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

• 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;

• B: the river isolated the two populations geographically so no alleles passed between them, and different conditions on each side (such as the presence or absence of gorillas as competitors) favoured different heritable traits, so the populations diverged;

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

• C: 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;

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

• C: channel/carrier protein, passive, down the gradient;

• D: the membrane-bound compartment concentrates hydrolytic enzymes on the target and protects the cell's own structures from digestion; products are then absorbed across the vacuole membrane;

• C: 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: for any shape, surface area scales with the square of the length (3² = 9) and volume with the cube (3³ = 27), so the ratio changes by 9/27, to one third; B has the ratio moving the wrong way; C and D use the wrong powers of 3;

• B: in quiet exhalation the diaphragm and external intercostals relax, so the diaphragm domes upwards and the ribs fall, reducing the volume of the thorax and raising the pressure in the lungs above atmospheric, so air flows out; at the end of each inhalation and exhalation air flow stops because the pressures are equal;

• A: faster evaporation from leaf cell walls draws more water out of the xylem, so the tension (negative pressure) rises and, through cohesion, the whole column is pulled up faster; root pressure is a push from the roots that transpiration does not raise, and xylem pressure becomes more negative, not positive;

• C: killing fungi cuts the mass lost from 58 % to 24 % and killing bacteria cuts it only to 44 %, while killing both leaves 6 %, so both groups of saprotrophs decompose the litter and fungi contribute more; A reverses this, only 6 % is lost without saprotrophs (B), and the mesh kept soil animals out (D);

• A: 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;

The two adjacent powers of ten are the distractors, so the tube must be counted, not the transfers.

• D: aerobic respiration requires oxygen and has a much higher ATP yield, producing carbon dioxide and water; anaerobic respiration in human muscle needs no oxygen, yields far less ATP, and produces lactate;

• D: 30 × 30 kJ = 900 kJ is transferred to ATP, and 900 ÷ 2870 × 100 = 31 %; the other 69 % is released as heat; ADP and phosphate hold less energy than ATP, since energy must be supplied to convert them to ATP, so the rest cannot be stored in them;

• B: separation reflects the balance between solubility (mobile phase) and adsorption (stationary phase); Rf = distance moved by pigment ÷ distance moved by solvent;

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

• B: 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;

• B: 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;

• C: 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);

• D: the table of the genetic code is a table of mRNA codons, so the amino acids come from the codons AUG, GCU and CAA (methionine, alanine, glutamine), not from the anticodons, which are complementary to them; steps 1 and 2 are correct, because the mRNA is complementary to the template (A) and an anticodon is complementary, not identical, to its codon (B); reading the anticodons backwards still does not give the codons (D);

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

• B: pollen lands on the stigma (II) (pollination), a pollen tube grows down the style to an ovule in the ovary (III), and a male gamete fuses with the female gamete inside the ovule (fertilization); A is wrong because the tube grows from the stigma to the ovary, C because the gametes fuse inside the ovule, not on the stigma, and D because pollen must land on the stigma, not on an anther;

• B: no child of an affected father in this pedigree is affected (III-1 and III-2 are the children of the affected II-2), so the condition is not passed on by males;

• D: 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;

• B: differences in adaptation lead to differential survival, then differential reproduction, so the trait becomes more common in the population; need or use does not direct variation;

• A: the allele was already carried by rats trapped in 1985, so the mutation (III) came before the first bait (II) and was not caused by it; rats with the allele can only out-survive other rats on the bait (I) once the bait is laid, and the rise in the allele's frequency (IV) is the result of that differential survival and reproduction; A has the allele arising only after the bait, which the 1985 samples rule out, B has rats surviving the bait before any was laid, and D puts the rise in frequency before its cause;

• D: once the algae died, saprotrophic bacteria multiplied as they decomposed them; their aerobic respiration increased the biochemical oxygen demand (BOD), so oxygen fell after the algal peak rather than during it, OWTTE;

A is contradicted by the graph: dissolved oxygen was still close to its highest (10.3 mg dm⁻³) when the algae were most abundant, and fell only as they died.

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

• (a) [3]
  • X-linked recessive;
  • all affected individuals are male, and the condition passes from an affected man, through an unaffected daughter, to an affected grandson (criss-cross inheritance / skips a generation);
  • it is recessive because unaffected parents (II-1 × II-2) have an affected child (III-1); autosomal recessive is less likely because it would affect both sexes roughly equally, OWTTE;
• (b) [2]
  • I-1 = XᵃY;
  • II-1 = XᴬXᵃ (an unaffected carrier / heterozygous);
• (c) [2]
  • a son is affected with probability ½ (he receives Xᵃ or Xᴬ from the carrier mother, and Y from the father);
  • a daughter is a carrier with probability ½ (she receives Xᴬ from her father and either Xᴬ or Xᵃ from her mother; a daughter cannot be affected here as the father is unaffected), OWTTE;

• (a) [1]
  • secondary consumer / third trophic level;
• (b) [1]
  • 140 ÷ 1300 × 100 = 10.8 % / 11 %;

  Accept 10.7–11.

• (c) [2 max]
  • not all limpets are eaten by sea stars (some die and pass to decomposers);
  • parts of the limpet (shell) are not digested / egested;
  • much of the energy absorbed is released as heat in cell respiration / used for movement rather than growth;
• (d) [2]
  • a prediction: dog whelks increase (at least at first) OR dog whelks decrease;
  • a reason matching the prediction: (increase) sea stars no longer eat mussels and barnacles, so more of this food is available to the whelks / there is less competition for mussels and barnacles OR (decrease) gulls, no longer able to feed on sea stars, eat more dog whelks, OWTTE;

  Content pivot. Award the first mark only when a matching reason is given. Also accept a later decrease if mussels crowd out barnacles, with reasoning.

• (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) [1]
  • 47 (species);

  Accept 45–49.

• (b) [2]
  • number of species increases with (island) area / positive correlation;
  • the increase levels off / each extra unit of area adds fewer species at larger areas, OWTTE;
• (c) [1 max]
  • larger (continuous) reserves protect more species (than small fragments) / fragmenting habitat reduces the number of species an area can support, OWTTE;
• (d) [2 max]
  • the island can hold only a small population, so a single event (storm, fire, disease or an introduced predator) could wipe out the whole species;
  • a small population loses genetic diversity (through inbreeding), so it is less able to survive new threats, OWTTE;
  • there is no other population to recolonize the island, so an ex situ population (captive breeding / stored germ plasm) is needed as insurance;
  • no single approach is sufficient: in situ protection needs to be combined with ex situ measures, OWTTE;

  Content pivot.