The chemical bonds which hold the molecules of ATP together are high energy bond, that means that, they possess a large amount of energy. When these bonds are broken energy is always release to perform work inside the cell. At the same time, reaction coupling also occur anytime an ATP molecule is broken down. Reaction coupling involves linking the hydrolyisis of ATP molecule, which is energetically favorable with another chemical reaction, which is energetically unfavourable. Thus a product of one reaction is picked up and use as a reactant in the second reaction.