Essential Cell Biology 5th edition

84 CHAPTER 3 Energy, Catalysis, and Biosynthesis
Figure 3–5 Living cells do not defy the
second law of thermodynamics. In the
diagram on the left, the molecules of both
the cell and the rest of the universe (the
environment) are depicted in a relatively
disordered state. In addition, red arrows
suggest the relative amount of thermal
motion of the molecules both inside and
outside the cell. In the diagram on the
right, the cell has taken in energy from
food molecules, carried out a reaction
that gives order to the molecules that the
cell contains, and released heat (yellow
arrows) into the environment. The released
heat increases the disorder in the cell’s
surroundings—as depicted here by the
increase in thermal motion of the molecules
in the environment and the distortion
of those molecules due to enhanced
vibration and rotation. The second law of
thermodynamics is thereby satisfied, even
as the cell grows and constructs larger
molecules.
sea of matter
cell
increased disorder
HEAT
increased order
The measure of a system’s disorder is called the entropy of the system,
and the greater the disorder, the greater the entropy. Thus another way
to express the second law of thermodynamics is to say that systems
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will change spontaneously toward arrangements with greater entropy.
Living cells—by surviving, growing, and forming complex communities
and even whole organisms—generate order and thus might appear to
defy the second law of thermodynamics. This is not the case, however,
because a cell is not an isolated system. Rather, a cell takes in energy
from its environment—in the form of food, inorganic molecules, or photons
of light from the sun—and uses this energy to generate order within
itself, forging new chemical bonds and building large macromolecules.
In the course of performing the chemical reactions that generate order,
some energy is inevitably lost in the form of heat (see Figure 3–2). Heat
is energy in its most disordered form—the random jostling of molecules
(analogous to the random jostling of the coins in the box). Because the
cell is not an isolated system, the heat energy produced by metabolic
reactions is quickly dispersed into the cell’s surroundings. There, the
heat increases the intensity of the thermal motions of nearby molecules,
thereby increasing the entropy of the cell’s environment (Figure 3–5).
To satisfy the second law of thermodynamics, the amount of heat released
by a cell must be great enough that the increased order generated inside
the cell is more than compensated for by the increased disorder generated
in the environment. In other words, the chemical reactions inside a
cell must increase the total entropy of the entire system: that of the cell
plus its environment. Thanks to the cell’s activity, the universe thereby
becomes more disordered—and the second law of thermodynamics is
obeyed.
Cells Can Convert Energy from One Form to Another
Where does the heat released by cells as they generate order come from?
To understand that, we need to consider another important physical law.
According to the first law of thermodynamics, energy cannot be created or
destroyed—but it can be converted from one form to another (Figure 3−6).
Cells take advantage of this law of thermodynamics, for example, when
they convert the energy from sunlight into the energy in the chemical
bonds of sugars and other small organic molecules during photosynthesis.
Although the chemical reactions that power such energy conversions
can change how much energy is present in one form or another, the first
law tells us that the total amount of energy in the universe must always
be the same.
Heat, too, is a product of energy conversion. When an animal cell breaks
down foodstuffs, some of the energy in the chemical bonds in the food