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Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in
wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of ligni ...
and
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, ...
, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.


History

Lignin was first mentioned in 1813 by the Swiss botanist
A. P. de Candolle Augustin Pyramus (or Pyrame) de Candolle (, , ; 4 February 17789 September 1841) was a Swiss botanist. René Louiche Desfontaines launched de Candolle's botanical career by recommending him at a herbarium. Within a couple of years de Candolle ...
, who described it as a fibrous, tasteless material, insoluble in water and alcohol but soluble in weak alkaline solutions, and which can be precipitated from solution using acid. He named the substance “lignine”, which is derived from the Latin word ''
lignum Lignum is Latin for wood and may refer to: * '' Gmelina lignum-vitreum'', plant endemic to New Caledonia * Lignum, common name of ''Muehlenbeckia florulenta'', plant native to inland Australia * Lignum Crucis, remnants of the True Cross * Lignum Lt ...
'', meaning wood. It is one of the most abundant organic polymers on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
, exceeded only by
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
. Lignin constitutes 30% of non-
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
organic
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
on Earth, and 20 to 35% of the dry mass of wood. Lignin is present in red algae, which suggest that the common ancestor of plants and red algae also synthesised lignin. This finding also suggests that the original function of lignin was structural as it plays this role in the red alga '' Calliarthron'', where it supports joints between calcified segments.


Structure

Lignin is a collection of highly heterogeneous polymers derived from a handful of precursor lignols. Heterogeneity arises from the diversity and degree of crosslinking between these lignols. The lignols that crosslink are of three main types, all derived from phenylpropane: coniferyl alcohol (4-hydroxy-3-methoxyphenylpropane) (G, its radical is sometimes called guaiacyl), sinapyl alcohol (3,5-dimethoxy-4-hydroxyphenylpropane) (S, its radical is sometimes called syringyl), and paracoumaryl alcohol (4-hydroxyphenylpropane) (H, its radical is sometimes called 4-hydroxyphenyl). The relative amounts of the precursor "monomers" (lignols) vary according to the plant source. Lignins are typically classified according to their syringyl/guaiacyl ratio. Lignin from gymnosperms (softwoods, grasses) is derived from the coniferyl alcohol, which gives rise to guaiacol upon pyrolysis. In angiosperms (hardwoods) some of the coniferyl alcohol is converted to sinapyl alcohol. Thus, lignin in angiosperms has both guaiacyl and syringyl components. Lignin's molecular masses exceed 10,000 u. It is hydrophobic as it is rich in aromatic subunits. The degree of polymerisation is difficult to measure, since the material is heterogeneous. Different types of lignin have been described depending on the means of isolation. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.


Biological function

Lignin fills the spaces in the cell wall between
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
, hemicellulose, and pectin components, especially in vascular and support tissues:
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived fr ...
tracheids, vessel elements and sclereid cells. Lignin plays a crucial part in conducting water and aqueous nutrients in
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
stems. The polysaccharide components of plant cell walls are highly
hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are ...
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. Lignin is present in all
vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
s, but not in
bryophyte The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants ( embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited ...
s, supporting the idea that the original function of lignin was restricted to water transport. It is covalently linked to hemicellulose and therefore cross-links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole. Its most commonly noted function is the support through strengthening of wood (mainly composed of
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived fr ...
cells and lignified sclerenchyma fibres) in vascular plants. Finally, lignin also confers disease resistance by accumulating at the site of pathogen infiltration, making the plant cell less accessible to cell wall degradation.


Economic significance

Global commercial production of lignin is a consequence of papermaking. In 1988, more than 220 million tons of paper were produced worldwide. Much of this paper was delignified; lignin comprises about 1/3 of the mass of lignocellulose, the precursor to paper. Lignin is an impediment to papermaking as it is colored, it yellows in air, and its presence weakens the paper. Once separated from the cellulose, it is burned as fuel. Only a fraction is used in a wide range of low volume applications where the form but not the quality is important. Mechanical, or high-yield pulp, which is used to make newsprint, still contains most of the lignin originally present in the wood. This lignin is responsible for newsprint's yellowing with age. High quality paper requires the removal of lignin from the pulp. These delignification processes are core technologies of the papermaking industry as well as the source of significant environmental concerns. In sulfite pulping, lignin is removed from wood pulp as lignosulfonates, for which many applications have been proposed. They are used as dispersants,
humectant A humectant is a hygroscopic (water-absorbing) substance used to keep things moist. They are used in many products, including food, cosmetics, medicines and pesticides. When used as a food additive, a humectant has the effect of keeping moisture ...
s, emulsion stabilizers, and sequestrants ( water treatment). Lignosulfonate was also the first family of water reducers or superplasticizers to be added in the 1930s as admixture to fresh
concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
in order to decrease the water-to-cement (''w/c'') ratio, the main parameter controlling the concrete porosity, and thus its mechanical strength, its diffusivity and its hydraulic conductivity, all parameters essential for its durability. It has application in environmentally sustainable dust suppression agent for roads. Also, lignin can be used in making biodegradable plastic along with cellulose as an alternative to hydrocarbon-made plastics if lignin extraction is achieved through a more environmentally viable process than generic plastic manufacturing. Lignin removed by the kraft process is usually burned for its fuel value, providing energy to power the paper mill. Two commercial processes exist to remove lignin from
black liquor In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers. The equivalent material i ...
for higher value uses: LignoBoost (Sweden) and LignoForce (Canada). Higher quality lignin presents the potential to become a renewable source of aromatic compounds for the chemical industry, with an addressable market of more than $130bn. Given that it is the most prevalent biopolymer after
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
, lignin has been investigated as a feedstock for biofuel production and can become a crucial plant extract in the development of a new class of biofuels.


Biosynthesis

Lignin biosynthesis begins in the cytosol with the synthesis of glycosylated monolignols from the
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
phenylalanine. These first reactions are shared with the phenylpropanoid pathway. The attached
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...
renders them water-soluble and less toxic. Once transported through the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...
to the apoplast, the glucose is removed, and the polymerisation commences. Much about its anabolism is not understood even after more than a century of study. The polymerisation step, that is a radical-radical coupling, is catalysed by oxidative enzymes. Both peroxidase and laccase enzymes are present in the
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
, 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 catalyses the formation of monolignol
radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
. These radicals are often said to undergo uncatalyzed coupling to form the lignin
polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...
. An alternative theory invokes an unspecified biological control.


Biodegradation

In contrast to other bio-polymers (e.g. proteins, DNA, and even cellulose), lignin resists degradation. It is immune to both acid- and base-catalyzed hydrolysis. The degradability varies with species and plant tissue type. For example, syringyl (S) liginl is more susceptible to degradation by fungal decay as it has fewer aryl-aryl bonds and a lower redox potential than guaiacyl units. Because it is cross-linked with the other cell wall components, lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes, leading to a reduced digestibility of biomass. Some ligninolytic enzymes include heme peroxidases such as lignin peroxidases, manganese peroxidases, versatile peroxidases, and dye-decolourizing peroxidases as well as copper-based laccases. Lignin peroxidases oxidize non-phenolic lignin, whereas manganese peroxidases only oxidize the phenolic structures. Dye-decolorizing peroxidases, or DyPs, exhibit catalytic activity on a wide range of lignin model compounds, but their ''in vivo'' substrate is unknown. In general, laccases oxidize phenolic substrates but some fungal laccases have been shown to oxidize non-phenolic substrates in the presence of synthetic redox mediators.


Lignin degradation by fungi

Well-studied ligninolytic enzymes are found in '' Phanerochaete chrysosporium'' and other white rot fungi. Some white rot fungi, such as '' Ceriporiopsis subvermispora'', can degrade the lignin in lignocellulose, but others lack this ability. Most fungal lignin degradation involves secreted peroxidases. Many fungal laccases are also secreted, which facilitate degradation of phenolic lignin-derived compounds, although several intracellular fungal laccases have also been described. An important aspect of fungal lignin degradation is the activity of accessory enzymes to produce the H2O2 required for the function of lignin peroxidase and other heme peroxidases.


Lignin degradation by bacteria

Bacteria lack most of the enzymes employed by fungi to degrade lignin, and lignin derivatives (aliphatic acids, furans, and solubilized phenolics) inhibit the growth of bacteria. Yet, bacterial degradation can be quite extensive, especially in aquatic systems such as lakes, rivers, and streams, where inputs of terrestrial material (e.g. leaf litter) can enter waterways. The ligninolytic activity of bacteria has not been studied extensively even though it was first described in 1930. Many bacterial DyPs have been characterized. Bacteria do not express any of the plant-type peroxidases (lignin peroxidase, Mn peroxidase, or versatile peroxidases), but three of the four classes of DyP are only found in bacteria. In contrast to fungi, most bacterial enzymes involved in lignin degradation are intracellular, including two classes of DyP and most bacterial laccases. In the environment, lignin can be degraded either biotically via bacteria or abiotically via photochemical alteration, and oftentimes the latter assists in the former. In addition to the presence or absence of light, several of environmental factors affect the biodegradability of lignin, including bacterial community composition, mineral associations, and redox state.


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-substituted 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 compounds, which impart the characteristic aroma and taste to smoked foods such as barbecue. The main flavor compounds of smoked ham are guaiacol, and its 4-, 5-, and 6-methyl derivatives as well as 2,6-dimethylphenol. These compounds are produced by thermal breakdown of lignin in the wood used in the smokehouse.


Chemical analysis

The conventional method for lignin quantitation in the pulp industry is the Klason lignin and acid-soluble lignin test, which is standardized procedures. The cellulose is digested thermally in the presence of acid. The residue is termed Klason lignin. Acid-soluble lignin (ASL) is quantified by the intensity of its Ultraviolet spectroscopy. The carbohydrate composition may be also analyzed from the Klason liquors, although there may be sugar breakdown products (furfural and 5-hydroxymethylfurfural). A solution of hydrochloric acid and phloroglucinol is used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence of coniferaldehyde groups in the lignin.
Thioglycolysis Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the condensation of flavans. They do not contain sugar residues. They ...
is an analytical technique for lignin quantitation. Lignin structure can also be studied by computational simulation. Thermochemolysis (chemical break down of a substance under vacuum and at high temperature) with tetramethylammonium hydroxide (TMAH) or cupric oxide has also been used to characterize lignins. The ratio of syringyl lignol (S) to vanillyl lignol (V) and cinnamyl lignol (C) to vanillyl lignol (V) is variable based on plant type and can therefore be used to trace plant sources in aquatic systems (woody vs. non-woody and angiosperm vs. gymnosperm). Ratios of carboxylic acid (Ad) to aldehyde (Al) forms of the lignols (Ad/Al) reveal diagenetic information, with higher ratios indicating a more highly degraded material. Increases in the (Ad/Al) value indicate an oxidative cleavage reaction has occurred on the alkyl lignin side chain which has been shown to be a step in the decay of wood by many white-rot and some soft rot fungi. Lignin and its models have been well examined by 1H and 13C NMR spectroscopy. Owing to the structural complexity of lignins, the spectra are poorly resolved and quantitation is challenging.


Further reading

*


References


External links


Tecnaro website
{{Authority control Phenylpropanoids Papermaking Plants Polymers