Chemistry Reference
In-Depth Information
in cereal straws and in brans. Large-scale industrial preparations of furfural, for instance, are based on
these materials.
Xylan
, one of the better-known hemicelluloses, is a component of plant cell membranes. This
pentosan occurs in association with cellulose. The structure of xylan was shown to be 1,4-polyxylose [
3
]:
H
OH
HO
O
O
O
O
O
HO
H
OH
Another hemicellulose,
Galactan
, is a minor component of some coniferous and deciduous woods.
Larch wood was shown to contain about 8% of this polymer [
4
].
Araban
, or polyarabinose is found in plant saps. All
pectins
also belong to the family of
hemicelluloses. These are gelatinizing substances that are found in many plants, particularly in
fruit juices. Crude pectins contain pentosans, galactosans, and similar materials. Purified pectins
yield on hydrolysis galacturonic acid and methanol. These high molecular weight polymers are
believed to consists to a good extent of poly (galacturonic acid), partially esterified with methyl
alcohol. In addition, the polymers contain galactose and arabinose molecules. The polymer is
probably linear [
5
-
10
] with a 1,4-glucosidal linkage between monomers. The relative amount of
various components depends upon the source of the pectin. Citrus pectin, for instance, is rich in
galacturonic acid but poor in galactose and arabinose.
Plant gums
and mucilages are high molecular weight polysaccharides composed of hexoses and
pentoses. They also contain some uronic acid units. Among the gums there is gum arabic, gum
tragacanth, and many others.
8.2.2 Starch
This is the most widely distributed substance in the vegetable kingdom and is the chief reserve
carbohydrate of plants. Starch consists of single repeat units of
D
-glucose linked together through
1 and 4 carbons by
)[
10
]. There are two types of starch molecules,
amylose
and
amylopectin
. The first one is mainly a linear polymer. Its molecular weight can range from 30,000 to
1,000,000, though it is mostly 200,000-300,000. Amylose is often pictured in a spiral form due to the
conformation of the
-linkages (
cis
a
-glucoside bonds:
a
CH
2
OH
O
O
HO
OH
O
HO
CH
2
OH
HO
HO
O
O
HO
CH
2
OH
O
