At the beginning of the citric acid cycle, acetate, a 2-carbon molecule, is adde

Question

At the beginning of the citric acid cycle, acetate, a 2-carbon molecule, is added to oxaloacetate, a 4-carbon molecule, to form citrate, a 6-carbon molecule. At the end of the cycle, only oxaloacetate, the 4-carbon molecule is present. What happened to the 2 carbons added at the beginning of the cycle?

a. The two carbons became part of more oxaloacetate molecules.

b. Two carbons combined with coenzyme A to make acetyl-CoA.

c. They became acetic acid in the cytoplasm.

d. Two carbons became two carbon dioxide molecules.

e. Two carbon cycled back to become citric acids

Explanation / Answer

d. Two carbons became two carbon dioxide molecules.

Citric acid is decarboxylated twice and the four-carbon intermediate is ultimately converted back into oxaloacetic acid.

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