An exergonic reaction is one that gives up energy. If heat is the form of energy given off, another term used is exothermic. Spontaneous reaction is another synonymous term. An endergonic reaction is one that absorbs energy from its surroundings. If heat is the energy form absorbed, the reaction is endothermic. In these terms, erg refers to work or energy, and therm refers to heat.
Figure - 1 : endergonic and exergonic reactions
An example of a spontaneous reaction (A—B) is diagrammed in Figure 1. The energy content of the product B is less than that of the reactant A. The difference or change in energy is E1. In this example, .E is negative because the energy content of B (the product) is less than that of A (the reactant).
Reaction (C—D) is an example of an endergonic, nonspontaneous, energetically uphill reaction. Here, the energy level of the product is greater than that of the reactant. This reaction will not occur unless there is an energy input. Here, E2 is positive in sign because the energy content of D (product) is greater than that of C (reactant). Many important reactions and processes in the physical and biological world are endergonic—that is, require energy inputs. In the biological world, endergonic reactions such as C—D are linked or coupled to, and driven by, exergonic reactions such as A—B. In the example given, the overall process is A—D. Note that the energy level of D, the final product, is less than that of A, the initial reactant.
Although chemical reactions and processes in biological systems are governed by laws of energetics, the linkages between exergonic (energy yielding) and endergonic (energy-consuming) reactions is usually indirect. As we shall see in the following sections, part of the energy available from exergonic processes is captured in the form of a high-energy intermediate compound, adenosine triphosphate (ATP). In a complementary way, most endergonic reactions in mammalian systems are catalyzed by an enzyme called ATPase that releases the energy of ATP.
Even though reaction A—B is downhill and “spontaneous,” it is not likely to happen because an energy barrier, called the energy of activation, must first be overcome. In other words, even though the reaction A—B is classified as spontaneous, some energy has to be put in to activate the system and “prime the pump.” As we shall see, several important biochemical pathways, such as the conversion of glucose to glucose 6-phosphate in glycolysis, begin with such activating steps. We will also see that enzymes are important because they Lower the energy of activation.
Although the energy of activation impedes some processes, the world as we know it depends on other processes having very high energies of activation. For example, oxides of nitrogen and other automobile exhaust emissions are currently very much in the news. Oxides of nitrogen are formed from nitrogen and oxygen according to the following reactions:
N2+2 O2 —> 2 NO2
N+O2—>NO2
These are spontaneous reactions with high energies of activation. In the automobile combustion chamber, extremes of temperature and pressure activate the reaction and produce noxious products. Without very high energies of activation, these reactions might well cause the atmosphere to catch fire.
In metabolic processes, initial reactants must first be activated. Because enzymes can lower the energy of activation, there is enzymatic control over these processes.
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