Electron Transfer & ATP Synthesis — NEET Biology

Correct answer: C — S3, S4 and S5 are correct

The TCA cycle is considered aerobic not because it directly uses oxygen, but because its continued functioning is critically dependent on the regeneration of NAD⁺ and FAD⁺, which occurs during oxidative phosphorylation, a process that absolutely requires oxygen. S1: Incorrect. Oxygen is not directly consumed during the formation of succinyl-CoA from α-ketoglutaric acid. This step involves oxidative decarboxylation, reducing NAD⁺, but not direct O₂ usage. S2: Incorrect. The Electron Transport System (ETS) requires molecular oxygen as the final electron acceptor. Without oxygen, the ETS cannot function effectively to reoxidize NADH+H⁺ and FADH₂. S3: Correct. NADH+H⁺ and FADH₂ are produced throughout glycolysis and the TCA cycle. Their reoxidation back to NAD⁺ and FAD⁺ is essential for these cycles to continue, and this reoxidation primarily occurs in the ETS, where oxygen is the final acceptor. S4: Correct. As stated in NCERT, "Oxygen acts as the final hydrogen acceptor." It pulls electrons through the ETS, thereby driving the entire oxidative phosphorylation process. S5: Correct. If oxygen is absent, NADH+H⁺ and FADH₂ cannot be reoxidized. Their accumulation would lead to a depletion of NAD⁺ and FAD⁺, which are vital coenzymes for various steps in glycolysis and the TCA cycle. This depletion would feedback inhibit key enzymes, ultimately halting the metabolic pathways for complete glucose oxidation. The correct statements are S3, S4, and S5.