The process by which living organisms generate energy from food is called cellular respiration. This metabolic process occurs in the cells of organisms, where glucose is broken down in the presence of oxygen to produce energy in the form of ATP (adenosine triphosphate). Cellular respiration can be aerobic, utilizing oxygen, or anaerobic, when oxygen is not available. The process also produces byproducts such as carbon dioxide and water, which are released from the organism.