Les composantes du bois
A highly complex work of nature, the formation
of wood is through the process of
photosynthesis -- the reaction of carbon
dioxide, water with sunlight to produce glucose.
David Seabright explains
We all learnt about photosynthesis at school.
However, it was always slanted towards our benefit,
for the production of oxygen, rather than
for the tree's.
For the latter, it is the essential process
for producing glucose -- the woody material's basic
monomer -- which through a process of biosynthesis
produces its polysaccharide structure of
cellulose and hemicellulose.
A third macromolecular substance present,
which has a different structure to that of the latter two
elements, is lignin. Whereas cellulose and
hemicellulose are "chains" of high-molecular-weight
polymers, lignin is defined as having an
aromatic chemical structure composed of phenylpropane
units.
However, other compounds or extractive material
make-up the total composition of xylem, grouped
into either organic (tannins and terpenes)
or inorganic matter (potassium and silicon). Generally, trees
from temperate climates have high proportions
of cellulose, hemicellulose and lignin, accounting for
some 98% of the total chemical composition.
Tropical trees can have fewer proportions of the three
basic elements, reduced to 90%, and have
a higher quantity of organic and inorganic substances.
Cellulose
To describe cellulose, or in fact any of
the other chemical compounds of xylem, is no easy task. A
chemist or a wood technologist would easily
understand the term: Cellulose consists of
anhydroglucopyranose units joined to form
a molecular chain. The units are joined by ß-(1--4)-
glycosidic linkages. However, to the layman
you may as well be talking Greek!
Trying to explain cellulose in simplistic
terms, the use of analogies is essential. Cellulose is basically a
"chain" of polymers, mainly glucose. In
order to understand its structure, the simple necklace is a
suitable analogy with each link being the
glucose monomer. The glucose monomer comprises of
carbon, hydrogen, and oxygen atoms (the
basic building blocks of all life in the plant kingdom). It has
the chemical formulation C6 H12 O6, representing
six units of carbon, twelve of hydrogen, and six of
oxygen. From this it is apparent there are
two forms of glucose present, depending on the
position of its number one carbon atom and
respective hydroxylic (OH) group.
With the necklace, individual links cross
through each outer to form the joint. Within the cellulose
chain, individual glucose monomers have
chemically linked to form a cellobiose unit by a
phenomenon called `condensation', eliminating
one molecule of water between two adjoining
hydroxylic groups at carbon 1 and 4. The
linking of the cellobiose units' make-up the cellulose chain
structure.
The term `the degree of polymerization (DP)'
explaining the size of the cellulose chain, with xylem
being 10,000 -- by dividing the molecular
weigh of cellulose by the molecular weight of one glucose
unit, a DP unit is obtained. Due to the
abundance of hydroxyl groups along the cellulose chain, it has
the tendency to form intra- and intermolecular
hydrogen bonds with adjacent cellulose chain.
Intramolecular bonds form between OH groups
of linked glucose monomers, whereas intermolecular
bonds form between OH groups of adjacent
cellulose chains. Groups of cellulose chains become
microfibrils which, when on-mass, make-up
the cell wall layer. (See Asian Timber April 95 pg. 29).
Apparently, there are no direct chemical
bonds between cellulose and hemicellulose, but sufficient
mutual adhesion is present through hydrogen
bonds and other weaker electro-static bonds such as
van der Waals forces.
Hemicellulose (Polyoses)
Defining hemicellulose is just about as easy
as cellulose -- it is again chains of polysaccharide units.
Whereas cellulose has a homopolysaccharide
structure -- where all the polysaccharide units are the
same -- hemicellouse has a hetropolysaccharide
make-up. It has a DP less than cellulose and has the
addition of "branches" from the main polymeric
chain. The polysaccharides that make up the
structure of hemicellulose can be divided
into groups depending on the positioned in the chain. The
groups are: pentoses, hexoses, hexuronic
acids, and deoxy-hexoses.
The composition and structure of hemicellulose
differ in angiospermae (hardwood) and
gymnospermae (softwood) tree species. Empirically
shown, gymnosperms have higher proportions
of the sugar mannose and galactose than
angiosperms, which in turn have higher proportions of
xylose and acetyl groups than the former.
The basic structure of hemicellulose found in angiosperms
comprise mainly of xylose units (Xylans),
linked through a similar process found in cellulose, but with
acetyl units inter dispersed and methylglucuronic
acid branches. DP ranges from 100 to 200. Other
units found in association with the angiospermae
xylans are rhamnose and galacturonic acid.
Other hemicellulose units found in angiosperms
include glucomannans which are glucose and
mannose units forming chains (DP=60 to 70)
which are slightly branched. However these have very
sporadic dispersion. Xylans found in gymnosperms
lack acetyl groups, have arabinofuranose unit
side branches, and have a higher proportion
of methylglucuronic acid. The DP for gymnospermae
hemicellulose is much lower, ranging from
70 to 130.
Gymnosperms also contain glucomannans, but
in higher quantities than angiosperms, with the
additions of acetyl groups and galactose.
Other hemicellulose units found in both types include
glucans, galactans, and pectin.
Lignin
As with cellulose and hemicellulose, lignin
is also a by-product of photosynthesized glucose,
converted through a complicated biosynthesis
process of enzymatic reactions including oxidation and
reduction. The result is the formation of
a monomeric phenylpropane unit with three varieties:
p-coumaryl, coniferyl, and sinapyl.
Lignin found in angiosperms will take on
a different structure to that found in gymnosperms.
"Guaiacyl" lignin, found predominantly in
gymnosperms, is a polymerization of the coniferyl
phenylpropane unit. "Guaiacyl-syringyl"
lignin, typically found in many angiosperms, is a copolymer
of the coniferyl and sinapyl phenylpropane
units. Lignin takes on an amorphous structure leading to a
large number of possible inter-linkages
between individual units, and can be attributed to
`non-selective free radical addition and
condensation', random reactions. I
nter-linking between lignin units form differently
than those found in cellulose and hemicellulose.
Empirical studies have shown that ether
bonds are the predominate linking agent between
phenylpropane units, although carbon-to-carbon
bonds do exist. Studies have also shown that
covalent linking also exists between lignin
and polysaccharides.
Complete structure
During the formation of the xylem cell, the
last component to be added is lignin interpenetrating the
microfibrils thus strengthening the cell
wall. Cellulose, in conjunction with hemicellulose, are
predominantly the major component of xylem,
accounting for some 70% to 80% of the total, and
function as the main structural element
due to their superior chemical and physical properties.