lignins

Lignins are a class of complex, highly cross-linked phenolic polymers that constitute a major component of the secondary cell walls in vascular plants. They are amorphous and largely insoluble, filling spaces between cellulose and hemicellulose, and they provide rigidity, hydrophobicity, and resistance to microbial decay.

Lignin is derived from phenylpropanoid monolignols, primarily p-coumaryl, coniferyl, and sinapyl alcohols. In plants, these units assemble into three main types of structural units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). The relative abundance of these units varies with species, tissue, and developmental stage. Oxidative coupling, catalyzed by peroxidases and laccases, creates a highly irregular three-dimensional network that is difficult to hydrolyze.

Biosynthesis and deposition occur in association with cellulose synthesis in the cell wall. Monolignols are transported to the wall, where radical formation initiates polymerization and cross-linking, strengthening xylem and other supportive tissues. The composition and degree of lignification influence wood properties, digestibility of plant biomass, and the mechanical behavior of tissues.

Functions of lignin include providing mechanical support, facilitating water transport in vascular tissues, and contributing to defense against pathogens. Lignin also contributes to soil organic carbon upon plant litter decomposition and affects the environmental fate of plant-derived carbon.

Industrial relevance centers on its recalcitrance to chemical and enzymatic breakdown. In pulping and papermaking, lignin removal is essential, while in biorefineries, lignin is explored as a source of value-added aromatics through depolymerization and selective upcycling. Analytical methods such as thioacidolysis and pyrolysis-GC/MS are used to characterize monolignol composition.