701
CHAPTER EIGHTEEN
Nutrition and Metabolism
as prolonged starvation, other fuel sources may become
available for neurons.) Even a temporary decrease in the
glucose supply may seriously impair nervous system func-
tion. Consequently, the body requires a minimum amount
of carbohydrate. If adequate carbohydrates are not obtained
from foods, the liver may convert some noncarbohydrates,
such as amino acids from proteins or glycerol from fats,
into glucose—a process called
gluconeogenesis.
The require-
ment for glucose has physiological priority over the need to
synthesize certain other substances, such as proteins, from
available amino acids.
Cells also use carbohydrates as starting materials for
synthesizing such vital biochemicals as the 5-carbon sugars
ribose
and
deoxyribose.
These sugars are required for the
production of the nucleic acids RNA and DNA and the disac-
charide
lactose
(milk sugar), synthesized when the breasts
are actively secreting milk.
PRACTICE
3
List several common sources of carbohydrates.
4
In what form are carbohydrates used as a cellular fuel?
5
Explain what happens to excess glucose in the body.
6
How does the body obtain glucose when its food supply of
carbohydrates is insuF
cient?
7
Name two uses of carbohydrates other than supplying energy.
An adult’s liver stores about 100 grams of glycogen, and muscle tissue
stores another 200 grams, providing enough reserve to meet energy
demands for about twelve hours when the person is resting. Whether
these stores are ±
lled depends on diet. People consume widely vary-
ing amounts of carbohydrates, often re²
ecting economic conditions.
In the United States, a typical adult’s diet supplies about 50% of total
body energy from carbohydrates. In Asian countries where rice is a
staple, carbohydrates contribute even more to the diet.
Sugar substitutes provide concentrated sweetness, so fewer calories
are needed to sweeten a food. Aspartame, a dipeptide (two joined
amino acids), is 200 times as sweet as table sugar (sucrose); saccharin
is 300 times as sweet as sugar; and sucralose (Splenda) is 600 times
as sweet as sucrose. Sucralose is derived from sucrose, with chloride
attached. The body does not recognize it as a carbohydrate, and so it
has zero calories.
Cellulose
is a complex carbohydrate abundant in our
food—it provides the crunch to celery and the crispness to
lettuce. We cannot digest cellulose, and most of it passes
through the alimentary canal largely unchanged. However,
cellulose provides bulk (also called F
ber or roughage) against
which the muscular wall of the digestive system can push,
facilitating the movement of food.
Hemicellulose, pectin,
and
lignin
are other plant carbohydrates that provide F
ber.
Carbohydrate Use
The monosaccharides absorbed from the digestive tract
include
fructose, galactose,
and
glucose.
Liver enzymes cata-
lyze reactions that convert fructose and galactose into glu-
cose
(f g. 18.2)
. Recall that glucose is the carbohydrate most
commonly oxidized in glycolysis for cellular fuel.
RECONNECT
To Chapter 4, Cellular Respiration, pages 120–124.
Some excess glucose is polymerized to form
glycogen
(glycogenesis), which the liver and muscles store as a glu-
cose reserve. Glycogen can be rapidly broken down to yield
glucose (glycogenolysis) when it is required to supply energy.
However, only a certain amount of glycogen can be stored.
Excess glucose beyond what is stored as glycogen usually
reacts to form fat and is stored in adipose tissue
(f g. 18.3)
.
Many cells can also oxidize fatty acids to obtain energy.
However, some cells, such as neurons, normally require con-
tinuous glucose for survival. (Under some conditions, such
FIGURE 18.3
Monosaccharides from foods are used for energy,
stored as glycogen, or reacted to produce fat.
C
OH
H
C
Fructose
H
CO
C
OH
H
C
OH
H
HO
H
CH
2
OH
C
OH
C
Galactose
H
C
H
HO
C
OH
H
OH
H
CH
2
OH
C
OH
C
Glucose
H
C
O
H
C
O
H
C
C
OH
H
HO
H
HO
H
CH
2
OH
FIGURE 18.2
Liver enzymes catalyze reactions that convert the
monosaccharides fructose and galactose into glucose.
CO
2
+
H
2
O
+
Energy
Glycogen
Fat
Starch
Sucrose
Maltose
Lactose
Carbohydrates
Digestion
Glucose
Glucose
and
Fructose
Glucose
Glucose
and
Galactose
Monosaccharides
(Respiration)
(Glycogenesis)
(Lipogenesis)
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