Exergonic and endergonic reactions
Exergonic and endergonic reactions are fundamental concepts in bioenergetics, the study of energy transformations in biological systems. Exergonic reactions release energy, resulting in a negative change in Gibbs free energy, allowing these processes to occur spontaneously. In contrast, endergonic reactions require an input of energy, indicated by a positive change in Gibbs free energy, making them nonspontaneous. Organisms maintain their order and functionality by coupling exergonic reactions, which provide the necessary energy, with endergonic reactions that build complex molecules.
A key player in this energy transfer is adenosine triphosphate (ATP), which provides energy for endergonic reactions in cells. The production of ATP itself is an endergonic process, while its breakdown is exergonic, illustrating the interconnected nature of these reactions. For example, the synthesis of sucrose from glucose and fructose is an endergonic reaction that is facilitated by energy derived from exergonic reactions involving ATP. This energy coupling enables cells to perform essential functions despite the inherent energy inefficiencies in biochemical processes. Understanding these reactions highlights how life is sustained through intricate energy management and transformation.
Exergonic and endergonic reactions
Category: Cellular biology
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 bioenergetics 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 nonspontaneous (that is, endergonic reactions require a source of energy to enable them to occur).
Energy Coupling
Many cellular reactions are endergonic and cannot occur spontaneously. Nevertheless, cells can facilitate endergonic reactions using the energy released from other exergonic reactions, a process called energy coupling. As an example, consider a common endergonic reaction in plants in which glucose and fructose are joined together to make sucrose. To enable this reaction to take place, it is coupled with a series of other exergonic reactions as follows:
glucose + adenosine triphosphate (ATP) ®glucose-p + ADP
fructose + ATP ®fructose-p + adenosine diphosphate (ADP)
glucose-p + fructose-p ®sucrose + 2 Pi (inorganic phosphate)
Therefore, although producing sucrose from glucose and fructose is an endergonic reaction, all three of the foregoing reactions are exergonic. This is representative of the way cells facilitate endergonic reactions.
Role of ATP
The principal molecule involved in providing the energy for endergonic cellular reactions to take place is adenosine triphosphate, or ATP, the same molecule used in the example above. ATP is typically produced by joining an inorganic phosphate to adenosine diphosphate (ADP), which is an endergonic reaction. This, too, represents a characteristic of chemical reactions: If a reaction is exergonic in one direction, it will be endergonic in the opposite direction. Thus, the breakdown of ATP is exergonic, while the production of ATP is endergonic. The energy for production of most of the ATP in plant cells comes from the light reactions of photosynthesis and the electron transport system in the mitochondria.
The enigma is why ATP, and not any other molecule, is used. Although no complete justification is available, there are several points that support its significance. First, there is the high stability of the ATP molecule at the physiological pH (around 7.4) toward hydrolysis and decomposition in the absence of an enzyme catalyst. This stability allows ATP to be stored in the cell until needed. Second, ATP is one of the molecules (a nucleotide) that is used in synthesis of DNA. Finally, the magnitude of the change in free energy involved in the ATP-ADP transformation is of an amount useful for driving many of the endergonic reactions in the cell. As a result, it can play the role of an intermediate quite easily.
Bibliography
Hall, D. O., and K. K. Rao. Photosynthesis. 6th ed. New York: Cambridge University Press, 1999. Treats photosynthesis in a simple, methodical manner and explains complex concepts in an interesting and user-friendly way. Helps the student to think practically about the subject, pointing him or her toward the next stage of understanding of plant biology.
Harris, David A. Bioenergetics at a Glance. Cambridge, Mass.: Blackwell Science, 1995. Clear, concise introduction to the study of energy use and conversion in living organisms, with an emphasis on the biochemical aspects of plant science and physiology and cell biology.
Lawlor, David W. Photosynthesis: Molecular, Physiological, and Environmental Processes. 3d ed. New York: Springer-Verlag, 2001. Covers all aspects of photosynthesis, from the molecular level to plant production. Written for undergraduate or graduate students and nonspecialists who want a concise overview of the process.
Stumpf, Paul K. “ATP.” Scientific American 188 (April, 1953): 85-90. Describes ATP’s participation in biochemical reactions and cellular energy systems. Includes schemes and a picture of ATP crystals.