Water is an end product of aerobic respiration in living beings. The process of cellular respiration, occurring in the presence of oxygen, extracts energy from ingested foods by converting them to carbon dioxide and water.
Respiration is a multistep process that begins with the breakdown of glucose to pyruvic acid in cell cytoplasm. Pyruvic acid is then oxidized to carbon dioxide and water in mitochondria, which are organelles present within cell cytoplasm. Energy released in the process of oxidation is stored in the form of potential energy of chemical bonds in ATP (adenosine triphosphate). ATP serves as the energy source for cells. It is composed of a nucleic acid, adenine, that is bound to three phosphate groups. The third phosphate group is attached to adenosine via a high-energy bond. The energy needed to create this bond is derived from the oxidation of foods. This energy is released by hydrolysis of the ATP back to ADP (adenosine diphosphate). The released energy is then utilized by cells for energy-consuming processes.