An exergonic reaction releases energy which is used for cellular activity. Energy is released during exergonic reactions by breaking less stable bonds and forming more stable chemical bonds. Most exergonic reactions, such as those in cellular respiration and cellular metabolism, occur spontaneously, explains The Biology Project at the University of Arizona.
Exergonic reactions can couple with endergonic reactions. Endergonic reactions do not occur spontaneously; they require activation energy. An outside source must supply this small input of energy. The result is still thermodynamically favorable because endergonic reactions release more energy than what was required to initiate the reaction. However, the change in Gibbs free energy is negative because there is still a net loss in energy from the specific reaction system.