TheKrebs cycleis simply another name for theCitric Acid Cycle, so named for the researcher who identified the completecyclein 1937. Thiscycledescribes a series of chemical reactions that take place in all aerobic organisms (aerobic meaning that they use oxygen to facilitate their metabolism, like we do).TheKrebs cycleoccurs in the mitochondrial matrix and generates a pool of chemical energy (ATP, NADH, and FADH2) from the oxidation of pyruvate, the end product of glycolysis. Pyruvate is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA, a 2-carbon molecule.

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Thecitric acid cycle(CAC)– also known as theTCA cycle(tricarboxylic acid cycle)or theKrebs cycle – is a series ofchemical reactionsused by allaerobic organismsto release stored energy through theoxidationofacetyl-CoAderived fromcarbohydrates,fats, andproteins, intoadenosine triphosphate(ATP) andcarbon dioxide. In addition, the cycle providesprecursorsof certain amino acids, as well as thereducing agentNADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellularmetabolismand may have originatedabiogenically.Even though it is branded as a 'cycle', it is not necessary for metabolites to follow only one specific route; at least three segments of the citric acid cycle have been recognized.

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of the mitochondria. Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2. This is considered an aerobic pathway because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.