Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Bioenergetics
Thermodynamics
Criterion for Spontaneity of a Process

As we have seen, some spontaneous processes—such as the combustion of Organic compounds—are accompanied by the release of heat (∆H is negative), whereas others proceed with the absorption of heat from the surroundings (∆H is positive). An example of the second type is the melting of ice at a Temperature slightly above 0 °C. In this case, a large change in the Entropy of Water occurs during melting, and at equilibrium at 0 °C, the numerical value of T∆S turns out to be exactly equal to ∆H [equation (3-6)].

The fact that ∆H–T∆S = 0 at equilibrium led J. Willard Gibbs to suggest introducing a thermodynamic function that provides a criterion for spontaneity; this function is now known as the Gibbs Free energy

Class="center">G=H—TS.    (3-13)

For processes occurring at constant temperature and pressure, The change in G is

∆G_T,P = ∆H—T∆S.    (3-14)

Moreover, for reversible (equilibrium) processes involving only pressure-volume work,

∆G_{Т,обратим} = ∆H - T∆S = 0.  (3-15)

It is easy to show that for any spontaneous (irreversible) process, ∆G is negative. Such processes are called exergonic. If ∆G is positive, the reaction will not proceed spontaneously; such reactions are termed endergonic. The decrease in free energy (–∆G) is a measure of the maximum amount of work that can be performed by a given reaction, provided, of course, that the reaction is coupled with a system capable of performing work via a reversible process. This work may be electrical, muscular, or osmotic, driven by reactions taking place in biological systems. In any real system, the work performed is inevitably less than –∆G because real processes are irreversible, meaning they are accompanied by an increase in entropy.

Returning to our earlier assumption that ∆H can serve as a measure of useful work, we note that T∆S accounts for only a few kilocalories in most reactions. Consequently, when ∆H is large (for example, in the combustion of nutrients), it does not differ too greatly from ∆G for the same process. This justifies using the caloric value of food as an approximate measure of the work performed As a result of its utilization by the Organism.