Clinton Community College explains that pyruvate is converted into acetyl coenzyme A before entering the citric acid, or Krebs, cycle. A carbon atom is removed from each of the two molecules of pyruvate, yielding acetyl coenzyme A and carbon dioxide. Pyruvate is produced during the first stage of cellular respiration, known as glycolysis. During the process of converting pyruvate to acetyl coenzyme A, six adenosine triphosphate molecules are produced.
The citric acid cycle is the second step in cellular respiration for eukaryotic organisms. While glycolysis occurs in the cytoplasm of the cells, the citric acid cycle and the final step in the process, known as the electron transport chain, take place in the mitochondria of the cells. Prokaryotic organisms also engage in the citric acid cycle. However, as explained by About.com, bacteria conduct the citric acid cycle in the cytoplasm of the cells, rather than in mitochondria. Most of the adenosine triphosphate produced via cellular respiration occurs during the portions of the cycle that occur inside the mitochondria.
According to About.com, the citric acid cycle was discovered and described by a British biochemist, Sir Hans Adolf Krebs. The process was named in his honor, shortly after its discovery in 1937.