cellulose, chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood. Cellulose has been used for the manufacture of paper since the 2d cent. Insoluble in water and other ordinary solvents, it exhibits marked properties of absorption. Because cellulose contains a large number of hydroxyl groups, it reacts with acids to form esters and with alcohols to form ethers. Cellulose derivatives include guncotton, fully nitrated cellulose, used for explosives; celluloid (the first plastic), the product of cellulose nitrates treated with camphor; collodion, a thickening agent; and cellulose acetate, used for plastics, lacquers, and fibers such as rayon.

C₂H₃O₂⁻ ion, a salt, ester, or acylal derived from acetic acid. Acetates are important in the biochemical synthesis of fats from carbohydrates in plants and animals. Industrially, metal acetates are used in printing, vinyl acetate in plastic production, cellulose acetate in photographic films and textiles (one of the first synthetic fibres, often called simply acetate), and volatile organic esters as solvents.

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Complex carbohydrate (polysaccharide) consisting of 1,000–3,000 or more glucose units in a linear chain structure that can pack into fibres of great tensile strength. The basic structural component of plant cell walls, cellulose is the most abundant of all naturally occurring organic compounds (90percnt of cotton and 50percnt of wood). Mammals (including humans) cannot digest cellulose, but bacteria in the rumens of cattle and other ruminants and protozoans in the gut of termites produce enzymes that can break it down. Soil fungi can also break down cellulose. Its most important uses are in wood, paper, and fibre products, as an ethanol and methanol source, and specialized applications. Cellulose derivatives are used in plastics, photographic films, rayon fibres, cellophane, coatings, explosives (e.g., nitrocellulose), and foods (e.g., the stabilizer and thickener carboxymethylcellulose).

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Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units.

Cellulose is the structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most common organic compound on Earth. About 33 percent of all plant matter is cellulose (the cellulose content of cotton is 90 percent and that of wood is 50 percent).

For industrial use, cellulose is mainly obtained from wood pulp and cotton. It is mainly used to produce cardboard and paper; to a smaller extent it is converted into a wide variety of derivative products such as cellophane and rayon.

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

History

Cellulose was discovered in 1838 by the French chemist Anselme Payen, who isolated it from plant matter and determined its chemical formula. Cellulose was used to produce the first successful 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.

Commercial products

Cellulose is the major constituent of paper and cardboard and of textiles made from cotton, linen, and other plant fibers.

Cellulose can be converted into cellophane, a thin transparent film, and into rayon, an important fiber that has been used for textiles since the beginning of the 20th century. Both cellophane and rayon are known as "regenerated cellulose fibers"; they are identical to cellulose in chemical structure and are usually made from viscose, a viscous solution made from cellulose. A more recent and environmentally friendly method to produce rayon is the Lyocell process.

Cellulose is used in the laboratory as the stationary phase for thin layer chromatography. It is the raw material in the manufacture of nitrocellulose (cellulose nitrate) which was historically used in smokeless gunpowder and as the base material for celluloid used for photographic and movie films until the mid 1930s.

Cellulose insulation made from recycled newsprint is becoming popular as an environmentally preferable material for building insulation.

Cellulose is used to make hydrophilic and highly absorbent sponges, as well as water-soluble adhesives and binders such as methyl cellulose and carboxymethyl cellulose which are used in wallpaper paste. Microcrystalline cellulose (E460i) and powdered cellulose (E460ii) are used as inactive fillers in tablets and as thickeners and stabilizers in processed foods.

Cellulose source and energy crops

The major combustible component of non-food energy crops is cellulose, with lignin second. Non-food energy crops are more efficient than edible energy crops (which have a large starch component), but still compete with food crops for agricultural land and water resources. Typical non-food energy crops include industrial hemp, switchgrass, Miscanthus, Salix (willow), and Populus (poplar) species.

Some bacteria can convert cellulose into ethanol which can then be used as a fuel; see cellulosic ethanol.

Structure and properties

Cellulose has no taste, is odourless, is hydrophilic, is insoluble in water and most organic solvents, is chiral and it is biodegradable.

Cellulose is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds. This linkage motif contrasts with that for α(1→4)-glycosidic bonds present in starch, glycogen, and other carbohydrates. Cellulose is a straight chain polymer: unlike starch, no coiling occurs, and the molecule adopts an extended and rather stiff rod-like conformation. The multiple hydroxyl groups on the glucose residues from one chain form hydrogen bonds with oxygen molecules on another chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. This strength is important in cell walls, where they are meshed into a carbohydrate matrix, conferring rigidity to plant cells.

Compared to starch, cellulose is also much more crystalline. Whereas starch undergoes a crystalline to amorphous transition when heated beyond 60-70 °C in water (as in cooking), cellulose requires a temperature of 320 °C and pressure of 25 MPa to become amorphous in water.

Chemically, cellulose can be broken down into its glucose units by treating it with concentrated acids at high temperature.

Many properties of cellulose depend on its degree of polymerization or chain length, the number of glucose units that make up one polymer molecule. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibers as well as bacterial celluloses have chain lengths ranging from 800 to 10,000 units. Molecules with very small chain length resulting from the break down of cellulose are known as cellodextrins; in contrast to long-chain cellulose, cellodextrins are typically soluble in water and organic solvents.

Plant-derived cellulose is usually contaminated with hemicellulose, lignin, pectin and other substances, while microbial cellulose is quite pure, has a much higher water content, and consists of long chains.

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 to remove lignin, hemicellulose, and xylosans. The resulting cellulose is 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. Each RTC floats in the cell's plasma membrane and "spins" a microfibril into the cell wall.

The RTC's contain at least three different cellulose synthases, encoded by CesA genes, in an unknown stoichiometry. 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. A cellulase may function to cleave the primer from the mature chain.

Breakdown (cellulolysis)

Cellulolysis is the process of breaking down cellulose into smaller polysaccharides called cellodextrins or completely into glucose units; this is a hydrolysis reaction. Because cellulose molecules bind strongly to each other, cellulolysis is relatively difficult compared to the break down of other polysaccharides.

Mammals do not have the ability to break down cellulose directly. Some ruminants like cows and sheep contain certain symbiotic anaerobic bacteria (like Cellulomonas) in the flora of the gut wall, and these bacteria produce enzymes to break down cellulose; the break down products are then used by the mammal. Similarly, lower termites contain in their hindguts certain flagellate protozoa which produce such enzymes; higher termites contain bacteria for the job. Fungi, which in nature are responsible for recycling of nutrients, are also able to break down cellulose.

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; these complexes are in some cases referred to as cellulosomes.

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

See also

• Cell wall
• Cellulose insulation
• Cellulosic ethanol
• Cellulosic ethanol commercialization
• Hemicellulose
• Microbial cellulose

External links

• LSBU cellulose page
• Clear description of a cellulose assay method at the Cotton Fiber Biosciences unit of the USDA.
• Cellulose films could provide flapping wings and cheap artificial muscles for robots - TechnologyReview.com
• Using cellulase enzymes in the bioethanol process
• A list of cellulolytic bacteria