Chapter 2
Chemistry Comes Alive
37
2
reactions involve
complete transfer
of electrons—some simply
change the pattern of electron sharing in covalent bonds. For
example, a substance is oxidized both by losing hydrogen atoms
and by combining with oxygen. Te common factor in these
events is that electrons that formerly “belonged” to the reactant
molecule are lost. Te electrons are lost either entirely (as when
hydrogen is removed and takes its electron with it) or relatively
(as the shared electrons spend more time in the vicinity of the
very electronegative oxygen atom).
±o understand the importance of oxidation-reduction reac-
tions in living systems, take a look at the overall equation for
cel-
lular respiration
, which represents the major pathway by which
glucose is broken down for energy in body cells:
C
6
H
12
O
6
1
6O
2
S
6CO
2
1
6H
2
O
1
A±P
glucose
oxygen
carbon
water
cellular
dioxide
energy
As you can see, it is an oxidation-reduction reaction. Consider
what happens to the hydrogen atoms (and their electrons). Glu-
cose is oxidized to carbon dioxide as it loses hydrogen atoms,
and oxygen is reduced to water as it accepts the hydrogen atoms.
Tis reaction is described in detail in Chapter 24, along with
other topics of cellular metabolism.
Energy Flow in Chemical Reactions
Because all chemical bonds represent stored chemical energy, all
chemical reactions ultimately result in net absorption or release
of energy. Reactions that release energy are called
exergonic
reactions
. Tese reactions yield products with less energy than
the initial reactants, along with energy that can be harvested for
other uses. With a few exceptions, catabolic and oxidative reac-
tions are exergonic.
In contrast, the products of energy-absorbing, or
ender-
gonic
, reactions contain more potential energy in their chemi-
cal bonds than did the reactants. Anabolic reactions are typically
energy-absorbing endergonic reactions. Essentially this is a case
of “one hand washing the other”—the energy released when
fuel molecules are broken down (oxidized) is captured in A±P
molecules and then used to synthesize the complex biological
molecules the body needs to sustain life.
Reversibility of Chemical Reactions
All chemical reactions are theoretically reversible. If chemical
bonds can be made, they can be broken, and vice versa. Revers-
ibility is indicated by a double arrow. When the arrows differ in
length, the longer arrow indicates the major direction in which
the reaction proceeds:
In this example, the forward reaction (reaction going to the
right) predominates. Over time, the product (AB) accumulates
and the reactants (A and B) decrease in amount.
When the arrows are of equal length, as in
A
1
B
÷
AB
neither the forward reaction nor the reverse reaction is domi-
nant. In other words, for each molecule of product (AB) formed,
one product molecule breaks down, releasing the reactants A
and B. Such a chemical reaction is said to be in a state of
chemi-
cal equilibrium
.
Once chemical equilibrium is reached, there is no further
net
change
in the amounts of reactants and products unless more of
either are added to the mix. Product molecules are still formed
and broken down, but the balance established when equilib-
rium was reached (such as greater numbers of product mol-
ecules) remains unchanged.
Chemical equilibrium is analogous to the admission
scheme used by many large museums in which tickets are sold
according to time of entry. If 300 tickets are issued for the 9 ²M
admission, 300 people will be admitted when the doors open.
Terea³er, when 6 people leave, 6 are admitted, and when an-
other 15 people leave, 15 more are allowed in. Tere is a con-
tinual turnover, but the museum contains about 300 art lovers
throughout the day.
All chemical reactions are reversible, but many biological reac-
tions show so little tendency to go in the reverse direction that they
are irreversible for all practical purposes. Chemical reactions that
release energy will not go in the opposite direction unless energy
is put back into the system. For example, when our cells break
down glucose via the reactions of cellular respiration to yield car-
bon dioxide and water, some of the energy released is trapped in
the bonds of A±P. Because the cells then use A±P’s energy for vari-
ous functions (and more glucose will be along with the next meal),
this particular reaction is never reversed in our cells. Furthermore,
if a product of a reaction is continuously removed from the reac-
tion site, it is unavailable to take part in the reverse reaction. Tis
situation occurs when the carbon dioxide that is released during
glucose breakdown leaves the cell, enters the blood, and is eventu-
ally removed from the body by the lungs.
Factors Influencing the Rate
of Chemical Reactions
What influences how quickly chemical reactions go? For atoms
and molecules to react chemically in the first place, they must
collide
with enough force to overcome the repulsion between
their electrons. Interactions between valence shell electrons—
the basis of bond making and breaking—cannot occur long dis-
tance. Te force of collisions depends on how fast the particles
are moving. Solid, forceful collisions between rapidly moving
particles in which valence shells overlap are much more likely to
cause reactions than are collisions in which the particles graze
each other lightly.
Temperature
Increasing the temperature of a substance in-
creases the kinetic energy of its particles and the force of their
collisions. For this reason, chemical reactions proceed more
quickly at higher temperatures.
Concentration
Chemical reactions progress most rapidly
when the reacting particles are present in high numbers, be-
cause the chance of successful collisions is greater. As the con-
centration of the reactants declines, the reaction slows. Chemical
A
1
B
AB
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