In oxidative phosphorylation, which process generates the energy that ATP synthase uses to synthesize ATP?

• A. Establishment of a proton-motive force (electrochemical H${+}^{+}$ gradient) across the inner mitochondrial membrane by the electron transport chain
• B. Oxidation of NADH to NAD${+}^{+}$ in the mitochondrial matrix producing ATP without a membrane gradient
• C. Reduction of $O_2$ to $H_{2}O$ directly powering ATP synthase by releasing heat
• D. Direct transfer of a phosphate group from an electron transport chain intermediate to ADP (substrate-level phosphorylation)

Answer: (A)