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408
C
HAPTER
T
WENTY
Metabolism
ANAEROBIC
Nutrients absorbed from the digestive tract are used for
all the cellular activities of the body, which together
make up
metabolism
. These activities fall into two cat-
egories:
Glucose (6c)
+2 ATP
Catabolism
, which is the breakdown of complex com-
pounds into simpler compounds. Catabolism includes
the digestion of food into small molecules and the re-
lease of energy from these molecules within the cell.
Lactic
acid (3c)
Pyruvic
acid (3c)
Pyruvic
acid (3c)
Lactic
acid (3c)
Anabolism
, which is the building of simple compounds
into substances needed for cellular activities and for the
growth and repair of tissues.
Through the steps of catabolism and anabolism, there
is a constant turnover of body materials as energy is con-
sumed, cells function and grow, and waste products are
generated.
+34 ATP
Checkpoint 20-1
What are the two phases of metabolism?
3CO
2
3H
2
O
3CO
2
3H
2
O
AEROBIC
Cellular Respiration
Energy is released from nutrients in a series of reactions
called
cellular respiration
(see Table 20-1
and
Fig. 20-1)
.
Early studies on cellular respiration were done with
glu-
cose
as the starting compound. Glucose is a simple sugar
that is the main energy source for the body.
carbon atoms in one molecule of a sub-
stance.) In cellular respiration, glucose first yields two mole-
cules of pyruvic acid, which will convert to lactic acid under
anaerobic conditions, as during intense exercise. (Lactic acid
must eventually be converted back to pyruvic acid.) Typically,
however, pyruvic acid is broken down aerobically (using oxy-
gen) to CO
2
and H
2
O (aerobically).
ZOOMING IN
✦
What
does pyruvic acid produce in cellular respiration under anaerobic
conditions? Under aerobic conditions?
The Anaerobic Phase
The first steps in the break-
down of glucose do not require oxygen; that is, they are
anaerobic
. This phase of catabolism, known as
glycolysis
(gli-KOL-ih-sis), occurs in the cytoplasm of the cell. It
yields a small amount of energy, which is used to make
ATP (adenosine triphosphate), the cells’ energy com-
pound. Each glucose molecule yields enough energy by
this process to produce 2 molecules of ATP.
The anaerobic breakdown of glucose is incomplete
and ends with formation of an organic product called
pyruvic
(pi-RU-vik)
acid
. This organic acid is further me-
tabolized in the next phase of cellular respiration, which
requires oxygen. In muscle cells operating briefly under
anaerobic conditions, pyruvic acid is converted to lactic
acid, which accumulates as the cells build up an oxygen
debt (described in Chapter 8). Lactic acid induces muscle
fatigue, so the body is forced to rest and recover. During
the recovery phase immediately after exercise, breathing
restores the oxygen needed to convert lactic acid back to
pyruvic acid, which is then metabolized further. During
this recovery phase, reserves stored in muscles are also
replenished. These compounds are myoglobin, which
stores oxygen; glycogen, which can be broken down for
glucose; and creatine phosphate, which stores energy.
The Aerobic Phase
To generate enough energy for
survival, the body’s cells must break pyruvic acid down
more completely in the second phase
of cellular respiration, which requires
oxygen. These
aerobic
reactions occur
within the mitochondria of the cell.
They result in transfer of most of the
energy remaining in the nutrients to
ATP. On average, about 34 to 36 mol-
ecules of ATP can be formed aerobi-
cally per glucose molecule—quite an
increase over anaerobic metabolism.
Table 20•1
Summary of Cellular Respiration of Glucose
LOCATION
END
ENERGY
PHASE
IN CELL
PRODUCT(S)
YIELD/GLUCOSE
Anaerobic
Cytoplasm
Pyruvic acid
2 ATP
(glycolysis)
Aerobic
Mitochondria
Carbon dioxide
and water
34–36 ATP
Figure 20-1
Cellular respiration.
This diagram shows the
catabolism of glucose without oxygen (anaerobic) and with oxy-
gen (aerobic). (C
M
ETABOLISM
, N
UTRITION
,
AND
B
ODY
T
EMPERATURE
409
During the aerobic steps of cellular respiration, the
cells form carbon dioxide, which then must be trans-
ported to the lungs for elimination. In addition, water is
formed by the combination of oxygen with the hydrogen
that is removed from nutrient molecules. Because of the
type of chemical reactions involved, and because oxygen
is used in the final steps, cellular respiration is described
as an
oxidation
of nutrients. Note that enzymes are re-
quired as catalysts in all the reactions of cellular respira-
tion. Many of the vitamins and minerals described later in
this chapter are parts of these enzymes.
Although the oxidation of food is often compared to
the burning of fuel, this comparison is inaccurate. Burning
fuel results in a sudden and often wasteful release of energy
in the form of heat and light. In contrast, metabolic oxida-
tion occurs in small steps, and much of the energy released
is stored as ATP for later use by the cells; some of the en-
ergy is released as heat, which is used to maintain body
temperature, as discussed later in this chapter.
For those who know how to read chemical equations,
the net balanced equation for cellular respiration, starting
with glucose, is as follows:
Checkpoint 20-2
What name is given to the series of cellular re-
actions that releases energy from nutrients?
Metabolic Rate
Metabolic rate
refers to the rate at
which energy is released from nutrients in the cells. It is
affected by a person’s size, body fat, sex, age, activity, and
hormones, especially thyroid hormone (thyroxine).
Metabolic rate is high in children and adolescents and de-
creases with age.
Basal metabolism
is the amount of en-
ergy needed to maintain life functions while the body is
at rest.
The unit used to measure energy is the kilocalorie
(kcal), which is the amount of heat needed to raise 1 kilo-
gram of water 1
The Use of Nutrients for Energy
As noted, glucose is the main source of energy in the
body. Most of the carbohydrates in the diet are converted
to glucose in the course of metabolism. Reserves of glu-
cose are stored in liver and muscle cells as
glycogen
(GLI-
ko-jen), a compound built from glucose molecules. When
glucose is needed for energy, glycogen is broken down to
yield glucose. Glycerol and fatty acids (from fat digestion)
C
6
H
12
O
6
6O
2
6CO
2
6H
2
O
glucose
oxygen
carbon
water
dioxide
Box 20-1
A Closer Look
Calorie Counting: Estimating Daily Energy Needs
simple formula. An average woman requires 0.9
kcal/kg/hour, and a man, 1.0 kcal/kg/hour. Multiplying 0.9 by
body weight in kilograms* by 24 for a woman, or 1.0 by body
weight in kilograms by 24 for a man, yields the daily basal en-
ergy requirement. For example, if a woman weighed 132
pounds, the equation would be as follows:
132 pounds
The equation to calculate total energy needs for a day is:
Basal energy requirement
(basal energy requirement
20
2.2 pounds/kg
60 kg
activity level)
Using our previous example, and assuming light activity lev-
els, the following equations apply:
At 40% activity:
1,296 kcal/day
(1,296 kcal/day
40%)
1,814.4 kcal/day
0.9 kcal/kg/hour x 60 kg
54 kcal/hour
At 60% activity:
1,296 kcal/day
(1,296 kcal/day
60%)
2,073.6 kcal/day
Therefore, the woman in our example would require be-
tween 1,814 and 2,073 Kcal/day.
1,296 kcal/day
To estimate total energy needs for a day, a percentage based
on activity level (“couch potato” to serious athlete) must also be
added to the basal requirement. These percentages are shown in
the table below.
ACTIVITY LEVEL
MALE
FEMALE
Little activity (“couch potato”)
25–40%
25–35%
Light activity (
e.g.
, walking to and from class,
50–75%
40–60%
but little or no intentional exercise)
Moderate activity (
e.g.
, aerobics several
65–80%
50–70%
times a week)
Heavy activity (serious athlete)
90–120%
80–100%
*
To convert pounds to kilograms, divide weight in pounds by 2.2.
C. To estimate the daily calories needed
taking activity level into account, see Box 20-1.
Calorie Counting: Estimating Daily Energy Needs
B
asal energy requirements for a day can be estimated with a
54 kcal/hour x 24 hours/day
410
C
HAPTER
T
WENTY
and amino acids (from protein digestion) can also be used
for energy, but they enter the breakdown process at dif-
ferent points.
Fat in the diet yields more than twice as much energy
as do protein and carbohydrate (
e.g.
, it is more “fatten-
ing”); fat yields 9 kcal of energy per gram, whereas pro-
tein and carbohydrate each yield 4 kcal per gram. Calo-
ries that are ingested in excess of need are converted to fat
and stored in adipose tissue.
Before they are oxidized for energy, amino acids must
have their nitrogen (amine) groups removed. This re-
moval, called
deamination
(de-am-ih-NA-shun), occurs
in the liver, where the nitrogen groups are then formed
into urea by combination with carbon dioxide. The blood
transports urea to the kidneys to be eliminated.
There are no specialized storage forms of proteins, as
there are for carbohydrates (glycogen) and fats (adipose
tissue). Therefore, when one needs more proteins than are
supplied in the diet, they must be obtained from body sub-
stance, such as muscle tissue or plasma proteins. Drawing
on these resources becomes dangerous when needs are ex-
treme. Fats and carbohydrates are described as “protein
sparing,” because they are used for energy before proteins
are and thus spare proteins for the synthesis of necessary
body components.
metabolic reactions. These 11 amino acids are described
as
nonessential
because they need not be taken in as food
(Table 20-2)
. The remaining 9 amino acids cannot be
made by the body and therefore must be taken in as part
of the diet; these are the
essential amino acids
. Note that
some nonessential amino acids may become essential
under certain conditions, as during extreme physical
stress, or in certain hereditary metabolic diseases.
Essential Fatty Acids
There are also two essential fatty
acids (linoleic acid and linolenic acid) that must be taken
in as food. These are easily obtained through a healthful,
balanced diet.
Checkpoint 20-4
What is meant when an amino acid or a fatty
acid is described as essential?
Minerals and Vitamins
In addition to needing fats, proteins, and carbohydrates,
the body requires minerals and vitamins.
Minerals
are chemical elements needed for body struc-
ture, fluid balance, and such activities as muscle contrac-
tion, nerve impulse conduction, and blood clotting. Some
minerals are components of vitamins. A list of the main
minerals needed in a proper diet is given in
Table 20-3
.
Some additional minerals not listed are also required for
good health. Minerals needed in extremely small amounts
are referred to as
trace elements
.
Vitamins
are complex organic substances needed in
very small quantities. Vitamins are parts of enzymes or
other substances essential for metabolism, and vitamin
deficiencies lead to a variety of nutritional diseases.
The water-soluble vitamins are the B vitamins and vi-
tamin C. These are not stored and must be taken in regu-
larly with food. The fat-soluble vita-
mins are A, D, E, and K. These
vitamins are kept in reserve in fatty tis-
sue. Excess intake of the fat-soluble vi-
tamins can lead to toxicity. A list of vi-
tamins is given in
Table 20-4
.
Certain substances are valuable
in the diet as
antioxidants
. They de-
fend against the harmful effects of
free radicals
, highly reactive and un-
stable molecules produced from oxy-
gen in the normal course of metabo-
lism (and also from UV radiation, air
pollution and tobacco smoke). Free
radicals contribute to aging and dis-
ease. Antioxidants react with free
radicals to stabilize them and mini-
mize their harmful effects on cells.
Vitamins C and E and beta carotene,
an orange pigment found in plants
that is converted to vitamin A, are
Checkpoint 20-3
What is the main energy source for the cells?
Anabolism
Nutrient molecules are built into body materials by ana-
bolic steps, all of which are catalyzed by enzymes.
Essential Amino Acids
Eleven of the 20 amino acids
needed to build proteins can be synthesized internally by
Table 20•2
Amino acids
NONESSENTIAL AMINO ACIDS* ESSENTIAL AMINO ACIDS**
Name
Pronunciation
Name***
Pronunciation
Alanine
AL-ah-nene
Histidine
HIS-tih-dene
Arginine
AR-jih-nene
Isoleucine
i-so-LU-sene
Asparagine
ah-SPAR-ah-jene
Leucine
LU-sene
Aspartic acid
ah-SPAR-tik AH-sid Lysine
LI-sene
Cysteine
SIS-teh-ene
Methionine
meh-THI-o-nene
Glutamic acid
glu-TAM-ik AH-sid
Phenylalanine fen-il-AL-ah-nene
Glutamine
GLU-tah-mene
Threonine
THRE-o-nene
Glycine
GLY-sene
Tryptophan
TRIP-to-fane
Proline
PRO-lene
Valine
VA-lene
Serine SERE-ene
Tyrosine TI-ro-sene
*Nonessential amino acids can be synthesized by the body.
**Essential amino acids cannot be synthesized by the body; they must be taken in as part
of the diet.
***If you are ever called upon to memorize the essential amino acids, the mnemonic (memory
device) Pvt. T. M. Hill gives the first letter of each name.
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