Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 28

Fig. 1 Schematic diagram of plant cell walls

2.1 Cellulose

Cellulose is a polysaccharide composed of linear glucan chains that are linked
together by b-1,4-glycosidic bonds with cellobiose residues as the repeating unit at
different degrees of polymerization depending on resources, and packed into
microfibrils which are held together by intramolecular hydrogen bonds as well as
intermolecular van der Waals forces [ 8 ]. Although polymorphy has been docu-
mented for cellulose, native cellulose occurs as cellulose I, which is a mixture of
two polymorphs I a and I b [ 9 , 10 ]. Cellulose I a is synthesized simultaneously with
the extension of the microfibril network, and thus is dominant in lower plants to
form the primary wall, and also in some bacteria. While, cellulose I b is deposited
within the secondary wall of higher plants for strength. The decipherment of
crystalline structure indicates that cellulose I a is characterized by the triclinic unit
containing one chain, while there are two chains in the monoclinic unit of cellulose
I b providing more intramolecular hydrogen bonds, making it more stable [ 11 ].
Harsh conditions are therefore needed to transform cellulose I b of plant biomass
into amorphous polymorphs that can be attacked more efficiently by cellulases.

2.2 Hemicelluloses

Hemicelluloses are a heterogeneous group of polysaccharides with the b-(1?4)-
linked backbone structure of pentose (C5) sugars, such as xylose and arabinose,
and hexose (C6) sugars, including mannose, galactose and glucose as the repeating
units, which have the same equatorial configuration at C1 and C4, as illustrated in
Fig. 2 [ 12 ]. The structural similarity of hemicelluloses to the b-1,4-glycosidic
bonds of the cellulose molecule benefits from a conformational homology,
which can lead to a strong non-covalent association with cellulose microfibrils.