Cellulose

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Cellulose as polymer of β-D-glucose
Cellulose as polymer of β-D-glucose
Cellulose in 3D
Cellulose in 3D

Cellulose (C6H10O5)n is a polysaccharide of beta-glucose.[1][2] It forms the primary structural component of green plants. The primary cell wall of green plants is made of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which (as wood) is argued to be one of the most common biopolymers on Earth (chrysolaminarin is often argued to be the other). Only one group of animals, the tunicates, has the ability to create and use cellulose. Some acetic acid bacteria are also known to synthesize cellulose, as well as many forms of algae, and the oomycetes.

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[edit] Background

Cellulose is a common material in plant cell walls. It was discovered and isolated about 170 years ago by the French chemist Anselme Payen ("Cellulose. Structure, modification and hydrolysis", 1986). 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[1] with an estimated annual production of 1.5x1012 Tonnes.[3]

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

Cellulose is the major constituent of paper and textiles made of cotton, linen and other plant fibers; further processing can be performed to make cellophane, rayon, cigarette papers(transparent), 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.

Rayon is a very important fiber made out of cellulose and has been used for textiles since the beginning of the 20th century.

[edit] Chemistry

Cellulose monomers (β-glucose) are linked together through β(1→4)-glycosidic bonds by condensation. This is in contrast to the α(1→4)-glycosidic bonds present in other carbohydrates like starch. 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.

In contrast to starch, cellulose is also much more crystalline. Whereas starch has a crystalline to amorphous transition at 60 -70 °C in water as in cooking, it takes 320°C and 25 MPa for cellulose to become amorphous in water.[3]

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.

[edit] Biosynthesis

In higher plants cellulose is synthesized at the plasma membrane by rosette terminal complexes (RTCs). The RTCs are hexameric protein structures, approximately 25 nm in diameter, that contain the cellulose synthase enzymes that synthesise the individual cellulose chains.[4] The RTCs contain at least three different cellulose synthases, encoded by CesA genes, in an unknown stoichiometry.[5] Separate sets of CesA genes are involved in primary and secondary cell wall biosynthesis. Cellulose synthase utilizes UDP-D-glucose precursors to generate microcrystalline cellulose. Cellulose synthesis requires chain initiation and elongation, and the two processes are separate. CesA glucosyltransferase initiates cellulose polymerization using a steroid primer, 'sitosterol-beta-glucoside' and UDP-glucose.[6] A cellulase may function to cleave the primer from the mature chain.

[edit] Breakdown

Mammals do not have the ability to break down cellulose directly. Typically, this ability is possessed only by certain bacteria (which have specific enzymes) like Cellulomonas etc, and which are often the flora on the gut walls of ruminants like cows and sheep, or by fungi, which in nature are responsible for cycling of nutrients. The enzymes utilized to cleave the glycosidic linkage in cellulose are glycoside hydrolases including endo-acting cellulases and exo-acting glucosidases. Such enzymes are usually secreted as part of multienzyme complexes that may include dockerins and cellulose binding modules, referred to in some cases as cellulosomes.

[edit] 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 cellulose 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.

Many cellulolytic bacteria break down cellulose into shorter linked chains known as cellodextrins.

[edit] Cellulose source and energetic crops

[edit] References

  1. ^ a b Crawford, R. L. (1981). Lignin biodegradation and transformation. New York: John Wiley and Sons. ISBN 0-471-05743-6. 
  2. ^ Updegraff DM (1969). "Semimicro determination of cellulose in biological materials". Analytical Biochemistry 32: 420 – 424. 
  3. ^ a b Cooking cellulose in hot and compressed water Shigeru Deguchi, Kaoru Tsujii and Koki Horikoshi Chem. Commun., 2006, 3293 - 3295, DOI:10.1039/b605812d
  4. ^ Kimura, Laosinchai, Itoh, Cui, Linder, Brown, Plant Cell, 1999, 11, 2075-2085
  5. ^ Taylor, Howells, Huttly, Vickers, Turner, PNAS, 2003, 100, 1450-1455
  6. ^ Peng, Kawagoe, Hogan, Delmer, Nature, 2002, 295, 147-150.

[edit] See also

[edit] External links

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