The major homopolysaccharides are cellulose, chitin,
starches (amylose and amylopectin), glycogen, and
Cellulose, a linear glucose polymer linked β-1-4 is the
predominant natural product in the biosphere. The allequatorial
structure allows for extensive hydrogen bonding,
whereas the axial, somewhat hydrophobic faces favor
a nonaqueous environment. The resulting aggregates have
a highly ordered, quasi-crytalline arrangement (Fig. 21);
hence, the unusual stability exhibited by the molecule,
exemplified in trees and wooden artifacts. The broad distributionandlowcost
of cellulosehasmadeitamajor starting
material for industrial development while the unmodifiedpolymer
remains thebasis forwood, paper, andcotton.
Chitin, identical in linkage to cellulose but composed
-acetylglucosamine instead of glucose, is the major
structural component of insect and crustacean exoskeletons
as well as a cell wall component of molds and fungi.
The structural comments regarding cellulose also apply
generally to chitin, especially in terms of stability. Less
industrial development has been done with this polymer,
in part because shrimp shells may present a more difficult
starting material than trees.
Amylose and amylopectin are closely related, α-1-4-
|Figure 21 The repeating unit of cellulose showing
bond interactions. The extended structure allows
chains to stack
via the relatively hydrophobic axial faces of
the pyranose rings.
polymers. They are major constituents of
starches and hence key nutrients worldwide. Amylose is
a linear chain with up to a few hundred glucose units.
Amylopectin has the same backbone chain but α-1-6
|Figure 22 Schematic structure for
glycogen and details of a typical
branch point. The same linkages are
present in amylopectin,
which has less frequent branches.
branches approximately every 20 glucose units. The
branches have the same linkages as the main chain. This
ramified structure allows for efficient packing in cells.
Glycogen (Fig. 22), present in all higher animals, is
closely related to amylopectin in that it has the same fundamental
structure of a linear glucose chain with branches,
and the same linkages. In this case, however, branches
occur about every seventh residue, yielding a highly rebranched,
tree-like envelope. This is essential for both
packing in cells and for the rapid degradation of the polymer
to provide critical metabolic intermediates. Glycogen
serves as a primary energy reservoir in muscle and as a
source of circulating glucose in the liver. It is of interest
that the biosynthesis of glycogen initiates on a core protein
(glycogenin), which may be cleaved from the polysaccharide
subsequent to polymerization.
Xylans are a group of polymers based on a structure
analogous to that of cellulose wherein xylose is the repeating
unit. The simplest representative contains only D-
xylose with β-1-4 linkages and is a common component
of plant walls. Several heteropolysaccharides utilize
the xylan backbone and have various other saccharides
as branches. Xylans are often associated with cellulose in
plant cell walls.