Equilibrium is the natural ratio of the molecules involved in a chemical reaction. Consider this example

A + B <----> C + D

Molecules A and B on the left of the arrows are called substrates, whereas molecules C and D on the right of the arrow are termed products. The natural equilibrium is useful for determining whether a reaction will proceed strongly to either the left (more substrates) or to the right (forming more products). We could use a specific example of a generalized biochemical reaction found in most organisms

Carbohydrate + O2 <----> CO2 + H2O

Equilibrium is often difficult to understand. Most students have no difficulty identifying the above equilibrium must lie to the right, because this is the respiration that provides us with energy from food. However, this is also the same equation that is the basis for photosynthesis; that reaction clearly involves movement of water and carbon dioxide to the left to make carbohydrate. It is important to realize that living systems are not at equilibrium. The biosphere provides life because it is a disequilibrium powered by sunlight. Individually, we only reach equilibrium after we die.

Biochemists view living biological systems as in a transient, semi-stable state consisting of many reactions at disequilibrium.

Maintaining a living complex biological system, a constant input of energy is required. This constant input of life-sustaining energy is sunlight.