Under aerobic conditions, which stage of cellular respiration yields the most ATP?

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

Answer: (C)

Under aerobic conditions, most ATP is produced during oxidative phosphorylation. In this stage, electrons are transferred through the electron transport chain in the mitochondrial inner membrane by NADH and FADH2. As electrons move, protons are pumped across the membrane, creating a proton gradient. When protons flow back through ATP synthase, ADP is phosphorylated to ATP. This process yields far more ATP than the other stages, typically about 26–28 ATP per glucose (depending on shuttle systems), making oxidative phosphorylation the largest source of ATP in aerobic respiration. Glycolysis adds a small amount directly (about 2 ATP) and the citric acid cycle also contributes a small direct amount (about 2 ATP) but mainly provides NADH and FADH2 to fuel oxidative phosphorylation. Fermentation, on the other hand, occurs without oxygen and produces only about 2 ATP per glucose, so it isn’t the main source under aerobic conditions.