Final answer:
The correct answer is option a. The citric acid cycle, or Krebs cycle, involves the oxidation of acetyl-CoA and the formation of a six-carbon molecule called citric acid. With the acetyl group of acetyl-CoA included, there are six carbon units present among the metabolites in the citric acid cycle.
Step-by-step explanation:
The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a series of reactions that occurs in the mitochondria and is a central part of cellular respiration. It involves the oxidation of acetyl-CoA, which is a two-carbon molecule derived from glucose metabolism.
The acetyl group from acetyl-CoA is combined with a four-carbon molecule called oxaloacetate to form a six-carbon molecule called citric acid.
During each turn of the citric acid cycle, two carbon dioxide molecules are released, but these may not necessarily contain the most recently added carbon atoms. The two acetyl carbon atoms will eventually be released on later turns of the cycle.
In total, each acetyl group that enters the cycle contributes two carbon units, so with the acetyl group of acetyl-CoA included, there are a total of six carbon units present among the metabolites in the citric acid cycle.