Phosphorus is as essential for life as is nitrogen, but for a different reason. It is absent in proteins, but present in the backbones of nucleic acid chains such as DNA. Even more importantly, phospborus is at the heart of the central energy-storage molecule, adenosine triphosphate (ATP), shown at the left. Just as silicates can form polysificate chains by sharing comer oxygen atoms of silicate tetrahedra, so can polymerize into polyphosphates. Two linked tetrahedra build a pyrophosphate ion, . Metaphosphates have rings or long chains of tetrahedra. ATP consists of three linked phosphate tetrahedra, with ribose (a sugar ring) and adenine (a nitrogen-containing base) hooked onto one end. ATP is remarkable for the large amount of energy it gives off when one of the phosphate groups is hydrolyzed away by water:



ADP is an abbreviation for adenosine diphosphate, which has one phosphate tetrahedron removed from ATP. Most other hydrolysis reactions yield only two or three kilocalories of energy. When ATP is synthesized from ADP and inorganic phosphate, an unusually large amount of energy can be stored in the final phosphate bond, which then is available for later use:

ADP + phosphate + 7.3 kcal of energy ATP +