Polysaccharides: Definition, Types, Examples and Functions

Polysaccharides: We all love to eat the items like potato fritters, aloo bonda, aloo Tikki, burgers, pizzas, etc. Guess the nutrient abundant in them? The ingredients like potato, wheat etc., contain abundant starch and are hence also known as a starchy food. But what is starch? Is it sugar, carbohydrate or polysaccharide?  We have learnt something in general about carbohydrates and their types. In this article, we will know more about polysaccharides in particular.

What are Polysaccharides?

The linear or branched chains formed by the joining of monosaccharide units with glycosidic linkages are called polysaccharides. So, polysaccharides are the polymers of monosaccharides. The monosaccharide units are linked with each other by glycosidic bonds.

Features of Polysaccharides

1. These are commonly called glycans or non-sugars.
2. Polysaccharides are large, high molecular weight biological molecules.
3. Many of them are insoluble in water.
4. Polysaccharides are generally not sweet in taste.
5. They form amorphous powder on desiccation.
6. Polysaccharides are polymers of simple sugars linked with each other by covalent bonds called glycosidic bonds.
7. Terminal ends of polysaccharides can be reducing (terminal carbon atom not involved in glycosidic bond) or non-reducing (terminal atom involved in glycosidic bond)
8. The general formula of polysaccharides is \({\left( {{{\rm{C}}_6}{{\rm{H}}_{10}}{{\rm{O}}_5}} \right)_{\rm{n}}}\) where \({\rm{n}} = \) number of monosaccharide units and can vary from \(100\) to \(2500.\)

Types of Polysaccharides

Since the polysaccharides are the polymers of monomeric units, these monomeric units can be of the same type or different types. Depending on the type of constituent monosaccharide units, polysaccharides are classified as:
a. Homopolysaccharides/ Homoglycans: When the monomeric unit is of a single type, e.g. starch, cellulose, glycogen, chitin, etc.
b. Heteropolysaccharides/Heteroglycans: When the monomeric units are of different types. These polysaccharides may contain lipid or protein parts as well—examples: hyaluronic acid, gamma globulins, heparin, etc.
Based on the functional roles, polysaccharides can also be classified as:
a. Storage polysaccharides: which are used as storage of food material or reserve energy, e.g. starch and glycogen.
b. Structural polysaccharides: which mainly contribute to the formation of structural components of cells such as cell walls, e.g. cellulose, hemicellulose, lignin.

Polysaccharides Examples

Let’s look at a few examples of polysaccharides in plants and animals.

Fig: Structural Differences Between the Major Polysaccharides

1. Starch

i. This is the most common storage polysaccharide found in plants acting as a food reservoir.
ii. The photosynthetic end product glucose is converted into starch and stored in different parts of the plant.
iii. It is a polymer of \({\rm{\alpha – D}}\)- glucose subunits and has two forms as amylose and amylopectin.
iv. Amylose is an unbranched polymer where \({\rm{\alpha – D}}\)– glucose subunits are linked with each other by \({\rm{\alpha – 1,}}\,{\rm{4}}\) glycosidicbonds.
v. Amylopectin is the branched part of starch where \({\rm{\alpha – D}}\)– glucose subunits are linked with each other by \({\rm{\alpha – 1,}}\,{\rm{6}}\) glycosidic linkage.
vi. The branching appears approximately per \(20\) to \(30\) glucose residues in the chain.
vii. Starch is hydrolyzed rapidly with amylase enzymes found in saliva and the small intestine.
viii. The amylose is naturally present in a coiled fashion. In such a state, it has spaces to accommodate the iodine molecules which form amylose-iodine complexes. These complexes appear blue-black in colour, and hence starch appears blue-black when treated with iodine. 

Fig: Amylose and Amylopectin

2. Cellulose

i. This is another major polymer of \({\rm{\beta – D}}\)- glucose found in plants.
ii. It is a fibrous, tough and water-insoluble polymer.
iii. Glucose units are linked with each other by \({\rm{\beta – 1,}}\,{\rm{4}}\) glycosidic linkages as a straight chain.
iv. Hydrogen bonding takes place between the neighbouring chains,
v. Which together make a fibril with tensile strength.
vi. Humans do not have an enzyme ‘cellulase’ to digest the beta linkages of cellulose and hence cannot digest cellulose.

Fig: \({\rm{\beta – 1,}}\,{\rm{4}}\) Glycosidic Linkages Between Glucose Molecules in Cellulose

3. Glycogen

i. Glycogen is the primary storage polysaccharide present in animals.
ii. Glycogen is analogous to starch in plants and is many times referred to as animal starch.
iii. Glycogen is thus a polymer of glucose, just like starch.
iv. Glucose units are linked in straight chains by \({\rm{\alpha – 1,}}\,{\rm{4}}\) glycosidic linkage, and the branching points are linked with \({\rm{\alpha – 1,}}\,{\rm{6}}\) glycosidic linkages.
v. The difference between starch and glycogen is that in glycogen, the branching of chains is very extensive.
vi. The average chain length is \(8-10\) glucose residues.
vii. In animals, glycogen is primarily stored by the liver and muscles, while glycogenesis takes place mainly in the liver.
viii. It serves as the energy reserve of the animal body.
ix. Glycogen acts as a reservoir for glucose in the body.

Fig: Bonding of Glucose Residues in Glycogen

Apart from the three main polysaccharides, many minor ones also exist in nature. They are not as abundant as starch, glycogen and cellulose, but their roles cannot be ignored. Some of the examples of minor polysaccharides are:

4. Hemicellulose

i. It is a plant polysaccharide composed of many diverse sugars.
ii. The glucose and acetyl derivatives of glucose are found in hemicellulose.
iii. Hemicellulose is present in the plant cell wall along with cellulose and gives strength to the plant cells.

5. Inulin

i. Inulin is a naturally occurring polysaccharide produced by many plants.
ii. It is a polymer of beta fructose residues linked with each other by \({\rm{\beta – 1,}}\,2\) linkages.
iii. Naturally, inulin is a storage polysaccharide in plants that do not store starch.
iv. Chicory root is the main source for the extraction of commercial inulin.
v. Inulin is used in food manufacturing as a replacement for sugars, fat and flour and in medicine to help measure kidney functions.
vi. Inulin also has bactericidal properties

6. Chitin

i. Chitin is modified carbohydrate-containing nitrogen and is found mainly in arthropods and in fungi.
ii. Polymer of N-acetyl-D-glucosamine monomers linked with \({\rm{\beta – 1,}}\,4\) glycosidic linkage.
iii. It is the major component of the exoskeleton of insects like beetles, bees, cockroaches etc. and arthropod animals like crabs, prawns etc.
iv. Chitin is also a major component of the fungal cell wall.
v. Chitosan is a water-soluble derivative of chitin.
vi. It is used commercially in agriculture as biopesticide and in winemaking. It is also used in medicine due to its medicinal properties.

Fig: insects as natural sources of chitin

7. Pectin

1. Pectin is a heteropolysaccharide present in the cell wall of plant cells.
2. Its major component is galactose derivative-galacturonic acid.
3. It is present in the primary and middle lamella and cell walls of plant cells.
4. It is also present in fruits as fruit fibre.
5. Pectin is used commercially in jams and jellies as a gelling agent.
6. Pectin has medicinal uses in wound healing and speciality medical adhesives.

8. Bacterial Polysaccharides

1. Bacterial cell walls and cell membranes contain a variety of polysaccharides and carbohydrate derivatives together called bacterial polysaccharides.
2. Bacterial polysaccharides include peptidoglycans, lipopolysaccharides and exopolysaccharides.
3. Peptidoglycan is the major constituent of the bacterial cell wall.
4. Pathogenic bacteria secrete thick mucus-like polysaccharides that form a capsule-like structure around them.

Summary

Polysaccharides are the polymers of simple sugars that are joined by glycosidic bonds.  They are true macromolecules as they are polymers with high molecular weight. Polysaccharides are of two types- homopolysaccharides (with only one type of monomeric unit) and heteropolysaccharides (with more than one type of monomeric unit). Functionally we can classify them as structural and storage polysaccharides.

Storage polysaccharides act as reserve food material while structural polysaccharides form the major part of structures such as cell walls, fibrous tissue and exoskeleton. Starch and glycogen are the storage polysaccharides in plants and animals, respectively. Cellulose is the major structural polysaccharide in plants and gives us wood, paper and cotton. Chitin is an example of a structural polysaccharide in animals. Thus, storage of energy and structure formation are the two main functions of polysaccharides.

FAQs

Q.1. What are the four examples of polysaccharides?
Ans: The four main examples of polysaccharides are cellulose, starch, glycogen and chitin. Starch and glycogen are storage polysaccharides, while cellulose and chitin are structural polysaccharides in plants and animals, respectively.

Q.2. What are polysaccharides made of?
Ans: The linear or branched chains formed by the joining of monosaccharide units with glycosidic linkages are called polysaccharides.

Q.3. What are the functions of polysaccharides?
Ans: Polysaccharides have two main functions.
a. They serve as storage of reserve food material in plants and animals.
b. They form the structural components of the bodies of living organisms.

Q.4. What are the food sources of polysaccharides?
Ans: Following are few examples of food items rich in polysaccharides:
a. Tubers- Potato, sweet potato, tapioca contain starch
b. Seeds grains- such as rice, wheat, corn etc. contain starch
c. Fruits- Pectin 
d. Gums and resins from plants also contain ample amounts of polysaccharides.

Q.5. Are polysaccharides good or bad?
Ans: Polysaccharides are natural polymers found in plants, animals and microbes. They have high nutritive values and are essential for the good immune system, digestive health and detoxification of the human body.  But excess polysaccharides can lead to conditions like obesity.

We hope this detailed article on Polysaccharides helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.