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 C₆H₁₂O_5. 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

Monosaccharides are sugars, which cannot be further hydrolysed to simple sugars. However, they are classified according to number of carbon atoms in sugar molecules.

1. Bioses: They contain two carbon atoms. They do not occur free in nature.
2. Trioses: They contain three carbon atoms, but in the form of phosphoric esters, eg: glyceraldehyde.
3. Tetroses: They contain four carbon atoms, eg: erythrose.
4. Pentoses: They are very common in plants and are the products of hydrolysis of polysaccharides like hemicellulose, mucilage and gums. Eg: Arabinose, Ribose and Xylose.
5. Hexoses: They are the monosaccharides containing six carbon atoms and are abundantly available carbohydrates of plant kingdom. They are further divided into two types- aldoses and ketoses. They may be obtained by hydrolysis of polysaccharides like starch, inulin etc. Aldoses: glucose, mannose, inulin etc. Ketoses: Fructose and Sorbose.
6. Heptoses: They contain 7 carbon atoms vitally important in the photosynthesis of plants and glucose metabolism of animal and are rarely found accumulated in plants eg: glucoheptose and mannoheptose.

Disaccharides

Carbohydrates which upon hydrolysis yield two molecules of monosaccharides are called as disaccharides.

Trisaccharide

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.

Tetrasaccharides

Stachyose, a tetrasaccharide, yield on hydrolysis, four molecules of monosaccharides.

Polysaccharides

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.

Chemical Tests for Carbohydrates

1. Molisch’s test: The test is positive with soluble as well as insoluble carbohydrate. It consists of treating the compounds with alpha-naphthol & concentrated sulphuric acid which give purple colour.
2. Reduction of Fehling’s Solution: to the solution of carbohydrate, equal quantity of Fehling’s solution A & B is added. After heating brick red precipitate is obtained.