Which stage of cellular respiration yields the most ATP?

• A. Electron transport chain
• B. Glycolysis
• C. Krebs cycle
• D. Fermentation

Answer: (A)

Energy for most ATP in cellular respiration comes from the electron transport chain through oxidative phosphorylation. In this stage, NADH and FADH2 donate electrons to the chain, which pumps protons across the inner mitochondrial membrane and creates a proton gradient. ATP synthase uses that gradient to convert ADP into ATP as protons flow back through the enzyme. This yields far more ATP than the earlier stages: about 26–28 ATP per glucose in aerobic conditions, compared with roughly 2 ATP from glycolysis and a similar small amount from the Krebs cycle directly. Fermentation bypasses the electron transport chain entirely, so it doesn’t produce significant ATP beyond the few produced in glycolysis. That’s why the electron transport chain is where the bulk of ATP is generated.