Molecules and Cells
The Molecules and Cells domain tests the chemistry beneath life: the molecules that build cells, how enzymes and membranes control reactions, how cells trap and release energy, and how division makes new cells. Expect data-response items - a rate table, a water-potential graph, a diagram to interpret - not straight recall.
Biological molecules and food tests
- Carbohydrates: monosaccharides (glucose, C6H12O6), disaccharides (maltose, sucrose) joined by condensation into glycosidic bonds; starch, glycogen and cellulose are polysaccharides.
- Proteins: amino acids joined by peptide bonds, folded into primary to quaternary structure; fibrous (collagen) versus globular (haemoglobin, enzymes).
- Lipids: glycerol plus fatty acids joined by ester bonds.
- Tests: Benedict's - reducing sugars give a brick-red precipitate only on heating; iodine - starch, blue-black; Biuret - protein, purple; emulsion test - lipid, cloudy white.
Enzymes
- Globular proteins with a specific active site (lock-and-key and induced-fit models); they lower activation energy and are not used up.
- Rate rises to an optimum then falls sharply because hydrogen and ionic bonds break and the tertiary structure, hence the active site, denatures permanently.
- Raising substrate concentration increases rate only until all active sites are saturated; then enzyme concentration limits.
- Q10 = rate at (T + 10 °C) ÷ rate at T.
Membranes and transport
- Fluid mosaic: phospholipid bilayer with intrinsic and extrinsic proteins, cholesterol, glycolipids and glycoproteins.
- Diffusion and facilitated diffusion run down a gradient without ATP; osmosis is water moving down a water-potential gradient through a partially permeable membrane; active transport uses ATP against a gradient; co-transport couples glucose uptake to a Na+ gradient in the ileum.
Energy and division
- Photosynthesis: light-dependent stage - photolysis of water, ATP and reduced NADP produced; Calvin cycle - RuBP + CO2 → GP → triose phosphate → RuBP and hexose.
- Respiration: glycolysis in the cytoplasm, link reaction and Krebs cycle in the matrix, oxidative phosphorylation on the inner membrane; anaerobic respiration in mammals yields lactate only.
- Mitosis: prophase, metaphase, anaphase, telophase; two genetically identical diploid cells for growth and repair.
Worked example
A student collects 18 cm³ of oxygen in 3 minutes at 25 °C and 36 cm³ in 3 minutes at 35 °C. Convert to rates first: 6.0 and 12.0 cm³ min⁻¹. Q10 = 12.0 ÷ 6.0 = 2.0, so the rate doubles over that interval. At 45 °C the volume falls below the 35 °C value - that is denaturation, not a Q10 below 1.
Traps
- Saying an enzyme is 'killed' or 'used up'; it is denatured.
- Writing 'positive for sugar' instead of reducing sugar, or omitting that Benedict's needs heating.
- Swapping the light-dependent stage with the Calvin cycle.
- Mixing mm and µm in magnification calculations.