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.