Formerly, carbohydrates were defined as group of compounds composed of carbon, hydrogen and oxygen in which the later two elements are in the same proportion as in water and were expressed by a formula (CH2O)n, i.e. hydrates of carbon. The word carbohydrates can be traced back to Germans who called them ‘Kohlenhydrates’. It was then termed carbohydrates in English. The definition did not remain valid as it was misleading with few compounds like (1) acetic acid expressed as CH3COOH (i.e. C2H4O2) and lactic acid as CH3-CHOHCOOH (i.e. C3H6O3) which are not carbohydrates and (2) sugars like rhamnose and fucose represented by the formula C6H12O5. The carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or compounds that on hydrolysis produce either of the above. They are substances of universal occurrence and are much abundant in plants rather than in animal.
Carbohydrates are grouped into two major classes: simple sugars(saccharides) and polysaccharides. Low molecular weight carbohydrates are crystalline, soluble in water and sweet in taste e.g., glucose, fructose and sucrose. The high molecular weight carbohydrates (polymers) are amorphous, tasteless & relatively less soluble in water e.g., starch, cellulose, gums, pectin, inulin etc. Depending upon the chemical structure, saccharides are subdivided as monosaccharides, disaccharides and trisaccharide.
Monosaccharides are sugars, which cannot be further hydrolysed to simple sugars. However, they are classified according to number of carbon atoms in sugar molecules.
Carbohydrates which upon hydrolysis yield two molecules of monosaccharides are called as disaccharides.
As the name indicates these liberate three molecules of monosaccharides on hydrolysis.
Scillatriose (squill), Mannotride (Manna), and Phanteose (psyllium) are the other examples of trisaccharides.
Stachyose, a tetrasaccharide, yield on hydrolysis, four molecules of monosaccharides.
On hydrolysis, they give an indefinite number of monosaccharides. By condensation with the elimination of water, polysaccharides are produced from monosaccharides. Depending upon the type of product of hydrolysis these are further classified as pentosans and hexosan. Xylan is pentosan, whereas starch, inulin, and cellulose are the example of hexosans. Cellulose is composed of glucose units joined by beta-1,4 linkages, hydrolysable by an enzyme cellulose present in animals, whereas starch contains glucose units connected with alpha-1,4; alpha-1,6 units.
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