Carbohydrates • Figure 2.10
Carbohydrate monosaccharides, like glucose, galactose, and fructose, combine through
dehydration reactions to form disaccharides, like sucrose and lactose. Many monosaccha-
rides can combine to form polysaccharides like glycogen or cellulose.
In a dehydration synthesis
reaction, glucose and fructose
(both monosaccharides)
combine to form sucrose
(disaccharide) plus water. In
the opposite reaction
(hydrolysis), sucrose
combines with water and
splits into glucose and
fructose.
If many glucose molecules
are combined in long chains,
you get polysaccharides like
glycogen or cellulose.
a.
Dehydration syn th e sis and h yd ro lysis of su cro se
HO
r
O H
HOC
H2
O H
\ OH H
ЮНІ
H
T
c
OH
OH
Dehydration
H
synthesis
Hydrolysis
ho
O H
HOCH2
O
H
N
H
К
^ 4
\OH
HZ
\
h
hoA
— T - ^
l
T
O
OH
OH
H
HOC
h2OH+
CH2OH CH2OH
2
H H H H Water
Glucose
Fructose
(CeH12Oe)
(CeH12Oe)
Sucrose
(C12H22O11)
b. Lacto se
HOCH2 HOCH2
Galactose and
glucose combine
to form the
e. Ceiiuiose
2.
Disaccharides (di-SAK-a-rids;
di-
= two), or double
sugars, consist of two monosaccharides joined by a
covalent bond. When two monosaccharides combine
to
form
a
disaccharide,
a
molecule
of water
is
formed and removed in a reaction called dehydration
synthesis (de- = from, down, or out;
hydra- =
water).
Such reactions are common during synthesis of large
molecules. For example, glucose and fructose combine
via dehydration synthesis
to form sucrose
(table
sugar) (Figure 2.10a). Conversely, disaccharides can
be split into monosaccharides by adding a molecule
of water; you learned about this type of reaction,
called a hydrolysis reaction, earlier in the chapter. For
example, sucrose can be hydrolyzed into glucose and
fructose (Figure 2.10). Other disaccharides include
maltose, or malt sugar
(glucose
+
glucose), and
lactose, or milk sugar (glucose + galactose).
3.
Polysaccharides
(pol'-e-SAK-a-nds;
poly-
=
many)
are large, complex carbohydrates that contain tens
or
hundreds
of
monosaccharides
joined
through
dehydration synthesis reactions.
Like
disaccharides,
polysaccharides
can
be
broken down into monosaccharides through hydrolysis
reactions. The main polysaccharide in the human
body is
glycogen
(Figure 2.10d), which is made entirely
of glucose units joined together in branching chains.
Glycogen is stored in liver cells and skeletal muscle
cells. Glycogen is broken down when glucose is needed
Life Uses Important Chemicals 35
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