These are the carbohydrates which are made of large number of monosaccharide units. They are naturally occurring condensation polymers in which large number of monosaccharide units are joined together through glycosidic linkages. They are colourless, tasteless amorphous powders. They play vital role in plant and animal life.
Let us study the structure of three important polysaccharides starch, cellulose and glycogen.
1. STARCH (AMYLUM), (C6H10O5)n
It is a most abundant source of carbohydrate in human diet. It is a major food reserve material of plants and occurs mainly in seeds, fruits, tubers and roots of the plants. The important sources of starch are wheat, rice, cassava, cocoyam maize, potatoes, legumes and other vegetables. repared foods such as fufu, akple, omotuo, tapioca and kokonte have a high starch content.
Starch is the polymer of D-glucose. It consists of two components:
a) Amylose, the water soluble fraction, and a linear, polymer of D-glucose units. It gives blue colour with iodine.
b) Amylopectin, the water insoluble fraction, and consists of linear as well as branched chain polymers of D-glucose.
Natural starch contains approximately 15-20% of amylose and 80-85% of amylopectin. Amylopectin does not give blue colour with iodine. A molecule of amylose may contain 200-1000 glucose units whereas a molecule of amylopectin may contain 2000-3000 glucose units. In amylose. as well as amylopectin the D-glucose units are linked through a-glycosidic linkages between C-l of one glucose unit and C-4 of the next glucose unit. Branching in amylopectin occurs through C1 – C6 glycosidic linkage.
The section of amylose as well amylopectin has been given as follows:
On hydrolysis with dilute acids or enzymes, starch breaks down into maltose and finally glucose.
(C6H10O5)n → C12H22O11 → C6H12O6
Starch Maltose Glucose
Starch is a white amorphous powder. It is tasteless and is insoluble in cold water.
2. CELLULOSE, (C6H10O5)n
Cellulose occurs exclusively in plants. It is the most abundant organic substance in plant kingdom. It is a chief structural material of cell walls of all the plants. It is also the chief component of cotton, wood and jute. Wood contains 45-50% while cotton contains 90-95% cellulose. It may be noted that over 50% of the total organic matter of the living world is cellulose.
Structurally, cellulose is a linear polymer of glucose units joined by β-glycosidic linkage between C-1 of one glucose unit and C- 4 of the next glucose unit as shown below:
The chains are held together by hydrogen bonds between glucose units of adjacent strands. The chains are so arranged as to constitute the bundles. This lends rigidity to its structure
Cellulose does not reduce Tollen’s reagent or Fehling’s solution. It is not fermented by yeast. Cellulose cannot be hydrolysed easily. However, when it is heated with dilute sulphuric acid, under pressure, it undergoes hydrolysis and yields glucose.
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- Cellulose forms many useful products when treated with suitable chemical reagents. Some of the important products are:
- Celluloid, which is used in films.
- Gun cotton, which is an explosive.
- Cellulose acetate is used in plastics, for wrapping films and nail polishes.
- Methyl cellulose is used in cosmetics, pastes and for fabric sizing.
- Ethyl cellulose is used in plastic coats and films.
On treatment with concentrated sodium hydroxide, cellulose forms a transluscent mass which imparts a silky lustre to cotton. The process is called mercerisation and cotton so produced is called mercerised cotton.
Glycogen is the principal reserve of carbohydrates in animals. The molecular structure of glycogen is similar to amylopectin. That is why it is also known as animal starch. However, one main difference between glycogen and amylopectin is that average chain length in glycogen is
10-14 while in amylopectin it is 25-30 glucose units.
Glycogen is present in liver, muscles and brain. When the body needs glucose, enzymes break glycogen into glucose.
In addition to starch and cellulose, a number of other polysaccharides are used as food components. These are the gums and pectins. Gums are polysaccharides made up of more than one type of monosaccharides. They are used for thickening and improvement of texture in food industry. Pectins are found in fruit skins and are extracted by boiling. Jelly preparations contain pectin dissolved in a fruit juice. The pectin causes jelly to set into a semi-solid.