Question

The two moles of CO₂ produced in the first turn of the citric acid cycle have their origin in the:

a) Carboxyl and methylene carbons of oxaloacetate.
b) Two carboxyl groups derived from oxaloacetate.
c) Carboxyl group of acetate and a carboxyl group of oxaloacetate.
d) Carboxyl group of acetate and the keto group of oxaloacetate.
e) Two carbon atoms of acetate.

Answer 1

The two moles of CO2 produced in the first turn of the citric acid cycle come from the carboxyl group of acetate and a carboxyl group of oxaloacetate, not from the acetyl group that enters the cycle.

Step-by-step explanation:

The two moles of CO2 produced in the first turn of the citric acid cycle originate from different sources. While two carbon atoms enter the cycle as part of an acetyl group, these are not the same carbon atoms that are immediately released as carbon dioxide. Instead, during the cycle, there is a series of reactions where citrate is oxidized, and two CO2 molecules are eventually produced from carbon atoms that were part of oxaloacetate that the acetyl group joined. Eventually, the carbon atoms from acetyl CoA will also be released as CO2, but this occurs in later cycles.

Based on this understanding, the correct answer is that the two moles of CO2 come from the carboxyl group of acetate and a carboxyl group of oxaloacetate, implying that they are not both derived directly from the acetyl group that enters the cycle. Therefore, the correct answer is:
c) Carboxyl group of acetate and a carboxyl group of oxaloacetate.