Free Energy

Free energy, abbreviated as G for the scientist Gibbs who discovered it is a measurement of useful energy that can be extracted from a reaction to do work. Depending on the direction of energy in the system reactions can be considered endergonic, exergonic, or at equilibrium.

If there's no net change in G, the reaction is at equilibrium. Which would cause cells to die because they would not have any leftover energy to do work. As a result they stay out of equilibrium by changing concentrations of reactants and products to keep metabolism running.

In plants the conversion of carbon dioxide and water to make glucose and oxygen requires chemical energy converted from sunlight. The energy put into the system is stored in the bonds of the glucose molecule making it an endergonic reaction. There is an input of energy into the system. The reverse reaction occurs in cellular respiration which breaks down glucose and oxygen to make carbon dioxide and water. This reaction is exergonic, energy stored in the glucose molecules is released.