Ligninu

Ligninu, commonly known as lignin, is a class of complex polyphenolic polymers that fills the spaces between cellulose and hemicellulose in the secondary cell walls of many vascular plants. It is one of the most abundant natural polymers, typically comprising about 15–30% of dry wood mass, and it plays a key role in providing rigidity and resistance to degradation.

Ligninu is formed from phenylpropanoid monomers—p-coumaryl, coniferyl, and sinapyl alcohols—that undergo radical coupling to create a highly cross-linked, three-dimensional network. The resulting polymer is heterogeneous and largely amorphous, with three major structural units: guaiacyl (G), syringyl (S), and hydroxyphenyl (H). This composition varies among species and wood types, influencing properties such as solubility and recalcitrance.

Biosynthesis occurs via the phenylpropanoid pathway in lignifying cells, with lignin deposited in the secondary cell wall alongside cellulose and hemicellulose. Ligninu contributes to cell wall strength, water management, and defense against pathogens, but its complex structure makes it resistant to enzymatic degradation.

Industrial relevance is dominated by pulp and paper production, where ligninu is removed from wood to access cellulose. Kraft and sulfite pulping use chemical treatments to break lignin–carbohydrate bonds. Residual lignin can be burned for energy or increasingly valorized into chemicals, materials, or fuels through methods such as organosolv processing and catalytic upgrading.

Degradation in nature is mainly carried out by white-rot fungi and certain bacteria, which employ enzymes like lignin peroxidases and laccases. This biological recalcitrance is a major consideration in biomass utilization and the carbon cycle. Ongoing research seeks to fractionate and convert ligninu into value-added products within sustainable biorefineries.