Polysaccharides, which are found in many different places in nature, serve different functions. Cellulose, the most abundant organic molecule on earth, is a polymer of glucose subunits and is the principal constituent of plant cell walls. Some bacteria synthesize cellulose in the form of fibrils that attach the bacteria to various surfaces. Glycogen, a carbohydrate storage product of animals and some bacteria, and dextran, which is also synthesized by bacteria as a storage product for carbon and energy, resemble cellulose in some ways.
These three polysaccharides, cellulose, glycogen, and dex-tran, are composed of glucose subunits, but they differ from one another in many important ways. These include (1) the size of the polymer; (2) the degree of chain branching, since the side chains of monosaccharides can branch from the main chain; (3) the particular carbon atoms of the two sugar molecules involved in cova-lent bond formation, such as a 1,4 linkage when the carbon atom number 1 of one sugar is joined to the number 4 carbon atom of the adjacent sugar; and (4) the orientation of the covalent bond between the sugar molecules. Thus, these subunits can yield a large variety of polysaccharides that have different properties but are composed of the same subunits. How these various aspects of the structure ofa polysaccharide fit into the structures ofcellulose, glycogen, and dextran are shown in figure 2.21.
Polysaccharides and oligosaccharides can also contain different monosaccharide subunits in the same molecule. For example, the cell walls of the Domain Bacteria contain a polysaccha-ride consisting of alternating subunit molecules of two different
2.6 Nucleic Acids 31
amino sugars. A polysaccharide consisting of different subunits is termed a complex polysaccharide.
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