IB Biology Paper 2 mock. The whole syllabus, at the real length.

• (a) [2]
  • volume of oxygen consumed = 44 (mm) × 1.0 (mm²) = 44 mm³ (in 10 minutes by 5.0 g);
  • rate = 44 ÷ (5.0 × 10) = 0.88 mm³ g⁻¹ min⁻¹;

  Accept 0.88 to 0.9. Award ECF for a correct method using a wrongly read distance. Units are required for the second mark.

• (b) [2 max]
  • rate increases with temperature from 10 °C to 30 °C (12 mm to 58 mm);
  • rate reaches a maximum at (about) 30 °C;
  • above 30 °C the rate falls, steeply between 35 °C and 40 °C (52 mm to 21 mm), OWTTE;

  Figures from the table are needed for at least one point.

• (c) [2]
  • 25 °C versus 30 °C: significant, because the ranges of mean ± SE (41.7 to 46.3 and 55.1 to 60.9) do not overlap;
  • 30 °C versus 35 °C: not significant, because the ranges of mean ± SE (55.1 to 60.9 and 47.4 to 56.6) overlap, OWTTE;

  Both the conclusion and the overlap reason are required for each mark. Accept "error bars would overlap / not overlap".

• (d) [2 max]
  • higher temperature gives molecules more kinetic energy, so substrates and enzymes (of cell respiration) collide more frequently;
  • more of the collisions have enough energy to react / the enzyme-catalysed reactions of respiration occur faster, OWTTE;
  • faster respiration uses oxygen faster (in the mitochondria / as the final electron acceptor), so the drop moves further, OWTTE;
• (e) [1]
  • individual larvae differ in their tolerance of high temperature, so some are damaged / stressed more than others, giving more variable results, OWTTE; OR larvae are more active / move about at high temperature, increasing variation between respirometers, OWTTE;

  Do not accept "human error" or "the equipment was less accurate" without a link to temperature.

• (f) [2 max]
  • the reduction in rate caused by 40 °C is not reversed on return to 25 °C / the damage is permanent, because the treated larvae still respire more slowly than the control at 25 °C;
  • longer exposure causes more damage, because 60 minutes reduced the rate (to 9 mm) more than 10 minutes (27 mm);
  • the differences are significant, because the mean ± SE ranges of the control and the treated groups do not overlap, OWTTE;

  Accept reference to denaturation of respiratory enzymes as a reason for the permanence.

• (g) [3 max]
  • cell respiration is a (series of) chemical reactions inside cells, whereas gas exchange is the physical process of diffusion of gases across a surface;
  • cell respiration releases energy from carbon compounds (glucose / fatty acids) to produce ATP, whereas gas exchange does not involve energy release / ATP production;
  • cell respiration is catalysed by enzymes (in the cytoplasm and mitochondria), whereas gas exchange occurs by passive diffusion without enzymes;
  • cell respiration consumes oxygen and produces carbon dioxide inside cells, whereas gas exchange moves oxygen into the organism and carbon dioxide out (across the body surface / tracheae / lungs);
  • anaerobic respiration can occur without gas exchange of oxygen, whereas gas exchange of oxygen is only useful if aerobic respiration is occurring, OWTTE;

  Content pivot. Award marks only for explicitly paired statements. Do not accept "respiration is breathing".

• (a) [3 max]
  • in both cases humans hunted the birds directly and destroyed their habitat;
  • both species were flightless or flocked predictably, making them easy to kill, and neither had evolved with human hunters;
  • the dodo was also affected by introduced species (pigs, rats, monkeys) eating eggs and young, whereas the passenger pigeon was not;
  • the passenger pigeon was killed on an industrial scale for markets, transported by rail and telegraph, whereas the dodo was taken by small numbers of sailors and settlers;

  Award marks only for explicitly comparative statements.

• (b) [2 max]
  • the species bred only in enormous colonies, so once numbers fell below a threshold breeding failed (small flocks could not raise young);
  • harvesting removed adults faster than the low reproductive rate (one egg per pair) could replace them;
  • loss of the mature forest that supplied its food acted at the same time as hunting, so numbers collapsed rapidly, OWTTE;

• carbon forms covalent bonds / bonds in which pairs of electrons are shared (between atoms);
• each carbon atom can form up to four bonds (with other atoms);
• the bonds can be single or double, or a combination of single and double bonds;
• carbon bonds to other carbon atoms, forming chains that can be branched or unbranched;
• carbon atoms can also form (single or multiple) rings;
• carbon bonds to atoms of other non-metallic elements (e.g. hydrogen, oxygen, nitrogen), so molecules of many shapes and sizes are possible, OWTTE;

Do not accept 'carbon forms ionic bonds'. Accept 'strong / stable bonds' as part of the first point.

• (a) [1]
  • the development of an (unspecialized) cell into a specialized cell (with a particular structure and function) by expressing some genes but not others, OWTTE;
• (b) [2 max]
  • the transplanted cells release a signalling chemical (that they would also have released in their original position);
  • the chemical diffuses away from the transplant, forming a concentration gradient in the surrounding host tissue;
  • host cells respond to the concentration they receive by switching on / expressing the genes for the structures of the donor region, OWTTE;
  • the host cells had the same genome as before but their pattern of gene expression was changed by their new position in the gradient;

  Accept 'morphogen' for signalling chemical. Do not accept answers based on the transplanted cells themselves forming the new structures.

• (a) [2 max]
  • the enzyme lowers the activation energy (peak is lower);
  • so at a given temperature more substrate molecules have enough energy to reach the transition state / react per unit time;
  • binding at the active site strains/weakens bonds in the substrate, OWTTE;
• (b) [1]
  • exergonic because the products are at a lower energy level than the substrates / energy is released;
• (c) [1]
  • energy released depends only on the difference between substrate and product energy levels, which the enzyme does not alter / enzyme affects only the route (transition state), not the start and end points, OWTTE;

• a (small) folded RNA (cloverleaf, held by base pairing) with an anticodon of three bases;
• the (3′) end carries the specific amino acid corresponding to its anticodon (attached by a tRNA-activating enzyme);
• the anticodon pairs (complementarily) with the mRNA codon at the ribosome, delivering the correct amino acid to the growing polypeptide;

• many species can play overlapping/similar roles (redundancy);
• if one species is lost, others can (partly) take over its function;
• genetic diversity within species allows populations to adapt (to disease/change);
• complex food webs offer alternative energy pathways, so a single loss does not collapse the web, OWTTE;

• (a) D4.1.7, D4.1.8 [7 max]
  • individuals vary in physical and behavioural traits, e.g. colour / size of ornaments / courtship displays / song;
  • some of these traits act as signs of overall fitness / health, and are used by the opposite sex (usually females) when choosing a mate;
  • individuals with the preferred traits are more successful in attracting mates / mate more often;
  • (so) they produce more offspring and pass on the alleles for the trait (and for the preference);
  • the trait becomes more common / more exaggerated in the population over generations;
  • sexual selection can favour traits that reduce survival, e.g. conspicuous plumage, because the mating advantage outweighs the survival cost;
  • example: the elaborate plumage and displays of male birds of paradise evolved through female choice;
  • Endler’s guppies: in pools with few predators males became more brightly coloured / more spots, because females prefer them;
  • in pools with predators, natural selection favoured duller males, which are less visible, so colour depended on the balance of the two pressures;
  • (so) sexual selection and natural selection can act in opposite directions and the outcome is a compromise, OWTTE;

  Accept competition between males (e.g. antlers, fighting) as sexual selection. A named example is required for full marks.

• (b) D3.2.14, D3.2.15 [4 max]
  • continuous variation: the trait can take any value across a range, whereas a discrete variable such as ABO blood group falls into distinct classes;
  • polygenic inheritance: several genes (at different loci) each contribute additively to the trait;
  • many combinations of alleles are possible, giving many intermediate phenotypes / a normal distribution;
  • environmental factors also affect the phenotype, e.g. exposure to sunlight (UV) darkens skin;
  • skin colour depends on the amount of melanin, determined by several genes and by sunlight;
  • data can be summarized by mean, median and mode and displayed in a box-and-whisker plot showing minimum, first quartile, median, third quartile, maximum and outliers, OWTTE;
• (c) D1.3.4, D1.3.7 [4 max]
  • a gene mutation is a (random) change to the base sequence of a gene;
  • mutations are caused by errors in DNA replication or DNA repair;
  • (also) by mutagens: chemicals, e.g. (benzopyrene in) tobacco smoke / mustard gas, and radiation, e.g. UV / X-rays / gamma rays;
  • mutations occur at random positions in the genome; no natural mechanism directs a change to a particular base to produce a desired trait;
  • most mutations are harmful or neutral to the individual, and only mutations in germ cells are passed to offspring;
  • (but) mutation is the original source of all genetic variation / new alleles, which is essential in the long term for evolution by natural selection, 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) D3.1.8, D3.1.12 [7 max]
  • male gametes are produced inside pollen grains (in the anthers), whereas female gametes / egg cells are produced inside ovules (in the ovary);
  • pollination: transfer of pollen from an anther to a stigma (of the same species), by insects / wind or other agents;
  • many species promote cross-pollination between different plants, increasing genetic variation;
  • the pollen grain develops, growing a pollen tube down the style to the ovule;
  • the male gamete (nucleus) travels down the tube and fuses with the egg cell / fertilization;
  • the zygote develops into an embryo (plant) inside the ovule;
  • the ovule becomes the seed, containing the embryo and food reserves, and the ovary becomes the fruit;
  • seeds are dispersed away from the parent by wind / animals / water / explosive fruits, reducing competition and colonizing new areas;
  • dispersal is distinct from pollination: seeds are moved rather than pollen;
  • germination: the seed absorbs water, food reserves are mobilized by enzymes / hydrolysis and the embryo grows, root first then shoot;
  • reproduction is sexual (meiosis and fusion of gametes) even when the plant is hermaphroditic, OWTTE;
• (b) C4.1.12 [5 max]
  • mutualism is an interspecific relationship in which both species benefit;
  • root nodules: (Rhizobium) bacteria in the roots of legumes / Fabaceae fix nitrogen, supplying the plant with ammonium / nitrogen compounds;
  • the legume supplies the bacteria with sugars / carbon compounds from photosynthesis and a protected (low-oxygen) habitat;
  • mycorrhizae: fungal hyphae associated with the roots of orchids / Orchidaceae absorb mineral ions (phosphate) and water for the plant, and supply carbon compounds to germinating orchid seeds;
  • the orchid supplies the fungus with sugars / carbon compounds once it photosynthesizes;
  • zooxanthellae in hard corals gain shelter, CO₂ and nitrogen compounds while supplying the coral with carbon compounds from photosynthesis / pollinators gain nectar while the plant gains pollen transfer;
  • because both partners survive and reproduce better, the relationship is maintained by natural selection, OWTTE;

  Accept common or scientific names. Any two of the three guide examples with benefits to both partners can reach full marks.

• (c) B4.2.9 [3 max]
  • physical structures such as thorns / spines / stings / tough (silica or lignified) leaves deter or injure herbivores;
  • toxic secondary compounds in leaves and seeds, e.g. alkaloids / caffeine / nicotine / cyanogenic glycosides, poison or deter herbivores;
  • some herbivores have metabolic adaptations, e.g. detoxifying enzymes, that break down these toxins;
  • leaf-eating insects such as caterpillars / locusts have chewing mouthparts (mandibles) to bite off and grind leaf tissue;
  • other insects such as aphids have piercing mouthparts (stylets) to penetrate tissue and suck sap, 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.