Cellular respiration is the process that releases energy from glucose and other molecules in the presence of oxygen. It occurs in three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis breaks down glucose into pyruvate, producing a small amount of ATP both with and without oxygen. The Krebs cycle and electron transport chain use oxygen to break down pyruvate completely, producing much more ATP through multiple biochemical reactions in the mitochondria. Overall, cellular respiration breaks down one glucose molecule to produce 36 ATP through these three stages, which cells then use for all their energy needs.