In chemistry, reactions are categorized by how they handle energy. Exergonic reactions release energy—typically as heat or light—while endergonic reactions consume energy to proceed.
Combustion of gasoline is a classic exergonic reaction. Octane molecules in gasoline contain more chemical energy than the water and carbon dioxide produced, so energy is liberated when the fuel burns. In contrast, photosynthesis in trees is an endergonic process that stores energy by converting carbon dioxide and water into complex organic molecules like cellulose.
Organisms rely heavily on endergonic reactions to build essential biomolecules such as amino acids, nucleotides, and fats. These processes draw energy from sugars or other high‑energy substrates. Because endergonic reactions cannot proceed spontaneously, cells supply the necessary energy via ATP or other co‑enzymes.
Even exergonic reactions often require an initial input of energy—known as activation energy—to overcome kinetic barriers. For example, charcoal needs a spark or a match to ignite. Once the reaction is triggered, the stored activation energy is released, and the reaction proceeds, releasing more energy than was initially invested.
Endergonic reactions are frequently reversible. For instance, burning a wooden log reverses the photosynthetic process that originally formed the tree: carbohydrates are oxidized to CO₂ and H₂O, releasing a modest amount of heat. The difficulty of reversing an exergonic reaction depends on how much additional energy would be required to drive the reverse process. This concept is emphasized by research from the University of Nebraska, Lincoln.
An energy hill diagram visually represents the energy profile of a reaction. The horizontal axis shows the reaction coordinate (time or progress), while the vertical axis displays the system’s total energy. For an exergonic reaction, the diagram rises to a peak—representing the activation energy—then falls below the initial energy level. For an endergonic reaction, the path climbs above the starting energy, indicating that the system must absorb energy before the reaction can complete.
