Cellular respiration is the process by which organisms use oxygen and food to produce energy in the form of ATP. It occurs in the mitochondria of plant and animal cells. During cellular respiration, food and oxygen are used to produce energy, water, and carbon dioxide. The three main stages of cellular respiration are glycolysis, the Krebs cycle, and oxidative phosphorylation. NAD+ and FAD are important coenzymes that accept electrons during the process. All living organisms undergo cellular respiration to power their metabolic functions.