78
1
General Principles
wall. The fibers are also quite resistant to chemical and enzymatic attack. Very pure cellulose
is found in the cotton boll and is also synthesized in a relatively pure form by some species of
bacteria, such as Acetobacter xylinum, Agrobacterium tumefaciens, and related bacteria [65].
When one thinks of cellulose, it is usually in connection with trees and wood that come to
mind. Cellulose is a major component of flax, comprising 80% (w/w), and jute, comprising
60–70% (w/w). Grasses, such a papyrus and bamboo have been important sources of cellulose
going back into ancient times. Papyrus was used as an early form of paper, made from the pith
of the papyrus plant, a wetlands sedge that grows to 5 meters (∼16 ft) in height and was once
abundant in the Nile Delta of Egypt. It was first known to have been used as a writing material
in ancient Egypt (at least as far back as the First Dynasty, 3,000 BC) but it was also widely
used throughout the Mediterranean, as well as in Europe and Southwest Asia, until about the
11th Century AD. Papyrus was prepared as a thin film from the outer bark that was glued
together with starch paste to give it body and the ability to hold ink. Bamboo also served man
from very early times, and continues to do so, as a building material to form houses, roofs, furniture, and so forth. Paper today is manufactured from several cellulose sources, such as wood
chips and sawdust, the fibers of the sugar cane plant (called bagasse), maize (corn) stalks,
and the straws of rye, oats, and rice. The β-(1→4) glycosidic linkage of cellulose is more
resistant toward acid hydrolysis than the α-(1→6) linkage of amylopectin. Cellulose is slowly
hydrolyzed by 1-M HCl at 100 °C. See > Fig. 6 for the structure of a segment of the cellulose
molecule.
6 Properties and Occurrence of Hemicelluloses
Hemicelluloses are a family of four basic types of polysaccharides, composed of two or more
monosaccharide residues. All have structural features similar to cellulose in that they have
their main chains that are β-(1→4) linked, with the exception of the arabinoglactans that are
β-(1→3) linked. The main chains are homopolysaccharides composed of a single monosaccharide residue, but they are highly branched by one or two different kinds of monosaccharides
that are linked for the most part to give single monosaccharide branches.
As previously mentioned, the cell walls of most plants contain 40–60% cellulose. The remaining carbohydrate, representing 40–50% (w/w) of the cell wall is composed of hemicelluloses.
The composition of the hemicelluloses varies from one plant type to another [66]. The four
basic types are
1. Xyloglucans composed of α-D-xylopyranose linked 1→6 to approximately every third
D-glucose residue of cellulose [66,67,68];
2. Xylan composed of D-xylopyranose linked β-(1→4) and glucurono-arabino-xylan, which
is composed of β-(1→4) D-xylopyranose chain with 4-O-methyl-α-D-glucopyranosyluronic acid linked 1→2 and α-L-arabinofuranosyl linked 1→3 to the xylan chain [66,67,68,69];
3. Mannan is composed of β-(1→4)-D-mannopyranose chains [70]; another type of D-mannan is galactomannan that has D-galactopyranose linked α-(1→6) to the D-mannan
chain [71].
4. Arabinogalactan is composed of β-(1→3) linked D-galactopyranose chain with β-(1→6)
linked D-galactopyranose branches and to a lesser degree a L-arabinofuranose disaccharide,
1
General Principles
wall. The fibers are also quite resistant to chemical and enzymatic attack. Very pure cellulose
is found in the cotton boll and is also synthesized in a relatively pure form by some species of
bacteria, such as Acetobacter xylinum, Agrobacterium tumefaciens, and related bacteria [65].
When one thinks of cellulose, it is usually in connection with trees and wood that come to
mind. Cellulose is a major component of flax, comprising 80% (w/w), and jute, comprising
60–70% (w/w). Grasses, such a papyrus and bamboo have been important sources of cellulose
going back into ancient times. Papyrus was used as an early form of paper, made from the pith
of the papyrus plant, a wetlands sedge that grows to 5 meters (∼16 ft) in height and was once
abundant in the Nile Delta of Egypt. It was first known to have been used as a writing material
in ancient Egypt (at least as far back as the First Dynasty, 3,000 BC) but it was also widely
used throughout the Mediterranean, as well as in Europe and Southwest Asia, until about the
11th Century AD. Papyrus was prepared as a thin film from the outer bark that was glued
together with starch paste to give it body and the ability to hold ink. Bamboo also served man
from very early times, and continues to do so, as a building material to form houses, roofs, furniture, and so forth. Paper today is manufactured from several cellulose sources, such as wood
chips and sawdust, the fibers of the sugar cane plant (called bagasse), maize (corn) stalks,
and the straws of rye, oats, and rice. The β-(1→4) glycosidic linkage of cellulose is more
resistant toward acid hydrolysis than the α-(1→6) linkage of amylopectin. Cellulose is slowly
hydrolyzed by 1-M HCl at 100 °C. See > Fig. 6 for the structure of a segment of the cellulose
molecule.
6 Properties and Occurrence of Hemicelluloses
Hemicelluloses are a family of four basic types of polysaccharides, composed of two or more
monosaccharide residues. All have structural features similar to cellulose in that they have
their main chains that are β-(1→4) linked, with the exception of the arabinoglactans that are
β-(1→3) linked. The main chains are homopolysaccharides composed of a single monosaccharide residue, but they are highly branched by one or two different kinds of monosaccharides
that are linked for the most part to give single monosaccharide branches.
As previously mentioned, the cell walls of most plants contain 40–60% cellulose. The remaining carbohydrate, representing 40–50% (w/w) of the cell wall is composed of hemicelluloses.
The composition of the hemicelluloses varies from one plant type to another [66]. The four
basic types are
1. Xyloglucans composed of α-D-xylopyranose linked 1→6 to approximately every third
D-glucose residue of cellulose [66,67,68];
2. Xylan composed of D-xylopyranose linked β-(1→4) and glucurono-arabino-xylan, which
is composed of β-(1→4) D-xylopyranose chain with 4-O-methyl-α-D-glucopyranosyluronic acid linked 1→2 and α-L-arabinofuranosyl linked 1→3 to the xylan chain [66,67,68,69];
3. Mannan is composed of β-(1→4)-D-mannopyranose chains [70]; another type of D-mannan is galactomannan that has D-galactopyranose linked α-(1→6) to the D-mannan
chain [71].
4. Arabinogalactan is composed of β-(1→3) linked D-galactopyranose chain with β-(1→6)
linked D-galactopyranose branches and to a lesser degree a L-arabinofuranose disaccharide,