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)?
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Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Citric Acid Cycle 1
Multiple Choice
Per molecule of glucose (two turns of the citric acid cycle), what are the net products of the citric acid (Krebs) cycle, excluding ATP/GTP and reduced cofactors generated by pyruvate dehydrogenase?
A
6 , 8 NADH, 2 FADH, and 2 GTP
B
4 , 6 NADH, 2 FADH, and 2 GTP
C
4 , 8 NADH, 2 FADH, and 2 GTP
D
2 , 3 NADH, 1 FADH, and 1 GTP
Verified step by step guidance1
Recall that one molecule of glucose produces two molecules of pyruvate through glycolysis, and each pyruvate enters the citric acid cycle after conversion to acetyl-CoA by pyruvate dehydrogenase. The problem asks to exclude products generated by pyruvate dehydrogenase, so focus only on the citric acid cycle itself.
Understand that one turn of the citric acid cycle processes one acetyl-CoA molecule. Since one glucose yields two acetyl-CoA molecules, the citric acid cycle turns twice per glucose molecule.
Identify the net products generated per single turn of the citric acid cycle: 2 CO\_2 molecules, 3 NADH molecules, 1 FADH\_2 molecule, and 1 GTP (or ATP equivalent).
Multiply each product by 2 to account for the two turns of the cycle per glucose molecule, giving the total net products per glucose molecule: 4 CO\_2, 6 NADH, 2 FADH\_2, and 2 GTP.
Confirm that these values exclude the NADH produced by pyruvate dehydrogenase, as requested, and represent only the direct output of the citric acid cycle per glucose molecule.
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