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57.2 CLASSIFICATION OF CARBOHYDRATES

Carbohydrates are classified into three major categories depending upon their behaviour towards hydrolysis:

 1.      MONOSACCHARIDES

These are simple carbohydrates which cannot be hydrolysed to simpler carbohydrates.

About 20 monosaccharides are known to occur in nature. Glucose and fructose are common examples.

2.      OLIGOSACCHARIDES

These are the carbohydrates which on hydrolysis give two to ten units of monosaccharides. Accordingly, they may be further divided into di, tri or tetrasaccharides depending upon the actual number of monosaccharide units formed by the hydrolysis of a particular oligosaccharide.

Disaccharides give two units of monosaccharides on hydrolysis. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. Common examples are sucrose and maltose. Both have molecular formula C12H22O11.

Sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose on hydrolysis gives two molecules of glucose only.

Trisaccharides give three units of monosaccharides on hydrolysis. Raffinose, C18H32O16 is a common example.

Tetrasaccharides give four units of monosaccharides on hydrolysis. Stachyose, C24H42O21 is a common example.

3.      POLYSACCHARIDES

These are the carbohydrates which are polymeric molecules and can be hydrolysed to give large number of monosaccharide units. The commonly occurring polysaccharides have the general formula (C6H10O5)n The common examples are starch, glycogen and cellulose.

It may be noted that the carbohydrates which are sweet in taste are collectively called sugars while those which are not sweet are called non-sugars. Monosaccharides and disaccharides are sugars but polysaccharides are non-sugars.

The relative degree of sweetness of various sugars is given below in tabular form:

 

Sugar

Lactose

Maltose

Galactose

Glucose

Sucrose

Fructose

Relative Sweetness

16

32

32

74

100

173

Do you know? 

Besides carbohydrates, some other compounds are far more sweet. For example,

  • Saccharin is about 500 times as sweet as sucrose.
  • Aspartame, a peptide, is about 160 times sweeter than sucrose.

REDUCING AND NON-REDUCING CARBOHYDRATES

The carbohydrates may also be classified as reducing and non-reducing sugars. The carbohydrates which can reduce Tollen’s reagent or Fehling’s or Benedict’s solution are classified as reducing sugars, while those which do not reduce these reagents are called non-reducing sugars:

Reducing sugars contain free aldehyde or ketonic group. All monosaccharides are reducing sugars. Disaccharides may be reducing or non-reducing. If the carbonyl groups of both the monosaccharides are involved in linkage, the disaccharide is non-reducing. On the other hand, if one of the carbonyl groups is free, the disaccharide is reducing: Sucrose is a non-reducing sugar while maltose is a reducing sugar.

SOURCES OF CARBOHYDRATES

Glucose occurs in sweet fruits such as grapes, mangoes, oranges, etc. Honey is also rich in glucose. In combined state it is present in maltose, sucrose, starch, cellulose, etc.

Fructose is found in ripe fruits and honey.

Sucrose. Major sources of sucrose are sugarcane and sugarbeet

Lactose is present in milk.

Starch. Major sources of starch are wheat, rice, cassava, root tubers such as potatoes, legumes and vegetables.

Cellulose. Cellulose is present in cotton, wood and jute.