Molecular and Cellular Biology
The Molecular and Cellular Biology domain tests how cells are built, how they move molecules and energy, and how they regulate reactions, signaling, and division. Expect items that connect structure to function: membrane composition to transport, organelle anatomy to metabolism, and protein shape to catalysis.
Core rules
- Membranes: phospholipid bilayers are selectively permeable. Small nonpolar gases diffuse freely; ions and polar solutes require channels or carriers. Water moves by osmosis toward the side with higher solute concentration (lower water potential).
- Energy flow: glycolysis occurs in the cytosol (net 2 ATP, 2 NADH); pyruvate oxidation and the Krebs cycle occur in the mitochondrial matrix and yield NADH, FADH2, and CO2; oxidative phosphorylation on the inner mitochondrial membrane uses chemiosmosis. Fermentation regenerates NAD+ without oxygen.
- Enzymes: they lower activation energy without changing ΔG or equilibrium. Km is the substrate concentration at half Vmax; a low Km means high affinity. Competitive inhibitors raise apparent Km, while noncompetitive inhibitors lower Vmax.
- Information flow: in eukaryotes, transcription occurs in the nucleus, followed by RNA processing and translation in the cytoplasm. Prokaryotes can couple transcription and translation.
- Cell cycle: G1, S, G2, and M phases, with checkpoints at G1/S and G2/M controlled by cyclins and CDKs; p53 responds to DNA damage.
Worked example
An enzyme has Vmax = 100 µmol min⁻¹ and Km = 5 mM. At [S] = 20 mM, V0 = Vmax[S]/(Km + [S]) = (100 × 20)/(5 + 20) = 2000/25 = 80 µmol min⁻¹. At [S] = 5 mM, V0 = 100 × 5/10 = 50 µmol min⁻¹, confirming that Km is the half-saturation concentration. With a competitive inhibitor, high substrate can still reach Vmax, but the apparent Km rises. With a noncompetitive inhibitor, Vmax falls because inhibitor binding reduces the proportion of active enzyme.
Traps
- Confusing competitive inhibition (raises Km, Vmax unchanged) with noncompetitive inhibition (lowers Vmax).
- Treating ATP yield as an exact whole number; actual oxidative phosphorylation yields vary by shuttle and cell type.
- Saying mitochondria “make most of the ATP” without noting that the proton gradient, not the Krebs cycle directly, drives ATP synthase.
- Mixing up osmosis direction: water moves toward lower water potential, which is usually the more concentrated solute side.
- Forgetting that enzymes change rates, not equilibrium constants or ΔG.