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Multiple Choice
In the citric acid (Krebs) cycle, how many molecules of ATP (or GTP, ATP-equivalent) are produced directly by substrate-level phosphorylation per turn of the cycle (per acetyl-CoA)?
A
0
B
2
C
3
D
1
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Verified step by step guidance
1
Understand that the citric acid cycle (Krebs cycle) processes one acetyl-CoA molecule per turn, generating energy-rich molecules.
Identify the step in the cycle where substrate-level phosphorylation occurs: this is the conversion of succinyl-CoA to succinate, catalyzed by succinyl-CoA synthetase.
Recognize that this step produces one molecule of GTP (or ATP equivalent) directly per turn of the cycle.
Note that other energy carriers produced in the cycle, such as NADH and FADH2, contribute to ATP production indirectly via oxidative phosphorylation, not substrate-level phosphorylation.
Conclude that the number of ATP (or GTP) molecules produced directly by substrate-level phosphorylation per acetyl-CoA in the citric acid cycle is one.