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Biopolymers 225

as in cellulose I and II, crystallites can form, but sam- of branch points (<0.5%) can be found in some amylose
ples of cellulose also contain amorphous regions where molecules. Amylose can adopt a helical conformation,
chain packing is less regular. Chemical modification of the ring conformations being stabilized by intramolecu-
cellulose takes place more easily in these amorphous lar hydrogen bonding between the OH on carbon 2 of a
regions. glucose residue and oxygen on carbon 3 of the adjacent
The cellulose derivatives of greatest commercial impor- glucose unit. Amylopectin probably has a structure where
tance are regenerated cellulose, and cellulose ethers and chains are arranged in clusters (Fig. 11), with the longer
esters. Regenerated cellulose can be prepared by acidi- α-(1 → 4)-linked glucose chains taking up double helical
fying cellulose solutions in cuprammonium hydroxide or conformations. The (1 → 6) bonds are more flexible than
alkaline solutions of cellulose xanthate, and by removal of (1 → 4) links, and this allows the amylopectin molecules
ester groupings from cellulose esters in organic solvents. to open up and give access to starch-degrading enzymes.
The regenerated cellulose can be spun as fibers for textiles, The action of such enzymes is necessary when energy (in
e.g., rayon, or cast as films such as cellophane which is the form of glucose) is required by the plant. Amylose
used for packaging. molecules in solution have a tendency to aggregate side
Paper consists of a network of tangled cellulose fibers by side and become insoluble. This phenomenon is known
and is made mainly from wood chips treated with alkali or as retrogradation and is important in the staling of baked
◦
sulfur dioxide in bisulfite solution to remove most of the products. At high temperatures (>80 C) starch gives vis-
noncellulosic constituents. Cellulose and cellulose deriva- cous solutions in water and, on cooling, forms a gel.
tives are used to make textiles and plastics, and as thicken- Starch is a natural energy source in the human diet.
ers and stabilizers. A variety of useful cellulose ethers are Because of this and its ability to form gels and highly vis-
known, varying in degree of substitution and nature of sub- cous solutions, it is widely used in the food industry, as a
stituents, and one of the most important is carboxymethyl
cellulose. Since it is considered safe for human consump-
tionandisnotdegradedinorabsorbedbythehumandiges-
tive tract, it is used in foodstuffs and pharmaceuticals. In
laboratories it has become important in modern techniques
for purifying proteins. Cellulose acetates and nitrates are
probably the most useful esters. The acetates can be spun
into fibers and used in textiles, but in bulk form the acetates
are good thermoplastics. Cellulose trinitrate was first used
as an explosive; nitrates are now used as membranes and
protective coatings and can be cast as films (celluloid).
Much interest is being shown in the possible utilization of
glucose, resulting from cellulose hydrolysis, as a feed-
stock for chemical manufacture, to reduce reliance on
petroleum as the major source of raw material for chemical
industry.
Starch is probably the second most abundant carbohy-
drate polymer, and is an energy-reserve material of higher
plants and some algae. It is granular in form and insol-
uble in cold water, and is a major constituent of cereal
grains, potatoes, peas, and beans. Starch is generally con-
sidered to consist of two glucan components, amylose and
amylopectin, with 15–30% of “normal” starches being
FIGURE 11 Cluster model for amylopectin wherein unbranched
made up of amylose. Genetic variants of plants such as
A-chains are attached through branch points to B-chains. Longer
maize can produce starches with no amylose or greater chain segments in clusters permit the formation of helical do-
than 30% of this polysaccharide. Amylose molecules are mains analogous to those in amylose. Ø = single reducing unit per
mainly linear chains of α-(1 → 4)-linked D-glucopyranose molecule. [Modified with permission from Robin, J. P., Mercier, C.,
residues while amylopectin has a highly branched struc- Duprat, F., Charbonniere, R., and Guilbot, A. (1975). Die Starke
27, 36–45. Copyright 1975 by Verlag Chemie, Weinhem. From
ture made up of chains of α-(1 → 4)-linked glucose units
Aspinall, G. O. “Polysaccharides” in the Encyclopedia of Physi-
joined by α-(1 → 6) bonds. About 4% of the residues of cal Science and Technology, Vol. 11, p. 178. Copyright 1987 by
amylopectin occur as branch points, while a small number Academic Press, Inc., New York.]