Pyruvate Oxidation & CO₂ Release — NEET Biology

Correct answer: B — S2 and S4 only

The question asks for the statements that are NOT correct about the products and events of one turn of the TCA cycle. S1: Correct. One turn of the TCA cycle generates 3 NADH + H+, 1 FADH2, and 1 ATP (or GTP) through substrate-level phosphorylation (NCERT page 159, 'Also there are three points in the cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH2. The conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised.'). S2: Incorrect. The two CO2 molecules released in one turn of the TCA cycle are from the oxidative decarboxylation of isocitrate and α-ketoglutarate, not directly from acetyl CoA itself, which has condensed with OAA to form citrate (NCERT page 158-159, Figure 12.3). S3: Correct. Regeneration of NAD+ and FAD+ from NADH and FADH2, respectively, is vital for the continued operation of the TCA cycle (NCERT page 159, 'it also requires regeneration of NAD+ and FAD+ from NADH and FADH2 respectively.'). S4: Incorrect. The immediate product of the condensation of acetyl CoA with oxaloacetic acid is citric acid, not succinyl-CoA (NCERT page 158, 'The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid...'). S5: Incorrect. From one glucose molecule, 2 CO2 are released during oxidative decarboxylation of pyruvate to acetyl CoA. Then, 2 turns of the TCA cycle release 2 CO2 per turn, totaling 4 CO2. So, a total of 4 carbon atoms from acetyl CoA are released as CO2 during two turns of the TCA cycle, which makes 6 CO2 in total from glucose breakdown (NCERT page 158-159, Figure 12.3). Therefore, statements S2 and S4 are NOT correct.