12.2.8—Oxidative phosphorylation

Respiratory ATP yield depends on how far the substrate is oxidised and whether oxygen allows oxidative phosphorylation to continue. Aerobic respiration uses glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation; anaerobic respiration continues with glycolysis and fermentation only.

• Glycolysis: uses substrate-level phosphorylation to give a small net ATP return and reduces coenzyme NAD while glucose becomes pyruvate. This is the ATP-producing stage that can continue without oxygen.
• Link reaction: prepares pyruvate for mitochondrial respiration, releases carbon dioxide and produces reduced NAD; it is not the main direct ATP-producing stage.
• Krebs cycle: gives a small substrate-level ATP return and produces reduced coenzymes as carbon compounds are oxidised. These coenzymes carry reducing power to oxidative phosphorylation.
• Oxidative phosphorylation: produces most aerobic ATP by using electron-transfer energy and a proton gradient; oxygen is required as the final electron acceptor. Its contribution is therefore much larger than the direct ATP made in glycolysis or the Krebs cycle.
• Per-glucose comparison: under the SME-supported textbook convention, aerobic respiration gives a high total of approximately 36 ATP, whereas anaerobic respiration gives a low total of approximately 2 ATP from glycolysis alone. Treat these as approximate teaching values, not universal measured constants.

Why the difference: aerobic respiration completely oxidises glucose and can transfer reducing power through the mitochondrial stages; anaerobic respiration partially oxidises glucose and cannot use oxygen as the final electron acceptor, so the mitochondrial ATP-producing stages do not proceed. The yield comparison is qualitative first and numerical only at the supported approximate level.

Do not add a fixed exact ATP number to every stage or treat the approximate total as independent of conditions. Distinguish substrate-level phosphorylation from oxidative phosphorylation; inhibitors and detailed causes of yield variation belong outside this card.