28
Chapter 2
It is the presence of numerous hydroxyl groups that makes
carbohydrates readily soluble in water.
Most carbohydrates taste sweet, particularly the carbo-
hydrates known as sugars. The simplest sugars are the
mono-
mers called
monosaccharides,
the most abundant of which is
glucose,
a six-carbon molecule (C
6
H
12
O
6
). Glucose is often
called “blood sugar” because it is the major monosaccharide
found in the blood.
Two ways to represent the bonds between the atoms of
a monosaccharide are illustrated in
Figure 2–7
. The fi rst is
the conventional way of drawing the structure of organic mol-
ecules, but the second gives a better representation of their
three-dimensional shape. Five carbon atoms and an oxygen
atom form a ring that lies in an essentially fl at plane. The
hydrogen and hydroxyl groups on each carbon lie above and
below the plane of this ring. If one of the hydroxyl groups
below the ring is shifted to a position above the ring, as shown
in
Figure 2–8
, a different monosaccharide is produced.
Most monosaccharides in the body contain fi ve or six
carbon atoms and are called
pentoses
and
hexoses,
respec-
tively. Larger carbohydrates can be formed by joining a num-
ber of monosaccharides together. Carbohydrates composed of
two monosaccharides are known as
disaccharides. Sucrose,
or table sugar, is composed of two monosaccharides, glucose
and fructose (
Figure 2–9
). The linking together of most
monosaccharides involves a dehydration reaction in which
a hydroxyl group is removed from one monosaccharide and
a hydrogen atom is removed from the other, giving rise to
a molecule of water and bonding the two sugars together
through an oxygen atom. Conversely, hydrolysis of the disac-
charide breaks this linkage by adding back the water and thus
uncoupling the two monosaccharides. Other disaccharides
frequently encountered are maltose (glucose-glucose), formed
during the digestion of large carbohydrates in the intestinal
tract, and lactose (glucose-galactose), present in milk.
When many monosaccharides are linked together to
form polymers, the molecules are known as
polysaccharides.
Starch, found in plant cells, and
glycogen,
present in animal
cells and often called “animal starch,” are examples of poly-
saccharides (
Figure 2–10
). Both of these polysaccharides are
composed of thousands of glucose molecules linked together
in long chains, differing only in the degree of branching along
the chain. Glycogen exists in the body as a reservoir of avail-
able fuel. Hydrolysis of glycogen, as occurs during periods of
CH
2
OH
C
OH
O
H
C
OH
C
H
H
H
OH
H
C
HO
OH
C
H
CO
C
C
C
C
C
H
H
H
H
H
H
OH
OH
OH
OH
Glucose
Figure 2–7
Two ways of diagramming the structure of the monosaccharide
glucose.
C
Glucose
Galactose
OH
OH
CH
2
OH
C
OH
H
C
OH
C
H
H
OH
H
OH
C
H
CO
CH
2
OH
C
OH
H
C
OH
C
H
H
H
OH
C
H
CO
Figure 2–8
The structural difference between the monosaccharides glucose and
galactose is based on whether the hydroxyl group at the position
indicated lies below or above the plane of the ring.
OH
C
C
C
OH
C
O
H
H
OH
Dehydration
H
C
C
C
OH
O
H
H
OH
H
C
Glucose
Fructose
Sucrose
Water
+
CH
2
OH
CH
2
OH
CH
2
OH
CH
2
OH
CH
2
OH
CH
2
OH
C
OH
H
C
OH
C
H
H
OH
H
OH
C
H
CO
C
H
C
OH
C
H
H
OH
H
OH
C
H
CO
H
2
O
O
+
+
+
Figure 2–9
Sucrose (table sugar) is a disaccharide formed when two monosaccharides, glucose and fructose, bond together through a dehydration reaction.
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