Biological Molecules and Cell Biology
Biological Molecules and Cell Biology is tested through structure–function relationships: biochemical tests, enzyme kinetics, membrane transport, organelle roles and the cell cycle. You must explain results and link molecular structure to cellular behaviour.
Core rules
- Condensation forms glycosidic, ester and peptide bonds; hydrolysis reverses them.
- Proteins have primary to quaternary structure; shape change at the active site explains denaturation and allosteric regulation.
- The fluid-mosaic membrane is selectively permeable; simple diffusion, facilitated diffusion and osmosis are passive, while active transport and endocytosis need energy.
- In mitosis, chromosomes condense in prophase, align at the metaphase plate, separate in anaphase, and nuclei reform in telophase; cytokinesis differs in plants and animals.
Worked example
An enzyme assay gives increasing initial rates as substrate concentration rises, then a plateau. Reasoning: at low [S], active sites are mostly free, so rate rises almost linearly; at high [S], all active sites are saturated, so adding substrate cannot raise the rate. The plateau is Vmax, and the substrate concentration giving half Vmax is Km. Raising enzyme concentration would raise Vmax but not change Km.
Traps
- Benedict’s test gives a brick-red precipitate with reducing sugars; sucrose needs hydrolysis first.
- Competitive inhibitors raise Km but leave Vmax unchanged; non-competitive inhibitors lower Vmax.
- Osmosis is water movement down a water potential gradient, not “suction”.
- Animal cells lyse in hypotonic solutions; plant cells become turgid because of the cell wall.