72
Chapter 3
steady state in which the anabolic and catabolic rates for the
synthesis and breakdown of most molecules are equal.
Chemical Reactions
Chemical reactions involve (1) the breaking of chemical bonds
in reactant molecules, followed by (2) the making of new chem-
ical bonds to form the product molecules. Take, for example, a
chemical reaction that occurs in the lungs, which permits the
lungs to rid the body of carbon dioxide. In the reaction shown
below, carbonic acid is transformed into carbon dioxide and
water. Two of the chemical bonds in carbonic acid are broken,
and the product molecules are formed by establishing two new
bonds between different pairs of atoms:
O
O
B
B
H—O—C—O—H
⎯⎯→
O
P
C + H—O—H
broken
broken
formed
formed
H
2
CO
3
⎯⎯→
CO
2
+ H
2
O + Energy
carbonic acid
carbon
water
dioxide
Because the energy contents of the reactants and products are
usually different, and because energy can neither be created
nor destroyed, energy must either be added or released dur-
ing most chemical reactions. For example, the breakdown of
carbonic acid into carbon dioxide and water releases energy
because carbonic acid has a higher energy content than the
sum of the energy contents of carbon dioxide and water.
The released energy takes the form of heat, the energy
of increased molecular motion, which is measured in units of
calories. One
calorie
(1 cal) is the amount of heat required to
raise the temperature of 1 g of water 1° on the Celsius scale.
Energies associated with most chemical reactions are several
thousand calories per mole and are reported as
kilocalories
(1 kcal = 1000 cal).
Determinants of Reaction Rates
The rate of a chemical reaction (in other words, how many
molecules of product form per unit of time) can be determined
by measuring the change in the concentration of reactants or
products per unit of time. The faster the product concentration
increases or the reactant concentration decreases, the greater
the rate of the reaction. Four factors (
Table 3–5
) infl uence the
reaction rate: reactant concentration, activation energy, tem-
perature, and the presence of a catalyst.
The lower the concentration of reactants, the slower the
reaction simply because there are fewer molecules available to
react. Conversely, the higher the concentration of reactants,
the faster the reaction rate.
Given the same initial concentrations of reactants, how-
ever, not all reactions occur at the same rate. Each type of chem-
ical reaction has its own characteristic rate, which depends
upon what is called the activation energy for the reaction.
In order for a chemical reaction to occur, reactant molecules
must acquire enough energy—the
activation energy
—to
enter an activated state in which chemical bonds can be bro-
ken and formed. The activation energy does not affect the
difference in energy content between the reactants and fi nal
products since the activation energy is released when the
products are formed.
How do reactants acquire activation energy? In most of
the metabolic reactions we will be considering, the reactants
obtain activation energy when they collide with other mole-
cules. If the activation energy required for a reaction is large,
then the probability of a given reactant molecule acquiring
this amount of energy will be small, and the reaction rate will
be slow. Thus, the higher the activation energy required, the
slower the rate of a chemical reaction.
Temperature is the third factor infl uencing reaction
rates. The higher the temperature, the faster molecules move
and the greater their impact when they collide. Therefore, one
reason that increasing the temperature increases a reaction rate
is that reactants have a better chance of acquiring suffi cient
activation energy from a collision. In addition, faster-moving
molecules collide more often.
A
catalyst
is a substance that interacts with a reactant by
altering the distribution of energy between the chemical bonds
of the reactant, resulting in a decrease in the activation energy
required to transform the reactant into product. Because less
activation energy is required, a reaction will proceed at a faster
rate in the presence of a catalyst. The chemical composition
of a catalyst is not altered by the reaction, so
a single catalyst
molecule can act over and over again to catalyze the conversion
of many reactant molecules to products
. Furthermore, a cata-
lyst does not alter the difference in the energy contents of the
reactants and products.
Reversible and Irreversible Reactions
Every chemical reaction is, in theory, reversible. Reactants are
converted to products (we will call this a “forward reaction”),
and products are converted to reactants (a “reverse reaction”).
The overall reaction is a
reversible reaction:
forward
Reactants
3::4
Products
reverse
As a reaction progresses, the rate of the forward reac-
tion will decrease as the concentration of reactants decreases.
Simultaneously, the rate of the reverse reaction will increase as
the concentration of the product molecules increases. Eventually
the reaction will reach a state of
chemical equilibrium
in
Table 3–5
Determinants of Chemical Reaction
Rates
1. Reactant concentrations (higher concentrations: faster
reaction rate)
2. Activation energy (higher activation energy: slower
reaction rate)
3. Temperature (higher temperature: faster reaction rate)
4. Catalyst (presence of catalyst: faster reaction rate)
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