INTRODUCTION:
These are polymerized products of many monosaccharide units.
They may be;
1)
Homoglycans, which are composed of single kind of monosaccharides
such as, starch, glycogen and cellulose.
2.
Heteroglycans, which are composed of two or more different
monosaccharides, hyaluronic acid, chondroitin sulphate.
STARCH:
It is the reserve carbohydrate of plant kingdom. Common sources
include; Potatoes, tapioca, cereals such as rice, wheat, and other
food grains. Starch is composed of amylose and amylopectin. When
starch is treated with boiling water, 10-20% is solubilized; this
part is called amylose. Amylose is made up of glucose units with
alpha-1,4 glycosidic linkages to form an unbranched long chain with a
molecular weight 400,000 D or more.
The
insoluble part absorbs water and forms paste like gel and this is
called amylopectin. Amylopectin is also made up of glucose units, but
is highly branched with molecular weight more than 1 million. The
branching points are made by alpha-1,6 linkage.
HYDROLYSIS
OF STARCH: Starch will form a blue colored complex with iodine;
this color disappears on heating and reappears when cooled. This is a
sensitive test for starch. Starch is non reducing because the free
sugar groups are negligible in number. When starch is hydrolysed by
mild acid, smaller and smaller fragments are produced. Thus
hydrolysis for a short time produces amylodextrin which gives violet
color with iodine and is non-reducing. Further hydrolysis produces
erythrodextrin which gives red color with iodine and mild reduction
of Benedict's solution. Later achrodextrins with no color with
iodine, but reducing, and further on, maltose with no color with
iodine, but powerfully reducing, are formed on continued hydrolysis.
ACTION
OF AMYLASES ON STARCH: Salivary amylase and pancreatic amylase
are alpha-amylases, which act at random on alpha 1,4-glycosidic bonds
to split starch into smaller units also known as dextrins, and
finally to alpha-maltose.
Beta-amylases
are of plant origin including almond, germinating seeds, which split
starch to form beta-maltose. They act on amylose to split maltose
units consecutively. Thus the enzyme starts its action from one end.
When beta-amylase acts on amylopectin, maltose units are liberated
from the ends of the branches of amylopectin, until the action of
enzyme is blocked at the 1,6-glycosidic linkage. The action of
beta-amylase stops at branching points, leaving a large molecule,
called limit dextrin or residual dextrin.
DEXTRANS:
These are highly branched homopolymers of glucose units with
1-6, 1-4 and 1-3 linkages. They are produced by microorganisms. They
have molecular weight 1 million to 4 millions. Since they will not
easily go out of vascular compartment, they are used for intravenous
infusion as plasma volume expander for treatment of hypovolemic
shock.
CHITIN:
It is present in exoskeletons of crustacea and insects. It is
composed of units of N-acetylglucosamine with beta-1,4 glycosidic
linkages.
HETEROGLYCANS:
These are polysaccharides containing more than one type of sugar
residues. Examples are: Agar which is prepared from sea weeds. It
contains galactose, glucose and other sugars. It is dissolved in
water at 100° C, which upon cooling sets into a gel. Agar cannot be
digested by bacteria and hence used widely as a supporting agent to
culture bacterial colonies. Agar is used as a supporting medium for
immuno-diffusion and immuno-electrophoresis.
Agarose
is made up of galactose combined with 3,6-anhydrogalactose units; it
is used as matrix for electrophoresis.
RELATED;
1.
STARCH
2.
GLYCOGEN
3.
CELLULOSE
4.
THE GLYCOSIDIC BOND
5.
PLASMA VOLUME EXPANDERS
REFERENCES
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