What does oxidative phosphorylation involve?

• A. Protein and electron carrier molecules embedded in the mitochondrial inner membrane
• B. Substrate-level phosphorylation in the cytosol
• C. Direct ATP synthesis by glycolysis in the cytosol
• D. Proton pumps in the plasma membrane

Answer: (A)

Oxidative phosphorylation relies on the electron transport chain complexes and ATP synthase that are embedded in the inner mitochondrial membrane. As electrons move through these protein and electron-carrier components, protons are pumped across the membrane, creating a proton gradient. The energy stored in this gradient drives ATP synthase to convert ADP and inorganic phosphate into ATP. This process depends on membrane-embedded proteins and their redox chemistry to power the gradient, then uses that gradient to synthesize ATP. It’s different from substrate-level phosphorylation, which makes ATP directly in glycolysis or the Krebs cycle without a proton pump-driven gradient. Proton pumps in the plasma membrane describe a different system, not the mitochondrial inner membrane machinery.