The citric acid cycle also called as the TCA cycle (tricarboxylic acid cycle) and the Krebs cycle or the Szent-Györgyi-Krebs cycle is a series of chemical reactions used by all aerobic organisms to make energy by the oxidization of acetate derived from carbohydrates, fats and proteins into carbon dioxide. Additionally, the cycle gives precursors involving particular amino acids as well as the decreasing agent NADH which is used in numerous biochemical reactions. Its middle significance too several biochemical pathways suggests in which it was one of the older established elements of cellular metabolism and should have originated abiogenically.