In cellular respiration, what is the role of NADH?

• A. Produces ATP directly
• B. Stores energy as NADH
• C. Converts NADH to NAD+
• D. Donates electrons to the electron transport chain

Answer: (D)

The main idea is that NADH serves as an electron carrier that powers the electron transport chain. NADH is formed when NAD+ accepts electrons during glycolysis, pyruvate oxidation, and the citric acid cycle, storing energy in its reduced form. Its crucial role is to donate those high-energy electrons to the electron transport chain, entering at the first complex and becoming NAD+ in the process. This electron transfer drives proton pumping across the inner mitochondrial membrane, creating a proton gradient that ATP synthase uses to make ATP. So NADH’s function is to supply the electrons that ultimately enable oxidative phosphorylation and ATP production. It doesn’t directly make ATP itself, and while NADH does represent stored reducing energy, the defining role here is delivering electrons to the chain (with NADH being oxidized to NAD+ as it donates).