AG3 = AGi
FIGURE 1-26 Energy coupling in mechanical and chemical processes. (a) The downward motion of an object releases potential energy that can do mechanical work. The potential energy made available by spontaneous downward motion, an exergonic process (pink), can be coupled to the endergonic upward movement of another object (blue). (b) In reaction 1, the formation of glucose 6-phosphate from glucose and inorganic phosphate (P|) yields a product of higher energy than the two reactants. For this endergonic reaction, AC is positive. In reaction 2, the exergonic breakdown of adenosine triphosphate (ATP) can drive an endergonic reaction when the two reactions are coupled. The exergonic reaction has a large, negative free-energy change (AC2), and the endergonic reaction has a smaller, positive free-energy change (AC1). The third reaction accomplishes the sum of reactions 1 and 2, and the free-energy change, AC3, is the arithmetic sum of AC1 and AC2. Because AC3 is negative, the overall reaction is exergonic and proceeds spontaneously.
actants, thus the reaction releases free energy, which is then available to do work. Such reactions are exergonic; the decline in free energy from reactants to products is expressed as a negative value. Endergonic reactions require an input of energy, and their AG values are positive. As in mechanical processes, only part of the energy released in exergonic chemical reactions can be used to accomplish work. In living systems some energy is dissipated as heat or lost to increasing entropy.
In living organisms, as in the mechanical example in Figure 1-26a, an exergonic reaction can be coupled to an endergonic reaction to drive otherwise unfavorable reactions. Figure 1-26b (a type of graph called a reaction coordinate diagram) illustrates this principle for the conversion of glucose to glucose 6-phosphate, the first step in the pathway for oxidation of glucose. The simplest way to produce glucose 6-phosphate would be:
Reaction 1: Glucose + Pi-> glucose 6-phosphate
(endergonic; AG1 is positive)
(Pi is an abbreviation for inorganic phosphate, HPO;j~. Don't be concerned about the structure of these compounds now; we describe them in detail later in the book.) This reaction does not occur spontaneously; AG
is positive. A second, very exergonic reaction can occur in all cells:
(exergonic; AG2 is negative)
These two chemical reactions share a common intermediate, Pi, which is consumed in reaction 1 and produced in reaction 2. The two reactions can therefore be coupled in the form of a third reaction, which we can write as the sum of reactions 1 and 2, with the common intermediate, Pi, omitted from both sides of the equation:
Glucose + ATP
glucose 6-phosphate + ADP
Because more energy is released in reaction 2 than is consumed in reaction 1, the free-energy change for reaction 3, AG3, is negative, and the synthesis of glucose 6-phosphate can therefore occur by reaction 3.
The coupling of exergonic and endergonic reactions through a shared intermediate is absolutely central to the energy exchanges in living systems. As we shall see, the breakdown of ATP (reaction 2 in Fig. 1-26b) is the ex-ergonic reaction that drives many endergonic processes in cells. In fact, ATP is the major carrier of chemical energy in all cells.
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