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A polysaccharide is a long molecule of carbohydrates, and is composed of repeated monomers that are joined together by bonds known as glycosidic bonds. The bonds are products of a condensation reaction. Polysaccharides range in structure, from highly branched structured polysaccharides to linear structured ones.
Often they are quite heterogeneous and may contain slight modifications with repeating units of the monomers. The macromolecules may be amorphous or water soluble and this largely depend on structure as well as the monosaccharide building blocks.
A polysaccharide is called a homopolysaccharide if it all the monosaccharide is of the same type and a heteropolysaccharide/ heteroglycan if it has been formed from more than one type of monosaccharide. (Stout and Taylor)
Polysaccharides are divided into two major categories namely:
- Structural polysaccharides
- Storage polysaccharides
Structural polysaccharides include
Cellulose– this is one of the main components of plant cell walls. As an un-branched polymer, cellulose is composed of about ten thousand units of glucose per chain. A hydroxyl group, which forms hydrogen bonds with other chains projects out from each of the chains, and forms rigid cross links in between the chains. This is what makes cellulose a strong support material. Cellulose is one of the most abundant organic substances in the world.
Chitin– this is closely related to cellulose in terms of the structure. The difference between the two is that while cellulose has a hydroxyl group, has (–NH.CO.CH3). Chitin may be found in some fungi and green algae cell walls and cuticles of arthropods.
Characteristics of storage polysaccharides
Insoluble to water,
Can fold into smaller shapes,
Can be converted to sugars easily
Glycogen– glycogens are the main form in which carbohydrates are stored in the body liver and muscles. Glycogen is branched structures and is broken down to produce sugars in cases where the body is starved.
Starches are found in plant cells, some bacteria and protists and are made up of amylopectin and amylose.
Pectins– this is a group of complex polysaccharides and present in a majority of primary cell walls and non- woody terrestrial pants. They also contain a 1, 4 linked alpha D- galactosyluronic acid residues (Dongowski, Gebhard & Flamme 2002).
These compounds are made up of D- glucose and contain beta glycosidic bonds. They are diverse vary in terms of:
Configuration in dimensions
They are better known as a class of soluble fibers found in many plants such as wheat, oats and barley. Beta glycans occur largely as cellulose and contribute positively in the nutrition of the human body, soluble fiber supplements and texturing agents.
Beta glycan derived from medicinal mushrooms and yeast is used to boost the immune system, and studies have showed the insoluble beta glycans to have a higher biological activity as compared to the soluble ones (Dongowski, Gebhard & Flamme 2002).
Some types of beta glycans such as those made from barley and yeast are used to reduce total cholesterol and LDL cholesterol for individuals with high cholesterol following some weeks of treatment (Patchen & MacVittie1986). Moreover, it is used to increase the chances of survival in individuals suffering from cancer, prevent infections and boost the immunity of HIV/ AIDS patients.
Green, Stout and Taylor, Biological Science 1&2, 2nd ed. Cambridge Cell and Molecular Biology by Sheeler and Bianchi, 3rd ed. Wiley
Dongowski, G; Huth M, Gebhardt E, Flamme W (December 2002). “Dietary fiber-rich barley products beneficially affect the intestinal tract of rats”. The Journal of nutrition (United States: American Society of Nutritional Sciences) 132 (12): 3704–3714
Patchen, ML; MacVittie TJ (February 1986). “Comparative effects of soluble and particulate glucans on survival in irradiated mice”. Journal of biological response modifiers (United States: Raven Press) 5 (1): 45–60