During which stage in the complete oxidation of glucose is the greatest number of ATP molecules formed from ADP –?(a)Glycolysis(b)Krebs cycle(c)Electron transport chain(d)Conversion of pyruvate acid to acetyl CoA

The correct answer is (c) Electron transport chain.
Detailed Explanation:
The complete oxidation of glucose to carbon dioxide and water involves several stages, and the production of ATP occurs at different points in these stages. Here is a breakdown of each stage in terms of ATP production:
1. (a) Glycolysis:
o Glycolysis is the first step in the breakdown of glucose and occurs in the cytoplasm. In this process, one molecule of glucose (6 carbon atoms) is split into two molecules of pyruvate (3 carbon atoms each).
o During glycolysis, a net total of 2 ATP molecules are produced (4 ATP molecules are generated, but 2 ATP molecules are consumed during the process). However, this is a small amount compared to other stages of cellular respiration.
2. (b) Krebs cycle (Citric acid cycle):
o The Krebs cycle occurs in the mitochondria. For each molecule of glucose, two molecules of pyruvate (produced from glycolysis) are converted to Acetyl-CoA, and each Acetyl-CoA enters the Krebs cycle.
o During one complete cycle, 2 ATP molecules are generated (1 ATP per turn of the cycle). Since each glucose molecule produces two Acetyl-CoA molecules, the total ATP production from the Krebs cycle is 2 ATP.
3. (c) Electron transport chain (ETC):
o The electron transport chain is the final stage of cellular respiration and occurs in the inner mitochondrial membrane. This is where the majority of ATP is produced.
o The NADH and FADH2 produced in glycolysis and the Krebs cycle donate electrons to the electron transport chain, which powers the movement of protons (H+) across the mitochondrial membrane, creating a proton gradient. The energy from this gradient drives ATP synthesis through oxidative phosphorylation.
o For each NADH molecule, approximately 3 ATP are produced, and for each FADH2 molecule, approximately 2 ATP are produced. Since many NADH and FADH2 molecules are generated in the earlier stages, the electron transport chain produces the greatest number of ATP molecules — typically around 28-30 ATP molecules per glucose molecule.
4. (d) Conversion of pyruvate to acetyl-CoA:
o This step occurs before the Krebs cycle, where each molecule of pyruvate is converted into Acetyl-CoA. This process releases NADH and produces no ATP directly. Therefore, it does not contribute to ATP production significantly.
The electron transport chain (ETC) is where the majority of ATP is produced during the complete oxidation of glucose, making it the stage where the greatest number of ATP molecules are formed. Therefore, the correct answer is (c) Electron transport chain.