Final answer:
In the citric acid cycle, the five-carbon molecule isocitrate is converted to the four-carbon molecule succinyl-CoA, through multiple reactions that include the release of a carbon dioxide molecule.
Step-by-step explanation:
The citric acid cycle, also known as the Krebs cycle or TCA cycle, begins with the combination of a four-carbon molecule, oxaloacetate, and a two-carbon molecule from acetyl CoA to form a six-carbon molecule called citrate. As the cycle proceeds, citrate is transformed through a series of reactions. The specific step where a five-carbon intermediate is converted to a four-carbon molecule involves isocitrate being oxidized and decarboxylated to yield α-ketoglutarate, and then another decarboxylation reaction converts α-ketoglutarate to succinyl-CoA, a four-carbon compound. This conversion releases one molecule of CO₂. The cycle continues to oxidize these compounds, eventually regenerating oxaloacetate, and during this process, it produces ATP (or GTP), three molecules of NADH, and one molecule of FADH₂ per acetyl CoA molecule that enters the cycle.