Figure 9.2 The citric acid cycle.
-Starting with one molecule of isocitrate and ending with fumarate, how many ATP molecules can be made through substrate-level phosphorylation (see Figure 9.2)?

Question

Quizplus · Accepted Answer

In the conversion of isocitrate to fumarate in the citric acid cycle, there is no ATP produced directly through substrate-level phosphorylation. However, the question might be slightly misleading as it seems to refer to a specific figure (Figure 9.2) not provided here. Based on general biochemical knowledge, the steps from isocitrate to fumarate involve the production of NADH (from the isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase steps) but not ATP directly. The only step in the citric acid cycle that involves substrate-level phosphorylation produces GTP (which can be considered equivalent to ATP in terms of energy transfer) occurs later in the conversion of succinyl-CoA to succinate, which is after the formation of fumarate. Therefore, the correct answer, assuming the question is about direct ATP production through substrate-level phosphorylation in the specific steps mentioned, is 0, which is not an option provided, suggesting a possible mistake in the question or answer choices. Given the options, A (1) is the closest, but it should be noted this is not strictly correct based on the biochemical pathway.

Figure 9.2 the Citric Acid Cycle