AskDefine | Define polysaccharide

Dictionary Definition

polysaccharide n : any of a class of carbohydrates whose molecules contain chains of monosaccharide molecules [syn: polyose]

User Contributed Dictionary

English

Noun

  1. A polymer made of many saccharide units linked by glycosidic bonds.
    Cellulose, starches, and complex carbohydrates, such as glycogen, are common polysaccharides in biology.

Translations

complex carbohydrate
  • Finnish: polysakkaridi
  • Italian: polisaccaride

Extensive Definition

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 homopolysaccharide, 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=.

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 corn are major sources of starch in the human diet.

Glycogen

Glycogen is a polysaccharide that is found in animals and is composed of a branched chain of glucose residues. It is stored in liver and muscles.

Structural polysaccharides

Cellulose

The structural component 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.

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

External links

homopolysaccharides : glucans
polysaccharide in Arabic: عديد السكاريد
polysaccharide in Belarusian (Tarashkevitsa): Полісахарыды
polysaccharide in Bosnian: Polisaharidi
polysaccharide in Bulgarian: Полизахарид
polysaccharide in Czech: Polysacharidy
polysaccharide in Danish: Polysakkarid
polysaccharide in German: Polysaccharide
polysaccharide in Spanish: Polisacárido
polysaccharide in Esperanto: Polisakarido
polysaccharide in Basque: Polisakarido
polysaccharide in French: Polysaccharide
polysaccharide in Scottish Gaelic: Poilisacaraid
polysaccharide in Galician: Polisacárido
polysaccharide in Ido: Polisakarido
polysaccharide in Italian: Polisaccaridi
polysaccharide in Hebrew: רב-סוכר
polysaccharide in Lithuanian: Polisacharidai
polysaccharide in Hungarian: Poliszacharid
polysaccharide in Malay (macrolanguage): Polisakarida
polysaccharide in Dutch: Polysacharide
polysaccharide in Japanese: 多糖
polysaccharide in Occitan (post 1500): Polisacarid
polysaccharide in Polish: Polisacharydy
polysaccharide in Portuguese: Polissacarídeo
polysaccharide in Romanian: Polizaharide
polysaccharide in Russian: Полисахариды
polysaccharide in Albanian: Polisakaridet
polysaccharide in Slovak: Polysacharid
polysaccharide in Sundanese: Polisakarida
polysaccharide in Finnish: Polysakkaridi
polysaccharide in Swedish: Polysackarider
polysaccharide in Turkish: Şekerler
polysaccharide in Ukrainian: Полісахариди
polysaccharide in Chinese: 多糖
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