Which process directly uses the energy stored in a proton gradient to produce ATP?

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

Answer: (B)

The energy stored in a proton gradient is used to make ATP through chemiosmosis during oxidative phosphorylation. In the mitochondria, the electron transport chain pumps protons across the inner membrane, creating a proton gradient (proton motive force). As protons flow back through ATP synthase, the enzyme uses that flow to drive the formation of ATP from ADP and Pi. This step directly converts the stored gradient energy into chemical energy in ATP. The other processes generate ATP without relying on a proton gradient: glycolysis and the citric acid cycle mainly produce ATP via substrate-level phosphorylation, and fermentation generates ATP in a similar way but without using mitochondria or a proton-motive force.