Biochemistry and Human Physiology: Enzymes, Metabolism and Organ Function
What the domain tests
This domain links molecular biochemistry to organ-system physiology. High-yield areas are enzyme kinetics, energetics of glucose oxidation, haemoglobin chemistry, acid–base balance, renal clearance and endocrine control, with clinical correlations such as diabetes and jaundice.
Core rules to hold
- Enzyme kinetics: v = Vmax[S]/(Km + [S]); Km is the substrate concentration at half Vmax; competitive inhibitors raise Km with unchanged Vmax; non-competitive inhibitors lower Vmax.
- Carbohydrate metabolism: glycolysis nets 2 ATP and 2 NADH; pyruvate dehydrogenase gives acetyl-CoA; the citric acid cycle and oxidative phosphorylation bring total yield to about 30–32 ATP per glucose.
- Other pathways: HMP shunt supplies NADPH and ribose-5-phosphate; beta-oxidation yields acetyl-CoA; the urea cycle clears ammonia; ketone bodies fuel the brain in starvation.
- Haemoglobin: oxygen dissociation shifts right with raised CO2, H+, temperature and 2,3-BPG (Bohr effect); fetal haemoglobin has higher oxygen affinity.
- Acid–base: Henderson–Hasselbalch for bicarbonate; respiratory compensation changes CO2 rapidly, renal compensation adjusts bicarbonate slowly.
- Renal physiology: clearance = (U × V)/P; inulin clearance measures GFR; PAH clearance measures renal plasma flow.
Worked example
An enzyme has Vmax 100 µmol/min and Km 5 mM. When the substrate concentration is 5 mM, v = 100 × 5/(5 + 5) = 50 µmol/min, exactly half of Vmax, which is the definition of Km. Doubling substrate to 10 mM gives 66.7 µmol/min, showing the diminishing return characteristic of Michaelis–Menten kinetics.
Traps that cost marks
Swapping competitive and non-competitive effects on Vmax; quoting 38 ATP as the modern glucose yield; confusing inulin with PAH clearance; forgetting that renal compensation for acidosis takes days, not minutes.