Explain how NADH and FADH2 contribute to ATP production during oxidative phosphorylation.

• A. They are used as substrates in the Krebs cycle.
• B. They convert glucose to glycogen for storage.
• C. They directly phosphorylate ADP to form ATP.
• D. They donate electrons to the electron transport chain, creating a proton gradient used to synthesize ATP.

Answer: (D)