Chapter Four

18
together form the most abundant biomaterial in nature and so studies of their
degradation and colonization are important ecologically (Hyde et al., 1998b).
Cellulose is the main constituent of wood with approximately 40-50% of the
dry weight of all wood species (Rayner and Boddy, 1988). It is located predominantly
in the secondary cell wall and least abundantly in the compound middle lamella
(Yuen, 1998). Cellulose comprises a long and linear homopolymer of β-D-glucose
residues connected by β-1,4 glycosidic linkages. The structural properties in the cell
wall and many physical and chemical properties of wood were controlled by the
hydroxyl group at the surface of the cellulose molecules (Zabel and Morrell, 1992).
Hemicellulose is composed of heteropolysaccharides. The dry weight of
hemicellulose in wood is between 20-30% (Yuen, 1998). Its polymers comprise
various pentose and hexose sugar units, i.e. glucose, xylose, mannose, galactose,
arabinose, rhamnose and uronic acids (Eriksson et al., 1990). In contrast with
cellulose, hemicellulose has short chain lengths, side chains that are sometimes
branched and sugar monomers other than glucose (Yuen, 1998).
Lignin is the most complex compound in the cell wall constituents that limit
microbial growth and retard wood decay. Lignin coats the cell wall polysaccharides
and chemically combines with them to form lignocellulose, which is very resistant to
microbial degradation, being a poly phenolic polymer formed from three types of
phenyl propane units (Yuen, 1998). Next to cellulose, lignin is the most important
renewable resource in the ecosystem (as wood component). Lignin is present in the
middle lamella and primary wall. It forms a barrier that must be degraded before most
cellulose is accessible to enzymes. Lignin-degrading enzymes are found in fungi that
cause soft rot, white rot and brown rot (Rayner and Boddy, 1988).