Biology: cells, physiology, genetics and taxonomy
Biology in this entrance examination tests whether you can move between molecular detail and whole-plant or whole-animal function, then apply genetic and taxonomic reasoning to unfamiliar cases. Expect definitions, process questions, crosses and classification of local flora and fauna.
Core concepts
- Cell biology: prokaryotes lack a membrane-bound nucleus and organelles; eukaryotes compartmentalize. Mitochondria and chloroplasts carry their own DNA. The plasma membrane is a phospholipid bilayer; small nonpolar molecules diffuse, while ions and polar molecules need channels or carriers.
- Plant physiology: light reactions occur in thylakoid membranes, split water and produce ATP and NADPH; the Calvin cycle fixes CO2 in the stroma via rubisco. C4 plants concentrate CO2 and limit photorespiration; CAM plants separate fixation in time. Water rises mainly by cohesion–tension driven by transpiration; sugars move in phloem by pressure flow.
- Animal physiology: the Na+/K+ ATPase maintains resting potential; an action potential is a transient Na+ influx followed by K+ efflux. The nephron filters, reabsorbs and secretes; hemoglobin shows cooperative, sigmoid oxygen binding.
- Genetics: Mendel's laws, test crosses, the dihybrid 9:3:3:1 ratio, linkage and Hardy–Weinberg equilibrium p² + 2pq + q² = 1.
- Taxonomy: binomial nomenclature uses genus plus specific epithet, italicized; ranks run from domain to species.
Worked example
A recessive disorder affects 1 in 10,000 individuals in an equilibrium population. What proportion are carriers? q² = 1/10,000 = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency = 2pq = 2 × 0.99 × 0.01 = 0.0198, about 2%. Reasoning: take the square root to obtain allele frequency, then apply 2pq; do not report 1 in 10,000 as the carrier rate.
Traps that cost marks
- Treating 'dominant' as 'most common'; dominance concerns expression, not frequency.
- Claiming root pressure drives water transport in tall trees; cohesion–tension is the main mechanism.
- Placing the Calvin cycle in the thylakoid rather than the stroma.
- Reversing genus and species, or failing to italicize binomials.