Which event creates the proton concentration gradient used in oxidative phosphorylation?

• A. Electron transport chain pumping protons across the inner mitochondrial membrane
• B. Breakdown of glucose to pyruvate in glycolysis
• C. Direct transfer of electrons to oxygen in the matrix
• D. Synthesis of NADH in the cytosol

Answer: (A)

Proton gradient formation is driven by the electron transport chain pumping protons across the inner mitochondrial membrane. As electrons move through the chain from NADH and FADH2, energy is released and used to push protons from the matrix into the intermembrane space. This builds a chemical and electrical gradient (proton-motive force) that ATP synthase uses to make ATP as protons flow back into the matrix. The other processes don’t create this gradient: glycolysis happens in the cytosol and doesn’t involve pumping protons across the inner membrane; breaking glucose to pyruvate yields NADH and ATP but doesn’t establish the proton gradient itself; while oxygen accepts electrons at the end of the chain, the gradient arises specifically from the active pumping of protons during electron transport; and NADH synthesized in the cytosol isn’t by itself what establishes the gradient.