Carbohydrates : Classification and its Basics

The carbohydrates are an important class of naturally occurring organic compounds These include glucose (grape sugar), fructose (Honey sugar), Sucrose (Cane sugar), Starch and cellulose (wood).  They are all composed of C, H and O. In general, carbohydrates can be represented by the formula C_m(H₂O)_n.

What are Carbohydrates?

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The carbohydrates are polyfunctional compounds. They contain the following functional groups.

1. Alcoholic hydroxy groups, -OH
2. Aldehyde group -CHO
3. Ketone group, -CO-

A precise definition of the term ‘Carbohydrate’ can be given as Polyhydroxyaldehydes or polyhydroxy ketones, and large molecules that produce these compounds on hydrolysis. This may be illustrated by citing the example of the two simplest carbohydrates containing three carbon atoms.

Classification of Carbohydrates

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The carbohydrates is divided into three major classes depending on the number of simple sugar units present in their molecule. In other words, the basis of classification of carbohydrates will be the number of simple sugar molecules produced on hydrolysis. The molecules so obtained may be of the same or different sugars.

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

Types of  Carbohydrate	Common Foods	Uses by Humans
Monosaccharides Glucose, Galactose, Fructose	Cereal grains and pasta, vegetables, fruits, and fruit juices, nuts and seeds	Fructose is a sugar found in Fruits.   All these are simple sugars which are easily digested by the body and a ready source of energy
Disaccharides Sucrose, Lactose and MAltose	Table sugar, Cane sugar, beer sugar, milk sugar	Sucrose is a source of fuel for our bodies and it adds flavor.   Maltose can be found in high maltose corn syrup which is the main sweetener of processed foods. Lactose found in milk provides energy for your body.
Polysaccharides Starches, Glycogen and Cellulose	Potatoes, Sweet potatoes, corn, cornflakes, plant material	Cellulose aids in digestion even though it can’t be digested. It is good for bulk.   Starches provide the body with energy Glycogen is needed as fuel for the muscles of humans and animals.

1. Monosaccharides

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These are simple sugars. The monosaccharides are single unit carbohydrates (polyhydroxy aldehydes or polyhydroxy ketones) that cannot be broken into simpler carbohydrates upon hydrolysis. Glucose and fructose are examples.

Glucose (C₆H₁₂O₆)  +  H₂O    →     No reaction

Further classification of Monosaccharides

The monosaccharides are again classified on the basis of two factors.

1. By the carbonyl function: Those containing the aldehyde function, (-CHO) are called Aldoses. Other containing the keto group (-CO-) are called Ketoses.
2. By the number of carbon atoms (3 to 8) in the molecule: The monosaccharides containing 3,4,5,6 etc., carbon atoms are designated as trioses, tetroses, pentoses, hexoses and so on.

The Sub-classes of monosaccharides based on the above factors are listed below.

No.of carbons in monosaccharides	Aldoses	Ketoses
3	Aldotriose	Ketotriose
4	Aldotetrose	Ketotetrose
5	Aldopentose	Ketopentose
6	Aldohexose	Ketohexose
7	Aldoheptose	Ketoheptose

Sugars and Non-Sugars

The monosaccharides and oligosaccharides are soluble crystalline substances having a sweet taste. They are collectively known as Sugars. Polysaccharides, on the other hand, are insoluble amorphous substances and are called Non-Sugars.

	Reducing Sugar	Non-Reducing sugar
1	Carbohydrates with a free aldehyde (at C-1) or a free ketone (at C-2) group	Aldehyde or ketone is not free in these sugars. But it is utilized in the bond formation
2	They are in hemiacetal or hemiketal form	They are in acetal or ketal form
3	It shows mutarotation	It does not show any mutarotation
4	Do form Osazone with phenylhydrazine	Do not form any osazones
5	Do form oximes with hydroxylamine	Do not form oximes

2. Oligosaccharides

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These are made of 2 to 10 units of monosaccharides or simple sugars. The oligosaccharides containing two monosaccharide units are called Disaccharides, and those containing three units Trisaccharides. Thus sucrose (C₁₂H₂₂O₁₁) is a disaccharide because on hydrolysis it gives one molecule of glucose plus one molecule of fructose.

Sucrose (C₁₂H₂₂O₁₁)  +  H₂O    →     C₆H₁₂O₆ (Glucose)  + C₆H₁₂O₆ (Fructose)

On the other hand, raffinose produces three simple sugars on hydrolysis and is designated as trisaccharide.

Raffinose (C₁₈H₃₂O₁₆)  +  2H₂O    →     C₆H₁₂O₆ (Glucose)  + C₆H₁₂O₆ (Fructose)  + C₆H₁₂O₆ (Galactose)

3. Polysaccharides

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Polysaccharides are polymeric carbohydrate molecule composed of long chains of monosaccharide units bound together by glycosidic linkages and o hydrolysis give the constituent monosaccharides or oligosaccharides. They range in structure from linear to highly branched. Examples include storage polysaccharides such as starch and glycogen and Structural polysaccharides such as Cellulose and Chitin.

They contain more than ten monosaccharide units in the molecule. Thus one molecule of starch or cellulose upon hydrolysis yields a very large number (n) of glucose units.

Starch (C₆H₁₀O₅)n  +  H₂O    →     nC₆H₁₂O₆ (Glucose) 

Homopolysaccharides:

Heteropolysaccharides:

On hydrolysis, it gives a mixture of monosaccharides. They are numerous in both plants and animals. These are also having not only simple sugars. They are mix with derivatives of sugars (such as amino sugars and uronic sugars).

• These are gelatinous substances with a high molecular weight
• These act as Cell supportive materials
• They serve as lubricant material