16.6 Polysaccharides 507
Hydrolysis
of glycosldic
bonds
Glycosidic bonds may be cleaved by hydrolysis reactions. We can take the
hydrolysis of an a(l-4) glycosidic bond between two hexosesas an example. For simplicity, only the carbon skeleton and the glycosidic bond are
shor,tm:
Fo,
Fo,
tl"t)+Ho
F",
l-o,
H*ti"*,,_(_)
The chemical hydrolysis of most complex sugarscan be done by heating an
aqueoussolution ofthe carbohydrate.Atrace ofacid is added as a catalyst.
Enzymes act as the catalyst in biological systems.Hydrolysis reactions will
be important aswe proceed into biochemistry, since they are the means by
which sugars, fats, and proteins are broken dornmto simple materials by
digestion.
'
'
PRACTICE
EXERCISE
I6.I I
The glycosidic bond in the following compound is hydrolyzed.\Mhat are
the structures of the products?
16.6Polysocchorides
AIM: To list the structures,sources,ond usesof the following
polysoccharides:storch,omylose,omylopectin,glycogen,
ond cellulose.
Focus
Polysaccharidesare composed
of many monosaccharide units
connected by glycosidic
linkages.
Individual sugar units may be connected to one another to form linear,
branched, or circular polymers, as shornmin Figure 16.8. Polysaccharides
haue many monosaccharidesbonded togetherto form a longpolymer chain.
The bonds connecting the sugar units are glycosidic.The l-4 and l-6
linkages are the ones most commonly found in natural polysaccharides
consistingof hexoses.
508
CHAPTER
t6 Carbohydrates
Figure16.8
Sugarunitsin a polysaccharide
canform linear,circular,
or
branchedstructures.
Starches are polryers conskting entirely of o-glucoseunits.They are the
major storage form of o-glucose in plants. T\nrostarches, amylose and amylopectin, are the most common. Amylose is composed of a [qgeI qaul
of o-glucopyranosemolecules linked a(I-->4) as sholvn in Figure 16.9.Complete amylose molecules may contain anywhere from a few to about 3000 oglucopyranose units. Alnyglgggg conslsts of chains of o-glucopyranose
molecules held together by 'a(,1.-+4)glycosidic linkages, but as showrrin Figure 16.10, amylopectin also contains a(l'6) cross-linking. Thepe crosslinks give 4mylopectin the appearance of a branched, bushy molecule.
There are uCually24to 30 o-glucopyranoseunits between the branch points
of amylopectin.
Glycogen-the main storageform of o-glucose in animal cells-has a
There are four human blood types,
A, B, AB, and O. Theseblood types
occur because of differences in
composition of relatively small
polysaccharide chains on the
surface ofred blood cells.
Linear chain
t.-6'
.cr-"r"N
Figure
15.9
Partial
structure
of the
starch
molecule
amylose.
,t)"t)"O"O"t)"O
a(I-->4) Linkage
A,-.rrVgeV4i ^
//-o
\
,/\
Figure16.10
Partialstructureof an
amylopectin
molecule.
n
A/
o \__J o
ct(lr4)
Linkage
16.6 Polysaccharides 509
Cellulose is a very important raw
material. The wood used in construction, the cotton used in textile production, and the
manufacture of paper and paper
products all have a large economic
impact.
structure similar to amylopectin. The main difference between
amylopectin and glycogen is that glycogen is even more highly branched_
there are onlyB to 12 n-glucopyrattose units between branih points.
Glycoq9" i! especially abundant in the liver, where it may amount io as much as
8% of the drymass of the organ.
Figure 16.11shows cellulose, the most important structural polysaccharide. celrulose is the material of plant ceil wails, and it is the major component inwood. cotton is about B0%pure cellulose.Like amylose, cellulose
is
a-linear polyrner consisting of r-glucose units. These o-glucose
ally from 300 to 15,000of them, arl connected by I__+4liniages that
""iir,
are "r"beta
rather than alpha. The shapesof amylose and cillulose moi6cules
are quite
different because of it-a(l-4)-linked
amylose tends to form loose spiral
structures (Fig. 16.12)and B(1-{)-linked cellulosetends to form striight
chains.
The difference in the shapesof amylose and cellulosehas a tremendous
L-o, J
f") n rt",,
{V-o-Vr,;L"
\\-o-l^ttFr
\o-l
Linearchain
/-",
/-",A /
|'/
tsor\ / ri -/
1-'$Yl
Y
fo:
{v
1-o, A
o-'\2r"$-o-
Figurel6.ll
Partialstructureof a cellulosemolecule.
\\,-o--}^r"e\
'o-/
\o-1
\/"^
ft"-,t)-,g}
\\-*--'itFr
'o-{/
\o-/
\/"
Figure16.12
Amyloseformsloosespiral
structures.
/
LP.'-''Linkage
'\___/
"-/
A
510
t6 Carbohydrates
CHAPTER
biological effect.The linear chains of cellulosepresenta uniform surfaceof
hydroxyl groups. These hydroxyl groups are involved in hydrogen bonding
between adjacent cellulose molecules; the large number of these weak
interactions give wood and cotton fibers their strength.A second effect of
the two different shapesis that en4lrnes capable of catalyzing the hydrol-vsis of starch are not capable of hydrolyzing cellulose. Human beings can
convert starch to its fuel form, n-glucose,but people lack enzyrnesto catalyze the hydrolysis of cellulose to glucose. Sawdust has little potential as
snack food for people. On the other hand, undigestible dietary carbohydratesdo seemto be important for human health as discussedin A Closer
Look: Dietary Fiber. The digestive systems of the cud-chewing animals
(ruminants) such as cows, sheep,and goats,as well as those of termites,
contain microorganisms whose enzyrnes catalyze the production of glucose from cellulose.These animals can thereforeuse celluloseas a nutritional source.
PRACTICE
EXERCISE
I6.T2
How does the structure of amylose differ from the structure of amylopectin?
PRACTICE
EXERCISE
I6.I5
What is the product of the complete hydrolysis of each of the following
polysaccharides?
(a) amylose
(b) amylopectin
(c) glycogen
(d) cellulose
i
Fiber
Have you hadyour fiber today?Many experts recommend that peopleeat high-fiberdiets in order
to prevent colon cancer.The nondigestiblecarbohydrates that we eat constitute dietary fibers.
Dietaryfibers consist of polysaccharidesthat cannot be hydrolyzgd to monosaccharides and therefore cannot be absorbed into the bloodstream:
Cellulose is insoluble fiber. Cellulose from vegetable leaves and stalks provides you with dietary
bulk and helps to pr"u".tt constipation.there arl
also soluble fibers. The soluble dietary fibers are
noncellulosic. Pectins from fruits, which are used
to thicken jams and jellies,are examplesof soluble
fibers. The pectins are polyhydroxy compounds
that have a carboxylic acid group at one end of the
molecule and an aldehyde group at the other. Vegi etable"gums" are also soluble fibers.
l_,,,,.
Fruitsand vegetables
arehigh in dieia-rv
fiber..
\.Vhich foods can we eat to ensure that we get
enough d.ietaryfiber?\Mheat,brown rice, and br-an
cereals are high in insoluble fiber. Oats, barley,
carrots, and fruits are high in soluble flber. Peas
and beans are a good source of both soluble and
insoluble fiber.