Cellular respiration is a series of metabolic reactions that break down glucose (a sugar) in the presence of oxygen to produce energy in the form of ATP (adenosine triphosphate). This process occurs in the mitochondria of the cell and involves four main stages: glycolysis, the Krebs cycle, the electron transport chain, and oxidative phosphorylation.

While cellular respiration is the primary way that most living cells obtain energy, some cells, such as bacteria, use other processes like fermentation, which does not require oxygen.