Under anaerobic conditions (a lack of oxygen), the conversion of pyruvate to acetyl-CoA stops. Which of the following best explains why this occurs?

• A. NAD\(^+\) is not regenerated efficiently without oxygen, halting the activity of pyruvate dehydrogenase.
• B. Pyruvate is directly converted to glucose in the absence of oxygen.
• C. ATP cannot be produced from glycolysis without oxygen.
• D. Oxygen is required as a substrate for the pyruvate dehydrogenase complex.

Answer: (A)

Verified step by step guidance

1. Understand the role of oxygen in cellular respiration: Oxygen is the final electron acceptor in the electron transport chain (ETC), which is part of aerobic respiration. Without oxygen, the ETC cannot function, and NADH cannot be oxidized back to NAD\(^+\).
2. Recognize the importance of NAD\(^+\): NAD\(^+\) is a crucial coenzyme that accepts electrons during glycolysis and the citric acid cycle. It must be regenerated for these processes to continue.
3. Analyze the role of pyruvate dehydrogenase: Pyruvate dehydrogenase is an enzyme that converts pyruvate into acetyl-CoA, a substrate for the citric acid cycle. This process requires NAD\(^+\) as a cofactor.
4. Connect the lack of oxygen to the halting of pyruvate dehydrogenase activity: Without oxygen, the ETC cannot regenerate NAD\(^+\) efficiently. As a result, the activity of pyruvate dehydrogenase is halted because it depends on the availability of NAD\(^+\).
5. Clarify why the other options are incorrect: Pyruvate is not directly converted to glucose (this would require gluconeogenesis, which is a separate process). ATP can still be produced from glycolysis under anaerobic conditions, though less efficiently. Oxygen is not a direct substrate for pyruvate dehydrogenase; its role is indirect through the regeneration of NAD\(^+\).