Polysaccharides

Polysaccharides 

These are carbohydrates formed of more than 10 sugar units . They are classified into : homopolysaccharides  and heteropolysaccharides .

Homopolysaccharides: 

They contain repeated same sugar units and include :

1. Starch 
2. Glycogen 
3. Cellulose
4. Inulin 
5. Chitin 
6. Dextrans

Starch :

• Structure :  Starch  granule is formed of inner (a) and outher (b) layers:

1. Inner layer:called amylase.It constitutes 15-20% of the granule and formed of non- branching helical Structure of glucose units linked together by a 1-4 glycosidic bond
2. Outer layer: called amylopectin .It constitutes 80- 85% of the granule and formed of branched chain.

• Each chain is composed  of 24-30 glucose units linked together by a 1-4 glycosidic bond and a 1-6 glycosidic bond at the branching points.
• Sources:  It is the most important food source of carbohydrate ,it is found cereals, potatoes,legumes and other vegetables.
• proprties :

1. Starch gives blue color with iodine.
2. Partial hydrolysis (digestion)by amylase enzyme gives various forms of dextrins

Glycogen :  (also called animal starch):

• Structure : It is highly branched chain homopolysaccharide. Each branch is composed of 12-14 glucose units ,linked together by a 1-4 glycosidic bonds and by a 1-6 glycosidic bond at branch point (like amylopectin) b.Sources:  Glycogen is the storage  form of carbohydrate in human and animals.It is synthesized and stored in liver , muscle and other tissues.
• proprties : It is gives reddish violet color with iodine .

Cellulose:

• Structure : It is long straight nonbranching chains of glucose units (b-D- glucopyranose)linked together by b 1-4 glycosidic bond.The chains are strengthening by cross linked hydrogen bonds .
• Sources:  Cellulose is the chief constituent of the framework of plant leafy vegetables, fruits ,wood,cotton,etc
• proprties :

1. Cellulose gives no color with iodine
2. Cellulose is insoluble in water
3. Many mammals including humans cannot digest cellulose of diet because of the absence of digestive hydrolase enzyme that attacks b- linkage
4. However the present of cellulose in diet is important because it increase the bulk of stool.This stimulates intestinal movement and prevents constipation
5. cellulose can be utilized and serve as a source of energy in herbivores because their gut contains bacterial enzyme than can attack b- linkage

Inulin :

• Structure : It is fructosan i.e.formed of repeated units of fructose linked together by b 1-2 bonds.

• Sources: Root of  plants
• proprties : Soluble in warm water
• Medical importance : Inulin clearance is one of diagnostic tests for investigation of glomerular filtration rate

Chitin :

• Structure : It is a polymer of N-acetylglucosamine liked together by b 1-4 glycosidic bonds
• Sources : It is important polysaccharide of invertebrates

Dextrans :

• Structure : It is branched chain homopolysaccharide .Each branch is composed of glucose units linked together by a 1-3 glycosidic bonds anda 1-6 glycosidic bond at branching point.
• Sources:   dextran is synthesized from sucrose by certain bacteria .
• Functions : dextran is used as plasma substitute and prevents thrombosis.
• proprties : dextran is highly vicous and slowly degraded.

Galactosaans (agar- agar)

• They are condensation of galactose units present in agar (algae)  .
• They are used to make cuture media .They are used medically to prevent constipation .

Heteropolysaccharides :

They contain repeated different sugar units and include glycosaminoglycans proteoglcans and glycoproteins.