Carbohydrates are also known as saccharides (Greek sakcharon means sugar). These are classified into the following three classes depending upon their behaviour towards hydrolysis.
1. Monosaccharides: These are the simplest carbohydrates which cannot be hydrolysed to smaller molecules. Their general formula is (CI-120)n where n = 3-7.
2. Oligosaccharides: (Greek, oligo means a few). These are carbohydrates which on hydrolysis give 2-10 molecules of monosaccharides. Depending upon the number of monosaccharide molecules actually obtained upon hydrolysis, they are further classified as di, tri, tetrasaccharides, etc. For example:
(i) Disaccharides: Carbohydrates which upon hydrolysis give two molecules of the same or different monosaccharides are called disaccharides. For example, sucrose, maltose, lactose, etc. Their general formula is Cl2H22O11.
(ii) Trisaccharides: Carbohydrates which on hydrolysis give three molecules of the same or different monosaccharides are called trisaccharides. For example, raffinose upon hydrolysis gives one molecule each of glucose, fructose and galactose. Their general formula is C18H32O16
(iii) Tetrasaccharides: Carbohydrates which upon hydrolysis give four molecules of the same or different monosaccharides are called tetrasaccharides. For example, stachyrose upon hydrolysis gives one molecule each of glucose and fructose and two molecules of galactose. Their general formula is C-24H42
3. Polysaccharides: Carbohydrates which upon hydrolysis give a large number of monosaccharide molecules are called polysaccharides. The most commonly occurring polysaccharides are starch, cellulose and glycogen. Their general formula is (C6fl1005)n where n = 100-3000.