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What are Polysaccharides? How to Classify?Polysaccharides are carbohydrates formed by more than 9 monosaccharides linked by glycosidic bonds.

Polysaccharides Classification:When they are formed by the same kind of monosaccharides, they are called homo polysaccharides, likestarch, glycogen and cellulose, formed each of them by hundreds of molecules of glucose linked by glycosidiclinkages.

If the polysaccharides molecules are formed by different kinds of monosaccharides, they are considered heteropolysaccharides. Hyaluronic acid, formed by thousands of alternative units of N-acetyl glucosamine andglucuronic acid, is an example of heteropolysaccharide.

I. HOMOPOLYSACCHARIDES:

Cellulose

Cellulose is a linear polymer of D-glucose residues bonded by b(1, 4)-O-glycosidic linkages. It is the mostabundant carbohydrate in nature.

It is formed by glucose units, linked by Beta-1, 4 O-glycosidic linkages. We can say then that, if we consider thekind of linkage, the repeating unit in cellulose is cellobiose, the disaccharide formed by two molecules of glucoselinked by Beta-D-O glycosidic bonds, (that is why some text books say that the monomer in cellulose iscellobiose).

The long fibers of cellulose are held together by intermolecular hydrogen bonds. Hydrogen bonding continues inthe same plane with other chains as well as in planes above and below this plane to form strong, fibrous bundles.It made cellulose very appropriate for its structural function in plants

Since cellulose is formed by glucose molecules, it can be a source of energy for certain species. The lack inhuman beings of appropriate enzymes for digesting cellulose make this polysaccharide unsuitable for humannutrition (Have you though about how hunger in the world could disappear if we had enzymes for digestingcellulose?).

Starch:Starch is the second most abundant carbohydrate in nature.

The biological functions include, in plants, the main way of storage of sugar, and consequently, of energeticsources; in humans, the first supply of glucose on diet (Answer to C-O7)

Starch is not really a molecule, but a grain formed by two different kinds of molecules: Amylose and Amylopectin

Amylose:

Amylose is a linear molecule formed by glucose units linked by alpha-1, 4 O glycosidic linkages. Taking inaccount the kind of linkage we can say that the repeating unit in amylose is maltose. (It explains that some booksindicate that the monomeric unit in amylose is maltose).

Amylose molecule is helicoidal

Amylopectin

Amylopectin is the second type of molecule that forms starch. It is a branched molecule, formed also by glucose.Amylopectin contains D-glucose residues bonded together by a(1, 4)-O-glycosidic linkages with branchingthrough a(1 6)-O-glycosidic linkages.

The disaccharides that can be obtained from the digestion of amylopectin are maltose and isomaltose.

Amylopectin shows a branch each 24-30 units of glucose,

Glycogen:

The structure of glycogen is very similar to amylopectin but more branched, with one branch every 8 to 12 glucoseunit

Glycogen is the way in which glucose is stored in animals. Glycogen is stored mainly in liver (to release glucoseto blood when necessary) and in muscle, where it is used as a reserve of energy for muscular contraction(Answer to C-o8)

II. HETERO POLYSACCHARIDES:

Hetero-polysaccarides contain two or more different kind of monosaccharides. Usually they provide extracellularsupport for organisms of all kingdoms: the bacteria cell envelope, or the matrix that holds individual cells togetherin animal tissues, and provides protection, shape and support to cells, tissues and organs.

Hetero-polysaccharides provide extracellular support to very different organisms, from bacteria to humans;together with fibrous proteins, like collagen, elastin, fibronectin, laminin and others, heteropolysaccharides are themost important components of the extracellular matrix. Hyaluronic acid, condroitin sulfates and dermatan sulfatesare important heteropolysaccharides in the extracellular matrix. These heteropolysaccharides usually are formed

by the repetition of a disaccharide unit of an aminosugar and an acid sugar.

A typical example

Other common constituents are sulfate groups linked to certain monosaccharides. Usually heteropolysaccharidesare associated with proteins forming proteoglycans, glycosaminoglycans or mucopolysaccharides (since they areabundant in mucous secretions). As a group, they perform diverse functions: structural, water metabolismregulation (as a reservoir of water), cellular cement, biological sieve, biological lubricant, docking sites for growthfactors, among other functions.

Established specific functions of some glycosaminoglycans are:

Hyaluronic Acid (Hyaluronate): It is a lubricant in the synovial fluid of joints,

give consistency to vitreous humor, contributes to tensile strength and elasticity of cartilages and tendons(Answer to C-O6)

Chondroitin Sulfates: contributes to tensile strength and elasticity of cartilages, tendons, ligaments and walls ofaorta.

Dermatan sulfate (former chondroitin sulfate B) is found mainly in skin, but also is in vessels, heart, lungs. It maybe related to coagulation and vascular diseases and other conditions.

Keratan sulfate: Present in cornea, cartilage bone and a variety of other structures as nails and hair.

Heparin:

It is a potent natural anticoagulant produced in the Mast Cells that causes antithrombin bind to thrombin andproduce inhibition of blood coagulation.

Glycosaminoglycans are synthesized in the ER and Golgi. They are degraded by lysosomal hydrolases. Adeficiency of one of the hydrolases results in a mucopolysaccharidosis. These are hereditary disorders in whichglycosaminoglycans accumulate in tissues, causing symptoms such as skeletal and extracellular matrixdeformities, and mental retardation.

Examples of these genetic diseases are Hunter and Hurler syndromes.

These diseases, caused by different enzyme deficits, are characterized by physical deformities, mentalretardation and disturbances in the degradation of heparan sulfate and dermatan sulfate.

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What are Polysaccharides? How to Classify?Polysaccharides Classification:I. HOMOPOLYSACCHARIDES:Starch:II. HETERO POLYSACCHARIDES: