Carbohydrates - Biomolecules

Carbohydrates

Carbohydrates, otherwise known as saccharides (derived from the Greek Sakcharon– sugar; many simple sugars taste sweet) are one of the abundant molecules in earth. They are essential to maintain life in both plants and animals. They are synthesised in plants by photosynthesis.

The compounds that we come across in our daily life such as table sugar, wood, cotton, starch, and honey etc... are all carbohydrates.

1. Importance

Carbohydrates are widely distributed in both plant and animal tissues. They occur as food reserves in the storage organs of plants and animals. They are the important source of energy which is required for the various metabolic activities of living organisms.

They provide raw material for many important industries including textiles, artificial silks, paper, films, plastics, lacquers, confectionary, drugs, fermentation and explosives.

2.  Definition

Carbohydrates are defined as poly-hydroxy aldehydes or ketones. They contain hydrogen and oxygen in the same ratio as in water (2:1)

The names of most of carbohydrates are characterised by the ending ‘-ose’. For example glucose, fructose, sucrose, cellulose, etc.

3. Classification

Carbohydrates are generally classified as Sugars and Non-sugars.

Sugars

Sugars are sweet, crystalline and soluble in water. They are classified further as below.

4. Monosaccharides

Mono saccharides have a general formula C_n(H₂O)_n. Based on the number of carbon atoms they are further classified into trioses, tetroses, pentoses, hexoses etc. They can also be classified as aldoses and ketoses based on the functional group present in them. They cannot be hydrolysed into simpler units. This can be further classified as aldoses and ketoses based on the functional group present in C1 position.

Aldoses:

Aldoses contain aldehyde group (-CHO) as a functional group along with two or more hydroxyl groups. Examples: glyceraldehyde, ribose, glucose galactose

Ketoses:

Ketoses contain keto group (>C=O) as a functional group along with two or more hydroxyl groups.

Examples: dihydroxy acetone, ribulose, fructose.

5. Oligosaccharides

Oligosaccharides are sugars that yield 2 to 10 monosaccharide molecules on hydrolysis. This can be further classified as di, tri, tetra saccharides etc. based on the number of monosaccharide units present. In these molecules monosaccharide units are interlinked by glyosidic bridges

Disaccharides

Disaccharides have a general formula C_n (H₂O)_n-1Example: sucrose, lactose and maltose. In these molecules monosaccharide units are inter linked by a glyosidicbridges.

Hydrolysis of disaccharides in presence of mineral acid yields corresponding monosaccharides. For example hydrolysis of sucrose gives glucose and fructose. Similarly maltose gives two molecules of glucose.

Trisaccharides

This will give three monosaccharide units on hydrolysis. These have a general formula as C_n(H₂O)_n-2 Example: raffinose, stachyose.

6. Polysaccharides (Non Sugars)

Polysaccharides are the carbohydrates that yield more than ten monosaccharide units upon hydrolysis.They are further classified into homopoly saccharides and heteropoly saccharides based on the monomeric units. Example: starch, cellulose, inulin

Homopolysaccharides

A homopolysaccharide yields the same type of monosaccharide units on hydrolysis. For example starch a homopolysaccharide yields only glucose upon hydrolysis. Similarly glycogen and cellulose also yield glucose on hydrolysis.

Heteropolysaccharides

A heteropolysaccharide yields more than one type of monosaccharide upon hydrolysis. Eg. Hyaluronic acid, heparin, keratan sulphate and chondroitin sulphate

These are present in extra cellular matrix and therefore they are called as mucopolysaccharides.

Hyaluronic acid is made up of glucuronic acid and N-acetyl glucosamine.

Starch:

Starch is the major form of stored carbohydrate in plants. Starch is composed of a mixture of two substances namely amylose, a linear polysaccharide and amylopectin, a branched polysaccharide. The detailed study of starch will be discussed in the unit 5.

Cellulose:

Cellulose is an unbranched polymer of β-D-glucose. Because of the absence of side chains, molecules stay together to form rigid structures in plants. Wood is largely made up of cellulose, and cotton is almost pure cellulose.

Cellulose may be modified in the laboratory by treating it with nitric acid to produce nitrocellulose or gun cotton which is an explosive component of smokeless powder. Partially nitrated cellulose, known as pyroxylin, is used in the manufacture of collodion, plastics, lacquers, and nail polish.

Glycogen:

Glycogen is a storage form of glucose which is mainly present in liver and muscle.

Glycogen is also known as animal starch. It is a multi branched polysaccharide of glucose.

The polysaccharide structure represents the main storage form of glucose in the body