Since biological systems are also chemical systems, these energy conversions involve chemical reactions. Most of the energy of a biological system is found in the chemical bond . The energy within chemical bonds is potential chemical energy. When a bond is broken, its potential energy is expressed as kinetic energy, usually in the form of heat, sometimes light. A chemical reaction changes atomic combinations.
All chemical reactions proceed until a balance is established between reactants and products.
A chemical reaction may be viewed as a system that is undergoing an irreversible change in state towards a state of equilibrium. The change that is occurring during such a reaction involves an alteration of the energy relationships of the system as well as the alteration of the particle relationships. The total energy of the universe remains constant - there is neither a gain nor a loss in the total amount of energy transferred or transformed. The conservation of energy permits a quantitative calculation of the energy relationships in a system. While the first law states that energy is conserved, it says nothing about the state of the energy resulting from an energy conversion in regards to its ability to do work.