LEARNING OBJECTIVES
In this article, author has explained the introduction, structure, characteristics, classification, functions and examples of polysaccharides
Polysaccharides definition
Table of Contents
Polysaccharides are high molecular weight carbohydrates that are made of repeating units of monosaccharides.
On hydrolysis, polysaccharide yield many units of monosaccharides and oligosaccharides. Polysaccharide has two types of structures; linear and branched. The branched structure of polysaccharide exists due to presence of free hydroxyl group at one monosaccharide. A polysaccharide is also known as “glycan”. Polysaccharide can be linear or branched depending upon the structure.
Structure of polysaccharide
A polysaccharide is made of repeating units of monosaccharides that are linked through the glycosidic bond. The 10 or more than 10 units are linked through 1, 4-glycosidic bond.
The glycosidic bond is formed with the elimination of water. Hydroxyl group is removed from one monosaccharide and hydrogen is removed from another monosaccharide to form polysaccharide.
Characteristics of polysaccharide
- They are high molecular weight carbohydrates.
- The molecular formula of polysaccharides is (C6H10O5)n where n can be 100 to 3000.
- They are insoluble in water.
- Like other carbohydrates, polysaccharides are not sweet in taste.
- They store high amount of energy.
- They provide support to plants and animals.
- Component of plant’s cell wall
- Component of fungi’s cell wall
- They are not crystalline (do not form crystals)
- Due to presence of many monosaccharides, polysaccharides are hydrophobic in nature
Classification of polysaccharides
Polysaccharides can be classified into two types on the basis of type of monosaccharide present.
1. Homopolysaccharide
The polysaccharides which are made up of single type of monosaccharides are called homopolysaccharide. Glucans are composed of glucose only. Fructosans are composed of fructose only.
For example, glycogen, cellulose and starch are some homopolysaccharides.
As shown from the structure, glycogen is made of single type of repeating unit that’s why it is known as homopolysaccharide.
2. Heteropolysaccharides
The polysaccharides which are made of different types of monosaccahrides and their derivatives are called heteropolysaccharides.
For example, hyaluronic acid, heparin and gamma globulin are heterpolysaccharides.
Functions of polysaccahrides
Polysaccharides play important role in storage of energy, cellular support and many other functions in cell.
1. Storage of energy
Polysaccharides are major class of carbohydrates that store energy. Starch stores energy in plants while glycogen stores energy in animals. Polysaccharides are made up of many monosaccharides so, they contain a large number of OH groups. This property of polysaccharides makes them hydrophobic. They remain together in cytosol and do not dissolve. Due to this reason, sugar content in cell decreases so, more sugar can be taken in.
More sugar means more energy. So, polysaccharides store energy in the form of starch and glycogen. Glycogen is present in liver, muscles and brain. When body needs energy, glycogen breaks down to glucose through the process of glycogenolysis.
2. Cellular communication
Polysaccharides are present in conjugated form (linked with other biomolecules). When proteins are attached with carbohydrates, they are called as glycoprotein. When lipids are attached with polysaccahrides, they are known as glycolipids.
Glycolipids and glycoprotein play role in cell signaling. They help in transport. When protein has to take inside the membrane, it attaches with the protein part of glycoprotein and thus moves inside the membrane. In this way, polysaccharide plays role in cellular communication and transport.
3. Cellular support
Polysaccahrides provides support to the plants and many other organisms. For example, cellulose is a major component of plants that provides support to the plants. It keeps the structure of plants stable. Cellulose is a major component of the cell wall of plants. Due to its high tensile strength, cellulose is responsible for the shape and rigid structure of plants.
Chitin is another polysaccharide that plays role in maintaining structure of fungi and other insects. The cell wall of fungi is composed of chitin which provides structural integrity to the fungi. Chitin is also found in the exoskeletons of insects.
Examples of polysaccharides
Starch
Starch is a homopolysaccharide made up of glucose monomers linked through 1, 4 glycosidic bond. It is found in the leaves, stem, roots and fruits of plants. Starch is a primary source of energy for the plants. Plants absorb direct energy sunlight and store it in the form of starch.
Starch is of two types, amylose and amylopectin. Amylose is a long unbranched starch which is insoluble in water. It has α 1, 4 glycosidic linkage. Amylase can be completely hydrolyzed by α amylase and β amylase.
Amylopectin is a branched starch which is soluble in water. It has α 1, 6 glycosidic linkage. Amylopectin cannot be hydrolyzed by α amylase and β amylase.
Glycogen
Glycogen is also a homopolysaccharide. It is also known as “animal starch”. It stores high amount of energy in animals especially in brain, liver and muscles. When glucose is deficient in body, glycogen breaks down to glucose through the process of glycogenolysis. Glycogen is made up of repeating units of glucose. The only difference between starch and glycogen is that glycogen is more branched as compare to starch.
Cellulose
Cellulose is major component of the plants cell wall. It is made of glucose units that are linked through β 1, 4 glycosidic bonds. Certain organisms including humans cannot digest cellulose due to absence of enzyme cellulose. This enzyme is present in some microorganisms which are present in the gut of herbivores. Although humans cannot digest cellulose, but it is needed in our diet in the form of fibers.