In cellular respiration, which stage produces the most ATP?

• A. Glycolysis
• B. Citric acid cycle
• C. Fermentation
• D. Oxidative phosphorylation

Answer: (D)

The stage that produces the most ATP is the oxidative phosphorylation stage, where the electron transport chain creates a proton gradient across the inner mitochondrial membrane and ATP synthase uses that gradient to make ATP. This step uses the high-energy electrons carried by NADH and FADH2 to power the production of a large amount of ATP, far more than the other stages.

Glycolysis and the citric acid cycle each contribute some ATP directly through substrate-level phosphorylation, but their main role is to generate NADH and FADH2 for oxidative phosphorylation. Fermentation can occur without oxygen and yields only the small amount of ATP produced during glycolysis, since it bypasses the electron transport chain. Therefore, the bulk of ATP in cellular respiration comes from oxidative phosphorylation.