As the name suggests, polysaccharides are substances built up by the condensationThe process in which a liquid forms from gas or vapor of the same substance. A chemical reaction in which two molecules combine to form a very small molecule and a larger molecule than either of the two reactants. of a very large number of monosaccharide units. Cellulose, for example, is a polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. of β-glucose, containing upwards of 3000 glucose units in a chain. Starch is largely a polymer of α-glucose.
These two substances are a classic example of how a minor difference in the monomerOne of the units that joins with other units to form a polymer. can lead to major differences in the macroscopic properties of the polymer. Good-quality cotton and paper are almost pure cellulose, and they give us a good idea of its properties. Cellulose forms strong but flexible fibers and does not dissolve in water. By contrast, starch has no mechanical strength at all, and some forms are water solubleAble to dissolve in a solvent to a significant extent.. Part of the molecular structure of cellulose and starch are shown in Fig. 1.
Note how the β-glucose units can condense in a straight chain because the C—O bonds on opposite sides of the ring (shown in color) are parallel to each other. Each of the chains shown in the figure is hydrogen-bonded (dashed lines) to other chains lying parallel to it and fitting snugly against it. This hydrogen bonding involves all the groups in the structure. As a consequence none are left exposed to hydrogen bond to solventThe substance to which a solute is added to make a solution. molecules except on the surface of the fiber. This explains why cellulose is insolubleUnable to dissolve appreciably in a solvent. in water.
Naturally occurring starch is usually a mixtureA combination of two or more substances in which the substances retain their chemical identity. of two different substances called amylose and amylopectin. Of these two, amylose has the simpler molecular structure. This is the structure shown in Fig. 1. Because amylose is a polymer of α-glucose, the C—O bonds at each end of the monomer are no longer parallel, and this prevents them from forming a straight chain. Instead, a spiral structure like that shown in the figure is produced. This spiral is stabilized to some extent by the occurrence of hydrogen bonding between each glucose ring and the rings immediately above it and below it in the spiral. Each spiral contains between 100 and 1000 glucose rings. The other form of starch, amylopectin, is also a polymer of α-glucose but contains many branch chains. Since these branches occur randomly in the molecule, amylopectin has no overall regular structure.
Cellulose and starch are different not only in overall structure and macroscopic properties. From a biochemical point of view they behave so differently that it is difficult to believe that they are both polymers of the same monosaccharide. Enzymes which are capable of hydrolyzing starch will not touch cellulose, and vice versa. From a plant’s point of view this is just as well since cellulose makes up structural material while starch serves as a storehouse for energyA system's capacity to do work.. If there were not a sharp biochemical distinction between the two, the need for a bit more energy by the plant might result in destruction of cell walls or other necessary structural components.
Bacteria, protozoa, termites, some cockroaches, and ruminant mammals (cattle, sheep, etc.) are capable of digesting cellulose. Ruminant mammals, termites and cockroaches themselves do not produce cellulase, the enzyme which breaks down cellulose, but rather, maintain a symbiotic relationship with bacteria in their guts which do breakdown the cellulose for digestion. Most organisms, including humans, are not capable of digesting cellulose, either through their own enzymes or through a symbiotic relationship with an organism which can. If our digestive enzymes could hydrolyzeTo react with water so that the water molecule is split into two parts. cellulose, humans would have available a much larger food supply. Quite literally we would be able to eat sawdust! It is possible to hydrolyze cellulose in the laboratory either with strong acidAn acid that ionizes completely in a particular solvent. or with cellulase, the enzyme used by bacteria. So far, however, such processes produce more expensive (and less tasty!) food than we already have available.