Krebs Cycle (also called Citric Acid Cycle and Tricarboxylic Acid Cycle), a series of chemical reactions occurring in humans and many other animals. In the Krebs cycle the carbon chains of carbohydrates, proteins, and fats are metabolized to yield carbon dioxide, water, and high-energy phosphate molecules. Energy released during the cycle is incorporated into molecules of adenosine triphosphate (ATP), a major source of energy for cellular metabolism. (See Adenosine Triphosphate.) Molecules that initiate many vital metabolic functions, such as amino acid synthesis, are also produced.

The Krebs cycle is named for Sir Hans A. Krebs, who first worked out the chemical details of the cycle in 1937. The name “citric acid cycle” is sometimes used because citric acid is an important intermediate substance produced in the cycle. The Krebs cycle is also called the tricarboxylic acid cycle because citric acid is a tricarboxylic acid, meaning it contains three carboxyl (CO2H) groups.