Module 2: Biomolecules - Water, Carbohydrates, Lipids, Proteins and Enzymes
Module 2 covers the molecules of life and the tests used to identify them. Questions target the link between molecular structure and biological function, condensation and hydrolysis, and the interpretation of enzyme-rate data.
Water
Water is dipolar, so it forms hydrogen bonds: high specific heat capacity and high latent heat of vaporisation (thermal stability, cooling by sweating), cohesion and surface tension (xylem transport), a good solvent for ions and polar molecules, and ice floating because it is less dense than liquid water.
Carbohydrates and lipids
Monosaccharides (trioses to hexoses; α- and β-glucose differ only in the position of the –OH on carbon 1) join by condensation, forming glycosidic bonds, to give disaccharides (maltose, sucrose, lactose) and polysaccharides. Starch (amylose helix plus branched amylopectin) stores energy in plants, glycogen is more branched for animals, cellulose is unbranched β-1,4 chains held by hydrogen bonds into microfibrils for strength. Reducing sugars (all monosaccharides, maltose, lactose) reduce Benedict's solution; sucrose is non-reducing. Triglycerides form by condensation of glycerol with three fatty acids into ester bonds; saturated chains pack tightly, unsaturated ones kink and lower melting point. Phospholipids are amphipathic and build bilayers.
Proteins and enzymes
Amino acids join by peptide bonds into polypeptides; primary sequence, secondary α-helix/β-sheet hydrogen bonding, tertiary ionic, disulfide, hydrogen and hydrophobic interactions, and quaternary subunit assembly. Fibrous proteins are structural; globular proteins are metabolic. Enzymes are globular catalysts that lower activation energy; the active site is specific, explained by lock-and-key or induced fit. Rate rises with temperature to an optimum, then falls as denaturation breaks bonds in the active site; pH shifts away from the optimum do the same. Competitive inhibitors bind the active site and their effect is reduced by more substrate; non-competitive inhibitors bind elsewhere and change active-site shape, so extra substrate does not help. Cofactors and coenzymes assist catalysis.
Worked example
Sucrose gives no colour with Benedict's reagent. After boiling with dilute HCl, cooling and neutralising with sodium hydrogencarbonate, the same solution gives a brick-red precipitate. Reasoning: sucrose is non-reducing, so it cannot reduce Cu²⁺; acid hydrolysis breaks its glycosidic bond, releasing glucose and fructose, both reducing sugars; neutralising restores the alkaline conditions Benedict's requires; the brick-red colour is Cu₂O. A positive control (glucose) and a negative control (water) confirm the reagent worked.
Traps that cost marks
Saying all polysaccharides are reducing; forgetting the neutralisation step. Describing an enzyme as 'killed' or 'used up'. Explaining a fall in rate after the optimum as substrate running out rather than denaturation. Calling starch and glycogen β-glucose. Forgetting phospholipids have a phosphate head plus two tails, not three fatty acids.