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(a) What are carbohydrates ? Name four important functions of carbohydrates. (b) How are carbohydrates classified ? |
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Answer» SOLUTION :(a) Carbohydrates. These are a class of compounds that contain the polyhydric aldehydes, polyhydric ketones and large polymeric substances which can be broken down to polyhydric aldehydes or ketones. In the earlier days, the carbohydrates were regarded as the hydrates of carbon with the general formula `C_x (H_2 O)_y`. For example, carbohydrates such as glucose `(C_6 H_(12) O_6 )`, fructose `(C_6 H_(12) O_6 )`, sucrose `(C_(12) H_(22) O_(11))` satisfied this definition. However, there are CERTAIN substances which have the formula of hydrates of carbon but were not carbohydrates. For example, formaldehyde (HCHO or `CH_2 O`), acetic acid `(CH_3 COOH` or `C_2 H_4 O_2 )`. Therefore, these days carbohydrates are defined as polyhydric aldehydes or ketones. Four important carbohydrates are : Ribose `(C_5 H_(10) O_5)`, glucose `(C_6 H_(12) O_6)`, fructose `(C_6 H_(12) O_6)`, sucrose `(C_(12) H_(22) O_(11))`. (b) Classification of carbohydrates. Carbohydrates in general may be classified into two classes : (i) Sugars. These are crystalline substances which are sweet and water SOLUBLE. For example, glucose, fructose and cane sugar. (ii) Non-sugars. These are tasteless, insoluble in water and amorphous. For example, starch, cellulose, etc. However, these days carbohydrates are systematically classified into three classes : (i) MONOSACCHARIDES. These are the simplest carbohydrates which contain a single carbohydrate unit and cannot be hydrolysed into simpler units. They contain upto six carbon atoms. They have the general formula `(CH_2O)_n`. They contain aldehydic or ketonic group. If the carbohydrates contain aldehydic group, they are named as aldoses and if they have ketonic group, they are commonly called ketoses. The common examples are ribose `(C_(5) H_(10) O_5)`, glucose `(C_6 H_(12)O)`, fructose `(C_6 H_(12)O)` etc. (ii) Disaccharides. These are the carbohydrates which hydrolysed to give two monosaccharides. Sucrose is the common example and on hydrolysis it gives glucose and fructose. The other examples are maltose and lactose. These are formed by the condensation reaction between two monosaccharides. For example, sucrose is made up of one unit of glucose and one unit of fructose, lactose is made up of one unit of each glucose and galactose. (iii) Polysaccharides. These are carbohydrates in which a large number of monosaccharide units are.linked to each other by oxygen bridges. These linkages are called glycosidic linkages. The common examples are cellulose, starch, etc. They get hydrolysed to give monosaccharides. |
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