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What is a Polysaccharide?

  • Polysaccharide is a long chain of carbohydrates whose molecules consist of numerous sugar molecules bonded together by glycosidic linkages. Polysaccharide examples: Starch, Cellulose or Glycogen are polysaccharides Carbohydrates. As we know carbohydrates are a major source of food and necessary for getting energy for the survival of living organisms. Consisting of Carbon, Hydrogen and Oxygen atoms, carbohydrates are made up of two basic compounds, namely aldehydes and ketones.

  • Polysaccharides are one of the three carbohydrates found in all natural and processed foods and the other two carbohydrates are monosaccharides and disaccharides.

  • Monosaccharide Examples: Glucose and Galactose

  • Disaccharide Examples: Sucrose and Maltose

  • Polysaccharide Examples: Starch, Glycogen and Cellulose

  • General Formula of Polysaccharides is Cx(H2O)y 

where, x is the number of carbon atoms (usually a large number between 200-2500)

  • Polysaccharides are an important class of biological polymers and generally contain more than 10 monosaccharide units, however, oligosaccharides are the polysaccharides with three to 10 monosaccharide units. Biological polysaccharides help in performing various functions in the living organisms, either structural functions or energy storage functions. Two examples of structural polysaccharides include cellulose and chitin; the cell walls of plants and other organisms are composed of cellulose which is considered as the most abundant organic molecule on Earth. 

  • Polysaccharide meaning: Poly means ‘many’ and saccharide means ‘sugar’; so a polysaccharide contains many sugar molecules.

See the below polysaccharide structure for clearer understanding.

[Image to be added Soon]

Types of Polysaccharides

Heteropolysaccharides and Homopolysaccharides are the two types of polysaccharides that are described in the following:

  • When more than one kind of monosaccharide is present in the polysaccharide structure, it is known as heteropolysaccharide or heteroglycan. In contrast, homopolysaccharide or homoglycan is the kind of polysaccharide which has the same type of monosaccharides. 

  • Examples of homopolysaccharides are glycogen, cellulose, starch and insulin. Glycogen is made up of a large chain of molecules and is found in animals and fungi. Cellulose is present in the cell wall of the plants and comprises long chains of beta glycosides. Starch is produced by the condensation of amylopectin and amylose; available largely in plants, fruits, seeds, etc. Insulin is composed of numerous fructofuranose molecules linked together in chains, it is found in tubers of artichoke, dahlia, etc.

  • Examples of heteropolysaccharides are hyaluronic acid, heparin, chondroitin-4-sulfate and gamma globulin. Hyaluronic acid is made up of N-acetyl-glucosamine and is found in connective tissues and skin. Heparin is made up of D-glucuronic acid, N-sulfo-D-glucosamine and L-iduronic acid and it is largely distributed in blood and mast cells. Chondroitin-4-sulfate sugars are D-glucuronic acid and N-acetyl-D-galactosamine-4-O-sulfate. It is found in the cartilages. Gamma globulin is composed of N-acetyl-hexosamine, D-galactose and D-mannose as the component sugars of the polysaccharide and is present in the blood.

Characteristics of Polysaccharides

  1. Many of the polysaccharides are insoluble in water.

  2. These are not sweet in taste.

  3. These are hydrophobic in nature.

  4. These do not form crystals on desiccation.

  5. Polysaccharides can be extracted in the form of a white powder.

  6. These have high molecular weight as carbohydrates.

  7. These are compact and osmotically active inside the cells.

  8. The hydrogen to oxygen ratio in them is 2:1 and they consist of hydrogen, carbon and oxygen. 

Functions of Polysaccharides

  1. Structural Functions- Common sources of energy are nutrition polysaccharides and most of the organisms can easily break down starch into glucose. However some complex polysaccharides are not very digestible that are known as dietary fiber and these provide important elements in the diet for humans. There are soluble fiber and insoluble fiber that are associated with various functions in the body. The former is responsible for lowering cholesterol levels (bad) in the blood, normalizes blood lipid levels and reduces sugar response after eating. The latter or insoluble fiber reduces the risk of diabetes.

  2. Storage Functions- Storage polysaccharides are starch, glycogen and insulin. A glucose polymer which is insoluble in water is the Starch. Both humans and animals have amylases to digest starches easily. Starch can be found in potato, rice, wheat and maize. Glycogen is the long-term store for energy and works the best in animal and fungal cells. The primary energy storage takes place in the adipose tissue and glycogen is made by the liver and muscles and also within the brain and stomach by glycogenesis. Glycogen serves as an energy reserve for animals, and is the main form of carbohydrate stored in the animal body. It is insoluble in water and yields glucose on hydrolysis.

FAQ (Frequently Asked Questions)

1. Describe the Classification of Polysaccharides.

Polysaccharides can be classified into two classes, namely, homopolysaccharides and heteropolysaccharides. The former type is made up of monosaccharide units and examples include cellulose, glycogen and starch. The latter type is made up of two or more types of monosaccharide units. Eg: Agar

2. What are the Examples of Structural Polysaccharides?

Some examples of structural polysaccharides are arabinoxylans, chitin, pectins and cellulose. Arabinoxylans are present in both the primary as well as secondary cell walls of plants and consist of two sugar arabinose and xylose. Cellulose is the structural component of plants; it is found largely in wood, paper and cotton. It is made up of repeated glucose units bonded by beta linkages. Chitin is a naturally occurring polymer and forms the structural component of many animals like exoskeletons. Pectins are complex polysaccharides and are present in most of the primary cell walls and non-woody parts of the plants.

3. Which are the Three Main Polysaccharides Associated with Human Nutrition?

Starch, Cellulose and Glycogen are the three main polysaccharides associated with human nutrition and the first two are obtained from plants; whereas glycogen is the storage polysaccharide made by the human liver and muscles.