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The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It is a central part of cellular respiration. In eukaryotes, the citric acid cycle takes place in the mitochondrial matrix. In prokaryotes, it occurs in the cytoplasm.

The cycle begins with the condensation of acetyl-CoA with oxaloacetate to form citrate. Through a series of enzymatic steps, citrate is oxidized and rearranged, releasing carbon dioxide molecules and producing high-energy electron carriers, NADH and FADH2. These reduced coenzymes then proceed to the electron transport chain, where their energy is used to produce a large amount of ATP, the primary energy currency of the cell.

The net result of one turn of the citric acid cycle is the production of 2 molecules of carbon dioxide, 3 molecules of NADH, 1 molecule of FADH2, and 1 molecule of GTP (which is equivalent to ATP). The cycle is amphibolic, meaning it is both catabolic and anabolic. It breaks down acetyl-CoA while also providing precursor molecules for the synthesis of various biomolecules, such as amino acids and fatty acids. This interconnectivity makes the citric acid cycle a crucial hub for metabolism. The regulation of the cycle is complex, involving feedback inhibition by ATP and NADH, as well as activation by ADP and NAD+.