Adenosine 5′ -triphosphate (ATP) is a multifunctional nucleotide primarily known in biochemistry as the "molecular currency" of intracellular energy transfer. It is the most abundant short-term energy store and immediate source of energy for cell work. In this role ATP transports chemical energy within cells. It is produced as an energy source during the processes of photosynthesis and cellular respiration. In order to maintain constant energy, a supply of ATP must be replaced as it is used.