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CLASSIFICATION OF POLYSACCHARIDES:
I•Based on Chemical Composition of Polysaccharides:
1•HOMOPOLYSACCHARIDES:- These are formed by the polymerisation of only one type of monosaccharide units. For example-,glycogen,starch and cellulose are formed by the polymerisation of glucose. Homopolysaccharides formed of glucose units are called glucans, of fructose units, fructose and of galactose galactans.
2•HETEROPOLYSACCHARIDES:- They are formed by polymerisation of two or more types of monosaccharide molecules. For example,mucopolysaccharides, glycoprotein, chitin, agar, pectin, hemicellulose.
BASED ON FUNCTION:
1•STORAGE POLYSACCHARIDES: These are polymers of glucose and are reserved energy sources for cell metabolism.They occur as starch in plants, as glycogen in animals and blue green algae and dextran in bacteria and yeast.
(i) STARCH: It is the most important source of carbohydrate in our food. It is formed of a-glucosidic chain having about 50,000 glucose molecules. Starch is a mixture of Amylase and Amylopectin polysaccharides.
(a)Amylase forms about 15-20 percent of starch and is in the form of unbranched coiled chain . Each chain is formed of about 300-400 glucose units, joined by 1-4 a-linkage. It give blue colour with iodine.
(b) Amylopectin: It forms 80-85 % of starch and it’s molecules and branched and coiled. Because of its more complex structure Amylopectin is less soluble in water than amylose . It gives red-purple colour with iodine solution.
(2) GLYCOGEN (Animal Starch):
It is found in liver and muscle cells of all higher animals and human beings. It is a branched polymer of glucose having 5,000-10,000 glucose units. In each glycogen molecule, a-D glucose units are joined by a-1-4 glucosidic linkage. The branches are connected with the main chain by a- 1-6 linkage.
Glycogen is soluble in water.it gives red colour with iodine. The synthesis of glycogen from glucose is called Glycogenesis and hydrolysis of stored glycogen into glucose is Glycogenolysis. Glucose from liver is distributed to all body cells for daily metabolism. Glucose produced in the muscle cells is utilised by muscles only.
(3) DEXTRINS: Dextrins are formed from partial hydrolysis of starch by enzyme salivary amylase. On hydrolysis they produce maltose which is further hydrolysed into glucose by enzyme maltase.
(4) DEXTRANS: These are storage polysaccharides of yeast and bacteria. Their molecules are in the form of branched chains. In dextrans D-glucose molecules are linked by a-1,6-glucosidic bond.
2• STRUCTURE POLYSACCHARIDES: They are also formed by the polymerisation of D-glucose molecules. cellulose and chitin are important structural Polysaccharides.
(i) CELLULOSE: It forms cell wall of plant cells and provides mechanical strength to the cells.Each cellulose fibre is made up of 10-15 thousand glucose molecules joined by beta-1, linkage.A number of cellulose molecules together form a microfibril and a bundle of microfibrils collectively form a macrofibril. They have great tensile strength.
•Cellulose is an important component of human diet and it adds bulk to intestinal contents and stimulate peristalsis in intestinal wall. Cellulose is not easily digested by amylase present in the digestive juices.
(ii) CHITIN: Chitin is found in the exoskeleton of insects and other arthropods and in the cell wall of fungi. It is a polymer of N-acetyl glucosamine.Chitin is soft and skin -like but because of the deposition of some proteins and calcium carbonate,it becomes hard. It is insoluble in water and gives definite form to the animals.
★AMYLOSE:- It forms 20% of starch and is a straight chain Polysaccharide. The chain is unbranded. ★Each amylose molecule is formed of 300-400 glucose monomers or units. ★It is soluble in hot water. ★its molecular weight ranges from 50,000-60,000. ★It gives blue with iodine. ★Amylose is hydrolysed with bita-amylase enzyme.
★AMYLOPECTIN:- It forms 80% part of starch and is a branched Polysaccharide and it’s chain is coiled. ★ An Amylopectin molecule is formed of thousands of glucose units. ★It is insoluble in water. ★Its molecular weight is more than 5,00,000. ★It gives pink purple colour with iodine. ★Amylopectin is hydrolysed into Dextrins by enzyme beta-amylase.
★STARCH:- It is a storage polysaccharide in plants. ★Amylose chains are unbranched but Amylopectin chains are branched. ★The chains of Amylopectin are branched after every 20-24glucose units. ★Molecular weight of amylose ranges from 50,000-60,000 and of Amylopectin is 5,00,000. ★Insoluble in water.
GLYCOGEN:- It is Animal Starch and is stored in animal cells like liver and muscles. ★Glycogen molecules are branched similar to Amylopectin. ★The glycogen chains are branched after every 8-10 glucose units. ★Molecular weight of each glycogen molecule is 60,00,000 and above. ★Soluble in water.
•CLINICAL SIGNIFICANCE OF GLYCOGEN:-
GLYCOGEN is stored in the muscles and liver cells. Muscle glycogen provides hexose sugars to muscle cells for glycolysis. Liver glycogen provides hexose sugars to all body cells for energy and maintains blood glucose level between meals. Liver glycogen is exhausted after 12-18 hours of fasting,whereas muscle glycogen is exhausted after prolonged vigorous exercise.
Inherited disorders result in glycogen storage diseases. They are caused due to insufficient mobilisation of glycogen and consequently the deposition of glycogen. This causes muscular weakness and may ultimately lead to death.
★SUCROSE★
Sucrose is a condensation product of glucose and fructose. Although glucose and fructose,which make up the structure of sucrose,are reducing sugars, sucrose itself is not a reducing sugar. In sucrose ,the oxygen bridge between the two monosaccharides occurs between carbon 1 of glucose and carbon 2 of fructose, resulting in the elimination of the free carbonyl groups of both these sugars.
★ACTION OF HEAT:- Sucrose melts on heating. Melted sugar on cooling solidifies into pale yellow glassy mass called barely sugar. When heated about 200⁰ C , it loses water to give brown coloured caramel which is used for colouring wines and confectionery.
★STARCH★
The starch is the second most abundant carbohydrate found in plants. It occurs in potatoes,rice, wheat,maize,and sago in the form of microscopic granules.
