In oxidative phosphorylation, which concentration gradient directly provides the energy (proton-motive force) that powers ATP synthase to synthesize ATP?

• A. The NADH concentration gradient between the mitochondrial matrix and intermembrane space
• B. The proton ($H^{+}$) electrochemical gradient across the inner mitochondrial membrane
• C. The oxygen ($O_2$) concentration gradient across the outer mitochondrial membrane
• D. The glucose concentration gradient between the cytosol and mitochondrial matrix

Answer: (B)