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. Converting cellulose from
energy crops into
biofuels such as
cellulosic ethanol is under investigation as an alternative fuel source.
Some animals, particularly
ruminants and
termites, can
digest cellulose with the help of
symbiotic micro-organisms that live in their guts.
Humans cannot digest cellulose; it is often referred to as '
dietary fiber' or 'roughage' (e.g. outer shell of
Maize) and acts 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 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 is used to make 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 is used in the laboratory as the stationary phase for
thin layer chromatography. Cellulose fibers are also used in liquid
filtration, sometimes in combination with
diatomaceous earth or other filtration media, to create a filter bed of inert material.
Cellulose is further used to make
hydrophilic and highly absorbent sponges.
Cellulose insulation made from recycled paper is becoming popular as an environmentally preferable material for
building insulation. It can be treated with
boric acid as a
fire retardant.
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.
A strand of cellulose (conformation I
α), showing the
hydrogen bonds (dashed) within and between cellulose molecules.
Structure and properties
Cellulose has no taste, is odourless, is
hydrophilic, is insoluble in
water and most organic
solvents, is
chiral and is
biodegradable. It can be broken down chemically into its glucose units by treating it with concentrated acids at high temperature.
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 or branching occurs, and the molecule adopts an extended and rather stiff rod-like conformation, aided by the equatorial conformation of the glucose residues. The multiple
hydroxyl groups on the glucose residues from one chain form
hydrogen bonds with oxygen molecules on the same or on a neighbor chain, holding the chains firmly together side-by-side and forming
microfibrils with high
tensile strength. This strength is important in cell walls, where the microfibrils are meshed into a carbohydrate
matrix, conferring rigidity to plant cells.
a triple strand of cellulose, showing the hydrogen bonds (cyan lines) between glucose strands
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.
[Cooking cellulose in hot and compressed water Shigeru Deguchi, Kaoru Tsujii and Koki Horikoshi Chem. Commun., 2006, 3293 - 3295, ]Several different crystalline structures of cellulose are known, corresponding to the location of hydrogen bonds between and within strands. Natural cellulose is cellulose I, with structures I
α and I
β. Cellulose produced by bacteria and algae is enriched in I
α while cellulose of higher plants consists mainly of I
β. Cellulose in regenerated cellulose fibers is cellulose II. The conversion of cellulose I to cellulose II is not reversible, suggesting that cellulose I is
metastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV.
Many properties of cellulose depend on its chain length or
degree of polymerization, 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 breakdown 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.
Cellulose is soluble in cupriethylenediamine (CED), cadmiumethylenediamine (Cadoxen), N-methylmorpholine N-oxide and lithium chloride / dimethylformamide. This is used in the production of regenerated celluloses (as viscose and cellophane) from dissolving pulp.Assaying cellulose
Given a cellulose-containing material, the carbohydrate 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
Location and arrangement of cellulose microfibrils in the plant
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 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. Cellulose synthase utilizes UDP-D-glucose precursors to elongate the growing cellulose chain. 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. 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;
