Molecular and Cellular Biology
This domain covers macromolecule structure, enzyme function, cell organelles, membrane transport, and the energy transformations of respiration and photosynthesis. Items reward structure–function reasoning: why a shape, a gradient, or a compartment position determines what happens.
Macromolecules and Enzymes
- Proteins are amino acid chains whose folded shape sets function; denaturation by heat or extreme pH destroys that shape irreversibly.
- Enzymes lower activation energy and are not consumed. Rate rises with substrate concentration until active sites saturate (Vmax); adding enzyme raises Vmax.
- Competitive inhibitors compete for the active site; noncompetitive inhibitors bind elsewhere and change shape.
- Phospholipids form bilayers because of their amphipathic nature; carbohydrates serve as energy and structural material.
Membranes and Transport
- Diffusion, facilitated diffusion, and osmosis are passive - down the gradient, no ATP.
- Pumps such as the Na⁺/K⁺ ATPase move ions against the gradient using ATP.
- Water moves toward the higher solute concentration. Hypertonic surroundings shrink a cell; hypotonic surroundings swell it; plant cells resist lysis via the wall.
Energy Conversions
- Glycolysis (cytoplasm, net 2 ATP) → Krebs cycle (matrix) → oxidative phosphorylation (inner membrane), roughly 30–32 ATP per glucose, with O₂ as the final electron acceptor.
- Light reactions in thylakoids make ATP and NADPH; the Calvin cycle in the stroma fixes CO₂ using rubisco.
Worked Example
Elodea cells are placed in 5% NaCl. The vacuole shrinks and the membrane pulls away from the wall (plasmolysis). Reasoning: the solution is hypertonic to the cell, so water exits the vacuole by osmosis down its concentration gradient; the rigid wall stays fixed while the protoplast shrinks. In distilled water the same cells would become turgid instead.
Traps
- Saying mitochondria "make energy" - they convert energy into ATP; energy is conserved.
- Confusing osmosis (water movement) with solute diffusion.
- Assuming facilitated diffusion needs ATP.
- Swapping thylakoid (light reactions) and stroma (Calvin cycle) locations.