Biomolecule: Carbohydrates

Carbohydrates

Importance

Carbohydrates are widely distributed in both plant and animal tissues. They are indispensible for living organisms and serve as skeletal structures in plants and also in insects and crustaceans. They occur as food reserves in the storage organs of plants and animals. They are the important source of energy required for the various metabolic activities of the living organisms.

Definition

Carbohydrates are defined as polyhydroxy aldehydes or ketones and are generally classified as follows.

Classification

Carbohydrates are generally classified into 4 major groups :

·           Monosaccharides

·           Disaccharides

·           Oligosaccharides and

·           Polysaccharides

Monosaccharides

These are carbohydrates that cannot be hydrolysed into more simpler form. These are otherwise known as simple sugars. The general formula is C_n(H₂O)_n. They may be subdivided into trioses, tetroses, pentoses and hexoses depending upon the number of carbon atoms they contain and also subdivided as aldoses and ketoses depending upon the presence of aldehyde or ketone groups (eg).

Functions

·           Ribose is a structural element of nucleic acids and also of some coenzymes.

·           Glucose on oxidation yield energy which is required for various metabolic activities.

·           Fructose is found in fruits, honey etc.which are responsible for sweetness and can be converted to glucose and utilised in the body.

·           Galactose is a component of milk sugar-lactose, glycolipids and glycoproteins

·            Mannose is a constituent of mucoproteins and glycoproteins which are essential for the body.

Disaccharides

These are carbohydrates that yield two molecules of same or different types of monosaccharides on hydrolysis. The general formula is C_n(H₂O)_n-1 (eg) Lactose, Maltose and Sucrose. The monosaccharide units are united by a glycosidic linkage.

Functions

1. Lactose is otherwise called as milk sugar. It is present in milk and is made up of monosaccharides - glucose and galactose.

Glucose + Galactose - - > Lactose

2. Maltose is otherwise known as ‘malt sugar’ and is present in germinating cereals, malt etc.It is the intermediate product in the hydrolysis of starch by amylase in the alimentary canal. It is made up of 2 molecules of glucose.

Glucose + Glucose - - > Maltose

3. Sucrose is otherwise called as ‘table sugar’ or ‘cane sugar’. It is the common sugar and is widely distributed in all photosynthetic plants. It does not exist in the body but occurs in sugarcane, pineapple, sweet potato and honey. It is made up of glucose and fructose. Glucose + Fructose - - > Sucrose

Oligosaccharides

These are carbohydrates that yield 2-10 monosaccharide units on hydrolysis.eg. Maltotriose.

Polysaccharides

These carbohydrates yield more than 10 monosaccharide units on hydrolysis. They are further classified into homopolysaccharides and heteropolysaccharides.

Homopolysaccharides

These on hydrolysis yield same type of monosaccharide units.

(eg). starch, glycogen, cellulose, inulin, pectin and hemicellulose yield only glucose on hydrolysis.

Heteropolysaccharides

These on hydrolysis yield a mixture of different types of monosaccharides. The heteropolysaccharides situated in extra cellular matrix are called as mucopolysaccharides.

(eg). hyaluronic acid, heparin, keratan sulphate and chondroitin sulphate.

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

Chondroitin sulphate is made up of either glucuronic acid (A and C type) or Iduronic acid (B type) and

Keratan sulphate consists of N-acetyl galactosamine, galactose and sulphuric acid

Functions

·           Starch is made up of repeated units of glucose moiety. It is the most important source of carbohydrate in our food. Such a compound which produces only glucose on hydrolysis is called a glucosan, and is found in cereals, potatoes, legumes and other vegetables.

·           Glycogen is the major carbohydrate reserve in animals and is often called animal starch. It is stored in liver and muscle of animals.

·           It is also found in plants which have no chlorophyll system [eg. fungi and yeasts] but not in green plants.

·           Cellulose is widely distributed in plant sources. It occurs in the cell walls of plants where it contributes to the structure. It is the main consituent of the supporting tissues of plants and forms a considerable part of vegetables.

·           Pectin and hemicellulose are present in fruits of many plants and serve as jelling agents.

·           Hyaluronic acid occurs in synovial fluid, in skin and in tissues. It acts as a cementing substance in tissues and also acts as a lubricant. It is also present in vitreous humor.

·           Heparin is used in medicine as an anticoagulant and prevents blood clotting.

·           Keratan sulphate is an important component of cartilage and cornea.