Saturday, May 03, 2008

Definitions for Polysaccharides

Definitions for Polysaccharides
Polysaccharides are complex polymers containing only one monosaccharide, homopolysaccharides, or several different monosaccharides or monosaccharide derivatives, heteropolysaccharides. Many polysaccharides exist in the plant and animal kingdoms. However, only a few of these are known to be significant in mammalian nutrition, either as dietary constituents or as human cell metabolites.

The most common digestible polysaccharides in plants is starch, a polymer of glucose. Starch is present primarily in the cells of grains, fruits and tubers in the form of granules that, under microscopes examinations, appear to be typical for each starch. The composition of starches also differs somewhat, but all types contain both amylase, a straight chain polymer of glucoses, and amylopectin, a branch chain polymer. The average chain contains 20 to 25 glucose units with approximately 5 to 8 glucose molecules between branching points within the chain. On hydrolysis in the intestinal tract, starch yields dextrin and maltose and, eventually glucose.

Cellulose is a straight chain polymer of glucose. It is a constituent of the cell walls of plants and gives rigidity to the plant structure much as the skeleton supports to human body. It is not attacked by digestive enzymes of the human and although it provides bulk to the diet it does not contribute significantly to the nutrition of body cells. Cellulose tends to be affected little by usual acid hydrolysis and requires the action of strong mineral acids. Plants also contain indigestible hemicellulose, which are unrelated chemically to cellulose and are homopolysaccharides containing D-xylose. Pectin, present in fruit, is an indigestible heteropolymer and contains arabinose, galactose, and galacturonic acid.
Definitions for Polysaccharides
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