Cellular respiration is the process by which living cells extract energy from glucose or other food molecules in the presence of oxygen. It has three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis breaks down glucose into pyruvic acid, producing a small amount of ATP. The Krebs cycle further breaks down pyruvic acid, producing more ATP and electron carriers. Finally, the electron transport chain uses these electron carriers to produce most of the cell's ATP through oxidative phosphorylation.