Carbohydrates were originally regarded as hydrates of carbon with general formula [Cx(H2O)y]. But this definition has certain limitations as Acetic Acid [CH3COOH = C2(H2O)2], Formaldehyde [HCHO = C1(H2O)1] are not Carbohydrates although can be expressed as [Cx(H2O)y], but Rhamnose (C6H12O5), deoxyribose (C5H10O4) etc are carbohydrates but cannot be represented by the general formula [Cx(H2O)y].
In the light of above contradictions are definition of Carbohydrates was modified as:
“Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis and contain at least one chiral carbon atom.” The Carbohydrates which contain aldehyde group are called aldoses, while the carbohydrates which contain ketonic group are called Ketoses.
The aldehyde or ketone group in carbohydrates is not present as such but usually exists in combination with one of the hydroxyl groups of the molecule in form of a hemiacetal or a hemiketal.
Classification of Carbohydrates:- Carbohydrates are classified as:
(1) Monosaccharides (2) Oligosaccharides (3) Polysaccharides
(1) Monosaccharides:- These are the simple carbohydrates which cannot be hydrolyzed to still simpler carbohydrates. Glucose and Fructose are common examples.
(2) Oligosaccharides:- These are the carbohydrates which on hydrolysis give two to ten units of monosaccharides. Depending upon the number of units given upon hydrolysis they may be categorized as Disaccharides (sucrose, maltose = C12H22O11); Trisaccharides (Reffinose = C18H32O16); Tetrasaccharides (Stachyose = C24H42O21) etc.
(3) Polysaccharides:- These are the carbohydrates which on hydrolysis give a large number of Monosaccharides units. These are the polymeric carbohydrates with general formula (C6H10O5)n. e.g. Starch, cellulose, glycogen etc.
It is to be noted that carbohydrates which are sweet in taste are collectively called Sugars while those which are not sweet are called Non-Sugars. Monosaccharides & Disaccharides are Sugars and Polysaccharides are non-sugars.