یکشنبه ۲۵ شهریور ۰۳ | ۲۰:۴۳ ۷ بازديد
The cell wall is composed of a great
number of microfibrils, as indicated by
the fine lines in this diagram. A
microfibril is a bundle of cellulose
polymer chains. Orientation of
microfibrils is very specific for each
layer. As shown here, microfibrils of
the S2 layer run more or less parallel
to the long axis of the cell,
whereas microfibrils of the S1 and S3
run more or less horizontally.
Orientation of microfibrils in the
primary wall is random. Minute
structure of the cell wall largely
determines properties of individual
fibers as well as wood as a whole.
Wood Chemistry
The polymers of wood can be classified into three
major types: cellulose, hemicellulose, and lignin. The
proportion of the three polymers varies between
species.
Lignin is a class of complex, high
molecular weight polymers whose
exact structure varies. It is an
amorphous, i.e., not crystalline,
polymer that acts as a binding agent
to hold cells together. Lignin also
occurs within cell walls to impart
rigidity. Like cellulose and
hemicellulose, lignin is made from
carbon, oxygen, and hydrogen.
However, these elements are
arranged differently so that they are
not classified as carbohydrates. They
are instead classified as phenolics,
and the polymer is based on the
phenylpropane unit.
original source: Adler, E. 1977. Lignin chemistry-Past, present and future.
Wood Sci. Technol. 11, 169-218.
There are many other chemical compounds in wood. They usually make up
only a small percent of the total composition of wood, but in some cases can
be considerably more. In most cases these compounds are not an essential
part of the structure of wood. One class of compo
If we look at still smaller units of structure, we discover
the elemental and organic composition of wood.
The three major elements of wood are carbon, oxygen,
and hydrogen. They are combined in complex molecules
that are then joined into polymers. These polymers
provide the structural integrity of wood. In addition,
wood contains small quantities of other orga
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