Why cellulose cannot be digested by humans

A large number of organisms ranging from the bacterium Acetobacter xylinum synthesizes cellulose from the forest trees. A good amount of cellulose is produced by A. Xylinum and also this bacteria is used in the study of cellulose biosynthesis. The protein named enzyme cellulose synthase present in the membrane synthesizes cellulose as well as catalyzes the glucose polymerization from UDP- glucose into the product of cellulose.

There are many bacteria, Dictyostelium disodium, and higher plants from which genes for cellulose synthesis can be identified. From the Cellulose which is present in plants to the bacterial cellulose, all are arranged in a similar way from the microfibrils as chains of polysaccharide and from ribbon bundles of microfibrils. In the human body, cellulose cannot be digested due to a lack of appropriate enzymes to break the beta acetal linkages.

The human body does not have the digestive mechanism to break the monosaccharide bonds of cellulose. Although cellulose is indigestible then also it helps in the smooth working of the intestinal tract. But, it plays a vital and necessary role in the human body because it is an excellent source of fiber. Every kind of sugar can be digested by the enzymes secreted from the human mouth, liver, and stomach except cellulose. There are some foods like fermented food, grains, and vegetables which are hard to digest or are indigestible.

By means of the symbiotic gut bacteria, cellulose can be digested by herbivores with the help of monogastric digestion. Herbivores are less efficient than ruminants in the case of extracting energy from the digestion of cellulose. Here, cellulose is digested by microbial fermentation. Herbivores eat plant materials as their food and the cell walls in plants contain cellulose. By these, the cellulose breaks into absorbable substances. Then it gets absorbed into the body and provides nutrition.

Termites are also unable to digest the cellulose then also they feed cellulose present within the wood. Some microorganisms live in the gut of termites. These microorganisms are named as mastigophorans.

Termites break down the wood and microorganisms produce the by-product which can be digested by both the organisms and termites. They have the required enzymes for the breakdown or hydrolysis of the cellulose; the animals do not, not even termites, have the correct enzymes. No vertebrate can digest cellulose directly. Even though we cannot digest cellulose, we find many uses for it including: Wood for building; paper products; cotton, linen, and rayon for clothes; nitrocellulose for explosives; cellulose acetate for films.

The structure of cellulose consists of long polymer chains of glucose units connected by a beta acetal linkage. The graphic on the left shows a very small portion of a cellulose chain. All of the monomer units are beta-D-glucose, and all the beta acetal links connect C 1 of one glucose to C 4 of the next glucose.

Cellulose - Chime in new window. Carbon 1 is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal. The Beta position is defined as the ether oxygen being on the same side of the ring as the C 6.

In the chair structure this results in a horizontal or up projection. This is the same definition as the -OH in a hemiacetal. Open graphic of hemiacetal in a new window Compare Cellulose and Starch Structures:. Cellulose: Beta glucose is the monomer unit in cellulose. As a result of the bond angles in the beta acetal linkage, cellulose is mostly a linear chain. Starch: Alpha glucose is the monomer unit in starch.

As a result of the bond angles in the alpha acetal linkage, starch-amylose actually forms a spiral much like a coiled spring. Dietary fiber is the component in food not broken down by digestive enzymes and secretions of the gastrointestinal tract. This fiber includes hemicelluloses, pectins, gums, mucilages, cellulose, all carbohydrates and lignin, the only non-carbohydrate component of dietary fiber. High fiber diets cause increased stool size and may help prevent or cure constipation. Cereal fiber, especially bran, is most effective at increasing stool size while pectin has little effect.

Lignin can be constipating. Fiber may protect against the development of colon cancer, for populations consuming high fiber diets have a low incidence of this disease. The slow transit time between eating and elimination associated with a low fiber intake would allow more time for carcinogens present in the colon to initiate cancer.

But constipated people do not have a higher incidence of colon cancer than fast eliminators, so fiber's role in colon cancer remains unclear. Dietary fiber may limit cholesterol absorption by binding bile acids.