<P> The citric acid cycle (CAC)--also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle--is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl - CoA derived from carbohydrates, fats, and proteins into adenosine triphosphate (ATP) and carbon dioxide . In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, 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 cellular metabolism and may have originated abiogenically . 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 . </P> <P> The name of this metabolic pathway is derived from the citric acid (a type of tricarboxylic acid, often called citrate, as the ionized form predominates at biological pH) that is consumed and then regenerated by this sequence of reactions to complete the cycle . The cycle consumes acetate (in the form of acetyl - CoA) and water, reduces NAD to NADH, and produces carbon dioxide as a waste byproduct . The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation (electron transport) pathway . The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP . </P> <P> In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion . In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol with the proton gradient for ATP production being across the cell's surface (plasma membrane) rather than the inner membrane of the mitochondrion . The overall yield of energy - containing compounds from the TCA cycle is three NADH, one FAD (2H), and one GTP . </P> <P> Several of the components and reactions of the citric acid cycle were established in the 1930s by the research of Albert Szent - Györgyi, who received the Nobel Prize in Physiology or Medicine in 1937 specifically for his discoveries pertaining to fumaric acid, a key component of the cycle . He was able to make this discovery successful with the help of pigeon breast muscle . Because this tissue maintains its oxidative capacity well after breaking down in the "Latapie" mill and releasing in aqueous solutions breast muscle of the pigeon was very well qualified for the study of oxidative reactions . The citric acid cycle itself was finally identified in 1937 by Hans Adolf Krebs and William Arthur Johnson while at the University of Sheffield, for which the former received the Nobel Prize for Physiology or Medicine in 1953, and for whom the cycle is sometimes named (Krebs cycle). </P>

Where does the citric acid cyle take place
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