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Every cell uses ATP for energy. It consists of a base (adenine) and three phosphate groups. One molecule of ATP contains three phosphate groups, and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP).
The ATP molecule is very versatile, meaning that it can be used for many things. Energy is stored in its chemical bonds.
When ADP binds with another phosphate, energy is stored that can be used later. In other words, when a bond is made, energy is stored. This is an endothermic reaction.
When ATP breaks a bond with a phosphate group and becomes ADP, energy is released. In other words, when a bond is broken energy is released. This is an exothermic reaction. The ATP phosphate exchange is a nearly never-ending cycle, stopping only when the cell dies.
Functions in cells
ATP is the main energy source for the majority of cellular functions. This includes the synthesis of macromolecules, including DNA and RNA (see below), and proteins. ATP also plays a critical role in the active transport of macromolecules across cell membranes, e.g. exocytosis and endocytosis.
DNA and RNA synthesis
In all known organisms, the deoxyribonucleotides that make up DNA are synthesized by the action of ribonucleotide reductase (RNR) enzymes on their corresponding ribonucleotides. These enzymes reduce the sugar residue from ribose to deoxyribose by removing oxygen.
In the synthesis of the nucleic acid RNA, ATP is one of the four nucleotides incorporated directly into RNA molecules by RNA polymerases. The energy driving this polymerization comes from cleaving off a pyrophosphate (two phosphate groups). The process is similar in DNA biosynthesis, except that ATP is reduced to the deoxyribonucleotide dATP, before incorporation into DNA.
- Stubbe J (5 April 1990). "Ribonucleotide reductases: amazing and confusing". J Biol Chem 265 (10): 5329–32. . http://www.jbc.org/cgi/reprint/265/10/5329.
- Joyce CM, Steitz TA (1995). "Polymerase structures and function: variations on a theme?". J. Bacteriol. 177 (22): 6321–9. . .