Carbohydrates 3 - Polysaccharides

 Let us understand what polysaccharides are, using 2 common examples of Polysaccharides, Cellulose and Glycogen.

 

Cellulose

Have you ever wondered how leaves, stems and roots of plants maintain their structure?  What help plants to create that rigid structure and give each plant their own unique shape? What gives them that immense strength to grow to such heights? And not to forget how does it protect itself from all that is deemed harmful to it?

Cellulose is a structural polysaccharide and the most abundant organic compound in the world and accounts for over ½ of the C in the biosphere. It is a primary structural component of plant cell walls. It is one of the main components responsible for all mentioned above.

Its structural formula is as follows,

It is composed of a linear chain of β – glucose molecules linked together by β (1 – 4) glycosidic linkages.

n = 15,000 β – D – glucose molecules

 

Several such cellulose chains which are positioned parallel to each other form H bonds between them. This form cross links resulting in the formation of cellulose microfibrils. Microfibrils contain around 80 such cellulose chains positioned parallel to each other.

Many microfibrils bundle together and form cellulose fibers.

Glycogen

We talked how glucose acts as the main source of energy in animals but does the body that assimilate glucose from the food we eat, use up all of it at once? If not, then it should be stored right? And used when needed. How does human bodies do this? This is where Glycogen comes in.

It is a storage polysaccharide found in animal cells. Specially in muscle and liver tissue of vertebrates. The glucose from food is converted to glycogen by hormones (Insulin) and stored in muscle and liver tissue. Then, when it is needed it is converted beck to glucose again by hormones (Glucagon).

 It is composed of a branched chain which is a result of 2 types of glycosidic linkages formed between glucose monomers.

The 2 types of chain molecules involved in forming glycogen are amylose and amylopectin.

Amylose is a linear unbranched chain containing α (1 – 4) glycosidic linkages.

While amylopectin is a branched chain containing α (1 – 4) glycosidic linkages and α (1 – 6) glycosidic linkages which is responsible for the branching.

• So, what can we learn about Polysaccharides by looking at these 2 examples?

We see that there are certain properties that are common to all polysaccharides. They are,

1.     1.   Each polysaccharide is a long chain polymer molecule composed of identical repeating units called monomers linked together by glycosidic bonds.

i)        Cellulose – monomer – β – Glucose.

ii)       Glycogen – monomer – α – Glucose.

Since they are made of the same monomer linked together, they are called homo-polysaccharides. Those which are made of different types of monomers units linked together are called hetero-polysaccharides.

 

1.       2.   They are all large polymer molecules having a high molecular weight.

Examples of Hetero-polysaccharides and their functions;

Hyaluronic acid act as a shock absorber and a lubricant. It is found in vitreous humor of human eye, connective tissue and synovial fluid found in joints.

It is made of 2 types of monomer units linked by glycosidic bonds and repeating, forming a long chain molecule. 

Heparin is an anti – coagulant and it prevents the formation of blood clots. It is formed in mast cells.