What is the role of the electron transport chain in mitochondria?

• A. Catalyzes substrate-level phosphorylation to generate ATP directly
• B. Generates a proton gradient that drives ATP synthesis
• C. Produces glucose via glycolysis
• D. Converts NADH to NAD+ without energy production

Answer: (B)

The electron transport chain uses the energy from electrons to move protons across the inner mitochondrial membrane, building a proton gradient and a membrane potential. This proton-motive force is what drives ATP synthase to convert ADP and inorganic phosphate into ATP as protons flow back into the matrix. Oxygen acts as the final electron acceptor, allowing the chain to keep functioning and forming water. So, its role is to generate the proton gradient that powers ATP synthesis. Substrate-level phosphorylation happens elsewhere, and the chain does not directly produce glucose or simply recycle NADH without energy capture.