Lignin

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of vascular plants and some algae. 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 cross-linked phenolic polymers.

Structure and composition

Lignin is a complex polymer of aromatic alcohols known as monolignols. It is most commonly derived from wood, and is an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose.

Biosynthesis

The biosynthesis of lignin is one of the most complex biochemical processes in plant metabolism. It involves the oxidative coupling of three different types of monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monolignols are synthesized in the cytosol and then transported to the cell wall, where they are oxidized by laccases or peroxidases to form radicals. These radicals can then couple to form the lignin polymer.

Functions

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.

Industrial uses

Lignin is used in a wide range of applications, from papermaking to the production of renewable materials. The global market for lignin is expected to reach USD 906.3 million by 2020, growing at a CAGR of 2.9% from 2015 to 2020.

See also

• Cellulose
• Hemicellulose
• Pectin
• Plant stem
• Wood

References

• 
  Structure of Lignin
• 
  General structure of Monolignols
• 
  Zellstoffwerk Rosenthal
• 
  Lignin Polymerisation