What are exergonic and endergonic reactions?

The amount of free energy available to do work within a cell is determined by calculating Gibb's free energy, or G.

During the course of a reaction, if energy is released, the reaction is exergonic. If energy is absorbed, the reaction is endergonic.

In complex cellular reactions, exergonic and endergonic chemical reactions are coupled. Exergonic reactions power the endergonic ones.