Structure of Different Polysaccharides
Structure of starch, cellulose and glycogen
Polysaccharides are the carbohydrates that are polymers of monosaccharide on which more than 10 sugar (monosaccharide) units are linked together through glycosidic linkage. They are one of the four major classes of biomolecules. The other three classes are nucleotides, proteins and lipids. They have varieties of roles and functions in the living system. They are of many types with different properties and functions. Cellulose, hemicelluloses, glycogen, starch, mucopolysaccharides, algal, bacterial and fungal polysaccharides, pectins and plant gums are of different examples of polycassharides. The monomeric units of polysaccharides are also of different types. Some are simple sugars and some are modified sugars. Glucose, fructose, galactose, mannose, ribose, xylose, arabinose are the simple sugars while glucuronic acid, galacturonic acid, N-acetyl glucosamine, N-acetyl galactosamine, N-acetylmuramic acids are some modified sugars. Some polysaccharides are linear in structure like cellulose and mucopolysaccharides, while some are branched like starch and glycogen. Some are acidic like hyaluronic acid, heparin sulfate while othere are neutral like glycogen, cellulose etc.
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Some Functions of Polysaccharides
- Storage: Some polysaccharides are used to store energy (as food reserve) like glycogen, starch, fructans and they are stored as food reserve for future uses. When there is need of energy they are hydrolyzed to provide energy and some biosynthetic precursors.
- Structure: Some act as structural unit protecting against environmental factors. These structural polysaccharides are cellulose, hemicelluloses, chitin etc. cellulose together with hemicelluloses is found in plant cell walls to provide rigidity to the cell while chitins are found in exoskeleton of insects.
- Recognition determinants: Some polysaccharides are also involved in recognitions like glycoproteins present at the surface of RBC are proteins linked to polysaccharides. This polysaccharide moiety is involved in recognition function and is recognized by special antibodies. Based on recognition and sugar moieties involved in recognition blood antigens are of three types making three blood group types.
- Natural lubricants: Mucopolyaccharides like keratan sulfate, chondroitin sulfate, hyaluronic acids are some polysaccharides that are found some tissues like joints, cornea, cartilage, extracellular fluids of animals and act as natural lubricants and also absorb mechanical shucks.
- Anticoagulants: Some polysaccharides like heparin, carrageenan are used as anticoagulants because of their special property. They are highly acidic mean while they carry highly negatively charged groups like carboxylic groups, sulfate groups and therefore they have ability to bind and scavenge the metallic ions. They are used as anticoagulants because they can bind ions (like Calcium ions, potassium ions) that can initiate coagulation of blood and thus prevents coagulation of blood.
- Secretion as gums: Some polysaccharides are secreted as excretions like plant gums. These plant gums (excretions) are secreted from injured tissues of plants which are sticky in nature to prevent the injured tissue from further infections and damages from external infectious agents like bacteria, fungi etc. Thus creates barrier in between the external environment and injured tissues.