Questions: If the TCA cycle doesn't require O2 as a reactant, why does the TCA cycle stop in the absence of oxygen? A. There's no oxygen to add to pyruvate to make acetyl CoA. B. The cell uses the O2 to generate the CO2 product made by the TCA cycle. C. NAD+ and FAD cannot be regenerated in the absence of O2 D. Without O2 the ETC will oxidize the acetyl CoA directly bypassing the TCA E. It doesn't stop in the absence of oxygen

The answer is C: \(\mathrm{NAD}^{+}\) and FAD cannot be regenerated in the absence of \(\mathrm{O}_{2}\).

Explanation for each option:

A. There's no oxygen to add to pyruvate to make acetyl CoA.

• This is incorrect. The conversion of pyruvate to acetyl CoA is catalyzed by the pyruvate dehydrogenase complex and does not directly require oxygen. Oxygen is not involved in this reaction.

B. The cell uses the \(\mathrm{O}_{2}\) to generate the \(\mathrm{CO}_{2}\) product made by the TCA cycle.

• This is incorrect. The \(\mathrm{CO}_{2}\) produced in the TCA cycle comes from the decarboxylation reactions within the cycle itself, not from oxygen.

C. \(\mathrm{NAD}^{+}\) and FAD cannot be regenerated in the absence of \(\mathrm{O}_{2}\).

• This is correct. The TCA cycle relies on \(\mathrm{NAD}^{+}\) and FAD to accept electrons and become reduced to \(\mathrm{NADH}\) and \(\mathrm{FADH_2}\). These reduced cofactors are then oxidized in the electron transport chain (ETC), which requires oxygen as the final electron acceptor. Without oxygen, the ETC cannot function, leading to a lack of \(\mathrm{NAD}^{+}\) and FAD regeneration, which in turn halts the TCA cycle.

D. Without \(\mathrm{O}_{2}\) the ETC will oxidize the acetyl CoA directly bypassing the TCA.

• This is incorrect. The electron transport chain does not oxidize acetyl CoA directly. The ETC functions to oxidize \(\mathrm{NADH}\) and \(\mathrm{FADH_2}\), not acetyl CoA.

E. It doesn't stop in the absence of oxygen.

• This is incorrect. The TCA cycle does stop in the absence of oxygen because the lack of \(\mathrm{NAD}^{+}\) and FAD regeneration halts the cycle.

In summary, the TCA cycle stops in the absence of oxygen because \(\mathrm{NAD}^{+}\) and FAD cannot be regenerated, which are essential for the cycle's continuation.