Biological molecules
Carbohydrates and lipids
Monosaccharides (glucose, fructose, galactose, ribose) join by condensation to form glycosidic bonds, releasing water; hydrolysis breaks them. Disaccharides: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose). α-glucose polymers store energy: amylose (unbranched, 1,4, helical, compact), amylopectin and glycogen (1,4 and 1,6 branches, many ends for rapid hydrolysis); insoluble, so no osmotic effect. β-glucose forms cellulose: straight chains, alternate glucose flipped, linked by hydrogen bonds into microfibrils: high tensile strength. Triglycerides = glycerol + 3 fatty acids (ester bonds): energy store, insulation. Phospholipids have a hydrophilic phosphate head and hydrophobic tails: membranes. Tests: Benedict's (reducing sugars), acid hydrolysis then Benedict's (non-reducing), iodine (starch), ethanol emulsion (lipids).
≈ 50 minEnzyme action and kinetics
Enzymes are globular proteins that lower activation energy by forming enzyme–substrate complexes at a specific active site. Induced fit: the active site changes shape slightly as the substrate binds. Rate rises with temperature (more collisions) up to an optimum, then falls as bonds break and the enzyme denatures. Away from the optimum pH, ionic and hydrogen bonds change, altering the active site. Rate increases with substrate concentration until all active sites are occupied (Vmax). Km is the substrate concentration at ½Vmax: a low Km means high affinity. Competitive inhibitors bind the active site (overcome by more substrate; Vmax unchanged, Km increases). Non-competitive inhibitors bind elsewhere, changing the active site (Vmax decreases). Immobilised enzymes can be reused and give product free of enzyme.
≈ 50 minDNA structure and replication
A nucleotide = phosphate + pentose sugar (deoxyribose in DNA, ribose in RNA) + nitrogenous base. Purines (A, G) have two rings; pyrimidines (C, T, U) one. Nucleotides join by phosphodiester bonds (condensation) into a sugar-phosphate backbone. DNA is a double helix of two antiparallel strands joined by hydrogen bonds: A–T (2), C–G (3). Semi-conservative replication: helicase breaks hydrogen bonds; free nucleotides pair with each exposed strand; DNA polymerase joins them 5′→3′ (continuous leading strand; fragments on the lagging strand joined by ligase). Each new molecule has one old and one new strand, shown by the Meselson–Stahl ¹⁵N/¹⁴N experiment. RNA is single-stranded with ribose and uracil.
≈ 45 minProteins and water
Amino acids (H₂N–CHR–COOH) join by condensation into peptide bonds. Primary structure: amino acid sequence. Secondary: α-helix or β-pleated sheet held by hydrogen bonds between C=O and N–H. Tertiary: further folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions between R groups. Quaternary: more than one polypeptide (and maybe a prosthetic group). Globular proteins (e.g. haemoglobin: 4 chains, haem with Fe²⁺, hydrophilic outside) are soluble and metabolic; fibrous proteins (e.g. collagen: three chains in a triple helix, glycine every third residue, cross-links) are insoluble and structural. Biuret test: purple. Water is polar; hydrogen bonding gives it high specific heat capacity, high latent heat of vaporisation, cohesion and solvent properties.
≈ 50 min