lignin n : a complex polymer; the chief non-carbohydrate constituent of wood; binds to cellulose fibers to harden and strengthen cell walls of plants

English

Noun

1. A complex non-carbohydrate aromatic polymer present in all wood.

Translations

External links

Lignin (sometimes "lignen") is a complex chemical compound most commonly derived from wood and an integral part of the cell walls of plants. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is the most abundant organic polymer on Earth after cellulose, employing 30% of non-fossil organic carbon 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 tracheids, sclereids and xylem. It is covalently linked to hemicellulose and thereby crosslinks different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole. It is particularly abundant in compression wood, but curiously scarce in tension wood.

Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls 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. Lignin is present in all vascular plants, but not in bryophytes, supporting the idea that the original function of lignin was restricted to water transport.

Lignin is indigestible by mammalian and other animal enzymes, but some fungi and bacteria are able to 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 radicals for depolymerization reactions. 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 pests.

• Dispersants in high performance cement applications, water treatment formulations and textile dyes

• Additives in specialty oil field applications and agricultural chemicals

• Raw materials for several chemicals, such as vanillin, 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 (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 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.

Structure

Lignin is a large, cross-linked, racemic macromolecule with molecular masses in excess of 10,000u. 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.

There are three monolignol monomers, methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 3). These are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringal (S) respectively.

Biosynthesis

Lignin biosynthesis (Figure 4) begins in the cytosol with the synthesis of glycosylated monolignols from the amino acid phenylalanine. These first 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. Much about its anabolism is not understood even after more than a century of study. 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 smoked foods.

References

External links

• Biosynthesis pathway of lignin
• The Lignin Institute A promotional site by a trade association of lignin manufacturers and users.