Starchy foods such as rice,wheat, corn, and potatoes provide us with readily available energy. They are also the main method of energy storage for the plants that produce them, as seedsor tubers. Starches are polymers of glucose, in either branched or unbranched chains; they are thus polysaccharides.Animals also produce a starch-like substance, called glycogen, that performs an energy storage function. Glycogen is stored in the muscles as a ready source of energy, and also in the liver, where it helps to regulate the blood glucose level.
We have, in our digestive tracts, very specific enzymes: one that breaks down starch and another that breaks down glycogen.However, the human digestive system does not have an enzyme to break down the other polymer of glucose: cellulose.Cellulose is a straight-chain, rigid polysaccharide with glucose–glucose linkages different from those in starch or glycogen. It provides structure and support for plants, some of which tower tens of metres in height. Wood is mainly cellulose; cotton fibres and hemp fibres are also cellulose. Indeed, it is because cellulose is indigestible that whole grains, fruits, and vegetables are good sources of dietary fibre.Herbivores such as cattle, rabbits, termites, and giraffes rely on some friendly help to do their digesting: They have specially developed stomachs and intestines that house enzymeproducing bacteria or protozoa to aid in the breakdown of cellulose.
It is the different glucose–glucose linkages that make cellulose different from starch or glycogen. Recall that, when glucose forms a ring structure, the functional groups attached to the ring are fixed in a certain orientation above or below the ring. Our enzymes are specific to the orientation of the functional groups, and cannot break the glucose–glucose linkages found in cellulose.
In starch and glycogen, glucose monomers are added at angles that lead to a helical structure, which is maintained by hydrogen bonds between–OH groups on the same polymer chain. The single chains are sufficiently small to be soluble in water. Thus, starch and glycogen molecules are both mobile and soluble—important properties in their role as readily available energy storage for the organism.
In cellulose, glucose monomers are added to produce linear polymer chains that can align side by side, favouring interchain hydrogen bonding. These interchain links produce a rigid structure of layered sheets of cellulose. This bulky and inflexible structure not only imparts exceptional strength to cellulose, it also renders it insoluble in water. It is, of course, essential for plants that their main building material does not readily dissolve in water.
