Endergonic and Exergonic Reactions in the Cell

If the free energy change is less than zero, the process is spontaneous and is called an exergonic reaction. In contrast, if the free energy change is greater than zero, the process is not spontaneous, and the reaction is endergonic.

To compare the energy changes of different reactions, the free energy change is usually reported under a standard set of conditions written as ΔG₀.

The hydration of fumarate to malate in the presence of fumarase is an exergonic reaction because the standard free energy change during the process is −3.8 kJ/mol.

For endergonic reactions to occur, energy needs to be added to the system. In cells, endergonic reactions are coupled with exergonic reactions.

For instance, the endergonic conversion of glucose to glucose-6-phosphate has a standard free energy change of 13.8 kJ/mol, and the exergonic hydrolysis of ATP has a standard free-energy change of  −30.5 kJ/mol.

These reactions are coupled inside the cell, and the entire process is exergonic, with a standard free-energy change of −16.7 kJ/mol, a total of the standard free energy of both processes.