98 Chapter 4
law) must still be present in the chemical bonds of glucose (first
law). It also follows that when the chemical bonds of glucose
are broken, converting the glucose back into carbon dioxide and
water, energy must be released. This energy indirectly powers
all of the energy-requiring processes of our bodies.Endergonic and Exergonic Reactions
Chemical reactions that require an input of energy are known
as endergonic reactions. Because energy is added to make
these reactions “go,” the products of endergonic reactions must
contain more free energy than the reactants. A portion of the
energy added, in other words, is contained within the product
molecules. This follows from the fact that energy cannot be
created or destroyed (first law of thermodynamics) and from
the fact that a more-organized state of matter contains more
free energy, or less entropy, than a less-organized state (second
law of thermodynamics).
That glucose contains more free energy than carbon diox-
ide and water can easily be proven by combusting glucose to
CO 2 and H 2 O. This reaction releases energy in the form of
heat. Reactions that convert molecules with more free energy
to molecules with less—and, therefore, that release energy as
they proceed—are called exergonic reactions.
As illustrated in figure 4.13 , the total amount of energy
released by a molecule in a combustion reaction can be released
in smaller portions by enzymatically controlled exergonic reac-
tions within cells. This allows the cells to use the energy to “drive”
other processes, as described in the next section. The energy
obtained by the body from the cellular oxidation of a molecule
is the same as the amount released when the molecule is com-
busted, so the energy in food molecules can conveniently be mea-
sured by the heat released when the molecules are combusted.
Heat is measured in units called calories. One calorie is
defined as the amount of heat required to raise the temperature
of 1 cubic centimeter of water 1 degree on the Celsius scale.
The caloric value of food is usually indicated in kilocalories
(1 kilocalorie 5 1,000 calories), which are often called large
calories and spelled with a capital C.Coupled Reactions: ATP
In order to remain alive, a cell must maintain its highly orga-
nized, low-entropy state at the expense of free energy in its
environment. Accordingly, the cell contains many enzymes
that catalyze exergonic reactions using substrates that come
ultimately from the environment. The energy released by
these exergonic reactions is used to drive the energy-requiring
processes (endergonic reactions) in the cell. Because cells
cannot use heat energy to drive energy-requiring processes,
the chemical-bond energy that is released in exergonic reac-
tions must be directly transferred to chemical-bond energy
in the products of endergonic reactions. Energy-liberating
reactions are thus coupled to energy-requiring reactions.(that’s why a perpetual motion machine is impossible in prin-
ciple). The total energy is conserved in these transformations
(first law of thermodynamics), but a proportion of the energy is
lost as heat. Therefore, the amount of energy in an “organized”
form—the energy available to do work—decreases in every
energy transformation. Entropy is the degree of disorganization
of a system’s total energy. The second law of thermodynamics
states that the amount of entropy increases in every energy
transformation. Because only energy in an organized state—
called free energy —is available to do work, this means that the
free energy of a system decreases as its entropy increases. A
hybrid car transforms chemical bond energy in gasoline to the
mechanical energy of turning gears, which is then transformed
into electrical energy that can later be used to turn gears. But
the second law dictates that the process cannot simply be
reversed and continued indefinitely; more gasoline will have to
be burned. The second law also explains why plants require the
continued input of light energy, and why we need the continued
input of the chemical bond energy in food molecules.
The chemical bonding of atoms into molecules obeys the
laws of thermodynamics. Six separate molecules of carbon diox-
ide and 6 separate molecules of water is a more disorganized
state than 1 molecule of glucose (C 6 H 12 O 6 ). To go from a more
disorganized state (higher entropy) to a more organized state
(lower entropy) that has more free energy requires the addition
of energy from an outside source. Thus, plants require the input
of light energy from the sun to produce glucose from carbon
dioxide and water in the process of photosynthesis ( fig. 4.12 ).
Because light energy was required to form the bonds of glucose,
a portion of that energy (never 100%, according to the second
Figure 4.12 A simplified diagram of
photosynthesis. Some of the sun’s radiant energy is captured
by plants and used to produce glucose from carbon dioxide and
water. As the product of this endergonic reaction, glucose has
more free energy than the initial reactants.
6 CO 2 + 6 H 2 OEnergy
Free energyC 6 H 12 O 6 (glucose) + 6 O 2