Chemistry Disaccharides, Polysaccharides and Importance of Carbohydrates
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### Topics Covered :

● Disaccharides
● Polysaccharides
● Importance of Carbohydrates

### Disaccharides :

=> Disaccharides on hydrolysis with dilute acids or enzymes yield two molecules of either the same or different monosaccharides.

● The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called color{green}("glycosidic linkage").

(i) color{green}("Sucrose ") : One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of color{red}(D-(+))-glucose and color{red}(D-(-)) fructose.

color{red}(undersettext(Sucrose)(C_(12)H_(22) O_(11)) +H_2O → undersettext{D- (+) - Glucose} (C_6H_(12)O_6) + undersettext{D-(-)-Fructose} (C_6H_(12)O_6))

● These two monosaccharides are held together by a glycosidic linkage between color{red}(C-1) of color{red}(α)-glucose and color{red}(C-2) of color{red}(β)-fructose. See fig.1.

● Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non reducing sugar.

● Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose.

● Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory.

● Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro color{red}((+)) to laevo color{red}((–)) and the product is named as color{green}("invert sugar").

(ii) color{green}("Maltose ") : Another disaccharide, maltose is composed of two color{red}(α-D)-glucose units in which color{red}(C-1) of one glucose color{red}((I)) is linked to color{red}(C-4) of another glucose unit (II). See fig.2.

● The free aldehyde group can be produced at color{red}(C-1) of second glucose in solution and it shows reducing properties so it is a reducing sugar.

(iii) color{green}("Lactose ") : It is more commonly known as color{green}("milk sugar") since this disaccharide is found in milk.

● It is composed of color{red}(β-D)-galactose and color{red}(β-D)-glucose.

● The linkage is between color{red}(C-1) of galactose and color{red}(C-4) of glucose. Hence it is also a reducing sugar. See fig.3.

### Polysaccharides :

=> Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages.

=> These are the most commonly encountered carbohydrates in nature.

=> They mainly act as the food storage or structural materials.

(i) color{green}("Starch ") : Starch is the main storage polysaccharide of plants.

● It is the most important dietary source for human beings.

● High content of starch is found in cereals, roots, tubers and some vegetables.

● It is a polymer of color{red}(α)-glucose and consists of two components— Amylose and Amylopectin.

● Amylose is water soluble component which constitutes about 15-20% of starch.

● Chemically amylose is a long unbranched chain with 200-1000 color{red}(α-D-(+))-glucose units held by color{red}(C1– C4) glycosidic linkage.

● Amylopectin is insoluble in water and constitutes about 80- 85% of starch.

● It is a branched chain polymer of color{red}(α-D)-glucose units in which chain is formed by color{red}(C1–C4) glycosidic linkage whereas branching occurs by color{red}(C1–C6) glycosidic linkage.

(ii) color{green}("Cellulose ") : Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom.

● It is a predominant constituent of cell wall of plant cells.

● Cellulose is a straight chain polysaccharide composed only of color{red}(β-D)-glucose units which are joined by glycosidic linkage between color{red}(C1) of one glucose unit and color{red}(C4) of the next glucose unit.

(iii) color{green}("Glycogen " ) : The carbohydrates are stored in animal body as glycogen.

● It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched.

● It is present in liver, muscles and brain.

● When the body needs glucose, enzymes break the glycogen down to glucose.

● Glycogen is also found in yeast and fungi.