01Four stages
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Glycolysis (cytosol): glucose → 2 pyruvate, net 2 ATP + 2 NADH. Pyruvate oxidation (matrix): pyruvate → acetyl-CoA + CO₂ + NADH. Krebs/citric acid cycle (matrix): releases CO₂, makes NADH, FADH₂ and a little ATP. Oxidative phosphorylation (inner membrane): ETC + chemiosmosis.
02Oxidative phosphorylation
NADH and FADH₂ drop electrons onto the ETC. As electrons move, proteins pump H⁺ into the intermembrane space. Oxygen is the final electron acceptor, forming water.
H⁺ flows back into the matrix through ATP synthase, producing most of the ~30–32 ATP per glucose. Without O₂, the chain backs up and stops.
03Fermentation
Without oxygen, cells regenerate NAD⁺ by fermentation so glycolysis can continue: lactic acid fermentation (muscle, bacteria) or alcoholic fermentation (yeast: ethanol + CO₂). Only 2 ATP per glucose.
04Notebook box
CO₂ is released in pyruvate oxidation and Krebs. O₂ is used only at the end of the ETC. Water is made at the ETC.
Worked example: A poison makes the inner membrane leaky to H⁺. Effect? → ETC still runs and O₂ is consumed, but ATP synthesis falls because the gradient dissipates (energy lost as heat).