Krebs Cycle Schematic Overview — NEET Biology

Correct answer: B — S3, S4 and S5 only

This question assesses the understanding of substrate-level phosphorylation and its occurrence across different respiratory pathways. S1: During glycolysis, 2 ATP molecules are consumed (for phosphorylation of glucose and fructose-6-phosphate), and 4 ATP molecules are directly synthesized (2 from 1,3-bisphosphoglycerate to 3-phosphoglycerate, and 2 from phosphoenolpyruvate to pyruvic acid). Thus, a net of 2 ATP is available from glycolysis. This statement is correct. S2: In the Krebs cycle, the conversion of succinyl-CoA to succinic acid synthesizes one molecule of GTP (which is equivalent to ATP) per acetyl CoA. Since one glucose molecule yields two acetyl CoA molecules, the Krebs cycle directly produces 2 ATP molecules per glucose via substrate-level phosphorylation. This statement is correct. S3: The overall net ATP production via substrate-level phosphorylation during aerobic respiration of one glucose molecule is the sum of net ATP from glycolysis (2 ATP) and net ATP from the Krebs cycle (2 ATP). This totals 4 ATP. Therefore, stating it is '6 ATP' is NOT correct. S4: The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate is an ATP-consuming step, not an ATP-producing step. It is catalyzed by phosphofructokinase and requires ATP phosphorylation, not substrate-level phosphorylation where a phosphate group is directly transferred from a substrate to ADP. This statement is NOT correct. S5: Fermentation produces ATP only through glycolysis, which involves substrate-level phosphorylation. The reoxidation of NADH+H+ to NAD+ in fermentation is essential to regenerate NAD+ for glycolysis to continue, but it does not directly produce ATP. Therefore, stating that ATP is produced 'solely through the reoxidation of NADH+H+ to NAD+' is NOT correct. Statements S3, S4, and S5 are NOT correct.