Multiple Choice
In eukaryotic cells, where does the Krebs (citric acid) cycle occur?
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Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Citric Acid Cycle 1
Multiple Choice
In the citric acid (Krebs) cycle, how many molecules of ATP (or GTP, as an equivalent) are produced directly by substrate-level phosphorylation per turn of the cycle (per acetyl-CoA)?
A
2
B
4
C
1
D
0
Verified step by step guidance1
Understand that the citric acid cycle (Krebs cycle) processes one molecule of acetyl-CoA per turn, producing energy-rich molecules including NADH, FADH2, and one molecule of GTP (or ATP) via substrate-level phosphorylation.
Recall that substrate-level phosphorylation refers to the direct synthesis of ATP (or GTP) from ADP (or GDP) during a metabolic pathway, without involving the electron transport chain.
Identify the specific step in the citric acid cycle where substrate-level phosphorylation occurs: the conversion of succinyl-CoA to succinate catalyzed by succinyl-CoA synthetase, which produces one molecule of GTP (or ATP) per cycle turn.
Note that other energy carriers produced in the cycle, such as NADH and FADH2, contribute to ATP production indirectly through oxidative phosphorylation, not substrate-level phosphorylation.
Conclude that per turn of the citric acid cycle (per acetyl-CoA), exactly one molecule of ATP (or GTP) is produced directly by substrate-level phosphorylation.
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