Biology  by Bradford
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IB Biology Paper 1 mock. The whole syllabus, at the real length.

A complete Paper 1 (Higher level) built from the 2025 guide: 75 marks across 44 questions, in 2 hours. It follows the real paper's structure and rubric below, and every question is original and tagged to the syllabus. Sit it on screen against the clock and mark yourself, print it for a class, or build a fresh one with nothing repeated.

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2 hours · 75 marks · 36% of the final grade.

  1. Paper 1A40 marks 40 multiple-choice questions. Answer all. One best answer per question; no marks lost for wrong answers.
  2. Paper 1B35 marks 4 data-based questions. Longer than SL, 8 to 10 marks each, and drawing on additional higher level content.

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  • Both parts are sat in one sitting.

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The 40 multiple-choice questions are spread across all 40 subtopics in proportion to each theme, and about 30% of them come from additional higher level material, the AHL share of teaching hours. The four data-based questions sit in the 8 to 10 mark range and are chosen to sum as close to 35 as the bank allows.

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Biology · Paper 1
Higher level · mock examination · Paper 1A multiple choice and Paper 1B data-based
120 minutes75 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers Whole syllabus: all 40 subtopics
  • Do not open this examination paper until instructed to do so.
  • Answer all questions.
  • A calculator is required for this paper.
  • Answers must be written within the answer boxes provided.
Name:
Paper 1A · Multiple choice

Answer all questions. Choose the one best answer for each question.

1.

How is the presence of large amounts of liquid water on the early Earth commonly explained?

[1]
  1. The oceans have always existed unchanged
  2. Water was delivered by asteroids and comets
  3. Water was made by early photosynthetic cells
  4. Water formed from the burning of methane
2.

During transpiration, water is pulled up through xylem vessels as a continuous column under tension. Which property of water prevents the column from breaking?

[1]
  1. Adhesion of water molecules to the lignin of the vessel walls
  2. Cohesion between water molecules resulting from hydrogen bonding
  3. The high specific heat capacity of water, which prevents it from vaporizing
  4. The low viscosity of water, which allows it to flow through narrow vessels
3.

Which bases are purines?

[1]
  1. Guanine and uracil
  2. Adenine and thymine
  3. Adenine and guanine
  4. Cytosine and thymine
4.

Which statements about DNA are correct?
I. It is (usually) a double helix.
II. It contains the base uracil.
III. It has a sugar–phosphate backbone.

[1]
  1. I, II and III
  2. I and III only
  3. II and III only
  4. I and II only
5.

During embryonic development, a signalling molecule released by neighbouring cells causes an unspecialized cell to develop into a muscle cell. What is the basis of this differentiation?

[1]
  1. The cell loses all genes that are not needed by muscle cells
  2. The signalling molecule causes mutations in muscle genes
  3. The pattern of gene expression in the cell changes
  4. The cell gains extra copies of the genes for muscle proteins
6.

A student drew a diagram of a cell from an electron micrograph and added the annotation shown for one structure. Which annotation follows the convention of including a function?

[1]
  1. Golgi apparatus — flattened membrane sacs near the nucleus
  2. Rough endoplasmic reticulum — synthesizes proteins for secretion
  3. Mitochondrion — approximately 1 μm wide, oval in section
  4. Cell wall — rigid outermost layer surrounding the plant cell
7.

The "regressive" (reduction) hypothesis proposes that viruses evolved from cells. Which observation is most consistent with this hypothesis?

[1]
  1. Some giant viruses carry genes for protein synthesis
  2. Viruses can be crystallized like simple chemicals
  3. All known viruses have identical genome sequences
  4. Viruses contain no nucleic acid of their own
8.

The diagram compares the forelimb bones of four mammals. What do these limbs provide evidence for?

[1]
human armbat wingwhale flipperhorse foreleghumerusradius and ulnacarpalsmetacarpal and phalangesForelimb skeletons of four mammals, not drawn to the same scale
  1. Convergent evolution, because the limbs carry out different functions
  2. Common ancestry, because the same bones occur in the same arrangement
  3. Analogous structures produced by the same selection pressures
  4. Structures that evolved independently in each of the four groups
9.

Which ecosystem is a case study of large-scale ecosystem loss driven by human activity?

[1]
  1. Mixed dipterocarp forest of Southeast Asia
  2. Caribbean mangrove forest
  3. Temperate deciduous forest in western Europe
  4. Boreal taiga forest in northern Canada
10.

Starch is digested to glucose in the gut. What type of reaction is this, and what is consumed?

[1]
  1. Condensation, consuming water
  2. Oxidation, consuming oxygen
  3. Condensation, releasing water
  4. Hydrolysis, consuming water
11.

Which of the following is NOT considered an organelle?

[1]
  1. A mitochondrion
  2. The cytoplasm
  3. A ribosome
  4. A lysosome
12.

Striated muscle fibres are much longer than typical cells and contain many nuclei. What is the advantage of this structure?

[1]
  1. It allows the fibres to change shape and crawl like white blood cells
  2. It provides enough nuclei to serve one long contractile unit
  3. It reduces the amount of ATP needed for each contraction
  4. It allows the fibres to divide rapidly to repair damage
13.

Which cell type is correctly matched with a size-related specialization?

[1]
  1. Red blood cell: very large, so it can carry more haemoglobin
  2. Egg cell: very small, so it can be moved easily along the oviduct
  3. Sperm cell: small and streamlined, so it can swim to the egg
  4. Motor neuron: very short, so that impulses arrive at the muscle sooner
14.

How is the concentration gradient of oxygen across the gill surface of a bony fish maintained?

[1]
  1. The fish holds water in its mouth until the oxygen concentrations in water and blood become equal, then releases it
  2. Cells of the gill surface actively transport oxygen from the water into the blood using ATP
  3. Water is continuously pumped over the gills while blood flows continuously through the gill capillaries
  4. Oxygen is stored in the gill tissue during periods of rest and released into the blood during activity
15.

More fluid leaves capillaries at their arterial ends than is reabsorbed at their venous ends. What normally happens to the excess tissue fluid?

[1]
  1. It accumulates in the tissues of healthy people
  2. It is broken down by the cells of the tissue
  3. It drains into lymphatic capillaries and returns to veins
  4. It passes into the arteries, raising blood pressure
16.

In a transverse section of the root of a young dicotyledonous plant, where is the xylem located?

[1]
  1. In a central star-shaped core with phloem between arms
  2. In a ring of bundles just beneath the epidermis
  3. Outside the phloem in scattered peripheral bundles
  4. Only in the root hairs, where water is absorbed
17.

Each autumn, monarch butterflies (Danaus plexippus) fly thousands of kilometres from southern Canada to overwintering sites in Mexico. Which reason for locomotion does this journey best illustrate?

[1]
  1. Foraging for food, because the butterflies feed on nectar along the route
  2. Escaping from danger, because predators are more abundant in Canada
  3. Migration, because the movement avoids unfavourable seasonal conditions
  4. Searching for a mate, because mating only takes place at the Mexican sites
18.

After a skeletal muscle has contracted, it cannot return to its full resting length by itself. Why?

[1]
  1. Titin locks the thick and thin filaments together until they are pulled apart by another muscle
  2. Calcium ions stay bound to troponin until an opposing force pulls them off the thin filaments
  3. The myosin heads remain attached to actin until an antagonistic muscle separates them
  4. Muscle tissue can only exert force when it contracts, so another muscle must stretch it
19.

Which set of features would you expect in a predatory mammal such as a wolf?

[1]
  1. Forward-facing eyes, canines and a short gut
  2. Photosynthetic skin and no digestive tract
  3. No teeth and a four-chambered stomach
  4. Side-facing eyes, grinding molars, long gut
20.

What is the role of NAD in cell respiration?

[1]
  1. It phosphorylates ADP directly to form ATP
  2. It pumps protons across the inner mitochondrial membrane
  3. It carries hydrogen from substrates to the transport chain
  4. It is the final electron acceptor of the transport chain
21.

In human muscle during vigorous exercise, pyruvate is reduced to lactate. What is the purpose of this reaction?

[1]
  1. To produce additional ATP directly from pyruvate
  2. To remove pyruvate because it is toxic to muscle fibres
  3. To oxidize NADH back to NAD⁺ so that glycolysis can continue
  4. To supply lactate as a substrate for the Krebs cycle
22.

During photosynthesis, carbon dioxide is converted to glucose. What is the source of the hydrogen needed for this conversion?

[1]
  1. Glucose stored in the chloroplast
  2. Atmospheric hydrogen gas absorbed by the leaves
  3. The breakdown of chlorophyll molecules
  4. The splitting of water by photolysis
23.

A drug binds to a receptor and triggers the same response as the natural ligand. Another drug binds the same receptor but produces no response and blocks the natural ligand. What are the two drugs?

[1]
  1. The first is an antagonist, the second an agonist
  2. Both are agonists
  3. The first is an agonist, the second an antagonist
  4. Both are antagonists
24.

A signalling chemical (ligand) produces a response only in certain cells of the body. What determines which cells respond?

[1]
  1. The ligand changes its shape in different tissues
  2. The ligand only enters cells with permeable membranes
  3. Only cells with a receptor for that ligand respond
  4. The ligand is delivered by the blood only to responding cells
25.

Local anaesthetics such as lidocaine block voltage-gated sodium channels in sensory neurons. Why does this stop pain?

[1]
  1. The anaesthetic destroys the pain receptors in the sensory endings
  2. The resting potential of the sensory neuron is abolished
  3. Potassium ions flood into the neuron and block depolarization
  4. Action potentials cannot be initiated or propagated without Na⁺ influx
26.

Where are synapses found?

[1]
  1. Only between the dendrites of one neuron and the axon terminal of another neuron in the brain
  2. Only between sensory receptors and the sensory neurons that carry impulses from them to the CNS
  3. Between two neurons and between a neuron and an effector cell such as a muscle fibre
  4. Between adjacent muscle fibres, so that they contract together as one unit
27.

What is positive phototropism?

[1]
  1. Directional growth of a shoot towards a source of lateral light
  2. Directional growth of a shoot away from a source of lateral light
  3. Rapid folding of leaves in response to being touched
  4. Faster germination of seeds when exposed to light
28.

Which organisms in the tundra food web are secondary consumers?

[1]
secondary consumersprimary consumersproducerslichensgrasses and sedgeslemmingarctic harecaribousnowy owlarctic foxwolf
  1. Snowy owl, arctic fox and wolf
  2. Lemming, arctic hare and caribou
  3. Lichens, grasses and sedges
  4. Snowy owl, lemming and lichens
29.

A ribosome has three tRNA binding sites. Which row describes their roles?

[1]
RowA siteP siteE site
A.holds the growing polypeptideaccepts the incoming tRNAmakes peptide bonds
B.accepts the incoming (aminoacyl-)tRNAholds the tRNA carrying the growing polypeptidereleases the empty (deacylated) tRNA
C.releases tRNAaccepts tRNAholds mRNA
D.binds mRNA capbinds poly-A tailbinds introns
  1. Row A
  2. Row B
  3. Row C
  4. Row D
30.

A neuron in an adult never divides, yet it transcribes the same gene many times a day for decades. Which statement explains why the polypeptide produced from this gene stays the same throughout the life of the cell?

[1]
  1. The template strand of the gene is used repeatedly without its base sequence being altered by transcription
  2. The gene is replicated each time it is transcribed, so a fresh, unused copy is always available
  3. The mRNA transcribed from the gene is stored permanently and re-used for every round of translation
  4. The template strand is rebuilt from the mRNA after each round of transcription, restoring the original base sequence
31.

In which stage are sister chromatids being separated?

[1]
CEAFBDSix stages of meiosis in a cell with 2n = 4, not in order. Maternal chromosomes green (solid), paternal pink (dashed).
  1. A
  2. B
  3. C
  4. E
32.

How does the rate at which an mRNA molecule is degraded affect gene expression?

[1]
  1. A short-lived mRNA is translated more quickly, so more of the protein is made
  2. The lifetime of an mRNA does not affect how much protein is made from it
  3. A short-lived mRNA is translated fewer times, so less of the protein is made
  4. A long-lived mRNA is translated only once, because ribosomes cannot reuse it
33.

Water moves from regions of higher water potential to regions of lower water potential. Which statement explains this movement in terms of energy?

[1]
  1. Water molecules gain potential energy as they move, making the movement irreversible
  2. Solute molecules chemically attract water molecules across the membrane
  3. Water tends to move so that its potential energy decreases, as such changes occur spontaneously
  4. Water at lower water potential has higher kinetic energy, drawing molecules towards it
34.

Two linked genes show 12 % recombinant offspring in a test cross. What does this frequency indicate?

[1]
  1. No crossing over ever occurs between them
  2. The genes are on different chromosomes
  3. The genes are alleles of one another
  4. They lie about 12 map units apart
35.

The pedigree chart shows the inheritance of a rare condition controlled by a single autosomal gene. Which individual must be heterozygous?

[1]
IIIIIII-1I-2II-1II-2II-3II-4II-5III-1III-2III-3III-4III-5unaffectedaffectedsquares male, circles female
  1. I-2
  2. II-3
  3. II-4
  4. III-5
36.

How does brown adipose tissue generate heat in cold conditions?

[1]
  1. It raises the metabolic rate of nearby white fat
  2. Its respiration is uncoupled, releasing heat
  3. It produces extra ATP for shivering muscles
  4. It insulates the body just like white fat
37.

In evolutionary biology, what does the "fitness" of an individual measure?

[1]
  1. Its physical strength and stamina
  2. Its overall body size
  3. Its total lifespan
  4. Its contribution of offspring
38.

In Endler's experiments, guppies were kept in ponds with or without predatory fish. In the ponds without predators, male guppies became more brightly coloured over several generations. Which conclusion is supported by this observation?

[1]
  1. Sexual selection by females favoured brightly coloured males
  2. Predation by other fish favoured duller, camouflaged males
  3. Bright colours were induced directly by the gravel in the pond
  4. Mutation rates increased in the absence of predatory fish
39.

Which changes are expected as primary succession proceeds on newly exposed rock? I. Species diversity increases II. Primary production increases III. Food webs become more complex

[1]
  1. I and II only
  2. I and III only
  3. II and III only
  4. I, II and III
40.

The diagram shows the concentration of DDT in a lake food chain. By approximately what factor is DDT more concentrated in the osprey than in the phytoplankton?

[1]
fish-eating bird (osprey)25.0ppm DDTtertiary consumerlarge fish (pike)2.1ppm DDTsecondary consumersmall fish (minnow)0.5ppm DDTprimary/secondary consumerzooplankton0.04ppm DDTprimary consumerphytoplankton0.005ppm DDTproducerlake water0.00002ppm DDTMean DDT concentration in the tissues of organisms from one lake, parts per million (ppm)
  1. 5
  2. 50
  3. 500
  4. 5000
Paper 1B · Data-based questions

Answer all questions. Answers must be written within the answer boxes provided.

41.

Three identical 2 dm³ glass jars were filled with pond water and sealed to make closed mesocosms. Jar A contained pondweed only, jar B contained pondweed and four pond snails, and jar C contained four pond snails only. The jars were kept on a windowsill in natural light. Every two days the dissolved oxygen concentration of the water was measured with an oxygen probe. The graph shows the results over 14 days.

0 2 4 6 8 10 12 14 0 3 6 9 12 Time / days Dissolved oxygen / mg dm⁻³ A: plants only B: plants + snails C: snails only
(a)State the dissolved oxygen concentration in jar C on day 8.[1]
(b)Explain the change in dissolved oxygen in jar A.[2]
(c)Deduce, with a reason, which jar is most likely to remain stable for the longest time.[2]
(d)Outline two features that a mesocosm must have for it to be a useful model of an ecosystem.[2]
(e)Explain why ecosystems require a continuous supply of energy but not a continuous supply of nutrients.[2]
42.

Researchers measured glucose uptake by cultured fat cells (adipocytes) exposed to increasing insulin concentrations for 30 minutes. A second set of cells was first treated with an antibody that binds to the insulin receptor's extracellular domain, blocking the hormone-binding site. Uptake is shown relative to untreated cells with no insulin (= 1.0).

Insulin concentration / nmol dm⁻³Relative glucose uptake (no antibody)Relative glucose uptake (+ receptor antibody)
01.01.0
0.11.61.0
13.11.1
104.81.2
1005.01.3
(a)Describe the effect of insulin concentration on glucose uptake in cells without the antibody.[2]
(b)Calculate the percentage increase in glucose uptake caused by 10 nmol dm⁻³ insulin in cells without the antibody, compared with no insulin.[1]
(c)Explain why glucose uptake plateaus at high insulin concentrations.[2]
(d)Explain the results for the cells treated with the receptor antibody.[2]
(e)State the general term for a chemical that binds specifically to a receptor and triggers its response.[1]
43.

The mean number of mitochondria per cell and the rate of oxygen consumption were measured for four human tissues.

TissueMean number of mitochondria per cellOxygen consumption / μmol g⁻¹ min⁻¹
cardiac (heart) muscle32009.4
liver11003.6
epidermis of skin2500.9
mature red blood cell00.0
(a)Identify the tissue with the highest rate of oxygen consumption per gram.[1]
(b)Deduce the relationship between mitochondrial number and oxygen consumption in these tissues.[2]
(c)Suggest why mature red blood cells consume no oxygen even though they transport it.[2]
(d)Describe two features of the inner mitochondrial membrane that increase the ATP produced by each mitochondrion.[2]
(e)Comment on the use of mitochondrial number alone as a measure of the respiratory activity of a tissue.[2]
44.

Each year a surveillance laboratory sequences the gene for haemagglutinin, a surface protein of influenza A, in viruses collected from patients. The number of amino acid differences between the circulating virus and the strain used in that year's vaccine was recorded, together with the effectiveness of the vaccine measured in the same season.

SeasonAmino acid differences from the vaccine strainVaccine effectiveness / %
1261
2452
3928
41219
5357
(a)Describe the relationship between the number of amino acid differences and vaccine effectiveness.[2]
(b)Estimate the vaccine effectiveness expected in a season with 6 amino acid differences.[1]
(c)Suggest why the number of differences from the vaccine strain increases from one season to the next.[2]
(d)Identify two variables, other than the vaccine strain, that could affect the measured effectiveness of a vaccine.[2]
1.
2.
(e)Suggest how surveillance data of this kind are used when choosing the strain for next season's vaccine.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-GAH______wAAZSwg

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. A1.1.7 [1]
  • B: bombardment by icy bodies plus outgassing supplied water; Earth's orbit (Goldilocks zone) and gravity let it persist as liquid;
2. A1.1.3 [1]
  • B: hydrogen bonds between water molecules hold the column together under tension; A adhesion helps water cling to the walls but does not hold water molecules to each other; C heat capacity affects temperature change, not the ability to withstand tension; D low viscosity eases flow but does nothing to stop the column separating;
3. A1.2.12 [1]
  • C: purines (A, G) have a double ring; pyrimidines (C, T, U) a single ring;
4. A1.2.2-A1.2.5 [1]
  • B — uracil occurs in RNA; DNA contains thymine;
5. A2.2.13 [1]
  • C: the genome stays the same; differentiation results from different genes being switched on/off, often triggered by signals from the cell's environment;
6. A2.2.11 [1]
  • B: an annotation must state the structure's function — only this option pairs the identification with what the structure does (protein synthesis for secretion); the others describe appearance or position only;
7. A2.3.5 [1]
  • A: giant viruses carrying cell-like genes suggest descent (with loss of functions) from cellular ancestors; other hypotheses are progressive (escaped genes) and virus-first;
8. A4.1.4 [1]
  • B: the bones correspond one to one in the same arrangement despite serving different functions, which is best explained by inheritance from a common ancestor with a pentadactyl limb;
9. A4.2.5 [1]
  • A: the dipterocarp rainforests (Borneo/Sumatra) have suffered severe loss to logging and oil-palm conversion, with orangutans among the species affected;
10. B1.1.3 [1]
  • D: glycosidic bonds are broken by hydrolysis, with one water molecule used per bond;
11. B2.2.1 [1]
  • B: organelles are discrete subcellular structures with specific functions; the cytoplasm (and the cell wall) are not organelles;
12. B2.3.9 [1]
  • B: fusion of many cells during development produces long fibres suited to contraction over a distance; multiple nuclei maintain protein synthesis throughout the large cytoplasm;
13. B2.3.5 [1]
  • C: sperm are among the smallest human cells, carrying minimal cytoplasm for efficient swimming; eggs are among the largest (nutrient store), red blood cells are small enough to pass along capillaries, and motor neurons can be over a metre long;
14. B3.1.3 [1]
  • C: ventilation with water and a continuous flow of blood keep bringing oxygen-rich water to the surface and carrying oxygenated blood away, so the gradient is never lost; A would abolish the gradient rather than maintain it; B fails because oxygen crosses by diffusion, not active transport; D fails because gills do not store oxygen;
15. B3.2.13 [1]
  • C: excess tissue fluid enters permeable, blind-ended lymphatic capillaries as lymph, which lymphatic vessels and ducts return to the venous blood; failure of this drainage causes oedema;
16. B3.2.10 [1]
  • A: roots have a central vascular cylinder (stele) with a star-shaped xylem core and phloem in the bays between its arms, surrounded by a wide cortex and an epidermis bearing root hairs;
17. B3.3.9 [1]
  • C: a long seasonal movement between regions to avoid winter is migration; feeding en route is incidental (A), the trigger is season not predation (B), and mating occurs after overwintering on the return journey rather than being the purpose of the flight (D);
18. B3.3.3 [1]
  • D: muscle generates force only by contracting, so an antagonist must lengthen it; titin is a spring that aids recoil and prevents overstretching, not a lock (A), calcium ions are pumped back into the sarcoplasmic reticulum (B), and myosin heads detach when ATP binds (C);
19. B4.2.10 [1]
  • A: predator adaptations: binocular vision for judging distance, piercing/slicing dentition, short gut (meat digests readily); prey species typically have lateral eyes for wide vision;
20. C1.2.7 [1]
  • C: NAD is reduced (to NADH) by accepting hydrogen from oxidation reactions and delivers electrons to the electron transport chain; oxygen is the final electron acceptor;
21. C1.2.9 [1]
  • C: the supply of NAD⁺ is limited; reducing pyruvate to lactate regenerates NAD⁺, allowing ATP production by glycolysis to continue without oxygen;
22. C1.3.2 [1]
  • D: water is split using light energy (photolysis), providing hydrogen for the reduction of CO₂ to glucose and releasing oxygen as a waste product;
23. C2.1.7 [1]
  • C: agonists activate the receptor; antagonists occupy it without activation, blocking the ligand;
24. C2.1.1 [1]
  • C: specificity comes from receptor distribution: the ligand's shape is complementary to a specific receptor, and only cells expressing that receptor respond;
25. C2.2.8 [1]
  • D: depolarization depends on voltage-gated Na⁺ channels; blocking them prevents impulse initiation/conduction in the treated region, so pain signals never reach the brain;
26. C2.2.5 [1]
  • C: synapses are junctions between neurons and between neurons and effectors (muscle, gland);
27. C3.1.18 [1]
  • A: positive phototropism is a directional growth response towards lateral (one-sided) light, shown by plant shoots;
28. C4.2.10 [1]
  • A: secondary consumers eat primary consumers; the owl, fox and wolf all feed on the herbivores (lemming, hare, caribou), which in turn eat the producers;
29. D1.2.17 [1]
  • B: Aminoacyl (in), Peptidyl (holding the chain), Exit (out), the tRNA relay through the ribosome;
30. D1.2.3 [1]
  • A: a single DNA strand serves as a template for RNA synthesis by complementary base pairing, and the DNA sequence is read but not changed, so a non-dividing somatic cell can conserve the same sequence for its whole life; B fails because replication occurs only before cell division, not during transcription; C fails because mRNA is short-lived and is broken down, not stored permanently; D fails because DNA is never rebuilt from mRNA in the cell, and there would be no need since the template is unchanged;
31. D2.1.9 [1]
  • B: in stage B (anaphase II) single chromatids are moving to opposite poles; in stage A (anaphase I) whole chromosomes, still made of two chromatids, are separated;
32. D2.2.3 [1]
  • C: degradation limits the number of rounds of translation; stability is a control point;
33. D2.3.9 [1]
  • C: water potential is a measure of the potential energy of water, and matter moves spontaneously from higher to lower potential energy;
34. D3.2.20 [1]
  • D: recombinant frequency measures separation: the closer the genes, the fewer crossovers between them (up to a maximum of 50 % for distant/unlinked genes);
35. D3.2.13 [1]
  • C: II-4 is affected, so carries the dominant allele, but has an unaffected (homozygous recessive) child III-3, so must also carry the recessive allele; I-2 and II-3 are unaffected (hh) and III-5 could be HH or Hh;
36. D3.3.5 [1]
  • B: uncoupling proteins let protons leak across the inner mitochondrial membrane, so the energy of fat oxidation is dissipated as heat instead of captured in ATP, important in infants and hibernators;
37. D4.1.9 [1]
  • D: fitness = reproductive success; a weak but prolific individual is fitter (in this sense) than a strong childless one;
38. D4.1.8 [1]
  • A: with no predators, the only selection pressure acting on colour is female mate choice, so the increase in brightness shows that bright males had greater reproductive success; predation was absent in these ponds, colour is inherited rather than induced, and mutation rate is not affected by predators;
39. D4.2.13 [1]
  • D: species diversity, primary production and food-web complexity all increase during primary succession (along with plant size and nutrient cycling);
40. D4.2.9 [1]
  • D: 25 ÷ 0.005 = 5000;
41. D4.2.4
  • (a) [1]
    • 1.9 (mg dm⁻³);

    Accept 1.8–2.0. Units required.

  • (b) [2 max]
    • pondweed photosynthesises (in the light) and releases oxygen;
    • oxygen released exceeds the oxygen used by the plants in respiration, so it accumulates;
    • the rise slows / levels off because carbon dioxide (or a mineral nutrient) becomes limiting / oxygen is saturated, OWTTE;

    Accept any two.

  • (c) [2]
    • jar B;
    • oxygen produced by the plants is balanced by oxygen used by the snails (and plants) / oxygen stays close to constant / carbon dioxide from the snails supplies the plants, OWTTE;

    Reason mark requires reference to the data or to the balance between the organisms.

  • (d) [2 max]
    • sealed / closed to matter but open to energy (light);
    • contains producers (autotrophs) to supply energy / food;
    • contains decomposers (saprotrophs) so nutrients are recycled;
    • large enough / diverse enough for interactions between organisms to occur;

    Accept any two.

  • (e) [2]
    • energy is lost as heat at each trophic level / in respiration and cannot be recycled;
    • nutrients (carbon, nitrogen) are recycled by decomposers / pass through cycles and are reused, OWTTE;

    Content pivot.

42. C2.1.10
  • (a) [2]
    • uptake increases with insulin concentration;
    • (but) levels off/plateaus at high concentrations (between 10 and 100 nmol dm⁻³), OWTTE;
  • (b) [1]
    • 380 % / a 3.8-fold increase, from (4.8 − 1.0)/1.0 × 100;
  • (c) [2]
    • (essentially) all receptors are occupied/saturated at high hormone concentration;
    • (so) further insulin cannot increase the response / the number of receptors (or transporters) becomes limiting, OWTTE;
  • (d) [2 max]
    • the antibody blocks the hormone-binding site (extracellular domain);
    • insulin cannot bind, so the (tyrosine kinase) receptor is not activated / no signalling cascade;
    • (so) GLUT4 is not moved to the membrane and uptake stays near 1.0, OWTTE;
  • (e) [1]
    • ligand / (an) agonist;
43. B2.2.4
  • (a) [1]
    • cardiac (heart) muscle;
  • (b) [2]
    • oxygen consumption increases as the number of mitochondria per cell increases (a positive relationship);
    • the relationship is approximately proportional, e.g. heart has about 3 times the mitochondria and about 2.6 times the oxygen consumption of liver, OWTTE;

    Accept any correct quantitative comparison for the second mark.

  • (c) [2]
    • they have no mitochondria, so they cannot carry out the Krebs cycle or oxidative phosphorylation;
    • they obtain ATP by (anaerobic) glycolysis instead, so none of the oxygen carried is used up in transit, OWTTE;
  • (d) [2 max]
    • folding into cristae gives a large surface area for electron transport chains and ATP synthase;
    • the membrane is impermeable to protons, so a steep gradient can be maintained across it;
    • electron carriers and ATP synthase are embedded in a fixed order/close together, so electrons and protons pass efficiently, OWTTE;
  • (e) [2 max]
    • the two variables are closely related here, so counting mitochondria gives a useful first estimate;
    • but mitochondria differ in size and in the area of cristae, so the same number can give different ATP output;
    • activity also depends on substrate and oxygen supply at the time, so a direct measure of oxygen use or ATP production is more reliable, OWTTE;
44. A2.3.6
  • (a) [2]
    • as the number of amino acid differences increases, vaccine effectiveness decreases (a negative relationship);
    • the fall is large, from 61 % at 2 differences to 19 % at 12 differences;
  • (b) [1]
    • about 42 %;

    Accept any value in the range 35-48 %.

  • (c) [2 max]
    • the influenza genome is RNA and is copied without proofreading, so mutations occur at a high rate;
    • variants whose surface proteins are not recognized by antibodies in the population survive and reproduce (natural selection);
    • these variants therefore spread, so the circulating virus drifts further from the vaccine strain each year, OWTTE;
  • (d) [2]
    • the age or health of the people vaccinated (immune response is weaker in the elderly);
    • the proportion of the population vaccinated / how early in the season they were vaccinated;
    • previous exposure of the population to related strains;
    • how effectiveness is defined and measured (laboratory-confirmed infection, hospital admission);

    Mark the first two only.

  • (e) [2 max]
    • sequences from viruses circulating now are compared to identify which variants are increasing in frequency;
    • the strain chosen is the one predicted to be dominant next season, so that the number of amino acid differences is as small as possible;
    • the decision must be made months in advance to allow time for manufacture, so it is a prediction that can turn out wrong, OWTTE;

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