In oxidative phosphorylation, what is the primary role of NADH?

• A. It transports oxygen from the cytosol into the mitochondrial matrix to support respiration.
• B. It directly phosphorylates ADP to ATP by transferring a phosphate group.
• C. It serves as the terminal electron acceptor, combining with electrons and protons to form water.
• D. It donates high-energy electrons to Complex I of the electron transport chain, helping drive proton pumping across the inner mitochondrial membrane.

Answer: (D)

Verified step by step guidance

1. Understand the context of oxidative phosphorylation, which is the process by which cells generate ATP using energy derived from electrons transferred through the electron transport chain (ETC) located in the inner mitochondrial membrane.
2. Recognize that NADH is a key electron carrier molecule produced during earlier metabolic pathways such as glycolysis and the citric acid cycle, and its role is to donate electrons to the ETC.
3. Identify that NADH donates its high-energy electrons specifically to Complex I (NADH dehydrogenase) of the electron transport chain, initiating the flow of electrons through the chain.
4. Know that the transfer of electrons from NADH to Complex I helps pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space, creating a proton gradient across the inner mitochondrial membrane.
5. Realize that this proton gradient is then used by ATP synthase to drive the phosphorylation of ADP to ATP, which is the main energy currency of the cell.