The exergonic reaction of hydrolysis of ATP must be coupled to an endergonic reaction to perform work or to power other reactions.
Explanation:
ATP (adenosin triphosphate) is hydrolized to give ADP and releases energy as follows:
ATP + H₂O ⇆ ADP + Pi + energy
The released energy itself is not used to power biochemical reaction. In order to power other reactions, ATP hydrolysis- a energetically favorable reaction- is coupled to another energetically disfavorable reaction. The linking or coupling is performed through a shared intermediate, which is often a phosphorilated molecule. For example, in the formation reaction of sucrose from glucose and fructose, an intermediate is formated by the transfer of a phosphate group (Pi) provided by ATP hydrolysis. Then, the phosphorilated intermediate (glucose-Pi)- wich is unstable- reacts with fructose in a spontaneous reaction to give sucrose.
