119
CHAPTER FOUR
Cellular Metabolism
Cellular respiration
is the process that transfers energy
from molecules such as glucose and makes it available for
cellular use. The chemical reactions of cellular respiration
must occur in a particular sequence, each one controlled
by a different enzyme. Some of these enzymes are in the
cell’s cytosol, whereas others are in the mitochondria. Such
precision of activity suggests that the enzymes are physi-
cally positioned in the exact sequence as that of the reac-
tions they control. The enzymes responsible for some of the
reactions of cellular respiration are located in tiny, stalked
particles on the membranes (cristae) in the mitochondria
(see chapter 3, p. 84).
Changes in the human body are a characteristic of life—
whenever this happens, energy is being transferred. Thus,
all metabolic reactions involve energy in some form.
ATP Molecules
Adenosine triphosphate (ATP)
is a molecule that carries
energy in a form that the cell can use. Each ATP molecule
consists of three main parts—an adenine, a ribose, and three
phosphates in a chain
(f g. 4.7)
. The second and third phos-
phates of ATP are attached by high-energy bonds, and the
chemical energy stored in one or both high-energy bonds
may be quickly transferred to another molecule in a meta-
bolic reaction. Energy from the breakdown of ATP powers
cellular work such as skeletal muscle contraction, active
transport across cell membranes, secretion, and many other
functions.
An ATP molecule that loses its terminal phosphate
becomes an
adenosine diphosphate (ADP)
molecule,
which has only two phosphates. ATP can be resynthesized
from an ADP by using energy released from cellular respira-
tion to reattach a phosphate, in a process called
phospho-
rylation
(fos
fo
¯r-ı˘-la
shun). Thus, as shown in
f
gure 4.8
,
ATP and ADP molecules shuttle back and forth between
the energy-transferring reactions of cellular respiration
and the energy-transferring reactions of the cell.
ATP is the primary energy-carrying molecule in a cell.
Even though there are other energy carriers, without enough
ATP, cells quickly die.
substrate. A cofactor may be an ion of an element, such as
copper, iron, or zinc, or a small organic molecule, called a
coenzyme
(ko-en
zı¯m). Many coenzymes are composed of
vitamin molecules or incorporate altered forms of vitamin
molecules into their structures.
Vitamins
are essential organic molecules that human
cells cannot synthesize (or may not synthesize in sufF cient
amounts) and therefore must come from the diet. Vitamins
provide coenzymes that can, like enzymes, function repeat-
edly, so cells require small amounts of vitamins. An example
is coenzyme A (derived from the vitamin pantothenic acid),
which is necessary for one of the reactions of cellular respira-
tion, discussed in the next section. Chapter 18 (pp. 710–716)
discusses vitamins further.
Factors That Alter Enzymes
Almost all enzymes are proteins, and like other proteins,
they can be denatured by exposure to excessive heat, radi-
ation, electricity, certain chemicals, or fl
uids with extreme
pH values. ±or example, many enzymes become inactive at
45°C, and nearly all of them are denatured at 55°C. Some
poisons denature enzymes. Cyanide, for instance, can inter-
fere with respiratory enzymes and damage cells by halting
their energy-obtaining reactions.
Certain microorganisms, colorfully called “extremophiles,” live in con-
ditions of extremely high or low heat, salinity, or pH. Their enzymes
have evolved under these conditions and are useful in industrial pro-
cesses too harsh to use other enzymes.
PRACTICE
4
How can an enzyme control the rate of a metabolic reaction?
5
How does an enzyme “recognize” its substrate?
6
How can a rate-limiting enzyme be an example of negative
feedback control of a metabolic pathway?
7
What is the role of a cofactor?
8
What factors can denature enzymes?
4.4
ENERGY FOR METABOLIC
REACTIONS
Energy
is the capacity to change something; it is the ability
to do work. Therefore, we recognize energy by what it can
do. Common forms of energy are heat, light, sound, electri-
cal energy, mechanical energy, and chemical energy.
Although energy cannot be created or destroyed, it can
be changed from one form to another. An ordinary incandes-
cent light bulb changes electrical energy to heat and light,
and an automobile engine changes the chemical energy in
gasoline to heat and mechanical energy.
ATP
Ribose
Adenosine
PPP
Adenine
Phosphates
FIGURE 4.7
ATP provides cellular energy currency. An ATP
(adenosine triphosphate) molecule consists of an adenine, a ribose,
and three phosphates. The wavy lines connecting the last two
phosphates represent high-energy chemical bonds.
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