Endoergic and Exoergic Reactions

FIELDS OF STUDY: Biochemistry; Genetics; Molecular Biology

ABSTRACT

The processes of endoergic and exoergic reactions are described, and their importance in biochemical transformations is elaborated. Exoergic reactions release free energy and can occur spontaneously, while endoergic reactions require energy from an outside source in order to proceed. Through the combined action of endoergic and exoergic reactions, many of the essential processes of biochemical systems such as photosynthesis, glycolysis, and respiration are carried out.

Energy Transfers in Chemical Reactions

All chemical reactions involve the transfer of energy, which can occur in the form of either heat or work. Chemical systems contain energy in the bonds between their atoms. The electrons that form these atomic bonds possess an energy level that is determined by various factors, including the nature of the bond, the extent to which the bond angles are deformed from their ideal angles, and the force of repulsion between the electrons in the bond. During a chemical reaction, the level of energy stored within a chemical system changes as bonds are formed or broken: the formation of a bond releases energy, some of which escapes from the system into its surroundings, while the breaking of a bond consumes energy. The overall change in the level of energy caused by a chemical reaction is called the enthalpy change of the reaction (ΔH). The enthalpy change is determined by subtracting the enthalpy, or total energy content, of the reactants from that of the products. If the enthalpy of the products of a reaction is lower than the enthalpy of the reactants, there is a negative change in enthalpy, meaning that the reaction is exothermic. If the enthalpy change of a reaction is positive, the reaction is considered to be endothermic. Endothermic reactions draw heat energy from their surroundings in order to be carried out, while exothermic reactions release heat energy.

Another important consideration in determining the energy transfer of a reaction is the change in entropy (ΔS) of the system. Entropy is a measure of the degree to which energy has been dispersed throughout the atoms and molecules of a system; high entropy indicates a wide dispersal of energy, causing the components of the system to move in a more disordered manner than in a system with lower entropy. As the enthalpy of a system increases, the entropy of the system also increases, causing the system to become less stable. The entropy change of a reaction is influenced by the temperature at which the reaction occurs; at lower temperatures, the change in entropy is greater than at higher temperatures. Furthermore, as the entropy of a system increases, the entropy of the system’s surroundings decreases, as energy flows from the surroundings into the system.

The entropy change of a reaction indicates whether the reaction is spontaneous or non-spontaneous. For a reaction to occur spontaneously, the total entropy change of both the system and its surroundings must be positive, representing a net increase in entropy. This in turn determines whether the reaction is exoergic or endoergic (or exergonic or endergonic; both sets of terms mean the same thing). An exoergic reaction is one that releases free energy, which is energy that is available to do work in a system, while an endoergic reaction requires an input of free energy in order to proceed. In other words, in an exoergic reaction, the change in free energy is negative, indicating that the system expended energy it already possessed in order to do work; in an endoergic reaction, the change in free energy is positive, indicating that energy was supplied to the system from an outside source. Thus, a spontaneous reaction is one that is exoergic, because it can occur at any time without an external energy supply. Spontaneous reactions can occur in one direction only, as reversing one requires an input of energy.

Entropy, Enthalpy, and Gibbs Free Energy

In thermodynamics, the basic characteristics of chemical reaction systems are described by three factors: entropy, enthalpy, and Gibbs free energy. Gibbs free energy is a measure of free energy as determined by the change of entropy within a system. (Free energy is essentially the total energy of a system minus the entropy, which represents energy that cannot be used to do work.) The three concepts are related by the expression

ΔG = ΔH − TΔS

where G is Gibbs free energy, H is enthalpy, T is temperature, S is entropy, and the delta symbol (Δ) represents the overall change in the associated factor. From this equation, it can be seen that the change in the Gibbs free energy is equal to the change in enthalpy minus the change in entropy, the effect of which is more significant at higher temperatures.

The relationship can be demonstrated using gas contained in a cylinder that is closed by a piston. Heating the gas increases its thermal energy and thus its enthalpy, causing the gas molecules to move more energetically and randomly, which in turn increases the entropy. As a result, the gas is able to exert more pressure against the piston using the extra energy, but it cannot use the full amount because some of the energy is being used to maintain the increased motion and randomness of the individual gas particles. The portion of the energy that is available to do work to move the piston is the free energy.

Gibbs free energy is named for American scientist Josiah Willard Gibbs (1839–1903), who first formulated the concept of the free energy, as well as several other fundamental principles of chemical thermodynamics.

Free Energy in Reactions

Endoergic reactions are chemical reactions that need the input of energy from an external source in order to take place. Endothermic reactions are superficially similar in this respect; the difference is that endoergic reactions do not occur unless energy is supplied, while endothermic reactions proceed normally, extracting energy from the surroundings in the process. Exoergic reactions, on the other hand, do not need any external input of energy and are therefore spontaneous. In multicomponent chemical reaction systems, the free energy released by an exoergic reaction is available to be used by non-spontaneous endoergic reactions. The Gibbs free energy accordingly plays a significant role in establishing equilibrium between reactants and products, as the energy released by the reactants in order to form the products can then be used by the products to reverse the reaction and re-form the reactants. An unfortunate side effect of this is that when other reaction mechanisms that might lead to undesired products are possible, the energy can be used by those as well. As the system tends toward equilibrium, the free energy in the system tends toward zero. At equilibrium, the free energy of the system is at its minimum value and does not increase unless the equilibrium state of the system is perturbed.

Catalysts function in chemical reactions by forming an activated complex with the reactants that decreases the activation energy of specific reactions. The most effective catalysts are those that ultimately reduce the required activation energy to zero, thus converting the overall reaction from endoergic to exoergic.

Endoergic and Exoergic Reactions in Biochemical Systems

Biological systems make great use of the combined action of exoergic and endoergic reactions. In cellular respiration, for example, glucose (C₆H₁₂O₆) is decomposed into carbon dioxide (CO₂) and water (H₂O), accompanied by the release of energy. The products of respiration contain less free energy than was stored in the reactants. The law of conservation of energy states that the energy released by the process must be stored or manifested in another form. While some is manifested as heat, which maintains a constant body temperature, the remainder is stored in the formation of chemical bonds through the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). The energy is recovered in the reverse process, which converts ATP back to ADP, and the energy released by this reverse reaction is used to drive many other endoergic biochemical reactions.

PRINCIPAL TERMS

• endergonic: synonym for endoergic; describes a reaction process that requires the input of energy in the form of work in order to proceed.
• equilibrium: the state that exists when the forward activity of a process is exactly equal to the reverse activity of that process.
• exergonic: synonym for exoergic; describes a reaction process that can occur spontaneously and releases energy in the form of work.
• Gibbs free energy: the energy in a thermodynamic system that is available to do work.
• spontaneous reaction: a chemical reaction that occurs without the input of energy from an outside source.

Bibliography

Abbott, David, ed. The Biographical Dictionary of Scientists: Chemists. New York: Bedrick, 1983. Print.

Crowe, Jonathan, and Tony Bradshaw. Chemistry for the Biosciences: The Essential Concepts. 2nd ed. Oxford: Oxford UP, 2010. Print.

Lafferty, Peter, and Julian Rowe, eds. The Hutchinson Dictionary of Science. 2nd ed. Oxford: Helicon, 1998. Print.

Lehninger, Albert L. Biochemistry: The Molecular Basis of Cell Structure and Function. New York: Worth, 1970. Print.

Lodish, Harvey, et al. Molecular Cell Biology. 7th ed. New York: Freeman, 2013. Print.

Reece, Jane B., et al. Campbell Biology: Concepts and Connections. 7th ed. San Francisco: Cummings, 2012. Print.