Like the synthesis of acetyl-CoA, the citric acid cycle takes place in the mitochondrial matrix. It is composed of eight reactions and is called a cycle because the starting molecule, oxaloacetate, is regenerated at the end (Fig. 7.8).

In the first reaction, the 2-carbon acetyl group of acetyl-CoA is transferred to a 4-carbon molecule of oxaloacetate to form the 6-carbon molecule citric acid or tricarboxylic acid (hence the variant names “citric acid cycle” and “tricarboxylic acid cycle”). The molecule of citric acid is then oxidized in a series of reactions. The last reaction of the cycle regenerates a molecule of oxaloacetate, joining to a new acetyl group and allowing the cycle to continue.

The citric acid cycle results in the complete oxidation of the acetyl group of acetyl-CoA. Since the first reaction creates a molecule with six carbons and the last reaction regenerates a 4-carbon molecule, two carbons are eliminated during the cycle. These carbons are released as carbon dioxide. Along with the release of carbon dioxide from pyruvate during pyruvate oxidation, these reactions are the sources of carbon dioxide released during cellular respiration and therefore the sources of the carbon dioxide that we exhale when we breathe.

The oxidation reactions that produce carbon dioxide are coupled with the reduction of the electron carrier NAD+ to NADH (Fig. 7.8). In this way, energy released in the oxidation reactions is transferred to NADH. More reduced electron carriers (NADH and FADH₂) are produced in two additional redox reactions. In fact, the citric acid cycle produces a large quantity of reduced electron carriers: three molecules of NADH and one molecule of FADH₂ per turn of the cycle. These electron carriers donate electrons to the electron transport chain, which leads to the production of ATP by oxidative phosphorylation.

One of the reactions of the citric acid cycle is a substrate-level phosphorylation reaction that generates a molecule of GTP (Fig. 7.8). GTP can transfer its terminal phosphate to a molecule of ADP to form ATP. This is the only substrate-level phosphorylation in the citric acid cycle.

Overall, two molecules of acetyl-CoA produced from a single molecule of glucose yield two molecules of ATP, six molecules of NADH, and two molecules of FADH₂ in the citric acid cycle.

Quick Check 3 At the end of the citric acid cycle, but before the subsequent steps of cellular respiration, which molecules contain the energy held in the original glucose molecule?