Which product of the electron transport chain reenters the Krebs cycle? A. ATP B. CoA C. NAD⁺ D. NADH E. Pyruvate

The correct answer is:

C. NAD⁺

Explanation:

During the electron transport chain, NADH is oxidized, donating electrons to the chain and becoming NAD+^++. This NAD+^++ is then available to re-enter the Krebs cycle (citric acid cycle), where it can pick up electrons again during the oxidation of substrates (such as acetyl-CoA), forming NADH once more.

Why the other options are incorrect:

• A. ATP: ATP is produced in the electron transport chain but is not directly recycled in the Krebs cycle.
• B. CoA: Coenzyme A (CoA) is involved in the formation of acetyl-CoA, which enters the Krebs cycle, but it does not re-enter the Krebs cycle after the electron transport chain.
• D. NADH: NADH is produced in the Krebs cycle, and it donates electrons to the electron transport chain, but it does not re-enter the Krebs cycle.
• E. Pyruvate: Pyruvate is converted to acetyl-CoA before entering the Krebs cycle, but it does not re-enter the cycle after the electron transport chain.