Environmental Engineering Reference
In-Depth Information
OH
OH
OH
OH
β
1
4
O
O
HO
HO
O
O
O
O
HO
HO
O
O
β
1
β
1
4
OH
4
OH
n
OH
OH
Cellobiose unit
H
O
H
O
O
H
O
O
O
O
O
H
O
O
H
H
O
FIGURE 2.3
Cellulose—structure (top) with both inter- and intramolecular hydrogen
bonding bridges (bottom).
2.2.2 Hemicellulose
Hemicelluloses (the word was derived from the Greek language hemisys = half )
are heteropolysaccharides, consisting of C
5
and C
6
sugars (hexosans, pentosans)
that are associated with the cellulose, and they are found in the cell wall regions
of plants. It is the second most abundant biopolymer species in plant biomass,
accounting for about 25
35% of dry wood, 28% of softwoods, and 35% of hard-
woods (Rowell, 1984). Hemicelluloses serve as a frame cementing material in
plant cell walls, holding together the cellulose micelles and fibers. This is why
they are called
-
together with cellulose and the less
abundantly present pectin, as opposed to starch and saccharose, which serve as
“
“
structural carbohydrates,
”
storage
”
molecules. Hemicelluloses consist of relatively small molecules contain-
ing 50
200 monosaccharide residues and contain linkages different from those in
cellulose, e.g., oligosaccharide side groups (branches) attached to the polysaccha-
ride backbone and acetyl groups. They are classified by the kind of main mono-
saccharide in their structure (e.g., xylose, arabinose, mannose, and galactose). The
types are
D
-xylans,
L
-arabino-
D
-xylans,
D
-mannans,
D
-galacto-
D
-mannans,
D
-gluco-
D
-mannans, and
L
-arabino-
D
-galactans, which can be grouped into xylans, gluco-
mannans, and arabinogalactans. The glucomannans in wood have small molecular
weights and are primarily linear in structure. Arabinogalactans are highly
branched, water-soluble polysaccharides and are diverse regarding their physico-
chemical properties. The distribution of hemicellulose in plant tissue varies with
different species.
Figure 2.4 shows an example of a hemicellulose structure composed of different
sugars.
Hemicelluloses are easier to decompose than cellulose, both thermochemically and
biochemically, due to the less stable intramolecular linkages.
-
Search WWH ::
Custom Search