HOMOPOLYSACCHARIDES - EASY NOTES

Definition -  

Made of only one type of monosaccharide unit

E.g. : Starch , Glycogen, Cellulose, Dextrin, Inulin and Chitin

1.Starch:

--Storage form of glucose in plants, e.g. in potato, in grains and seeds etc.

--Mainly composed of two constituents viz. Amylose (20%) and Amylopectin (80%)

--Amylose is a linear polymer of d-glucose units joined by α-1 → 4 glycosidic linkages

--Amylopectin - branch polymer of D-glucose units joined by  α-1 → 4 glycosidic linkages in straight chain and  α-1 → 6 glycosidic linkages in branched chain

Digestion of Starch

--Starch is digested by α amylase produced by salivary glands.

--α amylase (endoglycosidase) acts on α 1 – 4 linkage to produce maltose, isomaltose , and dextrin’s.

--β amylase (exoglycosidase) acts on α 1 – 4 linkage to produce glucose, maltose and limit dextrin’s.

--β amylase is present only in plants not present in humans.

--Dextrin’s occur in honey.

--All dextrin’s have few free aldehyde groups and can show mild reducing property.

Dextrin’s

--Incompletely digested products of starch and glycogen

--They are ultimately completely digested to maltose and glucose

2.Glycogen

--Major storage form of carbohydrate in animals

--also called as animal starch

--found mostly in liver & muscle

structure of glycogen is similar to that of amylopectin of starch but glycogen is highly branched when compared to amylopectin

--It has a branch after every 6-7 glucose residues and each chain has about 3-4 residues

Functions of glycogen

--The function of muscle glycogen is to act as a readily available source of glucose for energy within muscle itself.

--Liver glycogen is concerned with storage and maintenance of the blood glucose

3.Cellulose

--Most abundant carbohydrate found in plants.

--It is an unbranched polymer of glucose and consists of long straight chains which are linked by β-(1→4) glycosidic linkages and not α-(1→ 4) as in amylose of starch

--It has no calorific value in humans as it is not digested because of the absence of the enzyme cellulase

----But ruminant animals have bacteria that produce cellulase which digests the cellulose

----Importance of Cellulose

----For human cellulose has nutritional significance.

----Cellulose is a component of fiber in the diet.

----Cellulose is present in unrefined cereals. It increases bulk of stool, aids intestinal motility, acts as a stool softener and prevents constipation.

    4.Inulin

----Polymer of D-Fructose (Fructosans) linked together by β-(1 → 2) glycosidic linkage.

----It occurs in the tubers of some plants, e.g. chicory, bulb of onion and garlic.

----Inulin is hydrolysed by inulinase, which is not present in the humans and so it is not utilized as food

----Clinical Importance of Inulin

----Inulin has clinical importance as it is used in the studies of glomerular filtration rates (kidney function test).

    5.Chitin

----Found in protective outer skin (exoskeleton) of insects and crustaceans

----It is the 2^nd most abundant polysaccharide in nature after cellulose

----Made of N acetyl glucosamine monomers joined by β 1 – 4 linkages.