HOMOPOLYSACCHARIDE
DEFINTION
Homopolysaccharides are composed of single kind of monosaccharides.
Example: Starch, glycogen and cellulose.
Homopolysaccharides are also called as Homoglycans.
STARCH
It is the reserve carbohydrate of plant kingdom. Sources of
starch are potatoes, tapioca, cereals (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 400,000D or more.
The insoluble part absorbs water and forms paste like gel; this is
amylopectin. Amylopectin is also made up of glucose units, but
is highly branched with molecular weight more that 1 million. The
branching points are made by alpha-1,6 glycosidic linkages 9
(similar to isomaltose)
HYDROLYSIS OF STARCH
Starch will form a blue colored complex with iodine; this color disappears on heating and reappears when cooked.
This is a sensitive test for starch. Starch is nonreducing because the free sugar groups are negligible in number.
Hydrolysis
In short time gives
Amylodextrin violet color with iodine and is nonreducing
Further hydrolysis gives Red color with iodine and mild reduction
Erythrodextrin
of Benedicts solution
Further hydrolysis gives
Archodextrins No color with iodine but reducing
Further hydrolysis gives
Maltose No color on iodine, but powerfully reducing
Action of Amylase on Starch
salivary amylase and pancreatic amylase are alpha-
amylase,which act at random on alpha-1,4glycosidic bonds
to split starch into smaller units (dextrins) and finally to
alpha-maltose
Beta-amylase are of plant origin which split starch to
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. Beta amylase is important in plants in ripening
of fruites and germination of seeds.
Glycogen
it is the reserve carbohydrate in animals. It is stored in liver and
muscles.
About 5% of weight of liver is made up of glycogen. Excessive
carbohydrates are decomposed as glycogen.
glycogen is composed of glucose units joined by alpha-1,4 links in the
straight chains. It also has alpha-1,6 glycosidic linkages at the
branching points .
Molecular weight of glycogen is 5million.
Innermost core of glycogen consists a primer protein , Glycogenin.
Glycogen is more branched and more compact than amylopectin.
One glycogen molecule contains about 50,000 units of glucose.
Cellulose
it is the supporting tissue of plants .
Cellulose constitutes 99% of cotton, 50% of wood and is the most
abundant organic material in nature.
It is made up of glucose units combined with beta-1,4 linkage. It has a
straight line structure, with no branching points.
Molecular weight is in the order of 2 to 5 million.
cellulose has a variety of commercial applications, as it is the starting
material to produce fibers, celluloids, nitrocellulose, and plastics.
Beta-1,4 bridges are hydrolyzed by enzyme cellobiase. But the
enzyme is absent in animals and human digestive system, and hence
cellulose cannot be digested.
Herbivorous and animals have large cecum, which harbor bacteria.
These bacteria can hydrolyze cellulose, and the glucose produced is
utilized by the animals.
Inulin Dextrans Chitin
It is a long chain homoglycan These are highly branched It is present in exoskeletons
composed of D-fructose units homopolymers of glucose units of crustacea and insects.
with repeating beta-1,2 linkages. with 1-6,1-4 and 1-3 linkages. It is composed of units of
it is reserved carbohydrates They are produced by N-acetyl-glucosamine with
present in various bulbs and microorganisms. They have beta-1,4 glycosidic linkage.
tubers such as chicory, dahlia, molecular weight 1 million to 4
dandelion, onion, garlic. millions.
since they will not easily go out of
It was previously used to assess
vascular compartment, they are
renal clearance value and
used for intravenous infusion as
glumerular filtration rate in
plasma volume expanders for
patients.
treatment of hypovolemic shock
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