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Cellulose

Cellulose is an organic compound with the formula (C₆H₁₀O₅)_n, a polysaccharide derived from β-1,4 linked D-glucose units.^[1]^[2] It is the structural component of the primary cell wall of green plants, acetic acid bacteria, many forms of algae and the oomycetes.

Cellulose is the most common organic polymer, with an estimated annual natural production of 1.5x10¹² tonnes.^[3] It is mainly used to produce cardboard and paper; to a smaller extent it is converted into various synthetic products such as celluloid.

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 feces.

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1 History
2 Commercial products
- 2.1 Cellulose source and energy crops
3 Structure
4 Assaying cellulose
5 Biosynthesis
6 Breakdown (cellulolysis)
7 Hemicellulose
8 Derivatives
9 References
10 See also

History

Cellulose was discovered in 1838 by the French chemist Anselme Payen, who isolated it from plant matter and determined its chemical formula.^[1]^[4] Cellulose was used to produce the first thermoplastic polymer, celluloid, by Hyatt Manufacturing Company in 1870. Hermann Staudinger determined the polymer structure of cellulose in 1920. The compound was first chemically synthesized (without the use of any biologically-derived enzymes) in 1992, by Kobayashi and Shoda.^[3]

Commercial products

Cellulose is the major constituent of paper and textiles made from cotton, linen, and other plant fibers. Cellulose can be converted into cellophane, clear rolling papers made from Viscose film, rayon, and more recently cellulose has been used to make Modal, a bio-based textile derived from beechwood cellulose. Cellulose is used in the laboratory as the stationary phase for thin layer chromatography and cotton linters. It is the raw material in the manufacture of nitrocellulose, historically used in smokeless gunpowder and as the base material for photographic and movie films until the mid 1930s.

Rayon is an important fiber made out of cellulose and has been used for textiles since the beginning of the 20th century. Cellulose is used to make hydrophilic and highly absorbent sponges and water-soluble adhesives and binders such as methyl cellulose and carboxymethyl cellulose which are used in wallpaper paste.

Cellulose source and energy crops

Main article: Energy crop

The major combustible component of non-food energy crops is cellulose, with lignin second. Non-food energy crops are preferred to edible energy crops (which have a large starch component) because they do not cause inflation of food prices.

Typical non-food energy crops include Switchgrass, Miscanthus, Salix (Willow) and Populus (Poplar) species.

Structure

Cellulose is derived from (β-glucose), which condense through β(1→4)-glycosidic bonds. This linkage motif contrasts with that for α(1→4)-glycosidic bonds present in starch and other carbohydrates. 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, conferring rigidity to plant cells.

In contrast to starch, cellulose is also much more crystalline. Whereas starch undergoes a crystalline to amorphous transition at 60 -70 °C in water (as in cooking), cellulose requires 320 °C and 25 MPa to become amorphous in water.^[5]

Assaying cellulose

Given a cellulose-containing 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. Acidification of the extract precipitates β cellulose. The portion that dissolves in base but does not precipitate with acid 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.

In addition, cellulose is represented by the difference between acid detergent fiber (ADF) and acid detergent lignin(ADL).

Biosynthesis

In vascular plants cellulose is synthesized at the plasma membrane by rosette terminal complexes (RTC's). The RTC's are hexameric protein structures, approximately 25 nm in diameter that contain the cellulose synthase enzymes that synthesise the individual cellulose chains.^[6] The RTC's contain at least three different cellulose synthases, encoded by CesA genes, in an unknown stoichiometry.^[7] 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.^[8] A cellulase may function to cleave the primer from the mature chain.

Breakdown (cellulolysis)

Cellulolysis is the process relating to or causing the hydrolysis of cellulose (i.e. cellulolytic bacteria, fungi or enzymes).

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.

Many cellulolytic bacteria, fungi or enzymes break down cellulose into shorter linked chains known as cellodextrins.

Hemicellulose

Hemicellulose is a polysaccharide related to cellulose that comprises ca. 20% of the biomass of most plants. In contrast to cellulose, hemicellulose is derived from several sugars in addition to glucose, including especially xylose but also mannose, galactose, rhamnose, and arabinose. Hemicellulose consists of shorter chains - around 200 sugar units as opposed to 7,000 - 15,000 glucose molecules in the average cellulose polymer. Furthermore, hemicellulose is branched, whereas cellulose is unbranched.

Derivatives

The hydroxyl groups of cellulose can be partially or fully reacted with various reagents to afford derivatives 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;
Methylcellulose;
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.

References

^ ^a ^b Crawford, R. L. (1981). Lignin biodegradation and transformation. New York: John Wiley and Sons. ISBN 0-471-05743-6.
^ Updegraff DM (1969). "Semimicro determination of cellulose in biological materials". Analytical Biochemistry 32: 420 – 424.
^ ^a ^b Klemm, Dieter; Brigitte Heublein, Hans-Peter Fink, Andreas Bohn (2005). "Cellulose: Fascinating Biopolymer and Sustainable Raw Material". ChemInform 36 (36). Retrieved on 2007-12-31.
^ Young, Raymond (1986). Cellulose structure modification and hydrolysis. New York: Wiley. ISBN 0471827614.
^ Cooking cellulose in hot and compressed water Shigeru Deguchi, Kaoru Tsujii and Koki Horikoshi Chem. Commun., 2006, 3293 - 3295, doi:10.1039/b605812d
^ Kimura, Laosinchai, Itoh, Cui, Linder, Brown, Plant Cell, 1999, 11, 2075-2085
^ Taylor, Howells, Huttly, Vickers, Turner, PNAS, 2003, 100, 1450-1455
^ Peng, Kawagoe, Hogan, Delmer, Science, 2002, 295, 147-150.