Cells capture the energy released by cellular respiration in adenosine triphosphate, commonly abbreviated as ATP. ATP is the main energy currency of all cells, or the molecule that carries the energy needed for all the chemical reactions the cell performs.
ATP is generated during several stages of cellular respiration, including glycolysis and the following oxidation of pyruvate. Glycolysis takes place in the cytosol, the fluid that fills the cell, while the remainder of cellular respiration occurs in organelles known as mitochondria. Glycolysis is the separation of sugars with six carbon atoms each into two pyruvate molecules with three carbon atoms each. The major energy production occurs in the mitochondria and the oxygenation of pyruvate.
The mitochondria have internal membranes that are essential to the production of energy via the oxidation of pyruvate. These, combined with enzymes in their internal fluid, cause the pyruvate to undergo a series of chemical reactions, releasing energy as ATP at each step. As the process continues, carbon dioxide is released along with excess hydrogen ions. These are carried through a series of reactions known as the electron transport chain until they are combined with oxygen, which produces water molecules. The electron transport chain uses up some of the potential 38 ATP molecules cellular respiration could generate, so the net gain is generally closer to 30 ATP molecules.