Cell Biology and Biochemistry
What this domain tests
The Cell Biology and Biochemistry portion of KBIT-2 asks you to connect molecular structure to cellular function. Expect questions on membrane organization, organelle roles, energy metabolism, enzyme behavior, and the thermodynamic logic behind reactions. You should be able to interpret simple data tables and predict how a perturbation changes flux or transport.
Core rules and concepts
- Membranes are asymmetric phospholipid bilayers; hydrophobic tails face inward, and cholesterol modulates fluidity.
- Mitochondria generate most ATP via oxidative phosphorylation; the electron transport chain builds a proton gradient.
- Enzymes lower activation energy, not ΔG; they do not change equilibrium.
- Michaelis–Menten: v = Vmax[S]/(Km+[S]). Km is the substrate concentration at half Vmax and is inversely related to apparent affinity.
- Competitive inhibitors raise apparent Km with unchanged Vmax; noncompetitive inhibitors lower Vmax.
- ΔG determines spontaneity; ΔG°′ is at standard conditions and is not the same as actual ΔG.
Worked example
An enzyme has Vmax = 120 µmol/min and Km = 8 mM. At [S] = 8 mM, calculate v. Because [S] = Km, v = Vmax × 8/(8+8) = 60 µmol/min. If a competitive inhibitor is added at a concentration that doubles apparent Km to 16 mM, v at the same [S] becomes 120 × 8/(16+8) = 40 µmol/min. The reasoning: competitive inhibitor competes for active site, so more substrate is needed for half-maximal rate; Vmax is unchanged if substrate can outcompete.
Traps that cost marks
Do not say a lower Km always means higher Vmax. Do not treat ΔG°′ as the actual cellular ΔG. Do not confuse oxidation (loss of electrons) with reduction. Do not assume ATP is made only in mitochondria; glycolysis and photosynthesis also produce ATP. In transport questions, distinguish simple diffusion, facilitated diffusion, and active transport by energy source and direction.