Biology: Physiology, Genetics, and Data-Interpretation Skills
What the biology section tests
Cell structure and division, biomolecules and enzymes, plant physiology (photosynthesis, respiration, transport), human physiology (digestion, respiration, circulation, excretion, nervous and endocrine control), reproduction and development, genetics and molecular biology, evolution, and ecology. Questions mix diagram recall, data interpretation, and single-step genetic crosses.
Core rules
- Mitosis gives two diploid cells in one division; meiosis gives four haploid cells in two divisions, with crossing over in prophase I.
- A dihybrid cross of heterozygotes gives a 9:3:3:1 phenotypic ratio; a test cross reveals an unknown genotype.
- Enzymes lower activation energy, are not consumed, and denature above their optimum temperature or at extreme pH.
- Light reactions occur in thylakoid membranes and yield ATP and NADPH; the Calvin cycle fixes CO₂ in the stroma.
- In the ABO system, A and B alleles are codominant and O is recessive; group AB carries both antigens.
- Nitrogen fixers, nitrifiers, and denitrifiers occupy distinct steps of the nitrogen cycle.
Worked example
A man with blood group A (genotype Iᴬi) marries a woman with group B (Iᴮi). What groups can their children have? Gametes: Iᴬ or i from the father; Iᴮ or i from the mother. Offspring: IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B), ii (O). Each combination has a 25% probability, so all four blood groups are possible.
Traps that cost marks
- Claiming meiosis produces identical cells; crossing over and independent assortment make them genetically distinct.
- Confusing dominant with more common.
- Reversing the roles of NADPH and ATP in the Calvin cycle.
- Assuming an affected pedigree individual must be homozygous.
- Ignoring temperature when reading an enzyme-rate graph.