Cellular respiration is the aerobic harvesting of energy from food molecules. It has three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. Glycolysis occurs in the cytoplasmic fluid of the cell. Glycolysis begins respiration by breaking glucose into two molecules of pyruvate. The citric acid cycle takes place in the mitochondria and completes the breakdown of glucose by decomposing a derivative of pyruvate to carbon dioxide. Glycolysis and the citric acid cycle make small amounts of ATP; their purpose is to supply the electron transport chain with electrons. Oxidative phosphorylation includes the electron transport chain and chemiosmosis. NADH and FADH2 take electrons to the electron transport chain. Most of the ATP produced by cellular respiration is generated by oxidative phosphorylation, which uses the energy released by the downhill flow of electrons from NADH and FADH2 to O2 to phosphorylate ADP. My mom is going to use cellular respiration while running.