What drives protons back across the mitochondrial membrane in oxidative phosphorylation?

• A. Proton motive force
• B. Energy stored in ATP molecules
• C. The gradient of sodium ions across the plasma membrane
• D. The energy released by pyruvate oxidation

Answer: (A)

Protons moving back into the mitochondrial matrix are driven by the proton motive force, an electrochemical gradient across the inner mitochondrial membrane. The electron transport chain pumps protons from the matrix into the intermembrane space, creating both a chemical gradient (higher H+ concentration outside) and an electrical gradient (positive charge outside). Protons flow back through ATP synthase down this gradient, and the energy from that flow powers the synthesis of ATP from ADP and inorganic phosphate. This flow is the direct thermodynamic force for ATP production.

Other options don’t fit this specific mechanism. The energy stored in ATP molecules is the product of the process, not what pushes protons back across the membrane. The gradient of sodium ions across the plasma membrane is unrelated to mitochondrial ATP synthesis. And the energy released by pyruvate oxidation fuels the electron transport chain that establishes the proton gradient, but it isn’t the immediate driving force for protons returning through ATP synthase.