POLYSACCHARIDES

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P OLYSACCHARIDES

BY

DR. POULAMI ADHIKARY MUKHERJEE ASSISTANT PROFESSOR

DEPARTMENT OF ZOOLOGY NARAJOLE RAJ COLLEGE

ZOOLOGY: SEM- III, PAPER- C7T: FUNDAMENTALS OF BIOCHEMISTRY, UNIT 1: CARBOHYDRATES

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P OLYSACCHARIDES

 The term polysaccharide etymologically means multi saccharides.

 A saccharide refers to the unit structure of carbohydrates.

 Thus, a polysaccharide is a carbohydrate comprised of many saccharides, i.e. more than ten (mono)saccharide units.

 Polysaccharides are complex biomacromolecules that are made up chains of monosaccharides.

 When carbohydrates are made up of 10 or more monosaccharide units are called as polysaccharides. They are also known as glycans.

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P OLYSACCHARIDES

 The polysaccharides (or glycans) are composed of long chains of monosaccharides.

 The bonds that form these chains of monosaccharides are called glycosidic bonds.

 Thus, the polysaccharides are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages.

 The most commonly found monomer units in polysaccharides are glucose, fructose, mannose and galactose which are simple sugars.

 The polysaccharides can react with water (hydrolysis) using amylase enzymes at catalyst, which produces constituent sugars (monosaccharides, or oligosaccharides).

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C HARACTERS OF P OLYSACCHARIDES

 Polysaccharides are characterized by the following chemical properties:

(1) not sweet in taste,

(2) many of which are insoluble in water, (3) hydrophobic in nature,

(4) do not form crystals on desiccation,

(5) compact and not osmotically active inside the cells,

(6) high molecular weight carbohydrates

(5) can be extracted to form white powder, and (6) general chemical formula of C

_x

(H

₂

O)y.

 Polysaccharides consist of hydrogen, carbon, and oxygen, just as the other forms of carbohydrates., the ratio of hydrogen atoms to oxygen atoms being 2:1, that is why they are also described as hydrates of carbon.

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C HARACTERS OF P OLYSACCHARIDES

 The general chemical formula of polysaccharides is (C

₆

H

₁₀

O

₅

)n.

 They are considered as organic compounds similar to other carbohydrates due to the presence of carbon and C-C and C-H covalent bonds.

 Their structure ranges from a simple linear to the more complex, highly branched forms.

 Many of them are heterogenous. Depending on

their composition, they may

be amorphous or water-insoluble.

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C HARACTERS OF P OLYSACCHARIDES

 Polysaccharides differ

from oligosaccharides and disaccharides based on how many monosaccharide units are present.

 Disaccharides are made up of only two monosaccharides.

 Oligosaccharides have more than two i.e. 3 or more monosaccharides.

 The term oligosaccharide is commonly used to describe relatively shorter chains than polysaccharides.

 Polysaccharides are a type of biological macromolecule comprised of multiple monosaccharide units.

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S TRUCTURE OF P OLYSACCHARIDES

 All polysaccharides are formed by the same basic process: monosaccharides are connected via glycosidic bonds.

 When in a polysaccharide, individual monosaccharides are known as residues.

 The structure of the molecules being combined determines the structures and properties of the resulting polysaccharide.

 The complex interaction between their hydroxyl groups (OH), other side groups, the configurations of the molecules, and the enzymes involved all affect the resulting polysaccharide produced.

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 A polysaccharide used for energy storage will give easy access to the monosaccharides, while maintaining a compact structure.

 A polysaccharide used for support is usually assembled as a long chain of monosaccharides, which acts as a fiber. Many fibers together produce hydrogen bonds between fibers that strengthen the overall structure of the material.

 The glycosidic bonds between monosaccharides consist of an oxygen molecule bridging two carbon rings.

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 The bond is formed when a Hydroxyl group is lost from the carbon of one molecule, while the hydrogen is lost by the hydroxyl group of another monosaccharide.

 The carbon on the first molecule will substitute the oxygen from the second molecule as its own, and glycosidic bond is formed.

 Because two molecules of hydrogen and one oxygen is expelled, the reaction produced a water molecule as well. This type of reaction is called a dehydration reaction as water is removed from the reactants.

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ZOOLOGY: SEM- III, PAPER- C7T: FUNDAMENTALS OF BIOCHEMISTRY, UNIT 1: CARBOHYDRATES

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C LASSIFICATION OF P OLYSACCHARIDES

The polysaccharides are classified into:

POLYSACCHARIDES

HOMOPOLYSACCHARIDES HETEROPOLYSACCHARIDES

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1. Homopolysaccharides:

Those polysaccharides which contain only one kind of

monosaccharide unit are called

homopolysaccharides. e.g., starch, glycogen, cellulose, dextran, inulin, agar, chitin, etc.

2. Heteropolysaccharides:

Polysaccharides made up of two or more kinds of monosaccharide units, e.g., pectin’s and mucopolysaccharides.

Most of them are branched and exist in conjugation

with proteins and hence called proteoglycans. The

carbohydrate part is called glycosaminoglycan. They

have a repeating disaccharide unit which is acetylated

or sulphated.

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Examples of Homopolysaccharides:

Struture of starch:

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1. Starch:

It is the storage polysaccharide found in plant cells and exists in two forms: amylose is the helical form of starch comprised only of alpha-1,4 linkages and amylopectin that has a structure like glycogen except that the branched alpha-1,6 linkages are present on only about one in 30 monomers.

2. Glycogen:

This polysaccharide is the polysaccharide found in

animals to store energy and is composed of alpha-1,4-

glycosidic bonds with branched alpha-1,6 bonds

present at about every tenth monomer. It is mainly

produced by the liver and muscles, but it can also be

made during a process called glycogenesis.

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ZOOLOGY: SEM- III, PAPER- C7T: FUNDAMENTALS OF BIOCHEMISTRY, UNIT 1: CARBOHYDRATES

It is a structural polysaccharide that is found in the cell wall of plants and when consumed, it acts as a dietary fibre. Cellulose is said to be the most abundant organic molecule on earth. Wood, paper, and cotton are common forms of cellulose.

4. Dextran:

It is produced by yeasts and bacteria. It is made up of α-D-glucose linked by α-1 → 6 glycosidic linkages. The branching points are at 1-2, 1-3 and 1-4. It absorbs water to form gels. It is used as plasma substitute.

5. Inulin:

It is a fructosan. It cannot be metabolized by the body,

hence used to assay glomerular filtration rate (G.F.R.)

in the study of kidney function.

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It is a sulphated galactose. It dissolves in hot water. It gels on cooling, thereby forming a solidified medium in tissue culture studies.

7. Chitin:

N-acetylglucosamine (chitosamine) linked by β-1

→ 4 linkages. Present in the exoskeleton of

invertebrates like cockroach and crab.

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Heteropolysaccharides:

Most of them are branched and exist in conjugation with proteins and hence called proteoglycans. The carbohydrate part is called glycosaminoglycan. They have a repeating disaccharide unit which is acetylated or sulphated.

There are two major types of heteropolysaccharides:

(a) Pectins:

They are composed of galacturonic acid, galactose and arabinose.

(b) Mucopolysaccharides:

They are sticky polysaccharides (mucin like).

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These are found in different structural and functional roles in the human body.

Hyaluronic Acid: Acts as a lubricant in the synovial fluid of joints

Chondroitin Sulfate: It contributes to tensile strength and elasticity of cartilages, tendons, ligaments, and walls of the aorta.

Dermatan sulfate: It is found mainly in the skin, and also is in vessels, heart, lungs. It may be related to coagulation and vascular diseases and other conditions.

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

Heparin: Is present as an anticoagulant in the blood.

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ZOOLOGY: SEM- III, PAPER- C7T: FUNDAMENTALS OF BIOCHEMISTRY, UNIT 1: CARBOHYDRATES

and their composition is given below:

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The polysaccharides are also classified as:

POLYSACCHARIDES

STRUCTURAL

POLYSACCHARIDES NUTRIENT

POLYSACCHARIDES

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 Polysaccharides, just as the other carbohydrates, are a major source of energy.

 Hence they are also one of the main dietary components.

 Animals consume the polysaccharides to obtain monosaccharides that they can use to synthesize ATP.

 ATPs are chemical energy biologically synthesized through aerobic and anaerobic respirations.

B IOLOGICAL I MPORTANCE OF P OLYSACCHARIDES :

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ZOOLOGY: SEM- III, PAPER- C7T: FUNDAMENTALS OF BIOCHEMISTRY, UNIT 1: CARBOHYDRATES

 Plants store excess glucose in the form of starch.

 Thus, there are plants that are harvested to use the starch for food preparation and industrial purposes.

 Polysaccharides are also essential in living organisms as they serve as structural component of biological structures, such as cellulose and chitin.

 Plant cellulose is harvested for its multifarious uses in the industry.

B IOLOGICAL I MPORTANCE OF P OLYSACCHARIDES :

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Polysaccharides form a crucial part of cell function and structure:

1. Storage polysaccharides: Polysaccharides such as starch and glycogen are called storage polysaccharides because they are stored in the liver and muscles to be converted to energy later for body functions. Starch is found in plants whereas glycogen is found in animals.

2. Structural polysaccharides: Polysaccharides such as cellulose are structural polysaccharides which are found in the cell walls of plants.

Another structural polysaccharide is chitin.

F UNCTIONS OF P OLYSACCHARIDES :

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