Cell Biology and Biochemistry
What this domain tests
Cell Biology and Biochemistry examines how biological molecules build cells and how cells conserve and use energy. Expect questions on membrane transport, organelle function, enzyme kinetics, respiration, photosynthesis, and the properties of water and macromolecules.
Core rules and concepts
- The phospholipid bilayer is selectively permeable; small nonpolar molecules diffuse, while ions and polar molecules need channels or carriers. Osmosis is water movement down its water-potential gradient.
- Prokaryotes lack membrane-bound organelles; eukaryotes compartmentalize reactions: mitochondria for aerobic respiration, chloroplasts for photosynthesis, rough ER for secreted proteins, lysosomes for hydrolysis.
- Enzymes lower activation energy, are not consumed, and have an active site. Rate rises with substrate until saturation; temperature and pH extremes denature. Competitive inhibitors raise apparent Km; noncompetitive inhibitors reduce Vmax.
- ATP couples exergonic and endergonic reactions. Aerobic respiration yields more ATP than fermentation because the electron transport chain uses chemiosmosis.
Worked example
An enzyme has Vmax 100 µmol min−1 and Km 5 mM. At [S] = 5 mM, use Michaelis–Menten: v = Vmax[S]/(Km+[S]) = 100×5/(5+5) = 50 µmol min−1. At 15 mM, v = 100×15/(20)=75. Reasoning: at Km the enzyme is half-saturated; raising substrate beyond Km gives diminishing returns because active sites are increasingly occupied.
Traps
Do not say enzymes “provide energy” or are used up. Do not confuse diffusion with active transport. In respiration questions, count ATP per glucose carefully and distinguish substrate-level phosphorylation from oxidative phosphorylation.