what are carbohydrates? How to Classify?

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 Cm(H2O)n.

Thus glucose, C6H12Ocan be written as C(H2O)6. It was a french who gave them the class name Carbohydrates (carbon-Hydrates).

Carbohydrates are often referred to as Saccharides (Latin, Saccharum = sugar) because of the sweet taste of the simpler members of the class, the sugars.

what are carbohydrates? How to Classify?
what are carbohydrates? How to Classify?

What are Carbohydrates?

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

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


Glucose, Galactose, Fructose

Cereal grains and pasta, vegetables, fruits, and fruit juices, nuts and seedsFructose is a sugar found in Fruits.


All these are simple sugars which are easily digested by the body and a ready source of energy


Sucrose, Lactose and MAltose

Table sugar, Cane sugar, beer sugar, milk sugarSucrose 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.


Starches, Glycogen and Cellulose

Potatoes, Sweet potatoes, corn, cornflakes, plant materialCellulose 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

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 monosaccharidesAldosesKetoses

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

2. Oligosaccharides

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.

Figure 4. Formation of a disaccharide (top) by condensation and structure of two common disaccharides

Sucrose (C12H22O11)  +  H2O    →     C6H12O6 (Glucose)  + C6H12O(Fructose)

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

Raffinose (C18H32O16)  +  2H2O    →     C6H12O6 (Glucose)  + C6H12O(Fructose)  + C6H12O(Galactose)

common disaccharides

3. Polysaccharides

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) 


amylose and amylopectin
homopolysaccharide table


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