A nucleotide composed of three subunits: ribose (5-carbon sugar), adenine (a nitrogenous base composed of two carbon-nitrogen rings), and a triphosphate group (a chain of three phosphate groups, designated α, β, and γ) (see illus.). ATP is a vital energy-rich chemical found in all living cells. The oxidation of carbohydrates, fats, and proteins provides chemical energy that is used to synthesize ATP, which then serves as the source of the chemical energy used for biosynthesis, ion transport, and muscle contraction. The idea that ATP serves as the common currency of energy exchange in all cells is a cornerstone of biology that was first described by Fritz Lipmann in 1941.