Cellulose is a complex carbohydrate that is a primary constituent of plant cell walls. It occurs in both primitive and highly evolved plants. Cellulose comprises about 20-30% of primary cell walls and about 40-90% of secondary cell walls. It is the world's most abundant organic compound. The biological function of cellulose is thought to be skeletal, providing shape and strength to the cell wall. Cellulose differs from other polysaccharides found in plants by consisting of molecular chains that are very long, by containing only one repeating glucose subunit, and by occurring naturally in a crystalline state. The glucose subunits are connected by beta linkages. Most mammals, including man, do not have enzymes capable of promoting the hydrolysis of this beta linkage, so cellulose passes through the digestive tract unchanged. However, microorganisms found in the digestive tracts of herbivorous animals (especially in ruminants (cud-chewing animals)) can break down cellulose into products that can be absorbed and used as a food source. In the environment, brown rot fungi are able to degrade cellulose.
Of many widely utilized natural substances, cellulose, especially cellulose fiber from cotton and wood, is one the most important commercial raw materials for a large variety of chemical products. Cotton, flax, jute, and ramie fibers are comprised of nearly pure cellulose, while wood contains only about 42% cellulose. Since cellulose is insoluble in water, it can be readily separated from other constituents of plants. Cellulose can be produced from wood through the sulfite and sulfate pulping processes. Cellulose is treated with sodium hydroxide and exposed to carbon disulfide fumes to form sodium xanthate, an unstable ester. This solution, called viscose, can be forced through fine holes or slits into an acidic solution to form threads or sheets of rayon or cellophane.
Cellulose acetate esters are spun into fine filaments for the manufacture of some fabrics and are also used as photographic film, as a substitute for glass, in the manufacture of safety glass, and as a molding material. Cellulose ethers are used in paper sizings, adhesives, soaps, and synthetic resins. In mixtures of nitric and sulfuric acids, cellulose can form cellulose nitrates, which are flammable and explosive compounds. These compounds are used in various lacquers, plastics, medicines, and artificial leather. Guncotton, an explosive, is a type of cellulose nitrate.
Research has also been conducted to investigate the use of lignocellulosic biomass, which consists of cellulose, hemicellulose, lignin, and ash, to produce liquid fuels.
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