chemistry 12th class cbse notes

Polysaccharides

Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. They mainly act as the food storage or structural materials.

Starch

  • Main sources of starch are wheat, maize, rice, potatoes and barley.
  • It is a white amorphous powder, in-soluble in cold water.
  • It is a polymer of α-glucose and consists of two components- Amylose and Amylopectin.
  1. Amylose: It is a linear polymer of α‒ glucose. It is water soluble and gives blue colour with iodine solution.
  2. Amylopectin: It is a branched chain polymer of α‒ glucose. It is water in-soluble and does not give blue colour with iodine solution.

Cellulose

  • Cellulose is a linear polymer of β-glucose. It consists of bundles of polymeric chains which are held together by hydrogen-bonding.
  • Main sources are wood (45 –50%), cotton (90 – 95%). It is the main constituent of cell walls.
  • It is colourless amorphous solid that decomposes on heating.
  • It does not reduce Tollen’s reagent or Fehling’s solution.
  • It cannot be digested by human stomach.
  • Cell wall of bacteria and plants is made up of cellulose
  • Cellulose forms a number of important products, such as:
  1. Cellotape used for packaging.
  2. Viscose rayon used in textile industry.
  3. Gun cotton used as an explosive.

Glycogen

  • The carbohydrates are stored in animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched.
  • It is present in liver, muscles and brain.
  • When the body needs glucose, enzymes break the glycogen down to glucose.
  • Glycogen is also found in yeast and fungi.

 

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