ATP, ADP, and AMP are nucleotides consisting of the purine base adenine, the five-sided sugar ribose, and 3,2, or 1 phosphate groups respectively.

The transfer of a phosphate group is called phosphorylation. The most common phosphorylation reaction involves the addition of an inorganic phosphate group to ADP, forming ATP, the reverse of hydrolysis.

While ATP hydrolysis is very favorable thermodynamically, the process is very slow in the absence of a catalyst. ATP is kinetically stable. Kinetic stability is essential for the capacity of a biological system to control the flow of free energy by enzyme catalysis.

ATP is the principle immediate donor of free energy in biological systems. Many endergonic biochemical processes are coupled to the exergonic cleavage of ATP. The two primary reasons for the high negative free energy change corresponding to ATP hydrolysis are electrostatic repulsion and resonance stabilization. The high negative free energy change associated with electrostatic repulsion arises due to the fact that ATP is a strong anion, with a -4 total formal charge distributed along the three phosphoryl groups. The hydrolysis of the molecule is accompanied by the separation of like charge, a decrease in electrical potential energy (internal energy, enthalpy, free energy). Picture the three phosphate groups of ATP as a compressed spring. Furthermore, the products of cleavage enjoy greater resonance stabilization than the single molecule. Keep in mind that the energy is not located in the bonds linking the phosphate groups themselves but is a property of the difference between the initial and final states of the system.