Polysaccharide
Polysaccharides (sometimes called <html>glycans</html>) are relatively complex carbohydrates.
They are <html>polymers</html> made up of many <html>monosaccharides</html> joined together by glycosidic linkages. They are therefore very large, often branched, molecules. They tend to be amorphous, insoluble in water, and have no sweet taste.
When all the constituent monosaccharides are of the same type they are termed homopolysaccharides; when more than one type of monosaccharide is present they are termed heteropolysaccharides.
Examples include storage polysaccharides such as <html>starch</html> and <html>glycogen</html> and structural polysaccharides such as <html>cellulose</html> and <html>chitin</html>.
Polysaccharides have a general formula of Cn(H2O)n-1 where n is usually a large number between 200 and 500.
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Starches
Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. <html>Amylose</html> consists of a linear chain of several hundred glucose molecules. <html>Amylopectin</html> is a branched molecule made of several thousand of glucose units.
<html>Starches</html> are <html>insoluble</html> in <html>water</html>. They can be digested by hydrolysis catalyzed by enzymes called <html>amylases</html>, which can break the alpha-linkages. Humans and other animals have amylases, so they can digest starches. <html>Potato</html>, <html>rice</html>, <html>wheat</html>, and <html>maize</html> are major sources of starch in the human diet.
Glycogen
<html>Glycogen</html> is the storage form of glucose in <html>animals</html>. It is a branched polymer of glucose. Glycogen can be broken down to form substrates for respiration, through the process of <html>glycogenolysis</html>. This involves the breaking of most of the C-O-C bonds between the glucose molecules by the addition of a phosphate, rather than a water as in <html>hydrolysis</html>. This process yields phosphorylated glucose molecules, which can be metabolized with a saving of one <html>ATP</html> molecule.
Cellulose
The structural components of <html>plants</html> are formed primarily from <html>cellulose</html>. Wood is largely cellulose and <html>lignin</html>, while <html>paper</html> and <html>cotton</html> are nearly pure cellulose. Cellulose is a <html>polymer</html> 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 <html>termite</html>.
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 <html>molecular weights</html> on the order of 100-1000 <html>kDa</html>. 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 <html>E. coli</html> alone. Mixtures of capsular polysaccharides, either <html>conjugated</html> 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 <html>xanthan gum</html>, <html>dextran</html>, gellan gum, and <html>pullulan</html>.
