Endergonic reaction

In chemical thermodynamics, an endergonic reaction (also called a nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed. In layman's terms, the total amount of energy is a loss (it takes more energy to start the reaction than what you get out of it) so the total energy is a negative net result. For an overall gain in the net result see exergonic reaction. Another way to phrase this is that energy is absorbed from the surroundings into the workable system.

Under constant temperature and constant pressure conditions, this means that the change in the standard Gibbs free energy would be positive

for the reaction at standard state (i.e. at standard pressure (1 bar), and standard concentrations (1 molar) of all the reagents).

In metabolism, an endergonic process is anabolic, meaning that energy is stored; in many such anabolic processes energy is supplied by coupling the reaction to adenosine triphosphate (ATP) and consequently resulting in a high energy, negatively charged organic phosphate and positive adenosine diphosphate.

The equilibrium constant for the reaction is related to ΔG° by the relation:

where T is the absolute temperature and R is the gas constant. A positive value of ΔG° therefore implies

so that starting from molar stoichiometric quantities such a reaction would move backwards toward equilibrium, not forwards.

Nevertheless, endergonic reactions are quite common in nature, especially in biochemistry and physiology. Examples of endergonic reactions in cells include protein synthesis, and the Na⁺/K⁺ pump which drives nerve conduction and muscle contraction.

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