Polysaccharides

When several monosaccharides are linked together by glycosidic bonds, it forms a polysaccharide. It is a polymeric carbohydrate. It is one of the major classes of biomolecules and an important source of energy. It can be a homopolysaccharide or a heteropolysaccharide depending upon the type of the monosaccharides. It can be a straight chain of monosaccharides known as linear polysaccharides, or it can be branched known as a branched polysaccharide.

Characteristics of Polysaccharides

Polysaccharides have the following properties:

  1. They are high molecular weight carbohydrates.
  2. They consist of hydrogen, carbon, and oxygen, the hydrogen to oxygen ratio being 2:1
  3. They are hydrophobic in nature.
  4. They are not sweet in taste.
  5. They do not form crystals on desiccation.
  6. Many are insoluble in water.
  7. Inside the cells, they are compact and osmotically inactive.
  8. Can be extracted to form white powder.

Types of Polysaccharides

Polysaccharides are categorized into two types:

  • Homopolysaccharides
  • Heteropolysaccharides

Homopolysaccharides

A polysaccharide that contains the same type of monosaccharides is known as a homopolysaccharide. Some of the important homopolysaccharides are:

  • Starch: It is formed by the condensation of amylose and amylopectin. It is found largely in plants, fruits, seeds, etc.
  • Insulin: It is made up of a number of fructofuranose molecules linked together in chains. It is found in the tubers of dahlia, artichoke, etc.
  • Glycogen: It is made up of a large chain of molecules. It is found in animals and fungi.
  • Cellulose: The cell wall of the plants is made up of cellulose. It comprises of long chains of ꞵ-glycosides.

Heteropolysaccharides

A polysaccharide that contains different types of monosaccharides is known as a heteropolysaccharide. Some of the important heteropolysaccharides are:

  • Hyaluronic Acid: It is made up of D-glucuronic acid and N-acetyl-glucosamine. It is found in connective tissues and skin.
  • Heparin: It is made up of D-glucuronic acid, L-iduronic acid, N-sulfo-D-glucosamine and is largely distributed in mast cells and blood.
  • Chondroitin-4-sulfate: Its component sugars are D-glucuronic acid and N-acetyl-D-galactosamine-4-O-sulfate. It is present in the cartilages.
  • Gamma globulin: N-acetyl-hexosamine, D-mannose, D-galactose are the component sugars of this polysaccharide. It is found in the blood.

Functions of Polysaccharides

The polysaccharides serve a variety of function:

  1. They store energy in organisms.
  2. Due to the presence of multiple hydrogen bonds, the water cannot invade the molecules making them hydrophobic.
  3. They allow for changes in the concentration gradient which influences the uptake of nutrients and water by the cells.
  4. Many polysaccharides become covalently bonded with lipids and proteins to form glycolipids and glycoproteins. These glycolipids and glycoproteins are used to send messages or signals between and within the cells.
  5. They provide support to the cells. The cell wall of plants is made up of polysaccharide cellulose which provides support to the cell wall of the plant. In insects and fungi, chitin plays an important role in providing support to the extracellular matrix around the cells.

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