many mitochondria / acrosome for penetrating the egg;
• egg: large cytoplasm with food reserves for the early embryo;
• (any structural feature must be linked to its function; accept other valid examples, e.g. neurons with long axons, pneumocytes), OWTTE;

At least two named cell types with linked structure-function points.

• a skin cell still contains the whole genome / all the genes of the zygote but expresses only skin-cell genes, so the treatment must change which genes are expressed, not the genes present;
• to replace embryonic stem cells, iPSCs must regain both properties of stem cells: the capacity to divide endlessly and to differentiate along different pathways;
• embryonic stem cells (from the inner cell mass) are pluripotent / can form any body cell type, so iPSCs are a substitute only if they are equally pluripotent;
• adult stem cells such as those in bone marrow are only multipotent, so fully reprogrammed iPSCs would form a wider range of tissues than adult stem cells;
• advantage: iPSCs are made from an adult's cells, so no early-stage embryo is needed / the cells carry the donor's own genome;
• limitation: if reprogramming is incomplete, some skin-cell genes may stay switched on or others stay off, so the cells may differentiate along fewer pathways;
• judgement: iPSCs could replace embryonic stem cells only if they are shown to be fully pluripotent, OWTTE;

Balanced: at least one point for and one point against for full marks.

• both: surface area increases with the square of the side length and volume with the cube, so the surface area-to-volume ratio falls as size increases in the same way / the scale factors are the same;
• both: substances enter across the surface and must diffuse inwards, so the time to reach the centre increases with size;
• contrast: a cell is metabolically active, using materials and producing wastes throughout its volume, whereas an agar cube has no metabolism / no need for exchange that depends on its volume, OWTTE;
• contrast: cells are rarely cubes; many are flattened, elongated or folded, giving a higher ratio than a cube of the same volume;

Beyond this subtopic. Award if given; the marks can be earned without it.

• B2.1 contrast: a cell is bounded by a partially/selectively permeable plasma membrane that controls entry, whereas the agar surface is freely permeable;

At least one similarity and one difference for full marks. NOS: models are simplified versions of complex systems.

• for: the fibre is surrounded by a single continuous plasma membrane (sarcolemma);
• for: it contains cytoplasm (sarcoplasm) and organelles and functions as one unit, like a cell;
• against: it contains many nuclei, whereas a typical cell has a single nucleus;
• against: it is far longer/larger than typical cells / it forms by fusion of many cells, so it can be regarded as a multinucleate structure (syncytium) rather than one cell;
• judgement: striated muscle fibres challenge the standard idea of a cell / whether they count as cells depends on the definition used, OWTTE;

For full marks answers should include points on both sides and a judgement.

• stomata (mostly on the lower epidermis) allow CO₂ in and O₂ out;
• guard cells open/close stomata, controlling exchange (and water loss);
• spongy mesophyll has (interconnected) air spaces, giving a large internal surface area;
• mesophyll cell walls are moist, so gases dissolve before diffusing in;
• thin/flat leaf shape keeps diffusion distances short;
• waxy cuticle prevents (uncontrolled) water loss elsewhere, OWTTE;

• oxygen and carbon dioxide cross the alveolar wall by (passive) diffusion, whose rate depends on the steepness of the concentration gradient;
• ventilation replaces the alveolar air with fresh air, keeping the O₂ concentration in the alveoli high and the CO₂ concentration low;
• the branched network of bronchioles carries the fresh air to every one of the (millions of) alveoli;
• a dense network of capillaries surrounds each alveolus, so every alveolus is supplied with blood;
• continuous blood flow carries oxygenated blood away and brings deoxygenated blood, keeping the O₂ concentration in the blood low;
• blood arriving from the respiring tissues has a high CO₂ concentration, so CO₂ keeps diffusing into the alveoli, where ventilation removes it, OWTTE;

• oxygen reaches the tissues mainly by diffusion along the air-filled tracheae/tracheoles;
• the rate of diffusion depends on the surface area available and (inversely) on the distance, and falls off over longer distances (Fick's law);
• as body size increases, the distance from spiracles to the innermost tissues becomes longer, so diffusion becomes too slow to supply the core;
• volume (and so oxygen demand) increases faster than the tracheal surface area, i.e. the SA:V for exchange falls;
• there is no circulatory oxygen carrier (as in vertebrates) to speed transport over long distances;
• (so) very large insects could not supply their deepest tissues, setting an upper size limit; larger insects existed when atmospheric oxygen was higher, OWTTE;

Balanced discussion linking diffusion limits to size; an explicit SA:V link is expected.

• water evaporates from (the walls of) mesophyll cells and is lost through stomata / transpiration;
• (loss of water) generates tension / low (negative) pressure in the leaf (cell walls);
• water is drawn out of the xylem and through the leaf cell walls by capillary action;
• the pull is transmitted down the xylem / transpiration pull;
• cohesion between water molecules (due to hydrogen bonding) keeps the column continuous;
• adhesion of water to (cellulose/lignified) xylem walls helps prevent the column breaking;
• xylem vessels lack cell contents and have incomplete or absent end walls, so flow is unimpeded;
• lignified walls resist collapse under tension;
• pits allow water to pass between vessels / out of the xylem into the leaf tissue;
• root pressure can contribute (a small push), OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• B2.1 active transport of mineral ions into root cells raises their solute concentration;
• D2.3 (so) water enters root (hair) cells from the soil by osmosis;

HL candidates may express the solute-concentration points as water potential (ψ); accept either.

• both are closed circulations driven by a muscular heart, with blood kept within vessels;
• in a single circulation blood passes through the heart once per circuit (heart → gills → body → heart), whereas in a double circulation it passes through the heart twice (a pulmonary and a systemic circuit);
• in a single circulation blood loses much of its pressure at the gill capillaries before reaching the body, so delivery to the tissues is slower;
• in a double circulation blood returns to the heart to be re-pressurised before the systemic circuit, giving higher pressure and faster delivery to the tissues;
• a double circulation (with a four-chambered heart) keeps oxygenated and deoxygenated blood separate, and lets lung and body pressures be controlled separately;
• (so) a double circulation can support the higher metabolic rate of endothermic birds and mammals, OWTTE;

Comparative points required; award converse statements.

• during diastole the (relaxed) atria and ventricles fill with blood returning in the veins, with the atrioventricular valves open;
• atrial systole: the atria contract, pushing the final volume of blood into the ventricles;
• ventricular systole: the ventricles contract and ventricular pressure rises above atrial pressure;
• this pressure difference closes the atrioventricular valves, preventing backflow into the atria;
• when ventricular pressure exceeds the pressure in the arteries (aorta/pulmonary artery), the semilunar valves open;
• blood is ejected from the ventricles into the arteries;
• as the ventricles relax, ventricular pressure falls below arterial pressure, so the semilunar valves close, preventing backflow into the ventricles;
• when ventricular pressure falls below atrial pressure, the atrioventricular valves reopen and filling begins again;
• the cycle is initiated/timed by the sinoatrial node (pacemaker), so atria contract before ventricles;
• valves open and close passively according to the pressure gradients across them, so flow through the heart can only be one way, OWTTE;

Heart sounds may be credited as alternatives to the valve-closure points.

• at a source (e.g. a photosynthesizing leaf), sucrose is actively loaded into companion cells and then into sieve tube elements;
• this lowers the water potential inside the sieve tube at the source, so water enters by osmosis from the xylem;
• the resulting inflow of water raises the (hydrostatic) pressure inside the sieve tube at the source;
• at a sink (e.g. a root or growing tissue), sucrose is actively unloaded (removed) from the sieve tube, raising its water potential there;
• water leaves the sieve tube by osmosis at the sink, lowering pressure there;
• the resulting pressure difference (high at the source, low at the sink) drives a mass flow of sap through the sieve tube, from source to sink;
• sieve plates (with pores) allow sap to flow between adjacent sieve tube elements while the reduced cytoplasm and lack of a nucleus offer little resistance to flow;
• companion cells, with many mitochondria, provide the ATP needed for active loading/unloading of sucrose, OWTTE;

• ATP is needed for contraction: hydrolysis of ATP re-cocks the myosin head so that it can bind further along the actin and make another power stroke;
• each myosin head goes through many cycles in one contraction, so ATP is used continuously while the muscle contracts;
• in support of the claim, ATP binding is what detaches the myosin head from actin, so without ATP cross-bridges cannot be broken and the muscle cannot relax;
• in support of the claim, calcium ions are pumped back into the sarcoplasmic reticulum using ATP, so the binding sites on actin are covered again and cross-bridges stop forming;
• evidence: when ATP runs out after death the muscles become locked rigid (rigor mortis), OWTTE;
• against the claim, a relaxed muscle does not lengthen itself, because muscle tissue can only exert force when it contracts: it is stretched back to its resting length by the contraction of the antagonistic muscle, OWTTE;
• judgement: relaxation needs ATP to release the cross-bridges and remove calcium, but returning to resting length depends on the antagonist, so the claim is partly true, OWTTE;

Discuss: award a maximum of four marks without a judgement. Roles of ATP in both contraction and relaxation must appear for full marks.

• strength: with the membranes removed, the researcher can control the concentrations of ATP and Ca²⁺ directly, isolating the contractile machinery;
• strength: results are reproducible and show clearly that both ATP and Ca²⁺ are required for shortening;
• limitation: there is no intact sarcolemma / sarcoplasmic reticulum / nerve supply, so excitation–contraction coupling and neuromuscular control cannot be studied;
• limitation: the fibres are dead and cannot regenerate ATP, and the artificial solute levels may not match those inside a living fibre;
• judgement: the model is valuable for studying the contractile proteins themselves but is only a partial representation of whole living muscle, OWTTE;

NOS: evaluate a model, weighing strengths and limitations to a judgement.

• myosin molecules have (globular) heads with an actin-binding site and ATPase activity, and a hinged neck, allowing the head to attach, pivot (power stroke) and detach repeatedly;
• myosin tails aggregate into thick filaments with heads projecting along their length, so many cross-bridges can pull on each actin filament simultaneously;
• actin monomers polymerize into thin filaments bearing a (regularly repeating) myosin-binding site, providing a track that can be pulled toward the sarcomere centre;
• tropomyosin lies along actin covering the binding sites, and troponin changes shape when Ca²⁺ binds, moving tropomyosin aside, so the proteins form a molecular switch that couples contraction to nerve stimulation;
• titin is a giant elastic (spring-like) protein anchoring myosin to the Z-disc, centring the thick filament, resisting overstretching and providing elastic recoil;
• (judgement) each protein's shape/binding properties corresponds exactly to its role, so the sarcomere supports the structure–function principle (a Theme B unifying idea), OWTTE;

Cross-theme link to form and function; require at least one structure→function pairing for each named protein group for full marks.

• thick (waxy) cuticle reducing evaporation (from the epidermis);
• reduced/rolled leaves or leaves as spines, reducing surface area for transpiration, e.g. cacti / marram grass;
• stomata sunken in pits/grooves / surrounded by hairs, trapping humid air;
• deep or widespread roots reaching (scarce) water;
• water storage in succulent stems/leaves, e.g. cacti;
• (CAM-type) stomata opening at night when evaporation is low / vertical leaf orientation to reduce heating, OWTTE;

Credit any accurate named example; at least one named plant required for full marks.

• each species tolerates only a (limited) range of each abiotic variable / has a range of tolerance;
• outside the range the species cannot survive/reproduce, so the variable is limiting;
• combinations of variables define the habitats where the species is found;
• named variable + example, e.g. temperature/water limiting a named plant or animal (marram on dry dunes, corals in warm shallow water);
• second example or a described gradient (e.g. zonation up a rocky shore with exposure), OWTTE;
• (distribution may also be narrowed further by biotic factors, competition/predation);

• strength: a belt transect with regularly spaced quadrats systematically samples change along the environmental gradient and gives quantitative data;
• limitation: only one transect was used, which may be unrepresentative; replicate transects are needed to test whether the pattern is consistent / to calculate means;
• limitation: percentage-cover estimates from quadrats are subjective, and quadrat size/number affects the result;
• limitation: light intensity co-varies with other factors (soil moisture, temperature, competition), which are confounding variables, so a correlation does not establish that light is the cause;
• judgement: the method can reveal a correlation, but a controlled/manipulative experiment would be needed to show that light causes the distribution, OWTTE;

NOS: reward replication, confounds and the correlation-vs-causation point; must reach a judgement.

• autotrophs make their own carbon compounds, e.g. photosynthesis (plants/algae) using light energy;
• (some prokaryotes are) chemosynthetic: energy from oxidizing inorganic chemicals;
• heterotrophs obtain carbon compounds from other organisms;
• holozoic nutrition: ingestion then internal digestion (animals);
• saprotrophs: external digestion of dead matter (fungi/many bacteria);
• mixotrophs combine autotrophic and heterotrophic nutrition, e.g. *Euglena*, OWTTE;

• strength: in living species tooth shape strongly matches diet (flat ridged molars for grinding plants; pointed canines/carnassials for shearing meat), giving a reliable comparative baseline to apply to fossils;
• strength: jaw size and muscle-attachment scars indicate bite force, supporting inferences about tough or hard foods;
• limitation: teeth show what an animal could eat, not necessarily what it did eat, so omnivorous/opportunistic diets are ambiguous;
• limitation: the niche is more than diet (it also includes habitat, tolerances and interactions), so dentition reveals only part of it;
• judgement: dentition is a useful first line of evidence but should be corroborated (e.g. by tooth microwear, isotope analysis, gut contents or coprolites), OWTTE;

NOS: weigh strengths and limitations of the inference to a judgement.

• the final product of the pathway inhibits the first enzyme (unique to the pathway);
• the product binds to an allosteric site, not the active site;
• binding changes the shape of the active site so substrate cannot bind / non-competitive inhibition;
• as product accumulates the pathway slows / as product is used up inhibition is released;
• negative feedback prevents over-production / wasted resources;

Accept a named example (e.g. isoleucine inhibiting threonine deaminase).

• the loss of activity after heating and cooling is consistent with denaturation (an irreversible change to tertiary structure), so the conclusion is plausible;
• however a single result is weak evidence / the loss could have other causes (substrate or product broke down, evaporation, pH change, contamination);
• a valid conclusion needs a control kept at 30 °C throughout to show the assay can detect activity, plus repeats;
• leaving the cooled enzyme for longer / testing again at intervals after cooling would show whether any activity is regained (reversible inactivation) or none is (denaturation);
• measurements at several temperatures would locate the optimum and show the fall above it, strengthening the claim;
• overall the conclusion is reasonable but under-supported by one observation, OWTTE;

NOS: evaluating method + evidence. Evaluate requires strengths, limitations and a judgement.

• electrons (from NADH/FADH₂) pass along the electron transport chain (in the inner mitochondrial membrane);
• energy released pumps protons from the matrix into the intermembrane space;
• a proton gradient / proton-motive force is created (across the inner membrane);
• protons flow back into the matrix through ATP synthase;
• this flow drives phosphorylation of ADP to ATP / chemiosmosis;
• oxygen is the final electron acceptor, forming water;

• aerobic respiration requires oxygen, whereas anaerobic respiration does not;
• aerobic respiration fully oxidizes glucose to carbon dioxide and water, releasing a much higher yield of ATP per glucose molecule;
• anaerobic respiration only partially breaks glucose down (via glycolysis) to pyruvate, then to lactate, with a much lower net ATP yield (two ATP per glucose);
• aerobic respiration occurs partly in the mitochondria (as well as the cytoplasm), whereas anaerobic respiration occurs entirely in the cytoplasm;
• anaerobic respiration in humans produces lactate as a waste product, whereas aerobic respiration produces carbon dioxide and water;
• a wide range of respiratory substrates (carbohydrates, lipids, and to a lesser extent proteins) can be used aerobically, whereas only carbohydrate can be respired anaerobically, OWTTE;

• glycolysis (in the cytoplasm) splits glucose (6C) into two molecules of pyruvate (3C);
• with a net yield of two ATP and two reduced NAD per glucose;
• in the link reaction, pyruvate is oxidized and decarboxylated, releasing carbon dioxide and forming an acetyl group (2C), which combines with coenzyme A;
• this oxidation also reduces NAD (one reduced NAD formed per pyruvate, so two per glucose);
• the acetyl group is transferred to oxaloacetate (4C) to form citrate (6C), entering the Krebs cycle;
• the Krebs cycle includes four oxidation (dehydrogenation) reactions and two decarboxylations per acetyl group, regenerating oxaloacetate;
• these oxidations reduce NAD (and FAD), and release further carbon dioxide;
• a small amount of ATP is produced directly (by substrate-level phosphorylation) in the Krebs cycle;
• the reduced NAD produced by all three stages carries electrons (and protons) to the electron transport chain, where oxidative phosphorylation generates most of the ATP, OWTTE;

• light is absorbed by (pigments/chlorophyll in) photosystems (II and I) in the thylakoid membrane;
• excited electrons leave the (reaction centre) chlorophyll (of photosystem II);
• photolysis of water replaces these electrons, releasing H⁺ and O₂;
• electrons pass along an electron transport chain (between the photosystems);
• energy released pumps protons into the thylakoid space, creating a proton gradient;
• protons flow back through ATP synthase, phosphorylating ADP to ATP / photophosphorylation (chemiosmosis);
• photosystem I re-excites the electrons (with more light);
• electrons (plus H⁺) reduce NADP to NADPH (via NADP reductase);
• ATP and NADPH are (then) used in the Calvin cycle, OWTTE;

• the close match suggests the wavelengths chlorophyll absorbs are those that drive photosynthesis, supporting chlorophyll's role as the main photosynthetic pigment;
• this is a correlation between two spectra, and correlation alone does not prove causation;
• the match is not perfect because accessory pigments (chlorophyll b, carotenoids) also absorb and contribute, so absorption by chlorophyll is not the whole explanation;
• controlled experiments (e.g. measuring O₂ release at each wavelength, or aerobic bacteria clustering where photosynthesis is fastest) are needed to show absorbed light actually causes photosynthesis;
• balanced judgement: strong supporting evidence but not conclusive on its own, OWTTE;

NOS: correlation vs causation.

• photosystems (II and I) in the thylakoid membrane absorb light and emit excited electrons;
• photolysis of water in the lumen supplies (replacement) electrons and releases oxygen;
• electrons pass along carriers / an electron transport chain (in the membrane);
• energy released by electron flow pumps protons from the stroma into the (thylakoid) lumen;
• the small volume of the lumen means a steep proton gradient / proton-motive force builds up rapidly;
• the membrane is impermeable to protons, so they can return (to the stroma) only through ATP synthase;
• proton flow through ATP synthase drives phosphorylation of ADP to ATP (chemiosmosis);
• electrons from photosystem I (with protons) reduce NADP⁺ to NADPH on the stroma side;
• ATP and NADPH are released into the stroma, where the Calvin cycle uses them / the components are arranged in sequence in one membrane, making energy transfer efficient, OWTTE;

• both involve specific binding of a ligand to a (complementary) receptor protein;
• both change the activity/behaviour of the target cell (and depend on the cell possessing the receptor);
• surface receptors bind hydrophilic ligands (peptides/proteins/amines), whereas intracellular receptors bind hydrophobic ligands (steroids/thyroxine);
• surface signalling requires transduction across the membrane / second messengers, whereas intracellular complexes act directly (on DNA);
• surface pathways involve amplification cascades and can be very fast, whereas intracellular signalling changes gene expression and is slower/longer-lasting;
• in both, signalling ends when the ligand is removed/broken down, OWTTE;

Credit only explicitly comparative points for differences; award converse.

• both are transmembrane proteins in the plasma membrane of the target cell;
• both have a binding site on the outer surface of the membrane that is specific to / complementary in shape to their ligand;
• in both, the ligand is hydrophilic and remains outside the cell / does not enter the cell;
• in both, binding of the ligand changes the shape (conformation) of the receptor, which starts a response inside the cell;
• the acetylcholine receptor contains an ion channel, whereas the epinephrine receptor does not / is a G protein-coupled receptor;
• binding of acetylcholine opens the channel so that positively charged ions / sodium ions diffuse into the cell, whereas binding of epinephrine activates a G protein on the inner side of the membrane;
• the acetylcholine receptor changes the membrane potential / depolarizes the membrane directly, whereas the epinephrine receptor acts through a second messenger / causes adenylyl cyclase to make cAMP;
• the epinephrine signal passes through a cascade (cAMP activating protein kinase) that amplifies it, whereas the acetylcholine receptor needs no second messenger or enzyme cascade;
• the response to acetylcholine is very rapid (milliseconds) and brief, whereas the response to epinephrine is slower (seconds) and longer-lasting, OWTTE;
• acetylcholine is a neurotransmitter that diffuses across a synaptic gap from an adjacent neuron (a localized effect), whereas epinephrine is a hormone carried in the blood from the adrenal glands (a distant effect);
• acetylcholine binding leads to depolarization that can trigger an action potential / contraction of the muscle fibre, whereas epinephrine binding leads to activation of enzymes that break down glycogen / release of glucose from the liver cell;

Beyond this subtopic. Award if given; the marks can be earned without it.

• C2.2 acetylcholine is removed within milliseconds by acetylcholinesterase in the synaptic cleft, whereas epinephrine remains in the blood for minutes;

Award [5 max] if only similarities or only differences are given. Differences must be comparative (whereas); award the converse.

• the action potential arrives at/depolarizes the presynaptic terminal/knob;
• voltage-gated calcium channels open and Ca²⁺ diffuses in;
• vesicles (containing neurotransmitter, e.g. acetylcholine) fuse with the presynaptic membrane;
• neurotransmitter is released into the synaptic cleft by exocytosis;
• it diffuses across the (narrow) cleft;
• and binds to (specific) receptors on the postsynaptic membrane;
• ligand-gated (sodium) channels open, depolarizing the postsynaptic membrane;
• if threshold is reached, an action potential is initiated in the postsynaptic neuron;
• the neurotransmitter is (rapidly) broken down (e.g. by acetylcholinesterase) / recycled, ending the signal, OWTTE;

• an excitatory neurotransmitter (such as acetylcholine) binds to receptors on the postsynaptic membrane and opens channels for sodium / positive ions;
• sodium ions diffuse in, depolarizing the membrane / producing an excitatory postsynaptic potential (EPSP);
• a single EPSP is usually too small to reach the threshold potential;
• an inhibitory neurotransmitter (such as GABA) opens channels that let chloride ions in / potassium ions out;
• the membrane is hyperpolarized / an inhibitory postsynaptic potential (IPSP) moves the potential further from the threshold;
• a postsynaptic neuron has synapses with many presynaptic neurons, some excitatory and some inhibitory;
• the effects of all the EPSPs and IPSPs are added together / summed, so IPSPs cancel out some of the depolarization caused by EPSPs;
• inputs from several presynaptic neurons arriving at the same time are summed (spatial summation);
• repeated impulses from one presynaptic neuron arriving in quick succession are summed (temporal summation);
• if the summed depolarization reaches the threshold (about −55 mV) an action potential is generated and the impulse is transmitted;
• if the threshold is not reached no action potential is generated: the response is all-or-nothing;
• so inhibitory inputs can stop a neuron firing even while it receives excitatory input, OWTTE;

• both transfer information between (distant) parts of the body and can change target-cell activity;
• nervous signals travel as impulses along neurons, whereas hormones travel (dissolved) in the blood;
• nervous responses are (much) faster, whereas hormonal responses are slower (seconds to hours);
• nervous signals are precisely targeted (to specific cells), whereas hormones reach all cells but affect only those with receptors;
• nervous effects are brief, whereas hormonal effects are longer-lasting (growth/development/metabolism), OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• C2.2 both (ultimately) rely on chemical signals binding to receptors (neurotransmitters at synapses / hormones at target cells);

Credit only explicitly comparative points for differences; award converse.

• the nervous system detects and coordinates the response (medulla, chemoreceptors detecting CO₂/pH);
• the circulatory system raises heart rate and cardiac output, delivering more O₂ and glucose;
• the respiratory system increases ventilation rate and depth to supply O₂ and remove CO₂;
• the endocrine system (adrenaline) reinforces these changes and mobilises glucose;
• the muscular system contracts to produce movement, and no single system alone could meet the demand, so the coordinated (emergent) function depends on their integration, OWTTE;

• light from one side / lateral light is detected at the shoot tip;
• auxin is transported/redistributed towards the shaded side of the shoot;
• auxin diffuses freely into cells but cannot diffuse out;
• auxin efflux carriers transport auxin out of cells (by active transport);
• efflux carriers are positioned/coordinated on the same side of each cell, so auxin passes from cell to cell in one direction;
• a concentration gradient forms, with a higher auxin concentration on the shaded side;
• auxin promotes secretion of hydrogen ions (H⁺) into the apoplast/cell wall;
• acidification (of the wall) loosens cross-links between cellulose molecules, making the wall extensible;
• cells elongate/grow more where auxin concentration is higher;
• faster elongation on the shaded side causes the shoot to bend towards the light / positive phototropism, OWTTE;

Award marks for a coherent sequence; do not require photoreceptor names.

• the vaccine contains antigens (of the pathogen) / weakened or inactivated pathogen / mRNA coding for the antigen;
• the antigens are detected as foreign, triggering a primary immune response;
• (specific) helper T-cells are activated;
• B-cells with matching receptors are activated (with helper-T signals);
• activated B-cells divide (mitosis) / clonal expansion;
• plasma cells secrete antibodies specific to the antigen;
• memory (B- and T-) cells are produced and persist;
• on later infection, memory cells give a faster and stronger secondary response;
• the pathogen is destroyed before it can cause disease / antibody levels rise before symptoms, OWTTE;

• a zoonosis is an infectious disease that is transmitted to humans from another (animal) species;
• zoonoses are common: about 60 % of known human infectious diseases, and most newly emerging ones, are zoonoses, OWTTE;
• the modes of infection vary, e.g. bites, insect vectors, food or milk, direct contact with animals, inhaled droplets;
• rabies: the virus is in the saliva of infected mammals (e.g. dogs, bats) and enters the body through a bite or scratch;
• tuberculosis: (bovine TB) passes from cattle to humans in untreated (raw) milk / in droplets breathed out by infected animals;
• Japanese encephalitis: the virus multiplies in pigs/wading birds and is carried to humans by the bite of (*Culex*) mosquitoes acting as vectors;
• another valid example paired with its route, e.g. Lyme disease from tick bites, Ebola from contact with infected bats/primates/bushmeat;
• COVID-19 is caused by a coronavirus (SARS-CoV-2) that recently transferred to humans from another species (probably bats, possibly through an intermediate host);
• once in humans, SARS-CoV-2 spread from person to person in droplets/aerosols, so it caused a pandemic, OWTTE;
• COVID-19 caused millions of deaths worldwide / long-term illness in many people;
• health services were overwhelmed, and lockdowns disrupted economies, education and social life, OWTTE;

Credit an example only when it is paired with its route of transmission. Award up to [4] for points 3 to 7. For full marks the answer must include COVID-19.

• the surface glycoprotein of the virus acts as an antigen / is the molecule that antibodies (and the receptors of B-lymphocytes) recognize;
• antibodies bind only to an antigen whose shape is complementary to their binding site, so antibodies made after vaccination bind poorly or not at all to the changed glycoprotein, OWTTE;
• memory cells formed after vaccination are specific to the original antigen, so they are not activated by the variant;
• the variant triggers a new primary response: the few B-lymphocytes that match the new antigen must bind it and be activated by helper T-lymphocytes activated by the same antigen;
• activated B-lymphocytes must then divide by mitosis to form clones of plasma cells before antibody is secreted, which takes days;
• (meanwhile) the virus multiplies in the person, who develops the disease / vaccinated people are no longer immune to the variant because they cannot eliminate it rapidly;
• infected people release the variant and transmit it to others before it is eliminated, OWTTE;
• although 96 % are vaccinated, few people are immune to the variant, so the proportion immune is far below the level needed for herd immunity;
• transmission is no longer impeded: each infected person meets many susceptible people, so chains of transmission are not broken;
• (so) the number of cases rises rapidly / an epidemic occurs, as members of the population no longer protect one another (interdependence), OWTTE;
• people who cannot be vaccinated (e.g. infants, the immunocompromised), who were protected by the immunity of others, are now exposed as well / the whole population is at risk, not only the vaccinated, OWTTE;

For full marks the answer must include at least one point at each of the four levels: the molecule (points 1 and 2), the cells (points 3 to 5), the person (points 6 and 7) and the population (points 8 to 11). Do not accept "the virus has become resistant to the vaccine" without reference to the shape of the antigen or to memory cells.

• herbivory: an animal feeding on plants (e.g. zebra grazing grass);
• predation: one animal killing and eating another (e.g. lion and zebra);
• interspecific competition: two species using the same limited resource (e.g. barnacles competing for space);
• mutualism: both species benefit (e.g. pollinator and flowering plant / root nodule bacteria and legumes);
• parasitism: parasite benefits, host is harmed but (usually) not killed immediately (e.g. tapeworm);
• pathogenicity: a microorganism or virus causing disease in its host (e.g. chytrid fungus killing frogs / Mycobacterium tuberculosis causing tuberculosis in humans), OWTTE;

Interaction named + correct example needed for each mark; accept other valid examples.

• potential benefits: top-down control / a trophic cascade. Fewer deer reduces overgrazing, so vegetation recovers, increasing habitat and biodiversity;
• predators tend to remove weak or sick individuals, which may improve the prey population's health;
• risks/drawbacks: wolves may prey on livestock, causing conflict with people;
• the population may fail to establish, or prey may already be limited by other factors, so the intended effect is uncertain;
• unintended cascade effects on other species are hard to predict;
• a reasoned judgement weighing ecological benefit against human conflict and uncertainty, OWTTE;

Application + evaluation of trade-offs; Evaluate requires benefits, limitations and a judgement.

• photosynthesis fixes CO₂ (from air/water) into carbon compounds in producers;
• feeding passes carbon compounds along food chains;
• cell respiration (by all organisms) returns CO₂ to the atmosphere;
• decomposition: saprotrophs respire carbon from dead matter, releasing CO₂;
• combustion of biomass/fossil fuels releases CO₂;
• (some carbon is stored long-term in) fossil fuels / peat / carbonate rocks (removed from circulation), OWTTE;

• usefulness: they show feeding relationships and the routes of energy clearly, and help predict the effects of removing or adding a species;
• usefulness: they let trophic levels be identified and compared;
• limitation: they are a static snapshot, whereas real diets change with season, age and prey availability;
• limitation: they usually omit decomposers/detritivores and many microorganisms and rarer species;
• limitation: the arrows show the direction of energy flow but not the quantity transferred, and energy lost as heat is not shown;
• a pyramid of energy represents the quantities better; overall food webs are useful qualitatively but limited quantitatively, a reasoned judgement OWTTE;

NOS: evaluating a model's limitations.

• a pyramid of energy shows the flow of energy through each trophic level per unit area per unit time (kJ m⁻² yr⁻¹), not the amount present at one moment;
• all the energy at a higher level must come from the level below, and only ~10 % is transferred;
• (because) most energy is lost as heat from respiration, or in uneaten/undigested matter, and heat cannot be reused;
• (so) each level's energy flow must be smaller than the level below: an inverted energy pyramid would require energy to be created, which is impossible;
• biomass measurements are a snapshot of standing stock: a small mass of phytoplankton can support a larger mass of zooplankton if the phytoplankton reproduce/are replaced very rapidly (high turnover);
• (judgement) the apparent contradiction disappears when rate of production is distinguished from amount present, so energy pyramids represent ecosystem energetics more reliably than biomass snapshots, OWTTE;

NOS: choice of measurement (rate vs snapshot) affects the conclusions drawn from a model.

• helicase unwinds the helix and separates the strands (breaking hydrogen bonds);
• (single-strand binding proteins keep the strands apart) / gyrase/topoisomerase relieves the strain ahead of the fork;
• primase synthesizes (short) RNA primers;
• DNA polymerase III adds nucleotides (only) in the 5′ → 3′ direction, starting from a primer;
• the leading strand is synthesized continuously toward the fork;
• the lagging strand is synthesized in Okazaki fragments (away from the fork);
• complementary base pairing (A–T, G–C) ensures the correct sequence;
• DNA polymerase I removes the RNA primers and replaces them with DNA;
• DNA ligase joins the fragments (sealing the sugar–phosphate backbone);

• PCR amplifies DNA from tiny traces left at a crime scene (a drop of blood, a hair root, saliva), so a profile can be made from very little evidence;
• variable regions (short tandem repeats) differ in length between people, and gel electrophoresis separates the amplified fragments by length, so the band pattern is (almost) unique to one person;
• a crime-scene profile is compared with profiles from suspects or a database: bands matching at every region link that person to the sample;
• a mismatch at any region excludes a person with certainty, so profiling can clear innocent suspects / overturn wrongful convictions;
• in a paternity case, each band in the child's profile must come from the mother or the father, so every band not from the mother must appear in the true father's profile;
• testing more regions reduces the probability of a match by chance, so the conclusion is more reliable (the chances multiply);
• but identical twins have the same profile, and close relatives share many bands, so a match between relatives is less certain;
• contaminating DNA (for example from investigators) is amplified by PCR as well, which can give a misleading profile;
• degraded or very small samples may fail to amplify at some regions, giving an incomplete profile that is less able to discriminate;
• a mixture of DNA from several people gives many bands that are hard to assign to individuals;
• a match shows only that a person's DNA was present, not when or how it got there, so it does not by itself prove guilt;
• storing profiles in databases raises concerns about privacy, consent and misuse, OWTTE;

Discuss requires a balanced answer: award max [5] if only strengths or only limitations are given.

• transcription: RNA polymerase separates the DNA strands (at the gene);
• complementary (RNA) nucleotides pair with the template/antisense strand (U replacing T);
• mRNA is formed, carrying the gene's sequence as codons;
• (in eukaryotes the mRNA is processed and) mRNA travels to a ribosome (in the cytoplasm);
• translation: the ribosome reads codons in order (5′ → 3′), starting at AUG;
• tRNAs with complementary anticodons deliver specific amino acids;
• the ribosome catalyses peptide bonds between adjacent amino acids;
• the polypeptide grows until a stop codon is reached and is then released;
• (thus) the order of bases determines the order of amino acids / one gene → one polypeptide, OWTTE;

• a eukaryotic pre-mRNA contains coding exons separated by non-coding introns;
• during processing, introns are removed and exons joined together (splicing) to make the mature mRNA;
• in alternative splicing, different combinations or subsets of exons are joined from the same pre-mRNA;
• (so) one gene can yield several different mature mRNAs and hence several different polypeptides;
• the number of possible proteins can therefore greatly exceed the number of genes;
• different cell types or conditions splice the same transcript differently (a regulated process), further expanding the proteome, OWTTE;

• most (non-neutral) mutations are harmful: they disrupt proteins/regulation, e.g. genetic diseases;
• somatic mutations can cause cancer (uncontrolled cell division);
• mutagens (radiation/chemicals) raise these risks;
• however, mutation is the only source of new alleles / new genetic variation;
• variation is the raw material for natural selection, without mutation, no adaptation/evolution;
• rare beneficial mutations spread by selection (e.g. antibiotic resistance in bacteria, from the bacterium's perspective), OWTTE;

• hypothesis 1: the sequence has an essential function, so (almost) any change is harmful and is removed by natural selection (functional constraint);
• hypothesis 2: the rate of mutation in this region of DNA is unusually low (e.g. repair is more efficient there);
• hypothesis 1 predicts that new mutations arise in the sequence at a normal rate but carriers survive/reproduce less, whereas hypothesis 2 predicts that few new mutations arise at all;
• measure the mutation rate directly, e.g. by sequencing parents and offspring (or mutation-accumulation lines) and comparing the rate of new mutations in the conserved region with the rest of the genome;
• if the rate of new mutations is normal but the variants are (almost) absent from populations, selection must be removing them, supporting hypothesis 1;
• test function experimentally, e.g. use gene editing/knockout to alter or delete the sequence: a loss of function/fitness supports hypothesis 1 (functional constraint);
• if new mutations are genuinely rarer in the region (e.g. because repair is more efficient there), hypothesis 2 is supported;
• (NOS) both hypotheses make different testable/falsifiable predictions, and they are not mutually exclusive, both could contribute, so evidence for one does not automatically exclude the other, OWTTE;

NOS: distinguishing between competing hypotheses by their predictions.

• both are forms of nuclear division / both are preceded by DNA replication (S phase);
• both involve spindle microtubules moving chromosomes (through similar phases);
• mitosis has one division, whereas meiosis has two;
• mitosis produces two cells, whereas meiosis produces four;
• mitotic daughter cells are diploid/same ploidy as the parent, whereas meiotic cells are haploid (half);
• mitotic cells are genetically identical, whereas meiotic cells differ (crossing over, random orientation);
• mitosis serves growth/repair/asexual reproduction, whereas meiosis makes gametes/spores for sexual reproduction;

Credit only explicitly comparative points for differences; award converse.

• a tumour is a mass of cells formed by uncontrolled cell division (after mutations in genes that control the cell cycle);
• benign tumours remain at their site of origin and do not invade neighbouring tissue;
• benign tumours do not metastasize / do not spread to other parts of the body, so they are not cancers;
• benign tumours usually grow slowly / have a lower rate of cell division than malignant tumours;
• malignant tumours invade neighbouring tissue and can metastasize, so they are cancers;
• cells that break away from a malignant primary tumour can form secondary tumours elsewhere in the body;
• however, a benign tumour can still be harmful, e.g. by pressing on neighbouring organs, and further mutations can make it malignant;
• (judgement) the claim is false: only malignant tumours are cancers, although benign tumours may still need treatment or monitoring, OWTTE;

Discuss requires a supported conclusion; award the judgement point only if it follows from the benign/malignant distinction.

• spindle microtubules are required to attach to centromeres and separate sister chromatids in anaphase;
• blocking the spindle prevents mitosis being completed, cells arrest (at the spindle/metaphase checkpoint) and (usually) die;
• tumour cells divide far more frequently than most body cells (high mitotic index), so a drug targeting mitosis kills tumour cells disproportionately, justifying its use;
• however, some normal tissues also divide rapidly, hair follicle cells and the cells replacing the intestinal lining;
• (so) these tissues are also arrested/killed, causing hair loss and gut damage, the side effects;
• (judgement) the drugs are justified because the benefit of killing the tumour outweighs the (temporary/reversible) damage to other dividing tissues, though the therapeutic margin depends on the difference in division rates, OWTTE;

Justify: evidence linking spindle function, division rates and selective toxicity must lead to a supported conclusion.

• most regulation occurs at transcription;
• transcription factors bind specific (promoter/enhancer) sequences;
• (they) promote or block binding of RNA polymerase, raising or lowering transcription;
• DNA methylation (of cytosine, in promoters) silences genes;
• histone modifications alter chromatin packing, loosely packed (euchromatin) is expressible, tightly packed is not;
• these epigenetic patterns differ between cell types and are copied through cell division;
• the environment/signals (hormones) can change transcription-factor activity and epigenetic marks;
• post-transcriptional control: mRNA processing/stability/degradation adjusts how much protein is made;
• (translation rate and protein degradation add final control), OWTTE;

• epigenetic tags (methylation of DNA / of histones) can be added or removed in response to the environment during an organism's life, changing gene expression and phenotype;
• example of an environmental effect on tags, e.g. a change in diet or exposure to a pollutant altering methylation of particular genes;
• tags can persist through mitosis, so an acquired pattern of expression is kept in the descendants of that cell;
• most tags are removed from the genomes of the sperm and egg, but some escape removal;
• so some patterns of expression acquired by a parent can be passed to offspring without any change in base sequence, which supports the claim in a limited sense;
• against: because most tags are erased in each generation, such effects usually last for only one or a few generations;
• against: epigenetic changes are reversible and do not alter the base sequence / genotype, so no new alleles are produced;
• conclusion: epigenetics allows limited, usually short-lived inheritance of acquired patterns of gene expression, so it supports the claim only partly, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D4.1 against: long-term evolution depends on heritable changes in base sequence (new alleles) that arise by random mutation, not in response to need, and are acted on by natural selection;

Nature of Science; a balanced Discuss with a conclusion is required. The D4.1 point is creditworthy but does not count towards the marks.

• in hypotonic solutions water enters cells by osmosis (down the water-potential gradient);
• animal cells swell and may burst/lyse (no wall);
• plant cells become turgid, the wall resists expansion and prevents bursting (turgor supports the plant);
• in hypertonic solutions water leaves cells by osmosis;
• animal cells shrink/crenate;
• plant cells become flaccid and the membrane pulls away from the wall / plasmolysis;
• (hence isotonic fluids are used for animal tissues in medicine), OWTTE;

Award converse statements.

• when the plasma membrane is just beginning to pull away from the wall, the protoplast no longer presses on the wall, so ψp = 0;
• there is no net movement of water, so the water potential of the cells equals that of the solution, −1100 kPa;
• ψs = ψw − ψp = −1100 − 0 = −1100 kPa;
• cells vary: those that plasmolysed had a less negative ψs than −1100 kPa and those that did not had a more negative ψs, so −1100 kPa is a mean value;
• the value is slightly more negative than the ψs of the cells before immersion, because the water lost in reaching this point concentrated the cell sap, OWTTE;

• sperm are (very) small, whereas eggs are (much) larger, the egg stores food reserves/cytoplasm for the early embryo;
• sperm are motile (flagellum), whereas the egg is non-motile (moved by the oviduct), sperm must travel to the egg;
• sperm are produced in vast numbers (continuously), whereas (usually) one egg is released per cycle, many sperm compensate for losses on the journey;
• sperm have a midpiece packed with mitochondria to supply energy for swimming, whereas the egg does not move itself;
• at fertilization the sperm contributes (little more than) its nucleus/chromosomes, whereas the egg supplies the cytoplasm and organelles of the zygote (the sperm's tail and mitochondria are destroyed);

Beyond this subtopic. Award if given; the marks can be earned without it.

• D3.1.15 sperm have an acrosome (enzymes to penetrate the zona pellucida), whereas the egg has a zona pellucida and cortical granules (to prevent polyspermy);

Comparative phrasing expected; award converse statements.

• progesterone maintains the endometrium/uterine lining / maintains the continuity of pregnancy;
• progesterone is secreted initially by the corpus luteum;
• (hCG from the embryo prevents degeneration of the corpus luteum in early pregnancy);
• later in pregnancy the placenta takes over secretion of progesterone;
• progesterone inhibits contractions of the uterine muscle/myometrium during pregnancy;
• at the end of pregnancy progesterone levels fall/decrease;
• the fall in progesterone allows secretion of oxytocin (from the posterior pituitary) to increase;
• oxytocin stimulates contractions of the uterine wall;
• contractions stimulate further secretion of oxytocin: positive feedback;
• so contractions increase in strength/frequency until the baby is born;

• crossing over (prophase I) exchanges segments between non-sister chromatids of homologues;
• producing new combinations of (linked) alleles / recombinants;
• random orientation of homologous pairs at metaphase I;
• so alleles of unlinked genes assort independently, e.g. an AaBb parent produces AB, Ab, aB and ab gametes in equal proportions;
• gives 2ⁿ possible chromosome combinations per gamete (2²³ in humans);
• (so) each gamete is genetically unique;
• random fertilization: any sperm may fuse with any egg;
• multiplying the possible offspring genotypes (2²³ × 2²³ combinations);
• (hence) siblings differ from each other and their parents, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D1.3 mutation adds (entirely) new alleles;

• for: some characteristics are determined (almost) entirely by genotype, e.g. ABO blood group, where environment has no (practical) influence;
• many characteristics are polygenic: many genes each contribute a small (additive) effect, producing continuous variation (e.g. height, skin colour);
• against: the environment also affects phenotype, e.g. nutrition influences height / sunlight affects skin colour / temperature or soil conditions affect plant growth;
• (so) phenotype results from the interaction of genotype and environment, the genotype sets a potential range and the environment determines what is expressed within it;
• evidence: genetically identical individuals (identical twins, clones) can differ in phenotype, differences that can only be environmental;
• (judgement) the claim is an over-simplification: accurate for some discrete single-gene traits, but incomplete for most characteristics, OWTTE;

Discuss: require at least one point supporting and one challenging the claim, plus a judgement.

• the ABO blood group gene has multiple alleles (IA, IB and i), but each person inherits only two, giving a small number of distinct genotypes and phenotypes (A, B, AB and O);
• this produces discrete/discontinuous variation: individuals fall into a small number of separate, non-overlapping categories;
• skin colour, by contrast, is polygenic: several genes, each with a small additive effect, together influence the phenotype;
• skin colour is also influenced by environmental factors (such as exposure to sunlight);
• the combined effect of many genes and the environment produces a continuous range of phenotypes with no sharp boundaries between categories, OWTTE;
• (so) the number of genes controlling a characteristic, and the influence of environment, together determine whether its variation is discrete or continuous, OWTTE;

• osmoreceptors in the hypothalamus monitor blood solute concentration/osmolarity;
• when the blood is too concentrated (dehydration), the (posterior) pituitary releases (more) ADH;
• ADH acts on the collecting ducts (and distal tubules) of the kidney;
• making their membranes more permeable to water by insertion of aquaporins;
• (so) more water is reabsorbed from the filtrate into the blood (down the medullary gradient);
• a small volume of concentrated urine is produced;
• when the blood is too dilute, ADH secretion falls, less water is reabsorbed, and dilute urine is produced;
• the system is negative feedback: the response returns osmolarity toward the set point;
• thirst (also triggered by the hypothalamus) restores water by drinking, OWTTE;

• blood in the glomerulus is at high pressure because the afferent arteriole is wider than the efferent arteriole;
• ultrafiltration: water and small solutes (urea, glucose, amino acids, ions) are forced out of the blood into Bowman's capsule;
• the glomerular capillary wall has pores / is fenestrated;
• the basement membrane acts as the filter, holding back plasma proteins (and blood cells);
• podocytes of the inner wall of Bowman's capsule leave filtration slits for filtrate to pass through;
• filtration is not selective between useful and waste solutes, so the filtrate has the same concentration of glucose and urea as plasma;
• in the proximal convoluted tubule all the glucose (and amino acids) is reabsorbed by active transport;
• sodium ions are actively transported out of the tubule and water follows by osmosis / most of the filtered water is reabsorbed here;
• cells of the proximal convoluted tubule have microvilli, giving a large surface area for reabsorption;
• they have many mitochondria, supplying ATP for active transport;
• reabsorbed substances pass into the surrounding (peritubular) capillaries, returning to the blood;
• urea is not actively reabsorbed (only some diffuses back), so much of it stays in the filtrate, its concentration rises as water is removed, and it is excreted in urine, OWTTE;

• individuals in a population vary in their (heritable) characteristics;
• variation arises from mutation and sexual reproduction (meiosis + fertilization);
• organisms produce more offspring than the environment can support;
• (so) there is competition / a struggle for survival;
• individuals whose characteristics suit the environment survive better and reproduce more;
• their alleles are passed on to more offspring;
• over generations the population's (heritable) characteristics change;
• named example correctly used, e.g. antibiotic resistance / peppered moth / Galápagos finch beaks;
• (selection acts on the phenotype; evolution is the resulting change in the population’s heritable characteristics), OWTTE;

• strength: both changes have been observed directly, in real time, and measured (resistance %, morph frequencies), rather than only inferred from fossils;
• strength: both fit the mechanism. Heritable variation exists, a selection pressure (the antibiotic, or bird predation on the more conspicuous morph) changes survival, and the favoured type increases over generations;
• strength: the favoured type changes when the selection pressure changes (resistance rises with antibiotic use; the favoured moth form depends on bark colour), so predictions of the theory can be tested;
• strength: the observations can be / have been repeated in different places and by different scientists, OWTTE;
• limitation: in the field, other factors (e.g. other predators, migration of moths between areas) are hard to control, so the cause of the change is partly inferred from correlation;
• limitation: some early moth experiments were criticised for methodology (e.g. unnatural resting positions, release densities), showing the need for careful controls;
• limitation: each case involves one simple trait over a few decades, so extending the conclusion to complex structures over long time scales is an extrapolation;
• overall judgement: strong evidence that natural selection changes the heritable characteristics of populations, though on its own not evidence for every claim about evolution, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• A4.1 limitation: they show change within an existing species, not the origin of new species;
• A4.1 the conclusion is corroborated by independent lines of evidence (fossils, molecular data);

Nature of Science: evaluating the strength of evidence for evolution.

• a keystone species has a disproportionately large effect (relative to its abundance);
• named example, e.g. sea otter (sea urchin) kelp forest (or wolves in Yellowstone, sea stars eating mussels on rocky shores);
• the keystone controls (the population of) a strongly interacting species, e.g. otters eat urchins;
• when the keystone is removed, that species irrupts, e.g. urchin numbers explode;
• its resource is overexploited, e.g. kelp forests are grazed to "urchin barrens";
• many other species that depended on the (kelp) habitat decline, community structure collapses, OWTTE;

• a tipping point is a threshold beyond which change becomes self-reinforcing/irreversible (positive feedback);
• much of the Amazon's rainfall is recycled by the forest itself (transpiration → clouds/rain);
• clearing forest reduces transpiration, so rainfall falls, stressing/killing more forest, further reducing rainfall;
• (beyond the threshold) forest could shift to (drier) savannah/grassland even if clearing stops;
• consequences: loss of biodiversity/habitat, release of stored carbon (accelerating climate change), altered regional/global rainfall;
• uncertainty: the threshold's position (estimated ~20–25 % loss) is not known precisely, so precaution argues for halting deforestation early, OWTTE;

• DDT is persistent (not broken down) and fat-soluble, so it is stored in the tissues of organisms rather than excreted (bioaccumulation);
• its concentration increases at each trophic level because each consumer eats many contaminated prey, so top predators carry concentrations many times higher than the environment (biomagnification);
• in birds of prey (e.g. ospreys, pelicans) high concentrations caused eggshell thinning and breeding failure, and populations declined;
• for: indoor spraying kills the mosquitoes that transmit malaria, reducing a disease that kills many people, and DDT is cheap and long-lasting;
• spraying inside houses uses far less DDT than spraying crops and releases little into soil, water and food chains, so less is biomagnified, OWTTE;
• against: alternatives exist, e.g. insecticide-treated bed nets / less persistent insecticides, and any DDT that escapes persists in the environment for decades;
• conclusion: limited indoor use may be justified where malaria is common and alternatives fail, but widespread outdoor use is not, because of biomagnification, OWTTE;

Beyond this subtopic. Award if given; the marks can be earned without it.

• D4.1 against: mosquito populations have evolved resistance to DDT, so it becomes less effective;

A conclusion is required for the final mark. Award a maximum of three marks for a one-sided answer.

• burning of fossil fuels (and cement production) releases CO₂;
• deforestation reduces CO₂ uptake (and releases carbon when burned);
• agriculture/livestock/waste release methane (and nitrous oxide);
• (so) atmospheric greenhouse-gas concentrations have risen (far) above pre-industrial levels;
• greenhouse gases absorb long-wave radiation emitted by Earth and re-radiate it, retaining heat;
• the enhanced greenhouse effect raises global mean temperature;
• positive feedback: melting ice lowers albedo, so more radiation is absorbed → more warming;
• positive feedback: thawing permafrost releases CH₄/CO₂ → more warming;
• warmer oceans absorb less CO₂ (and hold more water vapour, itself a greenhouse gas), OWTTE;

• positive feedback: warming triggers a change that causes further warming (self-reinforcing);
• albedo/ice loss: melting sea ice replaces reflective white surface with dark ocean, which absorbs more radiation;
• permafrost thaw releases methane and CO₂ from decomposing organic matter;
• (warming oceans hold less dissolved CO₂ / droughts and fires release forest carbon);
• each loop's product feeds its cause, so change can accelerate once thresholds/tipping points are passed;
• (hence) warming may continue even if emissions fall, an argument for early action, OWTTE;