Biology - Cell Biology, Genetics, Human Physiology and Homeostasis
The biology section of the USJ Medical Sciences Concours tests cell biology, genetics, human physiology and homeostasis through short clinical or experimental scenarios, data tables and pedigrees. Most items require applying a principle to an unfamiliar case rather than reciting a definition.
Core rules to have automatic
- Membrane transport. Net flux follows the electrochemical gradient. The Na⁺/K⁺-ATPase moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed, holding the resting potential near −70 mV and stabilising cell volume. Water follows osmolarity, not the concentration of one solute.
- Enzyme kinetics. Kₘ is the substrate concentration at half Vₘₐₓ; a competitive inhibitor raises apparent Kₘ with unchanged Vₘₐₓ, while a non-competitive inhibitor lowers Vₘₐₓ.
- Genetics. Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, valid only with no selection, mutation, migration, drift or non-random mating. X-linked recessive traits appear at frequency q in males and q² in females.
- Integrative physiology. Cardiac output = heart rate × stroke volume; alveolar ventilation = (tidal volume − dead space) × respiratory rate; glomerular filtration depends on net filtration pressure.
Worked example
An autosomal recessive disorder affects 1 in 10 000 births in a population at equilibrium. Affected frequency = q² = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency = 2pq = 2(0.99)(0.01) ≈ 0.0198, about 1 in 50. The carrier frequency is roughly 200 times the disease frequency - a favourite exam point.
Traps that cost marks
Reading 'carrier frequency' as disease frequency; applying Hardy–Weinberg to a consanguineous or clinically selected population; swapping sympathetic and parasympathetic effects on the same organ; reporting a physiological rate without its time unit (mL/min versus mL/s).