A polysaccharide is the result of two or more monosaccharides combining via dehydration synthesis. Also known as glycans, most carbohydrates come in this complex form, such as glycogen or maltose. Depending on whether or not they've combined in a linear chain or a branched web, they're either very rigid or water-soluble and form pastes with water, respectively. For the most part they do not taste terribly sweet, with obvious exceptions, such as sucrose. The general formula for a polysaccharide is [Cx(H2O)y)]n where x is the number of carbons, y is the total number of water molecules that could be made out of the molecule's contents and n is the number of sugars in the polysaccharide. Note that y is usually equal to x-1.

An important sub-group of polysaccharides are oligosaccharides. Oligosaccharides are formed when anywhere from two to twenty monosaccharides combined to form a polysaccharide. While still polysaccharides, they're often grouped in this specific category since their size is limited and most of the commonly known polysaccharides (common sugars) are part of this group.

Common polysaccharides include glycogen, maltose, starch, cellulose, gum arabic, mannose, pectins, chitin, heparin and gum tragacanth. Structural example:

Sucrose (table sugar)

   H   CH2OH
    \ /
HO   C -- O   H   HOCH2  O    H
  \ /      \ /       \ /   \ /
   C        C -- O -- C     C--CH2OH
  / \      /          |     |
 H   C -- C -- OH     C --- C -- OH
    / \   |          / \     \    
  HO   H  H        HO   H     H    

When a polysaccharide is composed of units of the same sugar then it is known as a homopolysaccharide. Different sugars fusing to form a polysaccharide are known as heteropolysaccharides and usually no more than two different sugar types are found in the molecule. Homopolysaccharides are most often found in plants and animals as energy storage since glycogen and starch are both made exclusively out of glucose. As mentioned previously, linear polysaccharides have a rigid structure and this lends well to creating the cell walls present in plant cells. Heteropolysaccharides are usually part of proteins or lipids and found in animal tissues.

Sources:
"Polysaccharide." Encyclopædia Britannica. 2004. Encyclopædia Britannica Online. 4 Sept. 2004 <http://search.eb.com/eb/article?eu=62268>.
"Polysaccharide." Fact Index. 4 Sept. 2004 <http://www.fact-index.com/p/po/polysaccharide.html>.

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