Biomedical Engineering Reference
In-Depth Information
Fig. 1.1
Phenyl propane
structural units of lignin:
(
a
) p-hydroxyphenyl propane;
(
b
) guaiacyl propane;
(
c
) syringyl propane
region, the cellulose molecules are orderly arranged and are ruly, showing a clear
X-ray image, and usually hinder the biodegradation of cellulose. The amorphous
region of cellulose has a relaxed structure and is arranged irregularly. Generally, the
polymerization degree of natural cellulose molecules is higher than 1,000.
Hemicellulose is a heterogeneous glucan with a short side chain composed of two
or more monosaccharides. Sugar groups for hemicellulose are mainly xylose, glu-
cose, mannose, arabinose, and galactose and their various derivatives. As for straw
hemicellulose, it is mainly composed of poly-arab-4-O-methyl-glucuronxylans.
Hemicellulose is an amorphous material with a low polymerization degree (less
than 200, mostly 80-120). Generally, hemicellulose is the most complex of the
components in the cell wall of lignocellulose because since it forms covalent bonds
(mainly
-benzyl ether linkages) with lignin and an ester linkage with acetyl units
and hydroxycinnamic acids.
Lignin is a phenolic polymer with a three-dimensional structure. It is derived
primarily from three hydroxycinnamyl alcohols or monolignols (i.e.,
p
-coumaryl
alcohol, coniferyl alcohol, and sinapyl alcohol) by free radical generation, followed
by chemical coupling processes [
9
] (Fig.
1.1
). In accordance with its aromatic
core, the phenyl propane structural units of lignin can be divided into three types:
guaiacyl propane, syringyl propane, and
p
-hydroxyphenyl propane. The structure
of straw lignin is dominated by guaiacyl propane and syringyl propane, with a few
p
-hydroxyphenyl propane. Cellulose and hemicellulose are mainly linked through
hydrogen bonding. In addition to the strong hydrogen bonding internally in the
lignin, there exists a stable lignin-carbohydrate complex.
Molecular chains arrange regularly in the cell wall structure to form protofibrils,
which further form microfibrils, and then the microfibrils form fiber fines. Hemi-
cellulose fills between protofibrils. Lignin and hemicellulose wrapped around the
microfibrils, and a chemical connection exists between lignin and hemicellulose.
So, in the cell wall, cellulose constitutes a cellulose skeleton in the form of
microfibrils. Lignin and hemicellulose are cross-linked by a covalent bond to form a
three-dimensional structure, enwrapping microfibrils inside. The outside of the cell
wall (i.e., the intercellular layer) mainly is composed of lignin and pectin. Thus,
cellulose, hemicellulose, and lignin are intertwined to form a complex cell wall
structure to resist biodegradation.
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