What is the proton motive force in mitochondria?

• A. It is a gradient of electrons used to power ATP synthase
• B. It is the force generated by ATP synthase during hydrolysis
• C. It is the gradient of protons across the inner mitochondrial membrane that drives ATP synthase
• D. It is the flow of carbon dioxide out of the mitochondria

Answer: (C)

Proton motive force is the electrochemical gradient of protons across the inner mitochondrial membrane that drives ATP synthase. The electron transport chain pumps H+ from the matrix into the intermembrane space, creating both a chemical gradient (different proton concentration and pH) and an electrical gradient (membrane potential). This stored energy powers protons flowing back into the matrix through ATP synthase, causing the enzyme to synthesize ATP from ADP and inorganic phosphate. It’s the proton gradient, not a gradient of electrons, not the force generated by ATP hydrolysis, and not related to carbon dioxide movement.