ATP’s energy is utilized to couple an exergonic reaction to power an endergonic reaction during energy coupling. ATP provides the energy for both energy-releasing exergonic reactions and for energy-consuming endergonic reactions in energy coupling. When chemical bonds within ATP are broken, energy becomes released and can be utilized for cellular work. Without a source of energy, the Gibbs free energy of the reactants in an endergonic reaction will not be enough. The energy can only be supplied through ATP hydrolysis so that the endergonic reaction can occur.