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)

Verified step by step guidance

1. Understand that oxidative phosphorylation involves the generation of ATP using energy derived from an electrochemical gradient across the inner mitochondrial membrane.
2. Identify that the key gradient responsible for powering ATP synthase is the proton (H\(\textsuperscript{+}\)) electrochemical gradient, also known as the proton-motive force.
3. Recall that this proton gradient is established by the electron transport chain pumping protons from the mitochondrial matrix into the intermembrane space, creating both a concentration difference and an electrical potential difference.
4. Recognize that ATP synthase uses the energy stored in this proton gradient to drive the phosphorylation of ADP to ATP as protons flow back into the matrix through the enzyme.
5. Conclude that neither NADH, oxygen, nor glucose concentration gradients directly power ATP synthase; instead, it is the proton electrochemical gradient across the inner mitochondrial membrane that provides the necessary energy.