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. It is the energy in a chemical reaction that is available after entropy is accounted for. Reactions that take in energy are considered endergonic and reactions that release energy are exergonic. Plants carry out endergonic reactions by taking in sunlight and carbon dioxide to produce glucose and oxygen. Animals, in turn, break down the glucose from plants using oxygen and make carbon dioxide and water. When a system is at equilibrium, there is no net change in free energy. In order for cells to keep metabolism running and stay alive, they must stay out of equilibrium by constantly changing concentrations of reactants and products

Free Energy

The direction of energy flow through the system determines if the reaction is endergonic or exergonic. Systems with no net change in free energy are considered to be at equilibrium. Most chemical reactions are reversible—they can proceed in both directions. To stay alive, cells must stay out of equilibrium by constantly changing the concentrations of reactants and products so that metabolism continues to run.

Endergonic Versus Exergonic Reactions

If a reaction requires an input of energy to move forward, then the change in free energy, or the ΔG of the reaction is positive and the reaction is considered endergonic—energy has entered the system. In plants, the building of glucose molecules and oxygen from carbon dioxide and water—with the help of sunlight—is considered endergonic. The glucose molecules are considered as energy storage molecules.

Conversely, if energy is released in a reaction, then the change in free energy, or ΔG is negative and the reaction is considered exergonic. The products have less free energy than the reactants—energy has exited the system. This occurs in animals that break down glucose using oxygen to make carbon dioxide and water. The energy in the glucose molecules has been released.