Final answer:
Oxaloacetate, a four-carbon molecule, begins the citric acid cycle when it is coupled with acetyl CoA. This cycle involves a series of reactions that produce ATP and electron carriers NADH and FADH₂.
Step-by-step explanation:
The molecule that begins the citric acid cycle, also known as the Krebs cycle or TCA cycle, when coupled with acetyl CoA, is a four-carbon molecule called oxaloacetate (OAA). When acetyl CoA combines with OAA, it forms a six-carbon molecule called citrate, which is the first intermediate of the cycle. The citric acid cycle is a series of reactions that ultimately leads to the oxidation of acetyl-CoA to carbon dioxide and water, generating ATP and electron carriers NADH and FADH₂.