Agriculture Reference
In-Depth Information
b
1,4 glycosidic
Linkages
cellobiose
Fibrils
Glucan chains in
paracrystalline
structures
Microfibrils
Fig. 3.6.
Structure and organization of cellulose fibrils (from Esau, 1977). C
n
, O
n
= number of carbon/
oxygen atoms in glucose units.
HEMICELLULOSES
.
Hemicelluloses (or glycans)
are heterogenous polysaccharides divided into
different families according to the wall type and
botanical source (Joseleau, 1980). Their struc-
ture allows them to interact directly with cellu-
lose microfibres in addition to joining up with
other glycans. They can thus form both a
matrix and a reservoir of fibres. Among dicoty-
ledons, we find the xyloglucans that are to be
found mainly in the primary walls of flax and
hemp (Fig. 3.8a). The second large family of
hemicelluloses in dicotyledons is made up of
arabinoglucuronoxylanes (Fig. 3.8b). The sec-
ondary walls of hemp contain primarily glu-
curonoxylanes, of which a small proportion is
arabinoglucuronoxylanes. Finally, other non-
cellulose polymers containing mannose are
found to varying degrees among all the
angiosperms: these are the glucomannans,
galactomannans and galactoglucomannans
(Buchanan
et al
., 2000).
LIGNINS
.
Lignins are complex polymer struc-
tures resulting from the polymerization of three
phenylpropenoids (lignin-derived phenols): cou-
marylic, coniferylic and sinapylic alcohols. These
precursors give rise to the monomer subunits
p
-hydroxyphenyl (H), gaiacyl (G) and syringyl
(S), respectively, and can be distinguished by the
degree of methoxylation of their aromatic rings.
