Biology: Mechanisms, Inheritance and Human Physiology
What the biology paper rewards
The biology section of the Lebanese University Faculty of Public Health entrance exam tests mechanisms rather than lists: cell structure and transport, bioenergetics, molecular genetics and inheritance, human physiology, and the basics of immunity and microbiology. Questions are usually short and applied - you are given a situation (a solute gradient, a pedigree, a blood-gas value) and must predict the outcome.
Core rules to have automatic
- Membrane transport: water moves by osmosis toward the higher solute concentration; only active transport consumes ATP and can work against a gradient.
- Energy: glycolysis is cytosolic and yields 2 ATP net; the Krebs cycle and oxidative phosphorylation are mitochondrial. Modern textbook totals are about 30–32 ATP per glucose, not the older 36–38.
- Inheritance: the risk between two carriers of an autosomal recessive condition is 1/4 per child, and each child is an independent event. Hardy–Weinberg: p² + 2pq + q² = 1.
- Physiology: the action potential begins with Na⁺ influx and is terminated by K⁺ efflux; the proximal tubule reabsorbs most filtered water and glucose.
- Immunity: vaccination works through memory B and T cells, not by supplying ready-made antibodies.
Worked example with reasoning
Both parents are healthy carriers of an autosomal recessive CFTR mutation. Probability a child is affected = 1/4. Probability that, among two children, at least one is affected = 1 − P(none) = 1 − (3/4)² = 1 − 9/16 = 7/16 ≈ 43.8%. The complement rule avoids listing the three separate "at least one" cases.
Traps that cost marks
- Treating "dominant" as "common"; many dominant disorders are rare.
- Mixing genotype and phenotype ratios when codominance applies (ABO gives 1:2:1 phenotypes, not 3:1).
- Reversing the direction of osmosis.
- Believing an affected child changes the risk for the next child.
Productive practice
Redraw the nephron and the action-potential curve from memory, then work two-generation pedigrees involving sex-linked traits.