Cambridge IAL Biology cheat sheet

Magnification = image size ÷ actual size; actual size = image size ÷ magnification. Convert both to the same unit (1 mm = 1000 µm) before dividing.

Resolution is the smallest distance at which two points remain distinguishable; it depends on the wavelength of the radiation and the numerical aperture of the lens, not on magnification. Electron microscopes resolve better because electron wavelength is far shorter than visible light.

Water potential ψ = ψs + ψp. Pure water at atmospheric pressure has ψ = 0; solutes lower ψs below zero; turgor pressure raises ψp. Water moves from higher (less negative) to lower (more negative) ψ.

Surface area to volume ratio falls as an organism or cell gets larger, so exchange surfaces must be folded, flattened or ventilated. Fick's law: rate ∝ (surface area × concentration difference) ÷ diffusion distance.

Gas exchange in insects: spiracles, tracheae and tracheoles deliver air directly to tissues; water loss is limited by spiracle closure and a waxy cuticle. In fish, countercurrent flow in gill lamellae keeps a concentration gradient along the whole length.

Enzyme rate is affected by temperature (denaturation above optimum via hydrogen-bond breakage), pH (charge changes in the active site), and substrate concentration (plateaus when all active sites are occupied).

Competitive inhibitors raise the substrate concentration needed for a given rate but do not change Vmax; non-competitive inhibitors lower Vmax because some enzyme molecules are effectively removed.

In the light-dependent reaction, water is photolysed at photosystem II, electrons pass down the electron transport chain, protons are pumped into the thylakoid space, and ATP synthase makes ATP; NADP is reduced at photosystem I.

Practical skills: describe variables as independent, dependent and controlled; use a control to show the effect is due to the manipulated variable; choose a statistical test from the type of data (nominal, ordinal, interval) and whether the design is independent or matched.

Chi-squared test: χ² = Σ (O − E)² ÷ E, with degrees of freedom = number of categories − 1. Use it for frequencies in categories, not for measurements.

Spearman's rank: rs = 1 − (6Σd²) ÷ (n(n² − 1)). Use for correlation between two ranked variables; degrees of freedom = n − 2.

Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies. The model assumes no selection, mutation, migration, genetic drift, or non-random mating.

Genetic crosses: a test cross with a homozygous recessive reveals the genotype of a dominant phenotype; a 9:3:3:1 ratio in a dihybrid F2 indicates independent assortment, while deviations suggest linkage.

Sex linkage: a recessive allele on the X chromosome appears more often in males because they are hemizygous; affected fathers pass the allele to all daughters but no sons.

Evolution: allele frequencies change under natural selection, genetic drift, the founder effect, and the bottleneck effect. Speciation requires reproductive isolation, which may be geographical (allopatric) or within a population (sympatric).

Photosynthesis: the Calvin cycle uses ATP and reduced NADP from the light-dependent stage; RuBP is carboxylated by rubisco, and the products GP and TP are used to regenerate RuBP and to make carbohydrate.

Respiration: glycolysis yields net 2 ATP and 2 reduced NAD per glucose; the link reaction and Krebs cycle yield reduced NAD and reduced FAD; oxidative phosphorylation yields about 26–28 ATP in total.

Practical skills: use a t-test for comparing two means of interval data, a Mann–Whitney U test for ordinal data from independent groups, and a Wilcoxon test for matched pairs. Reject the null hypothesis when the calculated value exceeds the critical value at the chosen probability.

Magnification = image size ÷ actual size; actual size = image size ÷ magnification. Convert both to the same unit (1 mm = 1000 µm) before dividing.

Resolution is the smallest distance at which two points remain distinguishable; it depends on the wavelength of the radiation and the numerical aperture of the lens, not on magnification. Electron microscopes resolve better because electron wavelength is far shorter than visible light.

Water potential ψ = ψs + ψp. Pure water at atmospheric pressure has ψ = 0; solutes lower ψs below zero; turgor pressure raises ψp. Water moves from higher (less negative) to lower (more negative) ψ.

Surface area to volume ratio falls as an organism or cell gets larger, so exchange surfaces must be folded, flattened or ventilated. Fick's law: rate ∝ (surface area × concentration difference) ÷ diffusion distance.

Gas exchange in insects: spiracles, tracheae and tracheoles deliver air directly to tissues; water loss is limited by spiracle closure and a waxy cuticle. In fish, countercurrent flow in gill lamellae keeps a concentration gradient along the whole length.

Enzyme rate is affected by temperature (denaturation above optimum via hydrogen-bond breakage), pH (charge changes in the active site), and substrate concentration (plateaus when all active sites are occupied).

Competitive inhibitors raise the substrate concentration needed for a given rate but do not change Vmax; non-competitive inhibitors lower Vmax because some enzyme molecules are effectively removed.

In the light-dependent reaction, water is photolysed at photosystem II, electrons pass down the electron transport chain, protons are pumped into the thylakoid space, and ATP synthase makes ATP; NADP is reduced at photosystem I.

Practical skills: describe variables as independent, dependent and controlled; use a control to show the effect is due to the manipulated variable; choose a statistical test from the type of data (nominal, ordinal, interval) and whether the design is independent or matched.

Chi-squared test: χ² = Σ (O − E)² ÷ E, with degrees of freedom = number of categories − 1. Use it for frequencies in categories, not for measurements.

Spearman's rank: rs = 1 − (6Σd²) ÷ (n(n² − 1)). Use for correlation between two ranked variables; degrees of freedom = n − 2.

Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies. The model assumes no selection, mutation, migration, genetic drift, or non-random mating.

Genetic crosses: a test cross with a homozygous recessive reveals the genotype of a dominant phenotype; a 9:3:3:1 ratio in a dihybrid F2 indicates independent assortment, while deviations suggest linkage.

Sex linkage: a recessive allele on the X chromosome appears more often in males because they are hemizygous; affected fathers pass the allele to all daughters but no sons.

Evolution: allele frequencies change under natural selection, genetic drift, the founder effect, and the bottleneck effect. Speciation requires reproductive isolation, which may be geographical (allopatric) or within a population (sympatric).

Photosynthesis: the Calvin cycle uses ATP and reduced NADP from the light-dependent stage; RuBP is carboxylated by rubisco, and the products GP and TP are used to regenerate RuBP and to make carbohydrate.

Respiration: glycolysis yields net 2 ATP and 2 reduced NAD per glucose; the link reaction and Krebs cycle yield reduced NAD and reduced FAD; oxidative phosphorylation yields about 26–28 ATP in total.

Practical skills: use a t-test for comparing two means of interval data, a Mann–Whitney U test for ordinal data from independent groups, and a Wilcoxon test for matched pairs. Reject the null hypothesis when the calculated value exceeds the critical value at the chosen probability.