Cellular Structure, Proteins, and Metabolism
73
which the forward and reverse reaction rates are equal. At this
point there will be no further change in the concentrations
of reactants or products even though reactants will continue
to be converted into products and products converted into
reactants.
Consider our previous example in which carbonic acid
breaks down into carbon dioxide and water. The products of
this reaction, carbon dioxide and water, can also recombine
to form carbonic acid. This occurs outside the lungs and is a
means for safely transporting CO
2
in the blood in a nongas-
eous state.
CO
2
+ H
2
O + Energy
34
H
2
CO
3
Carbonic acid has a greater energy content than the sum of
the energies contained in carbon dioxide and water; there-
fore, energy must be added to the latter molecules in order to
form carbonic acid. This energy is not activation energy but
is an integral part of the energy balance. This energy can be
obtained, along with the activation energy, through collisions
with other molecules.
When chemical equilibrium has been reached, the con-
centration of products need not be equal to the concentration
of reactants even though the forward and reverse reaction
rates are equal. The ratio of product concentration to reactant
concentration at equilibrium depends upon the amount of
energy released (or added) during the reaction. The greater
the energy released, the smaller the probability that the prod-
uct molecules will be able to obtain this energy and undergo
the reverse reaction to reform reactants. Therefore, in such a
case, the ratio of product to reactant concentration at chemi-
cal equilibrium will be large. If there is no difference in the
energy contents of reactants and products, their concentra-
tions will be equal at equilibrium.
Thus, although all chemical reactions are reversible to
some extent, reactions that release large quantities of energy
are said to be
irreversible reactions
because almost all of the
reactant molecules have been converted to product molecules
when chemical equilibrium is reached. It must be emphasized
that the energy released in a reaction determines the degree to
which the reaction is reversible or irreversible. This energy is
not the activation energy and it does not determine the reac-
tion rate, which is governed by the four factors discussed ear-
lier. The characteristics of reversible and irreversible reactions
are summarized in
Table 3–6
.
Law of Mass Action
The concentrations of reactants and products play a very impor-
tant role in determining not only the rates of the forward and
reverse reactions, but also the direction in which the
net
reac-
tion proceeds—whether reactants or products are accumulat-
ing at a given time.
Consider the following reversible reaction that has
reached chemical equilibrium:
forward
A + B
3:::4
C + D
reverse
Reactants
Products
If at this point we increase the concentration of one of the
reactants, the rate of the forward reaction will increase and
lead to increased product formation. In contrast, increasing
the concentration of one of the product molecules will drive
the reaction in the reverse direction, increasing the formation
of reactants. The direction in which the net reaction is pro-
ceeding can also be altered by
decreasing
the concentration
of one of the participants. Thus, decreasing the concentration
of one of the products drives the net reaction in the forward
direction because it decreases the rate of the reverse reaction
without changing the rate of the forward reaction.
These effects of reaction and product concentrations on
the direction in which the net reaction proceeds are known as
the
law of mass action.
Mass action is often a major deter-
mining factor controlling the direction in which metabolic
pathways proceed because reactions in the body seldom come
to chemical equilibrium. More typically, new reactant mol-
ecules are added and product molecules are simultaneously
removed by other reactions.
Enzymes
Most of the chemical reactions in the body, if carried out in
a test tube with only reactants and products present, would
proceed at very low rates because they have high activation
energies. To achieve the high reaction rates observed in living
organisms, catalysts must lower the activation energies. These
particular catalysts are called enzymes. Enzymes are protein
molecules, so an
enzyme
can be defi ned as a protein catalyst.
(Although some RNA molecules possess catalytic activity,
the number of reactions they catalyze is very small, so we will
restrict the term
enzyme
to protein catalysts.)
Table 3–6
Characteristics of Reversible and Irreversible Chemical Reactions
Reversible Reactions
A + B
34
C + D + small amount of energy
At chemical equilibrium, product concentrations are only slightly higher than reactant concentrations.
Irreversible Reactions
E + F
⎯→
G + H + large amount of energy
At chemical equilibrium, almost all reactant molecules have been converted to product.
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