Lignin 190996 225356047 2008-07-13T07:09:00Z Closedmouth 372693 Reverted edits by [[Special:Contributions/59.180.163.175|59.180.163.175]] (using [[WP:HG|Huggle]]) {{distinguish|Lignan}} '''Lignin''' (sometimes "'''lignen'''") is a complex [[chemical compound]] most commonly derived from [[wood]] and an integral part of the [[cell wall]]s of [[plant]]s.<ref name=''kirk''>{{cite encyclopedia |last=Lebo |first=Stuart E. Jr. |author= |authorlink= |coauthors=Gargulak, Jerry D. and McNally, Timothy J. |editor= |encyclopedia=Kirk‑Othmer Encyclopedia of Chemical Technology |title=Lignin |url=http://www.mrw.interscience.wiley.com/emrw/9780471238966/kirk/article/lignlin.a01/current/pdf |accessdate=2007-10-14 |year=2001 |publisher= John Wiley & Sons, Inc |location= |doi=10.1002/0471238961.12090714120914.a01.pub2 }}</ref> The term was introduced in 1819 by [[A. P. de Candolle|de Candolle]] and is derived from the Latin word ''lignum'',<ref name="eero"> {{cite book |author= E. Sjöström |title= Wood Chemistry: Fundamentals and Applications |publisher= [[Academic Press]] |year= 1993}} </ref> meaning wood. It is one of the most abundant [[organic polymer]]s on [[Earth]], superseded only by [[cellulose]], employing 30% of non-[[fossil]] [[organic chemistry|organic carbon]]<ref name=boerjan>{{cite journal | author = W. Boerjan, J. Ralph, M. Baucher | month = Jun | year = 2003 | title = Lignin bios | journal = Ann. Rev. Plant Biol. | volume = 54 | pages = 519–549 | doi = 10.1146/annurev.arplant.54.031902.134938}} </ref> and constituting from a quarter to a third of the dry mass of [[wood]]. The compound has several unusual properties as a biopolymer, not least its heterogeneity in lacking a defined primary structure. == Biological function == Lignin fills the spaces in the [[cell wall]] between [[cellulose]], [[hemicellulose]] and [[pectin]] components, especially in [[tracheid]]s, [[sclereid]]s and [[xylem]]. It is [[covalent]]ly linked to [[hemicellulose]] and thereby crosslinks different plant [[polysaccharide]]s, conferring mechanical strength to the [[cell wall]] and by extension the plant as a whole.<ref> {{cite journal |author=M. Chabannes, ''et al.'' |year=2001 |title=''In situ'' analysis of lignins in transgenic tobacco reveals a differential impact of individual transformations on the spatial patterns of lignin deposition at the cellular and subcellular levels |journal=Plant J. |pages=271–282 |volume=28 |doi=10.1046/j.1365-313X.2001.01159.x |unused_data=|volume-28 }} </ref> It is particularly abundant in [[Reaction wood|compression wood]], but curiously scarce in [[Reaction wood|tension wood]]. Lignin plays a crucial part in conducting water in [[plant]] stems. The [[polysaccharide]] components of plant [[cell wall]]s are highly [[hydrophilic]] and thus permeable to water, whereas lignin is more [[hydrophobic]]. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes it possible for the plant's vascular tissue to conduct water efficiently.<ref name=sarkanen> {{cite book |author=K.V. Sarkanen & C.H. Ludwig (eds) |year=1971 |title=Lignins: Occurrence, Formation, Structure, and Reactions |location=New York |publisher=Wiley Intersci.}}</ref> Lignin is present in all [[vascular plant]]s, but not in [[bryophyte]]s, supporting the idea that the original function of lignin was restricted to water transport. Lignin is indigestible by mammalian and other animal [[enzyme]]s, but some [[fungi]] and [[bacteria]] are able to [[Biodegradation|biodegrade]] the polymer. The details of the reaction scheme of the biodegradation are not fully understood to date. These reactions depend on the type of wood decay - in fungi either [[brown rot]], [[soft rot]] or [[white rot]]. The enzymes involved may employ [[free radical]]s for depolymerization reactions.<ref> {{cite book |last= Carlile |first= Michael J. |coauthors= Sarah C. Watkinson |title= The Fungi |publisher= [[Academic Press]] |year= 1994 |isbn= 0-12-159959-0}} </ref> Well understood lignolytic enzymes are [[manganese peroxidase]], lignin peroxidase and [[cellobiose dehydrogenase]]. Furthermore, because of its cross-linking with the other cell wall components, it minimizes the accessibility of cellulose and hemicellulose to microbial enzymes. Hence, lignin is generally associated with reduced digestibility of the over all plant biomass, which helps defend against [[pathogens]] and [[Pest (organism)|pests]].<ref name=sarkanen/> Lignin peroxidase (also "ligninase", [[EC number]] 1.14.99) is a [[hemoprotein]] from the white-rot fungus ''[[Phanerochaete chrysosporium]] '' with a variety of lignin-degrading reactions, all dependent on [[hydrogen peroxide]] to incorporate molecular oxygen into reaction products. There are also several other microbial enzymes that are believed to be involved in lignin biodegradation, such as [[manganese peroxidase]], [[laccase]] and [[cellobiose dehydrogenase]]. == Ecological function == Lignin plays a significant role in the [[carbon cycle]], sequestering atmospheric carbon into the living tissues of woody perennial [[vegetation]]. Lignin is one of the most slowly decomposing components of dead vegetation, contributing a major fraction of the material that becomes [[humus]] as it decomposes. The resulting soil humus generally increases the photosynthetic productivity of plant communities growing on a site as the site transitions from disturbed mineral soil through the stages of [[ecological succession]], by providing increased [[cation exchange capacity]] in the soil and expanding the capacity of moisture retention between [[flood]] and [[drought]] conditions. == Economic significance == Highly lignified [[wood]] is durable and therefore a good raw material for many applications. It is also an excellent [[fuel]], since lignin yields more energy when burned than [[cellulose]]. Mechanical, or high yield [[wood pulp|pulp]] used to make [[newsprint]] contains most of the lignin originally present in the wood. This lignin is responsible for newsprint yellowing with age.<ref name="eero"/> Lignin must be removed from the pulp before high quality [[bleaching of wood pulp|bleached]] paper can be manufactured from it. In [[Sulfite process|sulfite pulping]], lignin is removed from wood pulp as [[sulfonate]]s. These [[lignosulfonates]] have several uses:<ref>{{cite web|url=http://www.lignin.org/whatis.html|title=Uses of lignin from sulfite pulping|accessdate=2007-09-10}}</ref> * [[Dispersant]]s in high performance [[cement]] applications, [[water treatment]] formulations and textile [[dye]]s * Additives in specialty [[oil field]] applications and [[agricultural]] chemicals * Raw materials for several chemicals, such as [[vanillin]], [[Dimethyl sulfoxide|DMSO]], [[ethanol]], [[torula yeast]], [[xylitol]] sugar and [[humic acid]] * [[Environmentally sustainable]] dust suppression agent for roads The first investigations into commercial use of lignin were done by [[Marathon Corporation]] in [[Rothschild, Wisconsin|Rothschild]], [[Wisconsin]] ([[USA]]), starting in [[1927]]. The first class of products which showed promise were [[leather tanning]] agents. The lignin chemical business of Marathon was operated for many years as Marathon Chemicals. It is now known as LignoTech USA, Inc., and is owned by the [[Norway|Norwegian]] company, [[Borregaard]], itself a subsidiary of the Norwegian conglomerate Orkla AS. Lignin removed via the [[kraft process]] (sulfate pulping) is usually burned for its fuel value, providing more than enough energy to run the mill and its associated processes. More recently, lignin extracted from shrubby willow has been successfully used to produce expanded polyurethane foam. <ref>[http://www.genesis.co.nz/Press+Releases/2007/GREEN+PLASTIC+PRODUCED+FROM+BIOJOULE+MATERIAL.html Green plastic produced from biojoule material] BioJoule Technologies Press Release, 12 July 2007.</ref> == Structure == [[Image:Lignin structure.svg|thumb|200px|'''Fig. 1''': An example of a possible lignin structure]] [[Image:LigninStructure.jpg|thumb|200px|'''Fig. 2''': A small piece of lignin polymer]] [[Image:MonolignolStructure.jpg|thumb|200px|'''Fig. 3''': The three common monolignols]] [[Image:LigninPolymerisation.jpg|thumb|200px|'''Fig. 4''': Polymerisation of [[coniferyl alcohol]] to lignin. The reaction has two alternative routes [[catalysed]] by two different oxidative enzymes, [[peroxidase]]s or [[oxidase]]s.]] Lignin is a large, cross-linked, [[racemic]] [[macromolecule]] with [[molecular mass]]es in excess of 10,000[[atomic mass unit|u]]. It is relatively [[hydrophobic]] and [[aromatic]] in nature. The [[degree of polymerisation]] in [[nature]] is difficult to measure, since it is fragmented during extraction and the [[molecule]] consists of various types of substructures which appear to repeat in a haphazard manner. Different types of lignin have been described depending on the means of isolation.<ref>{{cite web |url=http://www.lignin.org/01augdialogue.html |title=Lignin and its Properties: Glossary of Lignin Nomenclature |accessdate=2007-10-14 |last= |first= |coauthors= |date=July 2001 |work=Dialogue/Newsletters Volume 9, Number 1 |publisher=Lignin Institute}}</ref> There are three [[monolignol]] [[monomers]], [[methoxy]]lated to various degrees: [[paracoumaryl alcohol|''p''-coumaryl alcohol]], [[coniferyl alcohol]], and [[sinapyl alcohol]]<ref> {{cite book |author= K. Freudenberg & A.C. Nash (eds) |year=1968 |title=Constitution and Biosynthesis of Lignin |location=Berlin |publisher=Springer-Verlag}} </ref> (Figure 3). These are incorporated into lignin in the form of the [[phenylpropanoid]]s ''p''-hydroxyphenyl (H), guaiacyl (G), and syringal (S) respectively.<ref name=boerjan/> [[Gymnosperm]]s have a lignin that consists almost entirely of G with small quantities of H. That of [[Dicotyledon]]ic [[angiosperm]]s is more often than not a mixture of G and S (with very little H), and [[monocotyledon]]ic lignin is a mixure of all three.<ref name=boerjan/> Many grasses have mostly G, while some palms have mainly S.{{Fact|date=July 2007}} All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.<ref>{{cite journal |author=J. Ralph, ''et al.'' |year=2001 |title=Elucidation of new structures in lignins of CAD- and COMT-deficient plants by NMR |journal=Phytochem. |volume=57 |pages=993–1003 |doi=10.1016/S0031-9422(01)00109-1 }}</ref> == Biosynthesis == Lignin [[biosynthesis]] (Figure 4) begins in the [[cytosol]] with the synthesis of [[glycosylated]] monolignols from the [[amino acid]] [[phenylalanine]]. These first [[chemical reaction|reactions]] are shared with the phenylpropanoid pathway. The attached [[glucose]] renders them water soluble and less [[toxic]]. Once transported through the [[cell membrane]] to the [[apoplast]], the glucose is removed and the polymerisation commences.{{Fact|date=July 2007}} Much about its [[anabolism]] is not understood even after more than a century of study.<ref name=boerjan/> The [[polymerisation]] step, that is a radical-radical coupling, is [[catalysis|catalysed]] by [[oxidative enzyme]]s. Both [[peroxidase]] and [[laccase]] enzymes are present in the [[plant]] [[cell walls]], and it is not known whether one or both of these groups participates in the polymerisation. Low molecular weight oxidants might also be involved. The oxidative enzyme [[catalysis|catalyses]] the formation of monolignol [[Radical (chemistry)|radicals]]. These radicals are often said to undergo uncatalyzed coupling to form the lignin [[polymer]], but this hypothesis has been recently challenged.<ref>{{cite journal |author=Davin, L.B. |coauthors=Lewis, N.G. |year=2005 |title=Lignin primary structures and dirigent sites |journal=Current Opinion in Biotechnology |volume=16 |pages=407–415 |doi=10.1016/j.copbio.2005.06.011}}</ref> The alternative theory that involves an unspecified biologial control is however not accepted by most scientist in the field. ==Pyrolysis== [[Pyrolysis]] of lignin during the [[combustion]] of wood or [[charcoal]] production yields a range of products, of which the most characteristic ones are [[methoxy]] [[phenols]]. Of those, the most important are [[guaiacol]] and [[syringol]] and their derivatives; their presence can be used to trace a [[smoke]] source to a wood fire. In [[cooking]], lignin in the form of [[hardwood]] is an important source of these two chemicals which impart the characteristic aroma and taste to [[smoking (food)|smoked foods]]. == References == <references/> ==External links== *[http://www.genome.ad.jp/kegg/pathway/map/map00940.html Biosynthesis pathway of lignin] *[http://www.lignin.org The Lignin Institute] A promotional site by a trade association of lignin manufacturers and users. {{Phenylpropanoids}} [[Category:Phenylpropanoids]] [[bs:Lignin]] [[ca:Lignina]] [[cs:Lignin]] [[da:Lignin]] [[de:Lignin]] [[es:Lignina]] [[fa:لیگنین]] [[fr:Lignine]] [[id:Lignin]] [[it:Lignina]] [[he:ליגנין]] [[lt:Ligninas]] [[hu:Lignin]] [[nl:Lignine]] [[ja:リグニン]] [[no:Lignin]] [[pl:Lignina]] [[pt:Lignina]] [[ru:Лигнин]] [[su:Lignin]] [[fi:Ligniini]] [[sv:Lignin]] [[zh:木质素]]