The interconversion of ATP and ADP occurs in a cycle. The two outermost phosphates in ATP are held to the rest of the molecule by high-energy phosphoanhydride bonds and are readily transferred. Water can be added to ATP to form ADP and inorganic phosphate (Pi). This hydrolysis of the terminal phosphate of ATP yields between 11 and 13 kcal/mole of usable energy, depending on the intracellular conditions. The large negative delta G of this reaction arises from a number of factors. Release of the terminal phosphate group removes an unfavorable repulsion between adjacent negative charges; in addition, the inorganic phosphate ion (Pi) released is stabilized by resonance and by favorable hydrogen-bond formation with water. The formation of ATP from ADP and Pi reverses the hydrolysis reaction. Because this condensation is energetically unfavorable, it must be coupled to an energetically more favorable reaction to occur.  Fair Use and Copyright info