Final answer:
ATP is produced during the intermediate step of aerobic respiration known as the citric acid cycle, with 2 ATP molecules yielded directly from one glucose molecule. The overall ATP yield also includes contributions from NADH and FADH2 via the electron transport chain, with the pathways of glucose catabolism extracting about 34 percent of the energy from glucose.
Step-by-step explanation:
ATP, or adenosine triphosphate, is produced at several stages during aerobic respiration, including in the intermediate step known as the citric acid cycle (Krebs cycle). During this stage, each turn of the cycle produces one ATP molecule directly through substrate-level phosphorylation. However, the total ATP yield from the complete oxidation of glucose is further influenced by subsequent processes such as electron transport and oxidative phosphorylation, as well as the use of intermediates in other cellular processes.
The Krebs cycle itself directly yields 2 ATP molecules per glucose molecule, as the cycle goes around twice per molecule of glucose. However, the cycle also generates NADH and FADH2, which are used in the electron transport chain to produce a significant number of additional ATP molecules. The electron transport chain can lead to variations in the exact number of ATP molecules generated, depending on factors like the specificity of electron carriers (NAD* or FAD*). Overall, pathways of glucose catabolism extract about 34 percent of the energy contained in glucose.
It is also important to consider that ATP molecules are consumed and regenerated in various cellular processes. For instance, ATPs are used in anaplerotic reactions for the replenishment of intermediates in the Krebs cycle. Therefore, the net gain of ATP should account for any molecules utilized in such processes as well.