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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

224 Biopolymers

TABLE II Examples of Linear and Branched Homo- and Heteroglycans
Type Common name Monosaccharide sequence
Homoglycans
Linear, one linkage type Amylose 4)-α-D-Glc p-(1 4)-α-D -Glc p-(1
Cellulose 4)-β-D-Glc p-(1 4)-β-D-Glc p-(1
Linear, more than one linkage type Cereal β-glucan 3)-β-D-Glc p-(1 4)-β-D-Glc p-(1
Branched Amylopectin 4)-α-D-Glc p-(1 4)-α-D -Glc p-(1
6

4)-α-D-Glc p-1
Heteroglycans
Linear, one linkage type Glucomannans 4)-β-D-Man p-(1 4)-β-D-Man p-(1 4)β-D -Glc p-(1
Linear, more than one linkage type Hyaluronic acid 4)-β-D-Glc pA-(1 3)-β-D -Glc pNAC-(1
Branched Galactomannan 4)-β-D-Man p-(1 4)-β-D-Man p-(1
6

α-D-Gal p-1

Typical examples (Table II) of the first group would be of cellulose present on the earth at any one time. Some
amylose or cellulose, containing only (1 → 4)-glycosidic plant fibers, such as ramie, hemp, jute, and notably cotton
bonds, and of the second group, cereal β-glucans with seed hairs, contain from 60% to over 90% cellulose and
both β-(1 → 3) and β-(1 → 4)-bonds. The most com- can be used with minimum processing as textile fiber. In
plex homoglycans are branched, as is the case for amy- wood, however, cellulose exists in close association with
lopectin, also a glucan. Heteroglycans contain at least hemicelluloses and lignin (Section II.D) and harsh chem-
two kinds of sugar unit, and the systematic name re- ical treatments must be used to obtain cellulose, mainly
flects the monomer composition. Again, the polysaccha- for paper making.
ride molecules may be linear, with one or more linkage Cellulose molecules are unbranched chains of β-
type, or branched. Examples of linear heteroglycans (Ta- (1 → 4)-linked glucopyranose units up to 5000 residues
ble II) are glucomannans where glucose and mannose long and are essentially rigid and extended. Individual
4
residues are linked by β-(1 → 4)-bonds and hyaluronic β-D-glucopyranose units adopt the C 1 chair conforma-
acid (here a trivial name is commonly used) containing tion. Within one molecule, the ribbon-like shape is sta-
glucuronic acid and N-acetyl glucosamine joined by β- bilized by hydrogen bonding between the ring oxygen of
(1 → 3)-andβ-(1 → 4)-linkages.Branchedheteroglycans one glucose unit and the OH on carbon three of the ad-
can be much more complex, with one or more type of jacent monomer residue, and also between the oxygen
monosaccharide in the main chain and yet other sugars on carbon six of the first glucose and the OH on car-
in the branches, as is the case for plant pectins. With this bon two of the adjacent residue. Cellulose molecules can
degree of complexity, systematic nomenclature becomes align themselves side by side, with adjacent chains run-
difficult. ning in the same direction; these sheets are stabilized by
The functions and uses of the most widely studied and hydrogen bonds between the OH on carbon six of glu-
commerciallyimportantpolysaccharidesaredescribedbe- cose on one molecule and the oxygen on carbon three
low, grouped according to source. of the closest glucose residue on an adjacent molecule.
This structure is found in native, untreated cellulose and
is called cellulose I. The forces between sheets are van der
3. Plant Polysaccharides
Waals forces, but the sheets associate to form microfibrils.
Cellulose and starch are undoubtedly the plant polysac- These in turn aggregate to give fibers, which, in asso-
charides of greatest importance, both in abundance and ciation with other polysaccharides and proteins, are the
economic impact. Both are homopolymers of D-glucose, main structural components of plant cell walls. Cellulose
i.e., glucans. II is a form of cellulose obtained by chemical treatment
Cellulose is the most abundant biopolymer and is the with, for example, alkali. Here chains run antiparallel to
main structural material of the cell walls of higher plants. one another and intermolecular hydrogen bonding differs
It has been estimated that there are approximately 10 12 t from that in cellulose I. Where structure is well-ordered,