Exergonic And Endergonic Reactions

Category: Cellular biology

Exergonic reactions are spontaneous chemical reactions in which the products are at a lower energy level than the reac-tants; these reactions release energy. Endergonic reactions are nonspontaneous chemical reactions in which the products are at a higher energy level than the reactants; these reactions consume energy.

The primary source of energy for life on the earth is the sun, which is the energy source for photosynthesis: the biological process that transforms radiant energy into chemical energy. Chemical energy is stored in biological molecules, which can then be used as the fuel to provide an organism's energy needs. Such biological molecules include sugars (or carbohydrates), proteins, and lipids (or fats). In the reactions of metabolism, many types of molecules are synthesized (anabolism), and many are broken down (catabolism). Changes in energy content occur in all these reactions. Bioenergetics is the science that studies the description of the basic mechanisms that govern the transformation and use of energy by organisms. A basic tenet of bioener-

getics is that no chemical reaction can be 100 percent energy-efficient. In other words, in all reactions there is some transfer of energy, but some of it is always lost in the form of heat.

The energy (often measured in calories) contained in the molecular structure of a compound is called Gibbs free energy (after Josiah Willard Gibbs, 1839-1903, who founded the discipline of physical science) and is the energy available to perform work. The difference between the free energy of the products and the free energy of the reactants in a chemical reaction is called the change in free energy and is fundamental in determining if a reaction can occur spontaneously. If the change in free energy is negative, energy is released, and the free energy content is less in the products than in the reactants. Such reactions are considered exergonic. On the other hand, if the change in free energy is positive, the reaction is considered endergonic and is non-spontaneous (that is, endergonic reactions require a source of energy to enable them to occur).

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