Cellulose
Cellulose
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 is argued to be one of the most common biopolymers on
Earth . Only one group of animals, the
tunicates, has the ability to create and use cellulose.
Encyclopedia
Cellulosenis 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 is argued to be one of the most common biopolymers on
Earth . Only one group of animals, the
tunicates, has the ability to create and use cellulose.
History and applications
Cellulose is a common material in plant
cell walls and was first noted as such in 1833. 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 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; 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.
Rayon is a very important fiber made out of cellulose and has been used for textiles since the beginning of the 20th century.
Chemistry
Cellulose monomers are linked together through ß1?4
glycosidic bonds by
condensation. This is in contrast to the a 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 an 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 A cellulase may function to cleave the primer from the mature chain.
Breakdown
The ability to breakdown cellulose is not possessed by mammals. Typically, this ability is possessed only by certain bacteria 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.
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.
References
See also
External links
-
- at the Cotton Fiber Biosciences unit of the USDA.
-
- - TechnologyReview.com
