Cell Biology and Biochemistry: Membranes, Energy, and Information Flow
Core concepts
A candidate must be able to move between the molecular and the cellular scale. Know the four macromolecule classes and the bond that defines each (peptide, phosphodiester, glycosidic, ester). Understand membrane architecture: the fluid mosaic model, amphipathic phospholipids, cholesterol as a fluidity buffer, and the asymmetry of the bilayer. Be fluent in transport: simple and facilitated diffusion, primary active transport (Na+/K+-ATPase, 3 Na+ out per 2 K+ in), and secondary cotransport. Know glycolysis, the citric acid cycle, oxidative phosphorylation, and the chemiosmotic proton gradient, plus the ATP yield logic rather than memorized totals. Know organelle分工: rough ER for secreted and membrane proteins, Golgi for modification and sorting, lysosome for acid hydrolases, mitochondria for oxidative ATP. Understand the cytoskeleton triad (actin, microtubules, intermediate filaments) and motor directionality: kinesin toward the plus end, dynein toward the minus end. Enzyme kinetics belong here too: Km as an affinity proxy, Vmax, competitive versus noncompetitive inhibition, and allosteric regulation.
Traps
Questions love the direction of the proton gradient, the difference between ΔG and ΔG°, and the claim that a catalyst changes equilibrium (it does not; it changes rate). Another trap is confusing facilitated diffusion (saturable, no energy) with active transport. Watch for statements that cholesterol always decreases fluidity - it buffers, ordering at high temperature and maintaining fluidity at low temperature.
Worked example
An enzyme has Km = 5 mM. At [S] = 5 mM, velocity is exactly half of Vmax by the Michaelis–Menten definition. A competitive inhibitor raises apparent Km at unchanged Vmax; a noncompetitive inhibitor lowers Vmax at unchanged Km. Use that signature to classify inhibition data instantly.