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polysaccharide

  (pŏl'ē-săk'ə-rīd') pronunciation also polysaccharid (-rĭd) or polysaccharose (-rōs', -rōz')
n.

Any of a class of carbohydrates, such as starch and cellulose, consisting of a number of monosaccharides joined by glycosidic bonds.


 
 
Sci-Tech Encyclopedia: Polysaccharide

A class of high-molecular-weight carbohydrates, colloidal complexes, which break down on hydrolysis to monosaccharides containing five or six carbon atoms. The polysaccharides are considered to be polymers in which monosaccharides have been glycosidically joined with the elimination of water. A polysaccharide consisting of hexose monosaccharide units may be represented by the reaction below.
n{\rm C}_6{\rm H}_{12}{\rm O}_2 \rightarrow ({\rm C}_6{\rm H}_{10}{\rm O}_5)_n+ ( n - 1){\rm H}_2{\rm O}

The term polysaccharide is limited to those polymers which contain 10 or more monosaccharide residues. Polysaccharides such as starch, glycogen, and dextran consist of several thousand D-glucose units. Polymers of relatively low molecular weight, consisting of two to nine monosaccharide residues, are referred to as oligosaccharides. See also Dextran; Glucose.

Polysaccharides are often classified on the basis of the number of monosaccharide types present in the molecule. Polysaccharides, such as cellulose or starch, that produce only one monosaccharide type (D-glucose) on complete hydrolysis are termed homopolysaccharides. On the other hand, polysaccharides, such as hyaluronic acid, which produce on hydrolysis more than one monosaccharide type (N-acetylglucosamine and D-glucuronic acid) are named heteropolysaccharides. See also Carbohydrate.


 
Food and Nutrition: polysaccharides

Complex carbohydrates formed by the condensation of large numbers of monosaccharide units, e.g. starch, glycogen, cellulose, dextrins, inulin. On hydrolysis the simple sugar is liberated. See also non-starch polysaccharides.

 
Dental Dictionary: polysaccharide

n

A complex carbohydrate containing a large number of saccharide groups such as starch.

 

Any of a large class of long-chain sugars composed of monosaccharides. Because the chains may be unbranched or branched and the monosaccharides may be of one, two, or occasionally more kinds, polysaccharides can be categorized in various ways. Cellulose, starch, glycogen, and dextran are all polysaccharides of glucose, with different configurations. Pectins are composed of a galactose derivative, chitin of a glucose derivative. Connective tissues, joint fluid, and cartilage contain two-component polysaccharides, including heparin. See also oligosaccharide.

For more information on polysaccharide, visit Britannica.com.

 

A carbohydrate formed of long chains of monosaccharide units linked together. Polysaccharides may form linear or branched chains. They include the storage substances, glycogen and starch, and the structural substance cellulose (fibre or roughage). They are relatively insoluble, not sweet, and have a low osmotic effect.

 
Veterinary Dictionary: polysaccharide

A complex carbohydrate which, on acid hydrolysis, yields many monosaccharides.

 
Wikipedia: polysaccharide
3D structure of cellulose.
Enlarge
3D structure of cellulose.

Polysaccharides are relatively complex carbohydrates. They are polymers made up of many monosaccharides joined together by glycosidic bonds. They are therefore very large, often branched, macromolecules. They tend to be amorphous, insoluble in water, and have no sweet taste.

When all the monosaccharides in a polysaccharide are the same type the polysaccharide is called a homopolysaccharides, but when more than one type of monosaccharide is present they are called heteropolysaccharides.

Examples include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose and chitin.

Polysaccharides have a general formula of Cn(H2O)n-1 where n is usually a large number between 200 and 2500. Considering that the repeating units in the polymer backbone are often six-carbon monosaccharides, the general formula can also be represented as (C6H10O5)n where n=40-3000.

Storage polysaccharides

Starches

Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of Amylose and Amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units.
Starches are insoluble in water. They can be digested by hydrolysis, catalyzed by enzymes called amylases, which can break the alpha-linkages (glycosidic bonds). Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.

Structural polysaccharides

Cellulose

The structural components of plants are formed primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possessing the enzyme are present in their gut. The classic example is the termite.

Acidic polysaccharides

Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.

Bacterial capsule polysaccharides

Pathogenic bacteria commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria. Capsular polysaccharides are water soluble, commonly acidic, and have molecular weights on the order of 100-1000 kDa. They are linear and consist of regularly repeating subunits of one ~ six monosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular polysaccharides, either conjugated or native are used as vaccines.

Bacteria and many other microbes, including fungi and algae, often secrete polysaccharides as an evolutionary adaptation to help them adhere to surfaces and to prevent them from drying out. Humans have developed some of these polysaccharides into useful products, including xanthan gum, dextran, gellan gum, and pullulan.

Cell-surface polysaccharides play diverse roles in the bacterial "lifestyle". They serve as a barrier between the cell wall and the environment, mediate host-pathogen interactions, and form structural components of biofilms. These polysaccharides are synthesized from nucleotide-activated precursors and, in most cases, all the enzymes necessary for biosynthesis, assembly and transport of the completed polymer are encoded by genes organized in dedicated clusters within the genome of the organism. Lipopolysaccharide is one of the most important cell-surface polysaccharides, as it plays a key structural role in outer membrane integrity, as well as being an important mediator of host-pathogen interactions. The genetics for the biosynthesis of the so-called A-band (homopolymeric) and B-band (heteropolymeric) O antigens have been clearly defined, and a lot of progress has been made toward understanding the biochemical pathways of their biosynthesis. The exopolysaccharide alginate is a linear copolymer of ß-1,4-linked D-mannuronic acid and L-guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The pel and psl loci are two recently discovered gene clusters that also encode exopolysaccharides found to be important for biofilm formation. Rhamnolipid is a biosurfactant whose production is tightly regulated at the transcriptional level, but the precise role that it plays in disease is not well understood at present. Protein glycosylation, particularly of pilin and flagellin, is a recent focus of research by several groups and it has been shown to be important for adhesion and invasion during bacterial infection.[1]

See also

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References

Sutherland, I. W. (2002) Polysaccharides from Microorganisms, Plants and Animals, in: Biopolymers, Volume 5, Polysaccharides I: Polysaccharides from Prokaryotes (Vandamme, E. J., Ed.), Weiheim: Wiley VCH, pp. 1-19. ISBN: 978-3-527-30226-0

  1. ^ Cornelis P (editor). (2008). Pseudomonas: Genomics and Molecular Biology, 1st ed., Caister Academic Press. ISBN 978-1-904455-19-6 . 

External links



be-x-old:Полісахарыды


 
Misspellings: polysaccharide

Common misspelling(s) of polysaccharide

  • polysaccaride
  • polysaccharid

 
 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved.  Read more
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Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polysaccharide" Read more
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