ATP

ATP is a nucleotide made of the base adenine, the five-carbon sugar ribose and a chain of three phosphate groups. It is often called the energy currency of the cell because it links reactions that release energy, such as the breakdown of glucose, to processes that need energy, such as muscle contraction, building large molecules and pumping ions across membranes.

The bonds between the phosphate groups break easily. When water splits off the outermost phosphate, ATP becomes ADP (adenosine diphosphate) plus inorganic phosphate, and energy is released. Cells put that energy to work by transferring the phosphate to another molecule, which changes its shape or makes it more reactive. ADP is then recharged into ATP, mostly by the enzyme ATP synthase in mitochondria during oxidative phosphorylation, with smaller amounts made directly in glycolysis and the citric acid cycle, and in chloroplasts during photosynthesis.

A cell keeps only a small stock of ATP and recycles it constantly. ATP is also one of the building blocks used to make RNA, and outside cells it acts as a signaling molecule, including as a neurotransmitter.