Secondly, explain why energy is not just provided by the simple hydrolysis of ATP.

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.