Page 380 - Biosystems Engineering

Evaluation of Transgenic Wood for Pr oductivity & Quality 357

observed, the reduction of syringyl units is considered to be beneficial
to the kraft-pulping process because G-type lignin is more easily
extractable using chemical pulping techniques.

Cellulose and Hemicellulose
The perceivable shortage of petroleum resources demands novel
techniques to produce fuels and products from biorenewable
resources. The success of future bioenergy production will be depen-
dent on whether or not we can develop economically and environ-
mentally viable processes to convert biomass to biofuels. Celluloses
are the most abundant natural macromolecules that are promising for
production of biofuels and bioproducts. Current technologies of con-
verting lignocellulose into liquid fuels mainly convert cellulose and
hemicellulose into ethanol.
Cellulose is a high-molecular-weight linear homopolymer, consist-
ing of repeating β-D-glucopyranosyl units joined by (1-4) glycosidic
linkages in a variety of arrangements. The hydrogen bonds formed
between the ring oxygen atom of one glycosyl unit and the hydrogen
atom of the C-3 hydroxyl group of the preceding ring hinder the free
rotation of the rings on their linking glycosidic bonds, resulting in a
stiffening of the chain. The adjacent cellulose chains fit closely together
in an ordered crystalline region, so that high strength can be observed
in plants and some cellulose-constituted animals; cellulose is insoluble
in common solvents. In nature, cellulose chains have a degree of polym-
erization of approximately 10,000 glucopyranose units in wood cellu-
lose and 15,000 in native cellulose cotton (Sjöström 1993).
Hemicelluloses are heteroglycans, which consist of various sugar
units arranged in different proportions and with different substitu-
ents. Most hemicelluloses comprise two carbohydrate polymers: a
xylose- and a mannose-containing polysaccharide. The molecular
chains of these hemicelluloses are much shorter than that of cellulose.
Hemicelluloses have a side group and a branch group. Hemicellu-
loses are more associated with lignin than cellulose because of its
amorphous state. Hemicelluloses are relatively low-molecular-weight
polysaccharides. Xylan is the predominant type of hemicellulose in
hardwoods. Xylan is a polyose with a homopolymer backbone of
xylose units. It is linked by β-(1-4)-glycosidic bonds.
Our understanding of cellulose synthesis and deposition is still
limited; progress in this field has been slow due to our inability to
isolate active and intact cellulose synthase complexes and the intri-
cacy of the mechanistic processes involved in the wood cellulose
development (Joshi et al. 2004). It appears that hemicelluloses are
instrumental in the formation of cellulose fibril networks, and genetic
alterations of hemicelluloses can both disrupt or enhance cellulose
formation. Through expressed Aspergillus xyloglucanase constitutively
in Populus alba, the expression increased the length of the stem even