Polysaccharides are the complex carbohydrates. They are made up of chains of monosaccharides (the sugars) which are linked together by glycosidic bonds, which are formed by the condensation reaction. The linkage of monosaccharides into chains creates chains of greatly varying length, ranging from chains of just two monosaccharides, which makes a disaccharide to the polysaccharides, which consists of many thousands of the sugars.
As a group, the polysaccharides play diverse and important roles within the biology of life processes; however, they can be divided into two broad groups.
Structural polysaccharides
Cellulose
Cellulose is a major component of plant cell walls. It is an unbranched polymer with about ten thousand glucose units per chain. Hydroxyl groups (-OH) project out from each chain, forming hydrogen bonds with neighbouring chains which creates a rigid cross-linking between the chains, making cellulose the strong support material that it is.
Despite the combined strength of cellulose, it is fully permeable to water and solutes which makes it ideal for allowing water and solutes into and out of the cell.
It is the most abundant organic substance in the living world and it has been estimated that more than half the total organic carbon on the planet is in cellulose.
Chitin
Chitin is closely related in structure to cellulose, also being an unbranched polysaccharide. However, instead of the hydroxyl groups (-OH), the chains have the following structure –NH.CO.CH3 replacing it. Large amounts of chitin is found in the cuticles of arthropods, with smaller amounts being found in sponges, molluscs and annelids. Chitin can also be found in the cell walls of most fungi and in some green algae.
Polysaccharides used as energy stores
Polysaccharides make ideal storage molecules for energy for a number of reasons; a) they are large, this makes them insoluble in water and therefore they exert no osmotic or chemical effect on the cell; b) they fold into compact shapes; c) they are easily converted into the required sugars when needed.
Glycogen
Glycogen is a branched polysaccharide found in nearly all animal cells and in certain protozoa and algae. In humans and other vertebrates it is principally stored in the liver and muscles and is the main form of stored carbohydrate in the body, acting as a reservoir of glucose (which the glycogen can be broken down into) for when the body is being starved of food. In an animal that has been fed well, as much as 10% of its liver weight may be glycogen. The liver’s glycogen store can be almost completely depleted during a 24 hour fast or during muscular exertion but glycogen is quickly re-synthesised from newly ingested carbohydrate.
Starch
Starch is similar to glycogen, however it is found in plant cells, protists and certain bacteria. The starch granules can be seen in the cell through a light microscope. The starch granules are made up of two polysaccharides, amylose and amylopectin. Amylose is an unbranched molecule made up of several thousand glucose units, coiled helically into a more compact shape. Amylopectin is also compact but has a branched structure and is made up of twice as many glucose units as amylose.
Combining polysaccharides with proteins and lipids
Polysaccharides can be combined with proteins or lipids to form other biological molecules. For example, peptidoglycans, which are a combination of protein and polysaccharide and are found in the cell wall of certain bacteria. Glycolipids, a combination of polysaccharides and lipids are found in the cell membrane.
Sources
Biological Science 1&2 by Green, Stout and Taylor, 2nd ed. Cambridge
Cell and Molecular Biology by Sheeler and Bianchi, 3rd ed. Wiley
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