Biomedical Engineering Reference
In-Depth Information
of components in the cell wall. Some reports also indicated that xyloglucan is related
to plant morphogenesis. However, the generally accepted view is that the main
function of hemicellulose is to take part in building the cell wall structure and the
regulation of the cell growth process.
2.5
Lignin
Lignin is one of the most abundant organic polymers in plants, just behind cellulose.
It is the exclusive chemical composition of gymnosperm and angiosperm. The
content of lignin in wood and Gramineae is 20-40% and 15-20 %, respectively.
Lignin is the name of a group of substances; their inhomogeneity is manifested
in different species of plants, length of growing season, and different parts of the
plants. Even in the different morphologies of cells of the same xylem or different
cell wall layers, the structures of lignin are not the same [
29
].
Lignin is a complex composed of complicated phenylpropane units nonlinearly
and randomly linked; three main monomers are coumaryl alcohol, coniferyl alcohol,
and sinapyl alcohol. Because of the different monomers, lignin can be divided into
three types (Fig.
2.4
): syringyl lignin polymerized by syringyl propane, guaiacyl
lignin polymerized by guaiacyl propane, and hydroxy-phenyl lignin polymerized
by hydroxy-phenyl propane. Usually, gymnosperm mainly contains guaiacyl (G)
lignin; the dicotyledon mainly contains guaiacyl-syringyl (GS) lignin; the mono-
cotyledon mainly contains guaiacyl-syringyl-hydroxy-phenyl (GSH) lignin [
30
].
At one time, lignin in plant was divided into softwood, hardwood, and grass
lignins. Based on the structure of lignin, Gibbs divided lignin into G lignin
and GS lignin. G lignin is chiefly formed through dehydrated oligomerization of
coniferyl alcohol, and its structure is homogeneous. This kind of lignin has negative
Maule reaction because less than 1.5 % of syringaldehyde and about 5 % of
p-hydroxybenzaldehyde were generated when oxidized by nitrobenzene. Most
lignin in softwood belongs to G lignin, which is copolymerized by guaiacyl and has
a positive Maule reaction. GSH lignin is the result of the dehydrated oligomerization
of coniferyl alcohol and sinapyl alcohol; the content of lignin is 17-23 %. The ratio
of syringyl propane to guaiacyl propane is 0.5
0.1; it also contains 7-12 % ester
groups. p-Coumaryl alcohol in it is linked to lignin in the form of ester [
10
].
Fig. 2.4
Basic structural unit of lignin
