Biology: Cells, Systems, and Inheritance
Biology in the Cambodian health entrance exam tests cell structure and transport, enzyme action, human organ systems, basic genetics, and the biology of infection and immunity. Questions are often predictive: what happens to glomerular filtration when blood pressure falls, or what proportion of children in a carrier cross are affected.
Core rules and concepts
- Diffusion moves particles down a concentration gradient; osmosis moves water across a semipermeable membrane toward the side with more solute. Tonicity predicts whether a cell shrinks, swells, or stays stable.
- Enzymes lower activation energy and are not consumed. Rate increases with substrate until saturation, then plateaus; heat or extreme pH can denature the active site.
- The pulmonary artery carries deoxygenated blood to the lungs, while the pulmonary vein returns oxygenated blood to the left atrium. Aortic pressure is the highest in the systemic circulation.
- In the nephron, filtration is driven by hydrostatic pressure; the proximal tubule reabsorbs most filtered water and solutes; ADH increases water permeability in the collecting duct.
- In an autosomal recessive cross Aa × Aa, the offspring ratio is 1 AA : 2 Aa : 1 aa. Each child has a 25% chance of being affected and a 50% chance of being a carrier.
Worked example
Two carrier parents already have one unaffected child. What is the probability that the next child is affected? Each parent passes A or a with equal probability, independently of previous children. The four equally likely combinations are AA, Aa, aA, and aa. Only aa is affected, so the probability is 1/4, or 25%. The earlier child does not change the next child's risk; the answer is not 1/3. If the question asks for the chance that an unaffected child is a carrier, condition on the unaffected genotypes: Aa occurs in 2 of 3 cases, so the answer is 2/3.
Traps
Do not treat carriers as affected or confuse family totals with per-child risk. Do not reverse the pulmonary artery and vein. Do not assume all filtered water is excreted. Remember that enzymes change reaction rate, not the equilibrium position.