Cell Biology and Biochemistry
Cell Biology and Biochemistry
The Cell Biology and Biochemistry subarea asks you to connect molecular structure to cellular function. Expect questions on macromolecules, enzyme kinetics, membrane transport, organelle roles, and the energy pathways of photosynthesis and respiration, plus the cell cycle.
Core concepts
- Macromolecules: carbohydrates, lipids, proteins, and nucleic acids are polymers built by dehydration synthesis and broken by hydrolysis. Protein function depends on primary through quaternary structure; denaturation disrupts bonds but not peptide bonds.
- Enzymes: lower activation energy, do not change ΔG, and are sensitive to pH and temperature. Competitive inhibitors raise apparent Km without changing Vmax; noncompetitive inhibitors lower Vmax without changing Km.
- Membranes: the fluid mosaic model includes phospholipids, cholesterol, and integral/peripheral proteins. Simple diffusion and osmosis are passive; facilitated diffusion uses channels/carriers; primary active transport hydrolyzes ATP (for example, Na+/K+ ATPase); secondary active transport uses an electrochemical gradient.
- Energy: glycolysis occurs in the cytosol; the Krebs cycle and oxidative phosphorylation occur in mitochondria; light reactions occur in thylakoid membranes and the Calvin cycle in the stroma.
Worked example
A researcher measures an enzyme with and without an inhibitor. Without inhibitor, Vmax = 100 units and Km = 2 mM. With inhibitor, Vmax remains 100 units but Km rises to 6 mM. Because Vmax is unchanged and Km increases, the inhibitor is competitive: it competes with substrate for the active site, so more substrate is needed to reach half-maximal velocity. A noncompetitive inhibitor would instead lower Vmax while leaving Km unchanged. This is a frequent graph-reading question; compare both parameters, not just the curve shape.
Traps
Osmosis moves water, not solute; active transport is not always direct ATP hydrolysis; enzymes do not make a reaction more exergonic; and mitochondria do not perform glycolysis. Also distinguish prokaryotic from eukaryotic ribosomes and remember that chloroplasts and mitochondria contain their own DNA.