The composition of LCB depends on the plant species and consists primarily of cellulose, hemicelluloses and lignin, which are the integral part of cell wall in plant tissues (Fig. 3) (Fengel et al. 2003). Lignin is an amorphous aromatic biopolymer composed of phenyl propane structural units linked by ether and/or carbon-carbon bonds, supplying tissues stiffness, antiseptic, and hydrophobic properties amongst others (Fig. 4). The types of lignin structural units (p-hydroxyphenyl, guaiacyl and syringyl units), their abundance, types and frequency of inter-unit linkages vary significantly from plant to plant (Fengel et al. 2003). Lignin contributes to 15-30% of plant biomass and is the principal non-hydrolysable residue of LCB.

Cellulose and hemicelluloses are hydrolysable structural polymers of cell wall and the main sources of fermentable sugars (Lawford et al. 1993; Sanchez et al. 2008). Hemicelluloses contribute to 10-40% of plant material and are essentially heteropolysaccharides constituted

by pentoses, mainly D-xylose and L-arabinose, and hexoses, mainly D-mannose, D-galactose and D-glucose. These monosaccharides result from pentosans with a main backbone built by pentoses, and hexosans with a main backbone built by hexoses. Hemicelluloses possess an irregular structure and are chemically linked to lignins in the cell wall (Sjostrom 1993). The structure and the composition of hemicelluloses vary significantly among plant species. The most abundant hemicelluloses are xylans followed by mannans and galactans (Fig. 5). Hemicelluloses play an important structural role in cell wall regulating the spatial

distribution of principal macromolecular components (cellulose and lignin) and providing their compatibility.

Cellulose, the most abundant structural polysaccharide (30-50% abundance in the cell wall), is comprised by repeated P-D-glucopyranose units linked by P(1^4)-glycosidic bonds. In plant cell walls, cellulose chains aggregate into elementary fibrils (EF) which, in turn, are assembled into microfibrils (MF). MF are embedded into a matrix of lignin and hemicelluloses, thus becoming isolated of each other (Fig. 3). Plant cells assembled in different tissues are also separated by a layer enriched in lignin (middle lamella). This structural hierarchy hinders either chemical or enzymatic hydrolysis of cellulose, being the last one particularly difficult. Cellulose, the amorphous-crystalline polymer, is poorly accessible to hydrolysis due to the predominance of crystalline domains.

.OCH3

CH3O.

PiR link

HOH2C

J PhC ink

CH2OH

CH3O OCH3 OH

O-acetyl-4-O-methylglucuronoxylan

O-acetyl-galactoglucomannan

OH

arabinogalactan

Fig. 5. Schematic representation of major hemicelluloses in lignocellulosics