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The Krebs cycle, or the tricarboxylic acid cycle or citric acid cycle, is the second stage of cellular respiration. Electron tranport/oxidative phosphorylation and glycolysis are the the other two stages. This process happens in the cells of the body, in the cell's mitochondrial matrix. This stage is primarily fueled by the product of glycolysis, known as pyruvic acid. Fatty acids also fuel it. Two pyruvic acid molecules are produced from glycolysis, requiring the Krebs cycle to occur two times per glucose molecule.
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This phase occurs before the actual cycle begins. During this phase, pyruvic acid is converted into acetyl coenzyme A. The transitional phase has three steps. The pyruvic acid's carbon is removed and then released as carbon dioxide gas. The lungs will eventually expel this. The hydrogen atoms are then removed, via oxidation, which NAD+ will pick up. Lastly, acetyl CoA is produced when coenzyme A and acetic acid are combined.
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Beginning of the Cycle
Once the acetyl CoA is produced, it has to be completely broken down by entering the Krebs cycle. This cycle consists of eight steps. The first step involves a two-carbon acetyl group being transferred by coenzyme A to a four-carbon compound, oxaloacetate. This then become citrate, a six-carbon molecule.
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Steps Two Through Four
A six-carbon isocitrate is formed through the citrate rearranging. The carbon dioxide molecule is taken out once the isocitrate is oxidized. Alpha-ketoglutarate, a five-carbon molecule, is the result. Then, the NAD+ to NADH and H+ reduction. The fourth step involves carbon dioxide being taken out again, the oxidation of the alpha-ketoglutarate, and the adding of coenzyme A. Succinyl-CoA, a four-carbon compound is then formed.
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Steps Five Through Seven
Succinate is produced by taking away the coenzyme A from succinyl-CoA. Fumarate is then formed through the oxidation of the succinate. Malate is then produced by adding water to the fumarate.
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Finally, oxaloacetate is formed when the malate is oxidized. NAD+ is then reduced to NADH and H+ during the final oxidation. Oxaloacetate is regenerated and the formation of two carbon dioxide molecules occurs during each Krebs cycle. The next stage known as glucose oxidation is possible due to this cycle.
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Creation of the Krebs Cycle
Han A. Krebs, during the year of 1937, proposed that cells had a specific metabolic pathway. This cycle was created to explain how animal tissues oxidized carbohydrates. Later work completed by Krebs showed that this cycle is not exclusive to only animals, but that almost all aerobic cells have it. This cycle occurs within the mitochondria and provides all of the necessary energy for organism function.
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University of North Texas. (2010). The Citric Acid Cycle. Retrieved on November 15, 2010 from the University of North Texas: http://people.unt.edu/~hds0006/tca/index.htm
Kaiser, G. E. (2001). The Citric Acid (Krebs) Cycle. Retrieved on November 15, 2010 from Community College of Baltimore County: http://student.ccbcmd.edu/biotutorials/cellresp/cac.html