Essential Cell Biology 5th edition

The Use of Energy by Cells
83
(A) (B) (C) (D) (E)
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Biological Order Is Made Possible by the Release of
Heat Energy from Cells
The universal tendency of things to become disordered is expressed in
a fundamental law of physics called the second law of thermodynamics.
This law states that in the universe as a whole, or in any isolated system
(a collection of matter that is completely cut off from the rest of the
universe), the degree of disorder can only increase. ECB5 The e3.03/3.03 second law of
thermodynamics has such profound implications for living things that it
is worth restating in several ways.
We can express the second law in terms of probability by stating that
systems will change spontaneously toward those arrangements that have
the greatest probability. Consider a box in which 100 coins are all lying
heads up. A series of events that disturbs the box—for example, someone
jiggling it a bit—will tend to move the arrangement toward a mixture of
50 heads and 50 tails. The reason is simple: there is a huge number of
possible arrangements of the individual coins that can achieve the 50–50
result, but only one possible arrangement that keeps them all oriented
heads up. Because the 50–50 mixture accommodates a greater number
of possibilities and places fewer constraints on the orientation of each
individual coin, we say that it is more “disordered.” For the same reason,
one’s living space will become increasingly disordered without an
intentional effort to keep it organized. Movement toward disorder is a
spontaneous process, and requires a periodic input of energy to reverse
it (Figure 3–4).
Figure 3–3 Biological structures are
highly ordered. Well-defined, ornate,
and beautiful spatial patterns can be
found at every level of organization in
living organisms. Shown are: (A) protein
molecules in the coat of a virus (a parasite
that, although not technically alive, contains
the same types of molecules as those
found in living cells); (B) the regular array
of microtubules seen in a cross section of a
sperm tail; (C) surface contours of a pollen
grain; (D) cross section of a fern stem,
showing the patterned arrangement of
cells; and (E) a spiral array of leaves, each
made of millions of cells. (A, courtesy of
Robert Grant, Stéphane Crainic, and James
M. Hogle; B, courtesy of Lewis Tilney;
C, courtesy of Colin MacFarlane and
Chris Jeffree; D, courtesy of Jim Haseloff.)
“SPONTANEOUS“ REACTION
as time elapses
ORGANIZED EFFORT REQUIRING ENERGY INPUT
Figure 3–4 The spontaneous tendency
toward disorder is an everyday
experience. Reversing this natural tendency
toward disorder requires an intentional
effort and an input of energy. In fact, from
the second law of thermodynamics, we
can be certain that the human intervention
required will release enough heat to the
environment to more than compensate for
the reestablishment of order in this room.