Adenosine triphosphate (ATP) is the nucleotide known in biochemistry as the "molecular currency" of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ATP also plays an important role in the synthesis of nucleic acids. ATP molecules are also used to store the energy plants make in cellular respiration.
Chemically, ATP consists of adenosine and three phosphate groups. It has the empirical formula C10H16N5O13P3, and the chemical formula C10H8N4O2NH2(OH)2(PO3H)3H, with a molecular mass of 507.184 u. The phosphoryl groups starting with that on AMP are referred to as the alpha, beta, and gamma phosphates. The biochemical name for ATP is 9--D-ribofuranosyladenine-5'-triphosphate.
ATP can be produced by various cellular processes, most typically in mitochondria by oxidative phosphorylation under the catalytic influence of ATP synthase or in the case of plants in chloroplasts by photosynthesis. The main fuels for ATP synthesis are glucose and fatty acids. Initially glucose is broken down into pyruvate in the cytosol. Two molecules of ATP are generated for each molecule of glucose. The terminal stages of ATP synthesis are carried out in the mitochondrion and can generate up to 36 ATP.
The total quantity of ATP in the human body is about 0.1 mole. The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during a single day. ATP cannot be stored, hence its synthesis must closely follow its consumption.
Living cells also have other "high-energy" nucleoside triphosphates, such as guanosine triphosphate. Between them and ATP, energy can be easily transferred with reactions such as those catalyzed by nucleoside diphosphokinase : Energy is released when hydrolysis of the phosphate-phosphate bonds is carried out. This energy can be used by a variety of enzymes, motor proteins , and transport proteins to carry out the work of the cell. Also, the hydrolysis yields free inorganic phosphate and adenosine diphosphate, which can be broken down further to another phosphate ion and adenosine monophosphate. ATP can also be broken down to adenosine monophosphate directly, with the formation of pyrophosphate. This last reaction has the advantage of being an effectively irreversible process in aqueous solution.