Polysaccharides

Now you have heard of simple carbohydrates and complex carbohydrates. The most common form of carbohydrate that we find in the food we consume is Starch which is a complex carbohydrate. Now, these complex carbohydrates have a highly branched molecular structure and are named polysaccharide. Let us study them in detail.

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Polysaccharide

These are complex carbohydrates made up of repeating units of monosaccharides that are attached together by Glucosidic linkage. A molecule of a polysaccharide has n number of sugar molecules bound together to form a larger molecule. Another name for them is Glycans.

Polysaccharides are classified into two parts, namely

1. Homopolysaccharide: These molecules are made up of only one type of monosaccharides. A general theme for them is derived from the kind of monosaccharide units they have. A homopolysaccharide made up of only glucose molecules is named Glucans. One with only galactose molecules earns the name Galactus. In this given topic we will be focussing only on Glucans.
2. Heteropolysaccharide: These are polysaccharide molecules consisting of more than one type of monosaccharides.

Now let us focus on the three main polysaccharides commonly found in nature. They are the ones that we see every day in our day to day lives.

Browse more Topics under Biomolecule

• Carbohydrates
• Monosaccharides
• Disaccharides
• Structure of Proteins
• Amino Acids
• Enzymes
• Vitamins
• Nucleic Acids
• Structure of Nucleic Acids

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General Formula of Carbohydrate

Starch

(Source: Wikipedia)

Starch is an element present in all photosynthetic plants. We generally find starch in the plant’s roots and seeds. All plants when they synthesize glucose, the extra glucose is stored in the form of starch.

Starch is a glucan, meaning it only consists of glucose molecules all linked together. The general molecular formula for starch is ( C₆H₁₀O₅)n. The ‘n’ denotes the number of molecules linked together.

We find starch in the seeds of plants as granules. On heating these granules in the water we form a colloidal suspension. We obtain two components from this process. These two components are Amylose and Amylopectin.

Amylose

• Amylose themselves are also polysaccharides.
• Constitute about 10-20 % of a starch molecule
• They are made up of D-glucose units that connect with each other with the help of a α-glycosidic linkage.
• One glucose unit  connects to another glucose unit from the one-four position i.e. { α (1-4) }
• Amylose has the same basic structure of maltose, multiplied by ‘n’ number of times.
• In a basic amylose structure, there are almost 1000 upwards glucose molecules forming a link
• Although they are a big molecule they are very compact in size because they form an alpha-helical structure.
• Amylose molecules exist in form of a helix

Amylopectin

• They have the same basic structure that Amylose does which is D-glucose units combining in a { α (1-4) } form
• Constituent about 80-90% of a starch molecule
• They have a very interesting structure. They have a main branch similar to Amylose, but then also have branches.
• Branching in amylopectin occurs between C6 – C1, which means the sixth carbon in the chain connects with the first carbon of the branch.
• And the branching occurs every twenty to twenty-five glucose units.

Glycogen

(Source: Freedictionary.com)

Glycogen is also a Glucon i.e. it is made up exclusively of D-glucose units. It is a reserved carbohydrate source for animals as well as plants. Let us now see the structure and the functions of Glycogen.

Structure

The structure of glycogen is similar to that of Amylopectin. The only exception being that glycogen is very highly branched. In a glycogen molecule, the branching happens more frequently, almost after every six glucose units. This is the reason glycogen behaves differently to Amylopectin. This is the reason a glycogen molecule has a very high molecular weight. It is not compact in size either, it is a big molecule,

A hydrolysis experiment will suggest that in a glycogen molecule, one end group occurs after every ten to twelve units of glucose.

Functions

Glycogen performs some very important functions in plants and animals, It is able to perform these functions due to its unique structure and formation.

• Now as you are aware, glucose is found in the cell membranes of cells of plants and animals. These glucose molecules are very small and compact. They can easily diffuse out of a cell membrane. But glycogen is a big and complex molecule, so it will not diffuse out of the cell membrane. Hence it is an important function of glycogen, the storage of glucose within cells.
• If a large number of glucose cells are present inside the cells, the osmotic pressure in the cell will be very high. This can cause the cell membrane to burst. But if glucose combines into one big molecule of glycogen, the problem does not occur.
• As mentioned earlier glycogen is a glucose reserve for the cells of our body. If the glucose concentration is low, enzymes present in the cells can easily hydrolyze the end groups of glycogen to make glucose. This detachment process is made easy due to the structure of glycogen.
• The reverse of the above is true as well. If the glucose concentration is high, enzymes can attach glucose molecules to form glycogen.

Cellulose

(Source: Myorganicchemistry.com)

Cellulose is an important structural element of the cell walls of all photosynthetic plants. It is a fibrous kind of polysaccharide which is highly insoluble in water. Here again, Cellulose is a glucan. The D-glucose units connect in (1→4) fashion.

The connection though is different from starch and glycogen, it is a beta linkage. So the linkage is β-glucosidic linkage. The structure is not helical since the beta linkage confines the polysaccharide to a straight-chain form.

In the structure of cellulose -OH groups point outside the chain structure. Whenever two chains come close to each other they tend to form a stack on each other due to hydrogen bonding between these hydroxyl groups. As a result, we get a fibrous insoluble structure which is suitable for the functions of cellulose in the cell walls.

Learn more about Monosaccharides here in detail.

Solved Example for You

Q: The solution of iodine in KI is used to detect a solution of starch and glycogen, as it gives:

1. blue colour with starch and red colour with glycogen
2. a green colour
3. green colour with starch and red colour with glycogen
4. blue colour with starch and yellow colour with glycogen

Sol: The correct answer is option “A”. The solution of iodine in KI is used to detect a solution of starch and glycogen, as it gives blue colour with starch and red colour with glycogen.