Energy and respiration
ATP, glycolysis and anaerobic respiration
ATP (adenine + ribose + 3 phosphates) is the immediate energy source: hydrolysis to ADP + Pᵢ by ATP hydrolase releases a small, usable amount of energy; it is resynthesised by ATP synthase. Glycolysis (cytoplasm): glucose is phosphorylated (2 ATP used), split into 2 triose phosphate, then oxidised to 2 pyruvate, producing 4 ATP (net 2) and 2 reduced NAD. Without oxygen, NAD must be regenerated: in mammals pyruvate → lactate (lactate dehydrogenase); in yeast/plants pyruvate → ethanol + CO₂. Anaerobic respiration yields only 2 ATP per glucose. Respiratory quotient RQ = CO₂ produced ÷ O₂ used: carbohydrate 1.0, protein ~0.9, lipid ~0.7. Lipids release the most energy per gram (more H atoms).
≈ 50 minKrebs cycle and oxidative phosphorylation
Link reaction (matrix): pyruvate (3C) is decarboxylated (CO₂) and dehydrogenated (reduced NAD), forming acetyl (2C) which joins coenzyme A. Krebs cycle (matrix): acetyl CoA + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated twice and dehydrogenated four times back to oxaloacetate. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (substrate-level). Two turns per glucose. Oxidative phosphorylation (inner membrane, cristae): reduced NAD/FAD release electrons to the electron transport chain; energy pumps H⁺ into the intermembrane space; H⁺ flow back through ATP synthase (chemiosmosis) makes ATP. Oxygen is the final electron acceptor, forming water. Total ≈ 32 ATP per glucose. Redox indicators (DCPIP, methylene blue) accept electrons and lose colour.
≈ 55 min