Lignin is a complex, heterogeneous aromatic polymer that constitutes 15-30% of the dry weight of vascular plants, providing structural rigidity to cell walls and enabling water transport. It is biosynthesized from monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) via oxidative coupling, resulting in a three-dimensional network of phenylpropanoid units linked by ether and carbon-carbon bonds. Lignin is highly recalcitrant to degradation and varies in composition between softwoods (primarily guaiacyl units) and hardwoods (guaiacyl and syringyl units). Industrially, it is a byproduct of pulping processes and is used in adhesives, dispersants, and as a precursor for carbon fibers or biofuels. Its insolubility in most solvents stems from its high molecular weight and cross-linking.