Reactions, coupled energy change

We can predict whether pairs of coupled reactions will proceed spontaneously by simply summing the free energy changes for each reaction. For example, consider the reaction from glycolysis (discussed in Chapter 19)... 

The half-reactions and reduction potentials in Table 21.1 can be used to analyze energy changes in redox reactions. The oxidation of NADH to NAD can be coupled with the reduction of a-ketoglutarate to isocitrate ... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

What typically happens for an energetically unfavorable reaction to occur is that it is "coupled" to an energetically favorable reaction so that the overall free-energy change for the two reactions together is favorable. To understand what it means for reactions to be coupled, imagine that reaction 1 does not occur to any reasonable extent because il has a small equilibrium constant and is energetically unfavorable that is, the reaction has AG > 0. 

As an example of two reactions that are coupled, look at the phosphorylation reaction of glucose to yield glucose 6-phosphate plus water, an important step in the breakdown of dietary carbohydrates. The reaction of glucose with HOPO 2- does not occur spontaneously because it is energetically unfavorable, with AG° = + 13.8 kj/mol. (The standard free-energy change for a biological reaction is denoted AG0 and refers to a process in which reactants and products have a concentration of 1.0 M in a soiution with pH = 7.)... 

Additivity of Free Energy Changes Coupled Reactions... 

Because free energy changes are additive, it is ofien possible to bring about a nonsponta-neous reaction by coupling it with a reaction for which AG° is a large negative number. As an example, consider the preparation of iron metal from hematite ore. The reaction... 

ADP (adenosine diphosphate) and ATP (adenosine triphosphate) are complex organic molecules (Fig. 17.9) that, in essence, differ only hy the presence of an extra phosphate group in ATP. In the coupled reaction with glucose, about 38 mol of ATP are synthesized for every mole of glucose consumed. This gives an overall free energy change for the coupled reaction of... 

A coupled process links a spontaneous reaction with a nonspontaneous one. In this case, the negative free energy change of the acetyl phosphate reaction drives the conversion of ADP to ATP. 

Which redox couple in a redox reaction has the oxidizing role and which the redncing role depends on the relative abilities of the two couples to accept or donate electrons. For example O2 has a greater affinity for electrons than other potential oxidants in natnral systems, and is therefore reduced preferentially. The means of quantifying the relative abilities of redox conples to accept or donate electrons and the corresponding free energy changes is as follows. 

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