The Krebs cycle - also known as the tricarboxylic acid cycle (TCA cycle), or the citric acid cycle - consists of a series of chemical reactions used by aerobic (air breathing) organisms to generate energy through the oxidization of acetate, which is derived from foods consumed (carbohydrates, fats, and proteins), into carbon dioxide (CO₂).

This oxidization allows energy to be released that is chemically stored for later use through other chemical pathways that occur in aerobic cells (such as oxidative phosphoroylation).

The Kreb cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate). Citrate then goes through several chemical transformations, eventually losing two carboxyl groups as CO₂. The carbons lost as CO₂ originate from the oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. The transformation of acetyl-CoA-donated carbons as CO₂ requires several turns of the Kreb cycle.