Adenosine triphosphate or ATP is the chemical within a cell that allows it to store and transfer energy.
ATP is formed within the mitochondria, where it's precursor, adenosine diphosphate (ADP), is modified by adding an additional phosphate group. The working parts of a cell extract energy from ATP by breaking off the phosphate group again, which releases energy in the reaction.
Because the chemical reaction involved does not destroy either the ADP or the phosphate group, this reaction can be maintained indefinitely. Therefore, although the body's cells process their own weight in ATP every day, at any one time only about 220 grams of ATP is available in the human body.
In order to make ATP, mitochondria need simple sugars, such as glucose or fructose, or fat by-products such as fatty acids or glycerol. The reactions required to make ATP normally requires the presence of oxygen. The resulting chemical reaction releases molecular carbon dioxide.