Which energy-generating mechanism yields more ATP per glucose?

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

Answer: (C)

Oxidative phosphorylation harvests the energy from NADH and FADH2 produced during glucose metabolism through the electron transport chain and chemiosmosis. The resulting proton gradient powers ATP synthase to convert ADP and Pi into a large amount of ATP. In most cells, this step yields roughly 26–28 ATP per glucose (about 30–32 in some cases), which is far more than any other mechanism.

Fermentation, which occurs when oxygen is limited or mitochondria aren’t available, relies on glycolysis alone to produce ATP and regenerates NAD+ without any electron transport chain, giving only about 2 ATP per glucose. Substrate-level phosphorylation contributes a small amount of ATP directly in glycolysis and the TCA cycle (roughly 4 ATP per glucose in total), but it doesn’t use the vast energy transfer from NADH/FADH2 via a proton gradient. Glycolysis itself yields 2 ATP directly, with additional energy tied up in NADH that can, if routed through oxidative phosphorylation, add more ATP—but on its own it provides far less than oxidative phosphorylation.