​The citric acid cycle (TCA) operates only in the presence of molecular oxygen $$(O_2)$$  This is because:​

Correct option is B

Explanation of the correct answer:

The citric acid cycle, or TCA cycle, is tightly coupled with oxidative phosphorylation, a process that requires oxygen to function. Oxygen is the terminal electron acceptor in the electron transport chain (ETC). Here's why (2) is correct:

• (2): Oxygen is essential for the final step of the electron transport chain (ETC) in the inner mitochondrial membrane. It accepts electrons from NADH and FADH₂, allowing NAD⁺ and FAD to be regenerated. This reoxidation of NADH to NAD⁺ is crucial because NAD⁺ is required for the continuation of dehydrogenation reactions in the TCA cycle. Without oxygen, NAD⁺ cannot be regenerated, and the TCA cycle would halt because it needs NAD⁺ to accept electrons during the cycle’s dehydrogenation steps.

Information Booster:

• TCA cycle and Oxygen: The TCA cycle itself does not directly require oxygen, but it is highly dependent on the electron transport chain to regenerate the cofactors (NAD⁺, FAD) that are needed for key reactions in the TCA cycle, particularly during the dehydrogenation steps. Oxygen, by accepting electrons at the end of the electron transport chain, ensures that NADH can be oxidized back to NAD⁺ and FADH₂ can be oxidized to FAD, which is essential for the cycle's continuation.

Additional Information about Incorrect Options:

• Option 1: Oxygen does not directly activate enzymatic dehydrogenation reactions in the TCA cycle. These reactions are catalyzed by specific enzymes (such as NAD-dependent dehydrogenases), and oxygen's role is to regenerate NAD⁺ in the ETC, which is necessary for these reactions to proceed.

• Option 3: While oxygen is essential for the electron transport chain, which ultimately reduces the build-up of certain by-products like NADH and FADH₂, oxygen itself does not directly remove toxic by-products from the TCA cycle.

• Option 4: Oxygen does not directly activate ATP synthase. ATP synthase is activated by the proton gradient created by the ETC. While oxygen is involved in generating the proton gradient, it does not activate ATP synthase directly.