Essential Cell Biology 5th edition

82 CHAPTER 3 Energy, Catalysis, and Biosynthesis
Figure 3−1 A series of enzyme-catalyzed
reactions forms a linked pathway. Each
chemical reaction is catalyzed by a distinct
enzyme. Together, this set of enzymes,
acting in series, converts molecule A to
molecule F.
molecule molecule molecule molecule molecule molecule
A
CATALYSIS
BY ENZYME 1
B
CATALYSIS
BY ENZYME 2
C
CATALYSIS
BY ENZYME 3
D
CATALYSIS
BY ENZYME 4
E
CATALYSIS
BY ENZYME 5
F
molecule could in principle undergo. These enzyme-catalyzed reactions
are usually connected in series, so that the product of one reaction
becomes the starting material for the next (Figure 3−1). The long, linear
reaction pathways that result are in turn linked to one another, forming a
complex web of interconnected reactions.
ECB5 e3.01/3.01
Rather than being an inconvenience, the necessity for catalysis is a benefit,
as it allows the cell to precisely control its metabolism—the sum
total of all the chemical reactions it needs to carry out to survive, grow,
and reproduce. This control is central to the chemistry of life.
Two opposing streams of chemical reactions occur in cells: the catabolic
pathways and the anabolic pathways. The catabolic pathways (catabolism)
break down foodstuffs into smaller molecules, thereby generating
both a useful form of energy for the cell and some of the small molecules
that the cell needs as building blocks. The anabolic, or biosynthetic, pathways
(anabolism) use the energy harnessed by catabolism to drive the
synthesis of the many molecules that form the cell. Together, these two
sets of reactions constitute the metabolism of the cell (Figure 3−2).
The details of the reactions that comprise cell metabolism are part of the
subject matter of biochemistry, and they need not concern us here. But
the general principles by which cells obtain energy from their environment
and use it to create order are central to cell biology. We therefore
begin this chapter by explaining why a constant input of energy is needed
to sustain living organisms. We then discuss how enzymes catalyze the
reactions that produce biological order. Finally, we describe the molecules
inside cells that carry the energy that makes life possible.
THE USE OF ENERGY BY CELLS
Left to themselves, nonliving things eventually become disordered: buildings
crumble and dead organisms decay. Living cells, by contrast, not
only maintain but actually generate order at every level, from the largescale
structure of a butterfly or a flower down to the organization of the
molecules that make up such organisms (Figure 3–3). This property of life
is made possible by elaborate molecular mechanisms that extract energy
from the environment and convert it into the energy stored in chemical
bonds. Biological structures are therefore able to maintain their form,
even though the materials that form them are continually being broken
down, replaced, and recycled. Your body has the same basic structure it
had 10 years ago, even though you now contain atoms that, for the most
part, were not part of your body then.
useful
forms of
energy
Figure 3−2 Catabolic and anabolic
pathways together constitute the cell’s
metabolism. During catabolism, a major
portion of the energy stored in the chemical
bonds of food molecules is dissipated as
heat. But some of this energy is converted
to the useful forms of energy needed to
drive the synthesis of new molecules in
anabolic pathways, as indicated.
food molecules
CATABOLIC
PATHWAYS
lost
heat
the many
building blocks
for biosynthesis
ANABOLIC
PATHWAYS
the many
molecules
that form
the cell