Carbohydrates : Classification and its Basics
The carbohydrates are an important class of naturally occurring organic compounds. These compounds 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, C6H12O6 can 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). They are given this name because of the sweet taste of the simpler members of the class, the sugars.
What are carbohydrates?
The carbohydrates are polyfunctional compounds. They contain the following functional groups.
- Alcoholic hydroxy groups, -OH
- Aldehyde group -CHO
- Ketone group, -CO-
Polyhydroxyaldehydes or polyhydroxy ketones provide a precise definition of the term ‘carbohydrate’. These are large molecules that undergo hydrolysis to produce various compounds. We can illustrate this by using the example of the two simplest carbohydrates, each containing three carbon atoms.
Classification of Carbohydrates
The carbohydrates are 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.
- Monosaccharides
- Oligosaccharides
- 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
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
We again classify the monosaccharides based on two criteria.
- By the carbonyl function: Those containing the aldehyde function, (-CHO) are called Aldoses. Other containing the keto group (-CO-) are called Ketoses.
- 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
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)
Raffinose, on the other hand, hydrolyzes into three simple sugars and is known as a trisaccharide.
Raffinose (C18H32O16) + 2H2O → C6H12O6 (Glucose) + C6H12O6 (Fructose) + C6H12O6 (Galactose)
3. Polysaccharides
Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units. These units are bound together by glycosidic linkages. Upon hydrolysis, they 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. Monosaccharides are abundant in both plants and animals. These are not limited to simple sugars. They combine with sugar derivatives 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
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