8.12:
Products of the Citric Acid Cycle
In the presence of oxygen, pyruvate moves into the mitochondria after glycolysis, where oxidation of one molecule of pyruvate produces one acetyl-CoA, one molecule of carbon dioxide, or CO2, and one NADH. Then, acetyl-CoA enters the citric acid cycle and is modified to produce two molecules of CO2, three NADHs, one ATP, and one FADH2 in addition to the earlier products of glycolysis. So the product of a single glucose molecule, or two pyruvate molecules after oxidation of pyruvate and the citric acid cycle is six molecules of CO2, eight NADHs, two ATPs, and two molecules of FADH2.
8.12:
Products of the Citric Acid Cycle
The cells of most organisms—including plants and animals—obtain usable energy through aerobic respiration, the oxygen-requiring version of cellular respiration. Aerobic respiration consists of four major stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. The third major stage, the citric acid cycle, is also known as the Krebs cycle or tricarboxylic acid (TCA) cycle.
For every glucose molecule that undergoes cellular respiration, the citric acid cycle is carried out twice; this is because glycolysis (the first stage of aerobic respiration) produces two pyruvate molecules per glucose molecule. During pyruvate oxidation (the second stage of aerobic respiration), each pyruvate molecule is converted into one molecule of acetyl-CoA—the input into the citric acid cycle. Therefore, for every glucose molecule, two acetyl-CoA molecules are produced. Each of the two acetyl-CoA molecules goes once through the citric acid cycle.
The citric acid cycle begins with the fusion of acetyl-CoA and oxaloacetate to form citric acid. For each acetyl-CoA molecule, the products of the citric acid cycle are two carbon dioxide molecules, three NADH molecules, one FADH2 molecule, and one GTP/ATP molecule. Therefore, for every glucose molecule (which generates two acetyl-CoA molecules), the citric acid cycle yields four carbon dioxide molecules, six NADH molecules, two FADH2 molecules, and two GTP/ATP molecules. The citric acid cycle also regenerates oxaloacetate, the molecule that starts the cycle.
While the ATP yield of the citric acid cycle is modest, the generation of coenzymes NADH and FADH2 is critical for ATP production in the final stage of cellular respiration, oxidative phosphorylation. These coenzymes act as electron carriers and donate their electrons to the electron transport chain, ultimately driving the production of most of the ATP produced by cellular respiration.
Suggested Reading
Da Poian, A. T., El-Bacha, T. & Luz, M. R.M.P. (2010) Nutrient Utilization in Humans: Metabolism Pathways. Nature Education 3(9):11. [Source]
Williams, Niamh C., and Luke A. J. O’Neill. “A Role for the Krebs Cycle Intermediate Citrate in Metabolic Reprogramming in Innate Immunity and Inflammation.” Frontiers in Immunology 9 (February 5, 2018). [Source]