By Brooke Yool, Updated Aug 30, 2022
In chemistry, we refer to the reaction vessel as the "system" and everything else as the "surroundings." An endergonic reaction draws energy from the surroundings into the system, while an exergonic reaction releases energy from the system to the surroundings.
All reactions require an initial input of energy—the activation energy—to begin. For instance, wood combustion releases heat once it starts, but it still needs a flame to ignite and supply that initial energy.
To move from reactants to products, a reaction must overcome its unique activation energy barrier. The barrier’s height is independent of whether the reaction is endergonic or exergonic; a highly exergonic reaction can still have a substantial barrier, and vice‑versa.
Many reactions proceed through multiple steps, each with its own activation energy threshold.
Endergonic processes typically build larger structures. Protein synthesis and photosynthetic glucose formation both absorb energy. The reverse reactions—cellular respiration of glucose and the breakdown of proteins—are exergonic, releasing energy.
Catalysts lower the activation energy by stabilizing transition states, effectively creating a lower‑energy path for the reaction. Enzymes, the most common biological catalysts, exemplify this principle.
Only exergonic reactions occur spontaneously because they release energy. Endergonic processes, such as muscle building or DNA replication, are driven by coupling with exergonic reactions that supply the necessary energy difference.
