Lignin is a highly complex hydrophobic, cross-linked aromatic polymer synthesized by plants from three major phenylpropane units (syringyl, guaiacyl and p- hydroxyphenylpropane) linked together by ether and carbon-carbon bonds. It is tremendously complex and condensed structure together with high functionality and molecular weight, make almost impossible to certainly know how lignin is composed. Its main function in the cell wall is to achieve structural rigidity, holding the polysaccharides fibres together. It also serves as barrier against moisture, insects and diseases, protecting the more important carbohydrate structure of the plant.
Minor organic and inorganic components
Besides the structural constituents described above, the lignocellulosic biomass also contents minor organic and inorganic components. The content of these compounds can significantly vary among the lignocellulose source due genetic and environmental factors, as well as physiological and morphological differences between crops , . The minor organic oligo-constituents, usually so-called extractives, are formed by small size carbohydrates, proteins, hydrocarbons, oils, aromatics, lipids, fats, starches, phenols, waxes... synthesized by the plant itself with specific purposes (elasticity or permeability supplying, antifungal or disease against protection...).
The inorganic minerals (silicates, carbonates, sulphates, sulphites, nitrates...) are low molecular weight and simple structure compounds that can be found in soils. These components are absorbed by plants and fixed in their cell wall. The amount of inorganic matter varies not only among lignocellulose specimens (from 0.1 to 46%) but also within different parts of the plant, being usually more abundant in bark.