Cellulose

Cellulose (C6H10O5)n is a long-chain polymeric polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of green plants. The primary cell wall of green plants is made primarily of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which (as wood) is the most common biopolymer on Earth.

History and applications

Cellulose is a common material in plant cell walls and was first noted as such in 1838. It occurs naturally in almost pure form in cotton fiber. In combination with lignin and hemicellulose, it is found in all plant material. Cellulose is the most abundant form of living terrestrial biomass.

Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms. Cellulose is not digestible by humans, and is often referred to as 'dietary fibre' or 'roughage', acting as a hydrophilic bulking agent for faeces.

Cellulose is the major constituent of paper; further processing can be performed to make cellophane and rayon, and more recently Modal, a textile derived from beechwood cellulose. Cellulose is used within the laboratory as a solid-state substrate for thin layer chromatography, and cotton linters, is used in the manufacture of nitrocellulose, historically used in smokeless gunpowder.

Chemistry

Cellulose monomers (β-glucose) are linked together through 1-->4 glycosidic bonds by condensation. Cellulose is a straight chain polymer: unlike starch, no coiling occurs, and the molecule adopts an extended rod-like conformation. In microfibrils, the multiple hydroxyl groups on the glucose residues hydrogen bond with each other, holding the chains firmly together and contributing to their high tensile strength. This strength is important in cell walls, where they are meshed into a carbohydrate matrix, helping keep plant cells rigid.

Given a cellulose material, the portion that does not dissolve in a 17.5% solution of sodium hydroxide at 20 °C is α cellulose, which is true cellulose; the portion that dissolves and then precipitates upon acidification is β cellulose; and the proportion that dissolves but does not precipitate is γ cellulose.

Cellulose can be assayed using a method described by Updegraff in 1969, where the fiber is dissolved in acetic and nitric acid, and allowed to react with anthrone in sulfuric acid. The resulting coloured compound is assayed spectrophotometrically at a wavelength of approximately 635 nm.

Derivatives

The hydroxyl groups of cellulose can be partially or fully reacted with various chemicals to provide derivates with useful properties. Cellulose esters and cellulose ethers are the most important commercial materials. In principle, though not always in current industrial practice, cellulosic polymers are renewable resources.

Among the esters are cellulose acetate and triacetate, which are film- and fiber-forming materials that find a variety of uses. The inorganic ester nitrocellulose was initially used as an explosive and was an early film forming material.

Ether derivatives include

• Ethylcellulose, a water-insoluble commercial thermoplastic used in coatings, inks, binders, and controlled-release drug tablets;
• Hydroxypropyl cellulose;
• Carboxymethyl cellulose;
• Hydroxypropyl methyl cellulose, E464, used as a viscosity modifier, gelling agent, foaming agent and binding agent;
• Hydroxyethyl methyl cellulose, used in production of cellulose films.

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