Cellular and Molecular Biology
Domain 1 - Cellular and Molecular Biology tests whether you can move between the molecular and cellular scales: the four macromolecule families, enzyme behavior, membrane transport, organelle function, and how cells harvest and store energy. Items usually ask you to predict an outcome rather than simply name a structure.
Biochemistry and enzymes
- Dehydration synthesis joins monomers and releases water; hydrolysis splits polymers and consumes water.
- Enzymes lower activation energy and raise reaction rate; they do not change ΔG, the equilibrium constant, or the net energy yield.
- Michaelis–Menten: Km is the substrate concentration at ½Vmax, so a low Km means high apparent affinity.
- Allosteric and feedback inhibition (ATP inhibiting phosphofructokinase) regulate flux through pathways.
Membranes and transport
- Fluid mosaic: amphipathic phospholipids, integral and peripheral proteins, and cholesterol modulating fluidity in animal cells.
- Simple diffusion (O2, CO2), facilitated diffusion through channels and carriers (GLUT), primary active transport (Na+/K+ ATPase moves 3 Na+ out and 2 K+ in per ATP), cotransport (Na+/glucose in the gut), and osmosis.
- Water moves toward the higher solute concentration, which is the lower water potential.
Energy transformations
- Glycolysis in the cytosol → pyruvate oxidation → Krebs cycle in the matrix → electron transport and chemiosmosis on the inner mitochondrial membrane. O2 is the final electron acceptor; ATP synthase is a proton channel, not a substrate-level kinase.
- Current accounting gives roughly 30–32 ATP per glucose in eukaryotes; older textbooks state 36–38.
- Photosynthesis: light reactions in thylakoids split water and generate ATP and NADPH; the Calvin cycle fixes CO2 with RuBisCO in the stroma.
Worked example
An inhibitor raises Km from 5 mM to 15 mM while Vmax stays at 100 µmol/min. The enzyme now needs more substrate to reach half-maximal speed, which is the signature of competitive inhibition at the active site. A noncompetitive inhibitor leaves Km unchanged and lowers Vmax, because functional enzyme is removed rather than substrate binding blocked.
Traps that cost marks
- Reversing the water rule: cells swell in dilute (hypotonic) solutions, not salty ones.
- Saying mitochondria "make ATP" directly - the proton gradient drives ATP synthase.
- Confusing the thylakoid light reactions with the stromal Calvin cycle.