|Glucose dimer, shown in armchair conformation (cellobiose unit)|
|Monomer||β-D- glucose (monomer) cellobiose (dimer)|
|Molecular formula of the repeating unit||C 12 H 20 O 10|
|Molar mass of the repeating unit||324.28 g mol −1|
white odorless powder
~ 1.5 g / cm 3
insoluble in water
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
The cellulose (including cellulose ) is the main component of plant cell walls (mass fraction of about 50%) and thus the most common organic compound and also the most abundant polysaccharide (multiple sugar). Cellulose is also the most abundant biomolecule . It is unbranched and consists of several hundreds to tens of thousands (β-1,4- glycosidically linked ) β- D - glucose or cellobiose units. These high molecular weight cellulose chains assemble to form higher structures which, as tear-resistant fibers in plants, often have static functions. It differs from the polysaccharide chitin , which is also common in nature, in the absence of acetamide groups . Cellulose is important as a raw material for paper production , but also in the chemical industry and other areas.
Cellulose was discovered in 1838 by the French chemist Anselme Payen , who isolated it from plants and determined their chemical formula. Cellulose was used by the Hyatt Manufacturing Company in 1870 to make the first thermoplastic , celluloid . Hermann Staudinger determined the structure of cellulose in 1920. Cellulose was chemically synthesized for the first time by S. Kobayashi and S. Shoda in 1992 (without the help of biologically based enzymes ).
Cellulose with structures in the nanometer range (up to 100 nm in diameter) is called nanocellulose . The nanocellulose is divided into three categories: microfibrillated cellulose (MFC), nano-crystalline cellulose (NCC) and bacterial nanocellulose (BNC). The term was first coined in the late 1970s.
Cellulose is a polymer ( polysaccharide 'multiple sugar ') made from the monomer cellobiose, which in turn is a disaccharide ('double sugar') and a dimer of the monosaccharide ('simple sugar') glucose. The monomers are linked to one another by β-1,4-glycosidic bonds. Here, too, there is a β-1,4-glycosidic bond, so that glucose is often also defined as a monomer of cellulose.
The monomers are linked by a condensation reaction in which two hydroxyl groups (-OH) form a water molecule (H 2 O) and the remaining oxygen atom connects the ring-shaped basic structure ( pyran ring ) of the two monomers. In addition to this strong, covalent bond , the less strong hydrogen bonds are also formed intramolecularly . A cellulose molecule often consists of several thousand glucose units.
Cellulose is insoluble in water and most organic solvents . Solvents such as dimethylacetamide / lithium chloride , N-methylmorpholine-N-oxide , dimethylsulfoxide / tetrabutylammonium fluoride or ammonia / Cu 2 + ( Schweizer reagent ), as well as some ionic liquid , however, capable of dissolving cellulose.
Cellulose is of fundamental importance as a structural substance in most plants. Fibers in woody and non-woody plants consist of a large number of fibrils , which in turn consist of numerous cellulose molecules arranged parallel to one another. Cellulose microfibrils are synthesized in the plasma membrane of a cell in so-called rosette complexes. These contain the enzyme cellulose synthase , which produces β-D- glucans (D-glucose polymers with β-bond) and thereby links the first carbon atom of one D-glucose molecule with the fourth carbon atom of another D-glucose molecule. The production of the glucan chain requires two essential steps. First, sucrose synthase splits the disaccharide (double sugar) sucrose into its monomers glucose and fructose to provide glucose. The glucose is now linked by the cellulose synthase with uridine diphosphate (UDP) to form UDP-glucose . In a further step, the bound glucose is now transferred to the non- reducing sugar of the growing glucan chain. The glucan chain or the enzyme then moves on, so that a further synthesis step can take place.
An important feature of tunicates is a cuticular coat, which is separated from the single-layer epidermis and - unique in the animal kingdom - consists of cellulose.
Since plants build cellulose they produce themselves into their cell walls, they need endogenous cellulases to remodel cell walls, e.g. B. in growth processes . The plant cellulase gene is a very old gene.
Plant material consisting mainly of cellulose has been used by humans as fuel for cooking and heating at least since the Paleolithic Age . Cellulose is also an important raw material for material uses, but is also important as a natural or added component of food and feed . Since cellulose occurs in almost all types of plant biomass, it is also important in many other areas, such as B. in wood ( lignocellulose ) as building material etc.
Cellulose is an important raw material for paper production . The lignin and cellulose-rich wood is used as the raw material . Wood pulp is made from this , which is used for less high quality paper. By removing the lignin content, pulp can be produced, which consists mainly of cellulose and can be used for higher quality papers.
Another regenerated cellulose material is cellophane (cellulose hydrate), which is a common packaging material in the form of foils. Synthetic cellulose fibers (" rayon ") can also be produced. For this purpose, an alkaline solution of xanthogenized cellulose ("viscose solution") is processed into threads, the so-called regenerated fibers (e.g. viscose ).
A wide variety of cellulose derivatives are used in many ways, such as. B. methyl cellulose , cellulose acetate and cellulose nitrate in the construction, textile and chemical industries. Celluloid , the first thermoplastic , is derived from cellulose nitrate .
Since cellulose is naturally available in large quantities, attempts are being made to use this renewable raw material e.g. B. to make cellulosic ethanol available as a biofuel . Intensive research is currently underway to develop plant biomass , especially wood and straw, for this purpose.
Cellulose can also be used as a natural insulation material mixed with borax (fungicide and flame retardant) . For this purpose, sorted newsprint is first shredded in a mechanical process and treated with fungicides and flame retardants. The cellulose insulation material obtained can be blown in seamlessly and used for thermal insulation and soundproofing. The blowing process has been used in Canada and the USA since around 1940. The advantage of this insulation material is the environmentally friendly production and the further use of sorted newsprint.
In the laboratory it can be used as a filling material for column chromatography when separating mixtures of substances .
Almost all animals - with the exception of a few mollusks , such as a few snails, such as the Roman snail and a few species of termites - including most herbivores, unlike starch, cannot break down cellulose through their own metabolic functions , although both molecules are made up of glucose molecules. These animals only have the enzymes that can split α- 1,4- or α - 1,6 -glycosidic bonds (e.g. in starch) ( amylases ), but not β- 1,4-glycosidic bonds of cellulose ( Cellulases ). This is why these animals (e.g. cows) can only tap into the high energy content of this carbohydrate with the help of endosymbiotic microorganisms that live in their digestive organs.
Cellulose-eating animals then feed on the steadily growing mass of symbionts in their digestive system. Ruminants digest a large part of the cellulose and other polysaccharides in the rumen with the help of anaerobic microorganisms that convert the cellulose into fatty acids. The same applies to horses and water fowl , where processing takes place in the large intestine .
Humans also have no digestive enzymes to break down cellulose. With the aid of anaerobic bacteria of the colon in the first part, the cecum and the ascending colon ( Colon ascendens ) a portion of the cellulose from the food is broken down into short-chain oligosaccharides. They are absorbed through the colon mucosa and used by the metabolism . In addition to hemicelluloses , pectin and lignin , cellulose is an important vegetable fiber in human nutrition.
Some terrestrial crabs, such as the Isopoda, can break down cellulose with the help of endosymbiotic microorganisms. The same goes for insects like silverfish , almost all termites or cockroaches . More than 450 different endosymbionts were identified in 200 termite species examined . Endosymbionts of fossil termites have already been detected directly (in Burmese amber) from the Cretaceous period.
Cellulase detection from animals (termites)
However, reports of cellulase detection from termites contradict the view that animals generally lack cellulases. Some species of termites ( Reticulitermes speratus and Coptotermes formosanus ), the cancer Cherax destructor , the nematode Bursaphelenchus xylophilus and the clams Corbicula japonica and Lyrodus pedicellatus were cellulase genes detected.
Bacteria, fungi and flagellates
Many bacteria , fungi and flagellates can only break down cellulose through their cellulases down to glucose dimer cellobiose . A few protozoa and fungi such as Aspergillus - , Penicillium - and Fusarium TYPES additionally have the necessary β-1,4-glucosidase or Cellobiases which split the cellobiose to glucose. Some wood- decomposing fungi such as Ceriporiopsis subvermispora can also oxidatively break down cellobiose via cellobiose dehydrogenase ( CDH ), an extracellular hemoflavoenzyme . This creates gluconic acid instead of glucose .
The breakdown of cellulose by further hydrolytic enzymes is supported by carbohydrate-binding areas (CBMs) of the enzymes.
The green alga Chlamydomonas reinhardtii can split cellulose and use it to generate energy.
Cellulose or cellulose derivatives are also used in the food and pharmaceutical industries, e.g. B. as a thickener , carrier , filler , release agent , coating agent and foam agent . As a food additive , cellulose has the designations E 460 to E 466:
- E 460i - microcrystalline cellulose
- E 460ii - cellulose powder
- E 461 - methyl cellulose
- E 463 - hydroxypropyl cellulose
- E 464 - hydroxypropylmethyl cellulose
- E 465 - ethyl methyl cellulose
- E 466 - carboxymethyl cellulose
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