Products of the Citric Acid Cycle

One glucose molecule yields six carbon dioxide molecules, eight NADH molecules, two ATP molecules, and two FADH2 molecules after glycolysis, pyruvate oxidation, and the citric acid cycle. These products result from the cycle running twice, since glycolysis produces two pyruvate molecules that are converted into two acetyl-CoA molecules entering the cycle.

Each acetyl-CoA molecule produces two carbon dioxide molecules, three NADH molecules, one FADH2 molecule, and one ATP molecule during the citric acid cycle. Since one glucose generates two acetyl-CoA molecules through pyruvate oxidation, the cycle runs twice per glucose, doubling these individual products.

NADH and FADH2 function as electron carriers that donate electrons to the electron transport chain during oxidative phosphorylation. Although the citric acid cycle produces modest ATP directly, the coenzymes it generates are essential for producing most of the ATP generated by cellular respiration.

After glycolysis, pyruvate enters the mitochondria where it undergoes oxidation to produce one acetyl-CoA, one carbon dioxide molecule, and one NADH molecule. This pyruvate oxidation step prepares acetyl-CoA to enter the citric acid cycle for further energy extraction and coenzyme generation.

The citric acid cycle regenerates oxaloacetate, the molecule that combines with acetyl-CoA at the cycle's beginning to form citric acid. This regeneration allows the cycle to continue operating repeatedly, processing each acetyl-CoA molecule that enters from pyruvate oxidation in the mitochondria.

Glycolysis produces two pyruvate molecules from one glucose. Each pyruvate is converted into one acetyl-CoA through oxidation, generating two acetyl-CoA molecules total. Since each acetyl-CoA enters the citric acid cycle once, the cycle must run twice to process all products from a single glucose molecule.

The citric acid cycle directly produces only two ATP molecules per glucose, but generates six NADH and two FADH2 molecules that serve as electron carriers. These coenzymes drive the majority of ATP production through oxidative phosphorylation, making the cycle's coenzyme output far more significant than its direct ATP yield.