Biology by Bradford
IB Biology Paper 2 mock. The whole syllabus, at the real length.
A complete Paper 2 (Higher level) built from the 2025 guide: 81 marks across 13 questions, in 2 hours 30 minutes. 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.
What the real paper requires
2 hours 30 minutes · 80 marks · 44% of the final grade.
- Section A48 marks One long data question, then short-answer questions. Answer all. Longer than SL and drawing on additional higher level content.
- Section B32 marks Three extended-response questions. Answer two. Each is three parts from different areas summing to 15, plus 1 mark for construction.
On the front of the paper
- A calculator is required for this paper.
- Answers must be written within the answer boxes provided.
- Section B: answer two questions.
For the calculations, practise with IB Biology data booklet skills.
How this mock is built
Section A opens with a long data question, then fills to 48 marks evenly across the syllabus with about 30% additional higher level material. Section B offers three themes and marking on screen counts the best two you answer, plus their construction marks, so the total matches the cover.
The code on the paper rebuilds this exact mock and its markscheme. Build a fresh mock draws a new one from the same rules, so a second sitting is never the first one memorised.
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The mock
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.
- Do not open this examination paper until instructed to do so.
- Section A: answer all questions.
- Section B: answer two questions.
- A calculator is required for this paper.
- Answers must be written within the answer boxes provided.
Answer all questions. Answers must be written within the answer boxes provided.
Western mosquitofish (Gambusia affinis) were collected from a stream in which large predatory fish were present and were used to found two new populations in isolated ponds containing no predators. After 15 generations, researchers caught 30 adult males from the source stream and from each pond and recorded the number of dark pigment spots on the body and the body length of each male. To test whether any differences were heritable, offspring of each population were reared from birth in identical aquaria in the laboratory and the number of spots was counted when they became adults. Mean values are shown with the standard error (SE).
| Population | Mean number of spots per wild-caught male ± SE | Mean body length of wild-caught males / mm ± SE | Mean number of spots per laboratory-reared male offspring ± SE |
|---|---|---|---|
| Source stream (predators present) | 3.2 ± 0.4 | 24.8 ± 0.6 | 3.4 ± 0.5 |
| Pond 1 (no predators) | 9.1 ± 0.7 | 25.9 ± 0.7 | 8.6 ± 0.8 |
| Pond 2 (no predators) | 8.4 ± 0.6 | 25.4 ± 0.6 | 8.0 ± 0.7 |
Two species of treefrog, P and Q, look alike. Males attract females by calling, and the calls of the two species differ in pulse rate. In some areas both species breed in the same ponds; in other areas only species P occurs. Females of species P from both kinds of area were tested one at a time in a tank with two loudspeakers, one playing the call of a P male and the other the call of a Q male. The graph shows the percentage of females that moved to each loudspeaker (50 females from each kind of area).
Cabbage, kale, broccoli, cauliflower, kohlrabi and Brussels sprouts are all varieties of one species, Brassica oleracea, produced by farmers over the last few thousand years from a wild coastal plant.
Membrane proteins have hydrophobic amino acids on the surfaces that contact the bilayer core, while soluble cytoplasmic proteins have hydrophilic surfaces. Explain this difference.
Explain how the primary structure of a protein determines its three-dimensional shape.
The diagram shows a mammalian heart in the frontal plane. Identify the structures labelled V, VI and VII.
In a free-air carbon dioxide enrichment (FACE) experiment in a forest, CO₂ was released from rings of pipes so that experimental plots were maintained at 550 ppm CO₂, while control plots remained at the atmospheric concentration of about 415 ppm. Tree growth was monitored for ten years.
The herbicide DCMU blocks the transfer of electrons out of photosystem II. Predict its effect on oxygen production and on the Calvin cycle, giving reasons.
The same short region of a gene was copied by the polymerase chain reaction (PCR) from a normal allele and from three mutant alleles, P, Q and R. Each mutant allele carries one gene mutation, which lies within the copied region. The copies were separated by gel electrophoresis beside a ladder of DNA fragments of known lengths. Shorter fragments move further through the gel from the wells than longer fragments. The figure shows the gel.
Libraries of knockout organisms, in which a different single gene has been made inoperative in each strain, are available for model species such as the mouse (Mus musculus) and the yeast Saccharomyces cerevisiae. Suggest advantages to researchers of having access to such a library.
Answer two questions. Answers must be written within the answer boxes provided. One additional mark is available for the construction of your answers for each question.
Molecular techniques have transformed how biology is studied and applied.
The community found in a place depends on its abiotic conditions, and it changes through time both by succession and in response to a rapidly warming climate.
Tissues are supplied reliably only if blood is kept flowing under pressure, held constant in composition and sealed in when vessels are damaged.
Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
Rebuild or edit this exact paper (and its markscheme): biologybybradford.com/exam-maker?code=BbB-GQH______wAAZbiB
Show the markscheme
Markscheme BbB-GQH______wAAZbiB
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.
- (a) [1]
- presence or absence of predators (in the habitat / population);
Do not accept 'the population' or 'the pond' without reference to predators.
- (b) [2]
- (9.1 − 3.2) / 3.2 × 100;
- 184 %;
Accept 184–185 %. Award [2] for the correct answer with no working. ECF for a correct method with an arithmetic slip.
- (c) [2]
- not (shown to be) significant;
- because the ranges of mean ± SE overlap / 24.2–25.4 mm and 25.2–26.6 mm overlap, OWTTE;
The second mark requires reference to overlap of the standard-error ranges. Do not award for 'the difference is small' alone.
- (d) [2 max]
- to find out whether the difference in number of spots is heritable / genetic, rather than caused by the environment (e.g. diet, water, light) of each habitat;
- identical conditions control the environmental variables, so any remaining difference must be inherited, OWTTE;
- evolution is change in heritable characteristics, so only a heritable difference would count as evolution / acquired changes are not evolution;
- (e) [2 max]
- yes, it is evolution;
- laboratory-reared offspring of pond fish still have many more spots than offspring of stream fish (8.0–8.6 compared with 3.4) and the SE ranges do not overlap, so the characteristic is heritable;
- the change occurred in the characteristics of a population over generations (not within an individual), OWTTE;
- (f) [3 max]
- in the source stream, males with many spots are more conspicuous / more easily seen and eaten by predators (before they reproduce);
- so in the stream, alleles for many spots are removed / selected against, keeping the mean number low;
- in the ponds there are no predators, so spotted males survive as well as plain males;
Beyond this subtopic. Award if given; the marks above can be earned without it.
- D4.1 spotted males are preferred by females / have greater mating success, so they leave more offspring and alleles for many spots increase in frequency over the generations, OWTTE;
- (g) [3 max]
- in science a theory is a well-supported explanation that predicts and explains a broad range of observations (not a guess), OWTTE;
- it is impossible to formally prove that any scientific theory is true / to prove it corresponds exactly with reality;
- (because) future evidence could in principle contradict it / scientific knowledge is provisional;
- the theory is accepted as a pragmatic truth because it is consistent with all the evidence so far and works in practice;
- it is very unlikely ever to be falsified, given the breadth of the supporting evidence (e.g. sequence data, fossils, observed selection);
Content pivot (NOS). Do not accept 'because it has not been proved' as an unqualified statement.
- (a) [2]
- in both kinds of area most females moved to the call of a P male / their own species' call;
- the preference for the P call was stronger where both species breed together (96 % compared with 66 %) / fewer females moved to the Q call there (4 % compared with 34 %);
- in neither kind of area did every female choose the P call / some females from both kinds of area moved to the Q call;
Comparative points only. The second point is one mark however it is expressed: the P and Q percentages for one group add to 100, so 96 v 66 and 4 v 34 are the same comparison.
- (b) [2]
- the call is then the only difference between the two choices / other cues such as the male's size, appearance or smell are excluded;
- the same recorded call can be played to every female / calls are standardized in loudness and duration;
- so any preference shown must be a response to the call itself;
- live males might behave differently or move / might interact with each other or with the female;
- (c) [2]
- a P female that mates with a Q male produces only sterile offspring / has no grandchildren / wastes her eggs;
- so where Q is present, females that reject Q calls leave more offspring / selection favours stronger preference;
- the heritable preference for P calls therefore becomes more common in these populations;
- where Q is absent, females never meet Q males, so there is no such selection;
- (a) [3 max]
- farmers chose plants with desired traits (e.g. large leaves, swollen stems, enlarged flower buds) to breed from / selective breeding / artificial selection;
- the chosen traits are heritable, so they were passed on and became more pronounced over many generations, OWTTE;
- the heritable characteristics of each population changed / each variety diverged from the wild plant (and from the other varieties);
- large differences arose in only a few thousand years, showing how rapidly evolutionary change can occur;
- if selection by humans can produce this much change, selection by the environment (natural selection) over much longer times could produce even greater change, OWTTE;
- (b) [1 max]
- they can (still) interbreed / cross-pollinate to produce fertile offspring;
- they are not reproductively isolated from each other / gene flow between them is still possible, OWTTE;
- proteins fold so that surfaces match their (chemical) environment;
- in membranes, non-polar/hydrophobic R-groups face the (hydrophobic) hydrocarbon tails, anchoring the protein in the bilayer;
- in the cytoplasm, polar/charged R-groups face the (aqueous) surroundings, keeping the protein soluble (with hydrophobic R-groups buried inside), OWTTE;
- the primary structure is the sequence of amino acids (coded by the gene);
- the sequence determines where each type of R-group sits along the chain;
- R-group interactions (hydrogen bonds / ionic bonds / disulfide bridges / hydrophobic interactions) form between particular residues;
- (so) the chain folds into a specific/stable three-dimensional (tertiary) conformation, OWTTE;
- V, (left) atrioventricular valve / bicuspid (mitral) valve;
- VI, (aortic) semilunar valve;
- VII, septum;
- (a) [1]
- conditions (light/temperature/soil/rainfall/interactions with other organisms) are natural, so the results are more valid for predicting responses of real ecosystems / glasshouses alter conditions other than CO₂, OWTTE;
- (b) [1]
- to allow comparison with growth at current/ambient CO₂, so that differences can be attributed to the raised CO₂ (with other conditions the same);
- (c) [2 max]
- rate of photosynthesis increases;
- because CO₂ (concentration) is a limiting factor / more CO₂ is available for fixation (by Rubisco / in the Calvin cycle);
- (accept) the increase may diminish over time as other factors (light/nutrients/temperature) become limiting, OWTTE;
- oxygen production stops, because electrons are not being drained from photosystem II, so photolysis of water is not needed / halts;
- no electrons pass from photosystem II to photosystem I, so NADP is not reduced / no NADPH is made (and non-cyclic ATP production stops);
- so the Calvin cycle stops, as reduction of glycerate 3-phosphate needs NADPH (as well as ATP), OWTTE;
- (accept) some ATP can still be made using electrons emitted by photosystem I that return to it, but without NADPH the cycle cannot continue;
- (a) [2]
- Q: deletion, because its fragment is shorter than the normal fragment / has moved further than the normal fragment / has lost roughly 50 to 70 base pairs;
- R: insertion, because its fragment is longer than the normal fragment / has moved less far than the normal fragment / has gained about 100 base pairs;
Each mark needs both the type of mutation and a reason taken from the gel. Accept 'large/major deletion' and 'large/major insertion'. Do not accept 'frameshift' as the type of mutation.
- (b) [2]
- the mutation in P does not change the length of the fragment / is (probably) a (base) substitution;
- a substitution replaces one base with another, so the number of bases / the length of the fragment is unchanged;
- fragments of the same length move the same distance, so electrophoresis separates by length and cannot show a change in base sequence;
- (alternatively) an insertion or deletion of only one or a few bases changes the length too little for the two bands to be separated;
Do not award the first point for 'substitution' alone if the answer claims no other mutation could give this result; accept a very small insertion or deletion if it is linked to the gel being unable to resolve a difference of a few base pairs.
- (c) [2]
- determine the base sequence of (the PCR copy of) allele P / sequence it;
- compare / align it with the base sequence of the normal allele;
- a single base that differs, in a sequence of the same length, identifies a substitution (and its position);
- a strain lacking a gene of interest can be obtained without having to produce it, saving time and cost;
- the function of the gene can be investigated by comparing the phenotype of the knockout with that of the normal (wild-type) organism;
- the same strains are used by many laboratories, so results can be compared and repeated / reliability is improved;
- the effects of knocking out every gene in the genome can be surveyed systematically / genes of unknown function can be screened;
- knockouts of different genes can be combined by crossing to study interactions between genes, OWTTE;
Mark the first three only.
- (a) D1.1 [4 max]
- PCR amplifies a chosen sequence: cycles of denaturation (~95 °C), primer annealing, and extension;
- heat-stable (Taq) polymerase copies the target, doubling it each cycle (millions of copies from traces);
- primers determine which sequence is amplified;
- gel electrophoresis separates DNA fragments by size, DNA (negatively charged) moves toward the positive electrode;
- smaller fragments travel further, giving a band pattern;
- applications credited: DNA profiling (forensics/paternity) / diagnosis / sequencing preparation, OWTTE;
- (b) D2.2 [7 max]
- most control is at transcription;
- transcription factors bind specific (promoter) sequences;
- promoting or blocking the binding/activity of RNA polymerase;
- enhancers (possibly distant, looped into contact) increase transcription when bound;
- DNA (cytosine) methylation of promoters silences genes;
- histone modifications (acetylation/methylation) loosen or compact chromatin, changing accessibility;
- these epigenetic patterns differ between cell types and are inherited through cell division;
- the environment/signals can alter transcription-factor activity and epigenetic marks;
- mRNA stability/degradation (and translation rate) tune how much protein is made, OWTTE;
- (c) A3.2 [4 max]
- the same gene/sequence is aligned across the species being classified;
- (numbers of) base differences between each pair are counted;
- fewer differences indicate a more recent common ancestor / closer relationship;
- a cladogram is constructed (most parsimonious arrangement), grouping species into clades;
- differences accumulate at a roughly constant rate (molecular clock), so divergence times can be estimated / groups shown not to be clades are reclassified, OWTTE;
Plus 1 mark for the construction of the answer: clear enough to be understood without re-reading, succinct, with little or no repetition or irrelevant material.
- (a) D4.2.13, D4.2.12 [5 max]
- primary succession begins on newly formed substrate with no soil or previous community, e.g. bare rock after a volcanic eruption / moraine left by a retreating glacier / sand dunes;
- pioneer species (lichens / mosses / bacteria) colonize, tolerating harsh conditions, weathering the rock and adding organic matter when they die to form soil;
- as the soil deepens and holds more water and nutrients, larger plants replace the pioneers: grasses and herbs, then shrubs, then trees / increase in plant size;
- primary production increases as larger plants capture more light;
- species diversity increases (more habitats and niches) and food webs become more complex;
- nutrient cycling increases as biomass and decomposer activity increase;
- each community changes the abiotic environment (shade, soil, humidity) and the biotic environment, making conditions suitable for the next stage, until a climax community forms, OWTTE;
Accept any suitable terrestrial example.
- (b) B4.1.6, B4.1.7 [3 max]
- for any combination of temperature and rainfall (pattern) one type of natural ecosystem is likely to develop / biome distribution can be shown on a graph with these two variables as axes;
- a biome is a group of ecosystems with similar communities because of similar abiotic conditions (and convergent evolution);
- e.g. hot desert: very low rainfall and high temperatures / tundra: very low temperatures and low precipitation;
- e.g. tropical forest: high temperature and rainfall all year / temperate forest: moderate temperature and rainfall with seasons / taiga: long cold winters / grassland: rainfall too low to support forest;
- similar biomes on different continents contain unrelated species with similar adaptations because of convergent evolution, OWTTE;
Award the example points for any two biomes with both temperature and rainfall correctly characterized.
- (c) D4.3.10, D4.3.11 [7 max]
- phenology is the study of the timing of biological events, e.g. flowering / budburst / bird migration / nesting / emergence of insects;
- timing is cued by variables such as temperature and photoperiod (day length);
- warming advances events cued by temperature, e.g. earlier budburst / flowering / earlier peak of caterpillar biomass;
- events cued by photoperiod do not shift, because day length is unaffected by warming;
- (so) synchrony between interacting populations can be disrupted / a mismatch arises when one species uses temperature and the other uses photoperiod;
- e.g. Arctic mouse-ear chickweed grows earlier in warm springs but reindeer migration (cued by photoperiod) arrives at the same time, so calves miss the peak of food and survival falls;
- e.g. great tits: caterpillar biomass peaks earlier in warmer springs but egg-laying has advanced less, so chicks hatch after peak food and breeding success falls;
- warmer conditions allow some insects to complete more life cycles per year, e.g. spruce bark beetle two generations instead of one, so populations grow faster and outbreaks kill more trees;
- some species benefit (longer growing season / range expansion) whereas others decline, so effects are not uniformly negative;
- natural selection may allow populations to adapt, e.g. earlier breeding in some great tit populations, but change may be too fast for long-lived species, OWTTE;
Accept common or scientific names. A balanced discussion is expected: at least one point on species that benefit or adapt is needed for full marks.
Plus 1 mark for the construction of the answer: clear enough to be understood without re-reading, succinct, with little or no repetition or irrelevant material.
- (a) C3.2.3 [3 max]
- damage to a blood vessel exposes platelets to the damaged tissue / collagen, so platelets stick to the site and to each other;
- platelets release clotting factors;
- the clotting factors trigger a cascade of reactions, each step activating the next / amplifying the response;
- the cascade produces thrombin, which rapidly converts soluble fibrinogen into insoluble fibrin;
- fibrin forms a mesh of fibres that traps erythrocytes / red blood cells to form a clot, sealing the cut and preventing blood loss and entry of pathogens, OWTTE;
Accept "prothrombin is converted to thrombin" as part of the cascade point. No further details of the cascade are required.
- (b) B3.2.3, B3.2.5 [4 max]
- arteries have thick walls, whereas veins have thin walls (relative to the diameter of the vessel);
- arteries have a narrow lumen (relative to the wall), whereas veins have a wide lumen;
- arteries have thick layers of smooth muscle and elastic tissue to withstand high blood pressure, whereas veins have little muscle and elastic tissue because pressure is low;
- elastic tissue in arteries stretches during systole and recoils, maintaining / evening out the pressure between heartbeats, whereas veins do not maintain pressure;
- veins have valves to prevent backflow, whereas arteries have no valves because pressure keeps blood moving forward;
- the flexible wall of a vein allows it to be compressed by skeletal muscle action, squeezing blood towards the heart, whereas the wall of an artery resists compression / stays circular in section, OWTTE;
Award marks only for paired, comparative statements. Accept converse wording.
- (c) D3.3.3, D3.3.4 [8 max]
- blood glucose concentration is monitored by endocrine cells in the pancreas / islets of Langerhans;
- when glucose rises (after a meal) β-cells secrete insulin into the blood;
- insulin is transported in the blood to target cells, e.g. liver / muscle / adipose cells;
- insulin stimulates uptake of glucose by cells (more glucose transporters in their membranes) and conversion of glucose to glycogen in liver and muscle / to fat;
- (so) blood glucose falls back towards the set point;
- when glucose falls (between meals / during exercise) α-cells secrete glucagon;
- glucagon stimulates the liver to break down glycogen to glucose and release it into the blood, raising blood glucose;
- (this is) negative feedback: each hormone reverses the change that caused its secretion / the two hormones have antagonistic effects;
- type 1 diabetes: β-cells are destroyed by the person’s own immune system (autoimmune), so little or no insulin is produced, usually beginning in childhood;
- type 2 diabetes: target cells become resistant to insulin / receptors fail to respond, and insulin secretion may later decline; risk factors include obesity, diets rich in sugar and fat, lack of exercise, genetic factors and age;
- type 1 is treated with insulin injections (and monitoring of blood glucose), whereas type 2 is treated / prevented by diet, exercise and weight loss (and drugs); in both, persistently high blood glucose causes glucose in urine / thirst / damage to blood vessels, OWTTE;
Accept "glycogen breakdown" for glycogenolysis. Do not accept "glucagon converts glucose to glycogen".
Plus 1 mark for the construction of the answer: clear enough to be understood without re-reading, succinct, with little or no repetition or irrelevant material.
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