ATP Yield Assumptions in Cellular Respiration — NEET Biology

Correct answer: B — S4 only

Let's evaluate each statement regarding the fate and energy yield of NADH from glycolysis: S1: Oxidation of one molecule of NADH gives rise to 3 molecules of ATP. Since 2 NADH + H+ molecules are produced in glycolysis, they would yield 2 x 3 = 6 ATP molecules via oxidative phosphorylation in aerobic conditions. This statement is correct. S2: In animal cells, like muscles during exercise, pyruvic acid is reduced to lactic acid by lactate dehydrogenase, using NADH + H+ which is reoxidised to NAD+. This statement is correct. S3: One of the assumptions for calculating the net gain of ATP (e.g., 38 ATP) is that the NADH synthesized in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation. This statement is correct. S4: In alcoholic fermentation, pyruvic acid is first decarboxylated to acetaldehyde, and then acetaldehyde is reduced to ethanol by alcohol dehydrogenase. It is during the reduction of acetaldehyde to ethanol that NADH + H+ is reoxidized to NAD+. Acetyl CoA is not involved in alcoholic fermentation (it is a product of pyruvate oxidation before Krebs cycle). This statement is NOT correct. S5: In fermentation, there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid, and this is solely through substrate-level phosphorylation in glycolysis as oxidative phosphorylation does not occur under anaerobic conditions. This statement is correct. Therefore, the only statement that is NOT correct is S4.