Put another way, it is important to distinguish between G and G
. The latter is the
change in Gibbs free energy only when the reactants are all at concentrations of 1 M (if a
reactant is a gas, its partial pressure is used instead of concentration, and the partial pres-
sure is assumed to be 1 atm). A positive value for G
does not mean that the reaction
cannot proceed. It only means that the equilibrium is tilted toward the reactants. The reac-
tion can be made to proceed by keeping the concentration of one or more of the produc ts
low. On the other hand, G depends on the actual concentration of reactants and pro-
ducts. When the numerator of the quotient on the right-hand side of equation (5.6) is
less than the denominator, the logarithm will b e negative. If it is negative enough so
that its absolute value exceeds G
, G will be negative and the reaction will tend to
proceed spontaneously toward equilibrium. If, on the other hand, the numerator is more
than the denominator, G will be positive, and the reaction will tend to proceed in the
reverse direction.
As an analogy, G
is like the amount of potential energy that would be released
by a fluid falling through a standard elevation change, say, 1 meter. This is a property
of the fluid (a function of its specific gravity). On the other hand, G is like the
potential energy released by an actual change in elevation. This can be made arbitrarily
large, or reversed in sign, by changing the beginning and ending elevations for the
flow.
Equation (5.8) embodies what is known as Le Cha
ˆ
telier’s principle. This states that if
a reaction at equilibrium is distur bed, such as by adding one of the reacta nts or products to
the solution, the reaction will proceed in a direction so as to partially eliminate the dis-
turbance. For example, cells convert glucose-6-phosphate to fructose-6-phosphate as part
of glycolysis. If the fructose-6-phosphate were not subsequently consumed, allowing it to
accumulate, the reaction could stop or even reverse itsel f, so as to maintain the proper
ratio between product and reactant. This is an important mec hanism for the control of
biochemical reactions.
Also remember that the thermodynamic relationships say nothing about how fast a
reaction will proceed. The sucrose crystals in a sugar bowl are unstable in contact with
air in terms of equation (5.6), but do not react at a measurable rate. However, in the pre-
sence of microbial enzymes and other requirements of microbes, such as moisture and
nutrients, the sugar and oxygen are soon converted to carbon dioxide and water.
The enthalpy, H, is the actual amount of energy released by a reaction in a closed
system under constant pressure and temperature. So why do we use G, which is the
enthalpy reduc ed by a factor involving the change in entropy T S [equation (5.2)]
when we discuss the energy provided by a reaction? The answer is that the Gibbs free
energy is the energy available to do work. This work may include the driving of other
reactions ‘‘uphill’’ in a thermodynamic sense. For example, the Gibbs free energy
released in oxidizing sucrose can be used to synthesize amino acids. The H value for
the oxidation of glucose to carbon dioxide to water is 680 kcal/mol, whereas the G
value for this reaction is 686 kcal/mol, which gives K
eq
¼ 3:046.
The Calorie counts for fats, proteins, and carbohydrates represent values for H for
the complete oxidation (except for proteins, because the nitrogen is excreted mostly as
urea, not as nitrate). However, in humans the processes of digestion and absorption
also require energy, amounting to about 6%, 4%, and 30% of the energy in fats, carbohy-
drates, and proteins, respectively. This energy is wasted as heat. This accounts for why
people often want to eat les s in hot weather. It also suggests that they would be less
uncomfortable if they substituted pasta for meat on warm days.
84 ENERGY AN D METABOLISM