The Royal Society of Chemistry states that an endergonic reaction absorbs energy from its environment, while an exergonic reaction releases energy into its environment. Any chemical bonds formed during endergonic reactions tend to be weaker than those formed during exergonic reactions.
According to the Royal Society of Chemistry, each chemical reaction that occurs in an organism involves the breaking of bonds in one molecule and the formation of new bonds in another. Breaking chemical bonds requires energy, while forming new bonds releases energy. Whether the reaction is endergonic or exergonic depends on the relative strengths of the bonds being broken and the bonds being formed. In an endergonic reaction, the bonds being formed are weaker than the bonds being broken. In an exergonic reaction, the opposite is true.
Regarding cell metabolism, The Royal Society of Chemistry goes on to state that anabolic reactions, where energy is used and molecules are built up, are endergonic. An example is photosynthesis, when solar energy is absorbed to create glucose and oxygen from water and carbon dioxide. Catabolic reactions, where energy is given out and molecules are broken down, are exergonic. An example is cellular respiration, when glucose is converted to carbon dioxide and water.