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 and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by the cross-linking of phenolic precursors.
History Lignin was first mentioned in 1813 by the Swiss botanist A. P. 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, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose and chitin. Lignin constitutes 30% of terrestrial non-fossil organic carbon 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 may have been pre-adapted to synthesize lignin. This finding also suggests that the original function of lignin may have been structural as it plays this role in the red alga Calliarthron, where it supports joints between calcified segments.
Composition and structure The composition of lignin varies from species to species. An example of composition from an aspen sample is 63.4% carbon, 5.9% hydrogen, 0.7% ash (mineral components), and 30% oxygen (by difference), corresponding approximately to the formula (C31H34O11)n. 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 phenylpropene: coniferyl alcohol (3-methoxy-4-hydroxyphenylpropenol; its radical, G, is sometimes called guaiacyl), sinapyl alcohol (3,5-dimethoxy-4-hydroxyphenylpropenol; its radical, S, is sometimes called syringyl), and paracoumaryl alcohol (4-hydroxyphenylpropenol; its radical, H, is sometimes called 4-hydroxyphenyl). The relative amounts of the precursor "monomers" (lignols or monolignols) vary according to the plant source. Lignins are typically classified according to their syringyl/guaiacyl (S/G) ratio. Lignin from gymnosperms is derived from the coniferyl alcohol, which gives rise to G upon pyrolysis. In angiosperms some of the coniferyl alcohol is converted to S. Thus, lignin in angiosperms has both G and S 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, hemicellulose, and pectin components, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells. Lignin plays a crucial part in conducting water and aqueous nutrients 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. 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 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
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