Polysaccharides



 What are polysaccharides?

Polysaccharides:

The acid insoluble pellet also has polysaccharides ( carbohydrates) as another class of  macromolecules. Polysaccharides are long chains of sugars. They are threads ( literally a cotton thread) containing different monosaccharides as building blocks. For example, cellulose  is a polymeric polysaccharide consisting of only one type of monosaccharide i.e., glucose. Cellulose is a homopolymer. Starch is  variant of this but present as a store house of energy in plant tissues. Animals have another vaeiant called glycogen. Insulin is a polymer of fructose. In a polysaccharide chain ( say glycogen), the right end is called the reducing end and the left end is called the non__ reducing end. It has branches as shown form of a cartoon. Starch forms helical secondary structures. In fact, starch can hold I² molecules in the helical portion. The starch _ I ² is blue in colour .Cellulose does not contain complex helices and hence cannot hold I ².



Plant cell walls  are made of cellulose. Paper made from plant pulp and cotton fibre is cellulosic. There are more complex polysaccharides in nature. They have as building blocks, amino__ sugars and chemically modified sugars ( e.g., glucosamine,N__ acetyl galactosamine, etc.). Exoskeletons of arthropods, for example, have a complex polysaccharides called chitin. These complex polysaccharides are mostly homopolymers.


 

Composition:

The polysaccharides consist of more than six ( often numerous) molecules of a monosaccharide ( usually glucose). The latter are joined together by glycosidic bonds with a loss of  water each time a monosaccharide molecule is added. The number of constituent monosaccharide molecules is not known. The general formula for polysaccharides is C6H10O5)n where n stands for the unknown number of monosaccharide molecules. It is derived as under__ 



The polysaccharides are linear, branched or unbranched molecular chains.


Structure classification :

There are two important classes of polysaccharides regarding their composition: homopolysaccharides or homoglycans and heteroplysaccharides or heteroglycans.

 1): Homopolysaccharides:


These are composed of normal monosaccharide molecules of a single type. They include three biologically important substances: glycogen, starch and cellulose. All these are made up of glucose units. These differ in the arrangement of bonds between glucose units, in the branching pattern of the polymer, and in the total number of glucose units per chain.


  ● Nomenclature: Homopolysaccharides are named after their monosaccharide units.  For instance, glucan composed of glucose units, fructan formed of fructose units, galactan made of galactose units, xylan formed of xylose units, etc.


 (i) Glycogen:  It is the main reserve food material of animal cells. It is known as the animal starch. Its molecules is a long, much branched chain of about 30,000 glucose units. The successive glucose units are joined together by 1__ 4 œ__ glycosidic bonds¹ and branches are formed by 1__ 6 glycosidic bonds. Its molecular weight is about 4,000,000.Glycogen gives red colour with iodine solution. It is usually formed and stored in muscle cells and liver cells. These cells prepare it from glucose drawn from the blood stream. Formation of glycogen from glucose is called glycogenesis. Glycogen forms groups of tiny ellipsoidal, flattened granules in the cells usually associated with the smooth  endoplasmicreticulum. Glycogen readily breaks down into glucose units by hydrolysis when needed. Hydrolysis of glycogen to glucose is termed gkycogenolysis. Glucose, thus formed in the liver cells, is delivered by the  blood to the tissues for use in respiration in them. The glucose formed by hydrolysis of glycogen in the muscle cells is used in these cells only and not released into the blood. The hydrolysis of glycogen is brought about by enzymes called amylases.


   


 
Many bacteria and fungi also store glycogen. Liver of an adult human may store up to 0.91 kg. of glycogen. This glycogen can change to glucose that can last for about a day only.  

 (ii) Starch:   It is the reserve food material of plant cells. It  is also named amylum ( G.amylon = starch).It is found in abundance in the seeds ( legumes), grains ( fruits of rice, wheat, maize and other grasses), tubers ( potatoes), tapioca¹ and fruits ( bananas). Starch is formed as an end product of photosynthesis. Animals get it from the plants. Starch molecules accumulate to form starch grains. The starch grains are stored in chloroplasts or in special leucoplasts  called amyloplasts.The starch grains are of two kinds: simple that occur singly, and compound which occur in groups. A starch grain consists of several layers, the shells, arranged in concentric or eccentric manner around a point, the hilum.The starch grains from a particular source have a characteristic shape and can be identified by examination under the light microscope. Starch gives blue colour with iodine solution. It is also hydrolysed by amalyses.



When potatoes are boiled, water becomes cloudy because amylose dissolves in it. Amylopectin remains in the potatoes.

  Starch consist of two components: amylose and amylopectin, both glucose polymers. Amylose is more soluble in water than amylopectin.Generally, amylose forms only 20 to 30% of starch, the rest is amylopectin. Amylose molecule is an unbranched but spirally coiled chain of about 200 to 2,000 glucose units. Amylopectin molecule is a much branched chain of about 2,000 to 200,000 glucose units.The successive glycose units are joined by 1___ 4œ__ glycosidic bonds. Branches are formed by 1,6 glycosidic bonds.





 (iii) Cellulose ( Cellulin) : 
It is the most abundant  carbohydrate, in fact the most abundant organic compounds in nature. It is the main structural polysaccharide of the plants, certain fungi and some protists. It is specially abundant in wood ( 25__ 50%) and cotton fibres (90%). Cellulose molecule is a long, of about 6,000 glucose units, with molecular weight between 0.5 to 2.5 million. Several molecules ( glucose polymers) are held together by hydrogen bonds to form cellulose microfibrils, which are arranged in larger bundles, the macrofibrils. The latter have a great tensile strength. The cell wall contains many layers of cellulose fibrils that are often reinforced with other substances. Cellulose gives no colour with iodine solution. It is not broken down to glucose by animals and is, therefore, not useful as a food source.It simpley provides bulk, often called " fibre" or " roughage" necessary for proper functioning of the alimentary canal. Along the way, the cellulose fibres abrade the cells of the gut epithelium, stimulating them to secrete mucus that facilitates passage of food through the digestive tract.Thus, the fibre rich foods, such as bran, hasten the movement of faeces through the intestines. This results in rapid elimination of toxins from the body, and it may even prevent disease by reducing the time the pathogens stay in the digestive tract.




   Some animals can hydrolyse cellulose. These include farm animals and termites. The farm animals hydrolyse cellulose with the help of intestinal bacteria. The termites hydrolyse cellulose with the aid of intestinal flagellates. These symbiotic organisms produce an   enzyme cellulase for digesting cellulose. Snails also produce cellulase to digest cellulose. Certain fungi also can digest cellulose, thereby serving as decomposers, to recycle soil's chemicals. 


Cellulose differs from starch in having ß_ glucoside linkages that occur in  starch. 

The tunic of ascidians contains layers of supporting fibres of a kind of  cellulose called tunicin, or animal cellulose. 

About 100 billion tons of cellulose is prepared per year by the plants of the world.

● Uses of Cellulose: Cellulose is useful in many ways__ 

(i) Structural Compound : Cellulose is the main component of the cell walls in plants, most algae, certain fungi and some protists.

  (ii) Food Material.  Some animals, such as cattle, buffaloes and termites, use cellulose as food because they can digest it. Cellulose is an important food for bacteria and fungi also.

(iii) Roughage:  Cellulose serves as rough roughage in the human digestive tract.

(iv) Commercial Utility:   Cellulose provides many commercially useful materials. For example, cotton ,linen, jute, rayon, cellulose acetate ( used in fabrics, plastics, cigarette filters and shatterproof glass),  cellulose nitrate ( used in propellant explosives), cellophane and carboxymethyl cellulose ( added to icecream, cosmetics and medicines).

(v) Timber: Cellulose _ rich wood is used in the manufacture of furniture, sports goods, tool handles and many more articles.

(vi) Fuel: Poor quality wood and discarded pieces of good__ quality wood are used as fuel.

 (vii)  paper:  Cellulose _ rich wood is employed in the production of paper.


2): Heteroplysaccharides:

These are composed of modified monosaccharide molecules or of more than one type of monosaccharide units. They include mucopolysaccharides, glycoproteins or mucoproteins,chitin, peptidoglycan, agar, pectin and Hemicellulose.

(i) Mucopolysaccharides: These are complexes of proteins and polysaccharides in which the polysaccharides predominate. They contain polymers of modified * sugar ( galactose and mannose) molecules. They are slimy compounds such as that noticed on cutting okra or lady's finger ( bhindi) fruit. Hyaluronic acid, heparin, chondroitin sulphate and keratin sulphate are important examples. 

(a) Hyaluronic Acid: It is present in the matrix of connective tissues and in the various lubricating fluids of the body, such as synovial fluid of bone joints, cerebrospinal fluid of brain and spinal cord, vitreous  humour of the eye. It also occurs as a coat around the ovum and in tissue spaces acting as a cement material. 

(b) Heparin:It is an anticoagulant formed in the liver and mast cells. It is found in the blood and connective tissues.

 (c) Chondroitin Sulphate: It occurs among the ground substance materials in the matrix of connective tissue and cartilage. 

(d) Keratin Sulphate: It is found  in the connective tissue, cartilage, bone, cornea and skin as the strengthening material. 

( ii) Glycoproteins: These are complexes of polysaccharides and proteins in which proteins predominate. Blood group substances ( A,B,and Rh antigen of erythrocytes), luteinizing hormone, mucus and plant micilage are glycoproteins. 

(iii) Chitin: Chitin is the second most abundant natural polymer. It is present in the exoskeleton of arthropods ( crustaceans, spiders and insects) and in the cell walls of fungi. In the fungal walls, it is often called fungal cellulose. Chitin is a nitrogen __ glucosamine. Like cellulose, chitin molecules occur in unbranched chains in which monomers are joined by 1__ 4 ß linkages. Molecules occur as bundles, in which they are held together by hydrogen bonds. Chitin being soft and leathery, provides both flexibility and strength. It is hardened by deposition of calcium carbonate in crustaceans. 



  Chitin is used in water purification and waste treatment in Japan. 

 (vi) Agar (Agar _ Agar): It is a carbohydrate obtained from certain seaweeds ( various red algae such as Gracilaria and Gelidiella). It is composed of sulphated galactose monomers.It forms a gel with water. It is used to solidify culture media on which bacteria are grown. It is also used as a laxative, and as food in the East.


(v) Peptidoglycan: It is a  compound containing polysaccharide chains cross crosslinked by short peptides with the former predminating. It is present in the cell walls of bacteria and blue__ green algae.
 
(vi) Pectin: Pectin is composed of the sugars arabinose and galactose, the sugar acid galacturonic acids, and methanol. It forms long branching or straight molecules. It is soluble in water and may undergo sol gel interchange. Pectin occurs in the matrix of  cell wall and middle lamella. Pectin is used as a jellying  agent.

 (vii) Hemicellulose: Hemicelluloses  is composed of monosaccharide units, namely, galactose, mannose, xylose, glucose and glucomannose. It forms short, branched chains. Hemicellulose occurs in the cell wall. 

Functional Classification:

The polysaccharides are divided on the basis of  their function into two groups: Food _ storage polysaccharides predominant and Structural polysaccharides.

(i) Food __ storage polysaccharides: These include glycogen and starch. Both have been described already. They provide nourishment at the time of need.Their formation yields water which the cell can use in other reactions. 

 (ii) Structural Polysaccharides: These include cellulose, chitin, chondroitin sulphate, keratin sulphate and peptidoglycan. These have also been already discussed. 

● Properties: The polysaccharides are not sweetish. They have large molecules and high molecular weights. They are insoluble in water, but form colloidal solutions. They do not diffuse through the cell membranes. They can hydrolyse into simple sugars.

Functions:

 The polysaccharides have a number of diverse functions in living systems. 

( i)  Reserve Food Materials: Some polysaccharides  form an ideal storage product because  of many  advantages: 

(a) Their formation from monosaccharides removes many water molecules, thereby reducing their bulk to be stored.

(b) Their large molecules do not pass out through the cell membrane and remain in place till needed in the cell; 

(c) Being insoluble, they do not exert osmotic or chemical influence in the cell; 

(d) Their molecules fold into compact shapes; and 

(e) They are easily converted to sugars by hydrolysis when required. 

Two principal food reserve polysaccharides in  Eukaryotic cells are starch and glycogen.

(a) Starch : Starch is a major food reserve in algae and land plants. It is deposited as large granules in the chloroplasts of some green cells or in the colourless Leucoplasts  of other plant cells.

(b) Glycogen :Glycogen is  the principal food reserve in fungi and animals. It is deposited in the  cytoplasm rather than in organelles as is starch.It is particularly abundant in liver cells. It is a ready source of glucose for the blood.


( ii) Fuel Substances: Polysaccharides ( starch and glycogen) also serve as a source of energy. For this, they are hydrolyzed to their constituent glucose monomer units by specific enzyme actions. 

( iii) Structural Compounds: Polysaccharides act as structural compounds as well. Cellulose, pectin and chitin  are the main structural polysaccharides in eukaryotes and peptidoglycan in prokaryotes . 

(a) Cellulose occurs in the cell wall of most algae, all higher plants,  aand certain fungi and protists. 

 (b) Pectin, It forms matrix of cell wall and middle lamella in plants.

 (c)  Chitin  is found in the  cell wall  of many fungi and in the rigid exoskeleton of arthropods. Being hard, it provides protection and support. 


 (d) Peptidoglycan occurs in the cell wall of bacteria. 

(iv) Lubricant and Cement: Hyaluronic acid acts as a lubricant at the joints and as a cement substance to fix the follicle cells to the egg.

(v) Anticoagulant:Heparin prevents clotting of blood in the blood vessels. 

 (vi) Immunologic Reactions:Blood group substances are responsible for the major immunologic reactions of blood.

(vii) Protective Coats:Glycoproteins from a protective coat, called glycocalyx, on the intestinal lining. Mucus also protects the surface it covers. Mucilage forms protective coat around bacteria, algae and aquatic plants.

 (viii) Recognition of Stimuli : Glycoproteins and glycolipids present at cell surface recognize stimuli reaching the cells.

(ix) Medicinal Value: Mucilage in the husk of isabgol ( Plantago ovata) and in the leaves of Aloe vera has medical value. Agar, algin¹, carrageen², etc. derived from marine algae also have medicinal properties. 

 (x) Jellying Agents:  Pectins are used as commercial jellying agents. 

(xi) Roughage:Cellulose acts as an excellent roughage  in human intestine.

(xii) Food Cattle, buffaloes, termites use cellulose as food.

(xiii) Commercial Value: Cellulose has a great commercial importance. It has been discussed earlier.


Other polysaccharides:

Mannan: A homopolymer of mannose, is found in the cell walls of yeast and is also stored intracellularly in some plants.In yeast,  it has branched structure,  whereas in plants it is a linear molecule. 

Paramylum: It is nutrient Homopolysaccharide stored intracellularly as large granules in certain protozoa ( e.g.Euglena). It consist of unbranched chains of glucocyle units in 1,3 linkage.

Dextran: It is branched homopolymer of glucose produced by certain bacteria. The glycosidic linkages vary from one species to  another and may be 1,6., 1,4.,1,3., or 1,2.





































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