ATP

Adenosine triphosphate (ATP) is the principal energy carrier in the cell. It is an adenosine nucleotide composed of the adenine base, a ribose sugar, and three phosphate groups linked by high-energy phosphoanhydride bonds. The bonds between the second and third phosphates are especially energetic, and their hydrolysis releases usable free energy for cellular work.

Most cellular processes rely on energy transfer from ATP. Hydrolysis of ATP to ADP and inorganic phosphate (Pi) provides the driving force for endergonic reactions and powers activities such as active transport, muscle contraction, and the action of many enzymes. The standard free energy change is about -30.5 kJ/mol, and in cells it is often around -50 kJ/mol due to conditions such as Mg2+ and ionic strength.

ATP is synthesized mainly in mitochondria through cellular respiration, including glycolysis, the citric acid cycle, and oxidative phosphorylation, as well as in plants by photosynthesis. It is regenerated continuously from ADP and Pi by kinases, and cells maintain a small but rapidly replenished ATP pool to meet energetic demands.

Beyond energy transfer, ATP participates in signaling and metabolism. It serves as a substrate for phosphorylation reactions, acts on purinergic receptors when released extracellularly, and is the building block for RNA synthesis as one of the four ribonucleoside triphosphates.