Which pathway yields the most ATP per glucose under aerobic conditions?

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

Answer: (B)

Under aerobic conditions, the most ATP comes from oxidative phosphorylation in the mitochondria. Here’s why: glycolysis and the Krebs cycle generate NADH and FADH2, which carry high-energy electrons to the electron transport chain. As these electrons move through the chain, protons are pumped across the inner mitochondrial membrane, creating a proton gradient. ATP synthase then uses the flow of protons back across the membrane to convert ADP to ATP. Oxygen serves as the final electron acceptor, forming water. This chain of events produces far more ATP than the small amount made directly by substrate-level phosphorylation in glycolysis and the Krebs cycle. Overall, about 30–32 ATP per glucose are produced in aerobic respiration (the exact number depends on shuttle systems that transfer reducing equivalents into the mitochondria). Fermentation or relying only on substrate-level steps yields much less ATP and isn’t the main source when oxygen is available.