Cellular Respiration
cellular respiration , the process of cell catabolism in which cells turn food into usable energy in the form of ATP. In this process glucose is broken down in the presence of molecular oxygen into six molecules of carbon dioxide, and much of the energy released is preserved by turning ADP and free phosphate into ATP. Cellular respiration occurs as a series of chemical reactions catalyzed by enzymes, the first of which is glycolysis, a series of anaerobic reactions in which glucose (a 6-carbon molecule) is split into two molecules of lactate (a 3-carbon molecule), producing a net gain of two ATP molecules. In a series of aerobic reactions, lactate is converted to pyruvate, which enters the mitochondrion and combines with oxygen to form an acetyl group, releasing carbon dioxide. The acetyl group (CH3CO) is then combined with coenzyme A as acetyl coenzyme A, and enters the Krebs cycle. During this series of reactions, each acetyl group is oxidized to form two molecules of carbon dioxide