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

• A: capillary action depends on adhesion to polar surfaces (with cohesion following behind);

• C: the (near-)universal genetic code is most simply explained by inheritance from a universal common ancestor; a later change to the code would alter almost every protein and be lethal, so it has been conserved;

• C: 22 pairs plus X and Y would give 46; the extra copy of chromosome 21 makes 47;

• C: same underlying structure, different function = homology, evidence of divergence from a common ancestor; analogous = similar function, different structure (convergence);

• B: seeds are stored dried and cold so they remain viable for decades; this ex situ germplasm storage insures against loss of species and crop varieties (genetic diversity), allowing future reintroduction/breeding;

• C: glucose is small and polar (hydroxyl groups), so it dissolves and is transported easily; it is also chemically stable and readily oxidised;

• D: peptide bonds form between the amine group of one amino acid and the carboxyl group of the next, by condensation, during translation on ribosomes;

• A: small non-polar O₂ crosses freely; polar glucose needs a transporter; charged Na⁺ needs channels or pumps;

• D: in a eukaryote the nuclear envelope separates transcription from translation, so mRNA can be modified before it meets ribosomes; in a prokaryote ribosomes can bind the mRNA immediately, so the unmodified message is translated; A fails because bacteria have their own ribosomes; B fails because translation occurs inside the cell, not in the medium; C fails because the stem states that transcription did occur;

• A: volume scales with the cube of length, surface area with the square, so volume outpaces area and the SA:V ratio falls;

• C: CO₂ produced/present in mesophyll cells diffuses into the (spongy) air spaces and exits through open stomata; (during daylight net flow of CO₂ is inward, but any CO₂ leaving takes this path);

• A: temperature raises the evaporation rate and the air's capacity for vapour; wind removes the humid boundary layer at the leaf surface, so vapour diffuses out faster;

• C: facultative mixotrophs use whichever mode conditions favour (photosynthesis in light, feeding when prey is abundant); obligate mixotrophs must use both;

• B: tube 1 is the undiluted stock, so only four tenfold dilutions have been carried out: 1.0 × 10⁻⁴ mol dm⁻³; A and C are the off-by-one errors of counting three or five dilution steps;

Design move: near-miss discrimination — the two adjacent powers of ten are the distractors, so the tube must be counted, not the transfers.

• A: glucose + oxygen → carbon dioxide + water (with ATP produced);

• A: correlation between absorption and rate shows chlorophyll captures the light energy used; accessory pigments explain the differences;

• C: clearing the transmitter ends each signal; without it the postsynaptic membrane stays depolarized/blocked (as with nerve agents and some insecticides);

• C: CO₂/pH-sensitive chemoreceptors (in the medulla and arteries) feed the medulla's respiratory centre, which increases signals to the diaphragm and intercostal muscles, driving faster, deeper ventilation;

• D: a community is all the populations of different species living and interacting in an area; including the abiotic environment as well would make it an ecosystem;

• A: photoautotrophs (plants, algae, cyanobacteria) use light energy, whereas chemoautotrophs (e.g. iron-oxidizing or nitrifying bacteria) use energy released by exothermic oxidation of inorganic chemicals;

• B: pairing rules (A with T, G with C) mean the template dictates the new sequence exactly (new strand is complementary, so the copy of the partner strand is identical);

• A: each tRNA has an anticodon and carries the corresponding amino acid; codon–anticodon pairing at the ribosome orders the amino acids;

• B: a single-nucleotide polymorphism (SNP) is a position where a base substitution has produced two variants common in the population, and because the genetic code is degenerate the two codons can code for the same amino acid; A fails because a SNP is defined by the base difference, not by its effect; C fails because SNPs are substitutions and ribosomes cannot skip bases; D fails because the stem places the variant in the coding sequence and most codon changes at the third position are silent;

• B: nuclear division (mitosis/meiosis) distributes the replicated chromosomes; cytokinesis then splits the cytoplasm;

• B: pollination is only the transfer of pollen to the stigma; fertilization is the fusion of male and female gametes inside the ovule, producing the embryo;

• D — phenotypic plasticity is the capacity to develop different traits, from the same genotype, suited to the environment experienced;

• A: without insulin the target cells are not stimulated to take up glucose, so blood glucose stays high after a meal; the glucagon loop is unaffected;

• C: mutation is random and prior; the antibiotic changes the selection pressure, so pre-existing resistant variants are favoured;

• A: DDT is persistent and fat-soluble; each consumer eats many prey over its life and keeps their DDT, so the toxin becomes more concentrated with each transfer;

• A: warming extends vector/pest ranges and (usually) increases generations per year; III is backwards;

• (a) [2]
  • normal insulin lowers blood glucose strongly (4.6), whereas the mutant insulin lowers it only slightly (0.9);
  • saline causes almost no change (0.2), so the injection itself is not responsible, OWTTE;
• (b) [3]
  • insulin is a protein whose three-dimensional shape (including its binding site) is determined by its amino-acid sequence;
  • the single amino-acid change alters the shape of the receptor-binding region;
  • (so) the mutant insulin binds its receptor poorly, triggering little signalling, so cells take up/store less glucose, OWTTE;
• (c) [1]
  • to show that the injection procedure/fluid alone does not lower blood glucose, so any fall is due to the insulin, OWTTE;

• (a) [2]
  • rate of CO2 production increases with glucose concentration up to about 6–8 %;
  • above about 8 % glucose concentration, the rate levels off (no further increase), OWTTE;
• (b) [2 max]
  • at low glucose concentrations, glucose (substrate) availability limits the rate of respiration, so more glucose increases the rate;
  • at higher concentrations another factor becomes limiting, e.g. the amount/activity of respiratory enzymes (or oxygen for the aerobic component), OWTTE;
• (c) [1]
  • temperature (accept: yeast concentration/mass, volume of suspension, time allowed);
• (d) [2]
  • carbon dioxide;
  • produced by anaerobic respiration / alcoholic fermentation in yeast;

• (a) [1]
  • 24.1 ÷ 9.8 ≈ 2.5 (times);

  Accept 2.4–2.5.

• (b) [1]
  • lipids release (roughly) twice as much energy per gram as carbohydrates;
• (c) [2]
  • heat is lost to the surroundings/air/tube (not all transferred to the water);
  • incomplete combustion of the sample / some sample remains unburned / heat absorbed by the glass, OWTTE;

  Mark the first two answers only.

• (d) [2]
  • lipid stores (about) twice the energy per gram / is stored without (associated) water;
  • (so) the bird carries less mass for the same energy, which matters for flight, OWTTE;

  Content pivot.

• (a) [2 max]
  • reads from the diagram that 40 eyepiece divisions align with 0.5 mm of the stage micrometer;
  • converts to 500 µm and divides by 40;
  • 12.5 (µm);

  Award full marks for a correct answer with no working. Accept 12.5 µm per division. ECF from a misread alignment.

• (b) [1]
  • 325 (µm);

  ECF from part (a). Units not required.

• (c) [1]
  • ± 6.25 (µm) / ± 6.3 (µm);

  ECF from part (a). Accept ± half of the value found in (a).

• (d) [2 max]
  • the graticule divisions are arbitrary / have no fixed size of their own;
  • the size a division represents depends on the magnification (of the objective in use), so it changes between objectives;
  • so it must be compared with a scale of known length, the stage micrometer, before any reading means anything, OWTTE;

  Do not accept 'the graticule is not accurate'. Accept 'it is not to scale' only with a reason.