Under aerobic conditions, which mechanism produces the majority of ATP?

• A. Substrate-level Phosphorylation
• B. Oxidative Phosphorylation
• C. Fermentation
• D. Krebs Cycle

Answer: (B)

Oxidative phosphorylation is responsible for most ATP production when oxygen is present. In this step, NADH and FADH2 created earlier donate electrons to the electron transport chain in the inner mitochondrial membrane. As electrons move through the chain, protons are pumped across the membrane, building a proton gradient. ATP synthase uses this gradient to convert ADP and inorganic phosphate into ATP. This pathway yields far more ATP than direct substrate-level phosphorylation, which happens during glycolysis and the Krebs cycle but in much smaller amounts. Fermentation does not occur under aerobic conditions, and while the Krebs cycle generates NADH and FADH2, the bulk of ATP comes from oxidative phosphorylation driven by the electron transport chain.