Topic 2 covers the molecules of life: from the unique properties of water to the central dogma of molecular biology (DNA → RNA → protein). It also introduces metabolism through cell respiration and photosynthesis.
Water: hydrogen bonding gives cohesion, high specific heat capacity, solvent properties, and density anomaly (ice floats). Carbohydrates: monosaccharides (glucose), disaccharides (maltose, sucrose), polysaccharides (starch, glycogen, cellulose). Lipids: triglycerides (energy storage), phospholipids (membranes). Proteins: amino acid chains with 4 levels of structure; functions include enzymes, transport, structure, immunity.
Biological catalysts (proteins) with specific active sites. Lock-and-key model / induced fit model. Factors: temperature (denaturation above optimum), pH (each enzyme has optimal pH), substrate concentration (saturation). Competitive inhibitors bind to active site; non-competitive bind elsewhere and change active site shape.
DNA: double helix, A-T (2 H-bonds), G-C (3 H-bonds), antiparallel strands, sugar-phosphate backbone. Replication: semi-conservative (Meselson-Stahl experiment), helicase unwinds, DNA polymerase III adds nucleotides 5\'→3\'. Transcription: RNA polymerase copies sense strand → mRNA. Translation: ribosomes read mRNA codons, tRNA brings amino acids, polypeptide formed.
Cell respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ~36-38 ATP). Glycolysis (cytoplasm), link reaction, Krebs cycle (matrix), oxidative phosphorylation (inner membrane). Anaerobic: lactate in animals, ethanol + CO₂ in yeast. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light-dependent (thylakoids) and light-independent (stroma, Calvin cycle) reactions.
At SL, no — you need to know the overall inputs, outputs, and location (mitochondrial matrix), not individual steps. At HL, you need more detail but still not every intermediate. Focus on: pyruvate → acetyl-CoA (link reaction), Krebs cycle produces CO₂, reduced NAD/FAD, and a small amount of ATP. Oxidative phosphorylation produces most ATP using the electron transport chain and chemiosmosis.
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