Polysaccharides
Polysaccharides are complex carbohydrates formed from monosaccharides. A number of monosaccharide molecules such as those of glucose, become linked by glycosidic bonds with the elimination of a molecule of water for each monosaccharide added. When a polysaccharide has multiple molecules of the same type, it is described as homopolysaccharide. For example, starch and glycogen are composed of only glucose. When a polysaccharide molecule is formed by more than one type of monosaccharide molecules, it is described as a heteropolysaccharide. For example, chitin and agar.
fig. 14.1 - Structural Appearance of Polysaccharides
fig. 14.2 - Starch Grains from Different Sources
Based on their functional significance, polysaccharides can be distinguished under two categories namely storage polysaccharides (e.g., starch and glycogen) and Structural polysaccharides (e.g., cellulose, lignin and chitin).
The following table lists the biologically important polysaccharides and their functions.
| Name of the Polysaccharide | Composition | Occurrence | Functions |
|---|---|---|---|
| 1. Starch | Polymer of glucose containing a straight chain of glucose molecules (amylose) and a branched chain of glucose molecules (emylopectin) | In several plant species as main storage carbohydrate | Storage of reserve food |
| 2. Glycogen | Polymer of glucose | Animals (equivalent of starch) | Storage of reserve food |
| 3. Callose | Polymer of glucose | Different regions of a plant, In the sieve tubes of phloem | Formed often as a response to wounds |
| 4. Insulin | Polymer of fructose | In roots and tubers (like Dahlia) | Storage of reserve food |
| 5. Cellulose | Polymer of glucose | Plant cell wall (most abundant organic molecule on the_Earth) | Cellwall matrix |
| 6. Pectin | Polymer of galactose and its derivatives | Plant cellwall | Cellwall matrix |
| 7. Hemicellulose | Polymer of pentoses and sugar acids | Plant cellwall | Cellwall matrix |
| 8. lignin | Polymer of glucose | Plant cellwall (dead cells like sclerenchyma) | Cellwall matrix |
| 9. Chitin | Polymer of glucose | Bodywall of arthropods. In some fungi also | Exoskeleton Impermeable to water |
| 10. Murein | Polysaccharide cross linked with amino acids | Cell wall of prokaryotic cells | Structural, protection |
| 11. Hyaluronic acid | Polymer of sugar acids | Connective tissue matrix. Outer coat of mammalian eggs | Ground substance, protection |
| 12. Chondroitin sulphate | Polymer of sugar acids | Connective tissue matrix | Ground substance |
| 13. Heparin | Closely related to chondroitin | Connective tissue cells | Anticoagulant |
| 14. Gums and mucilages | Polymers of sugars and sugar acids | Gums - barks of trees. Mucilages-flower | Retain water in dry seasons |
fig.14.3 - Biologically Significant Polysaccharides
