Chemistry Reference
In-Depth Information
a
14 residues (
Figure 3.24
), which allows its rapid degradation to simultaneously
release glucose units from the end of each branch point.
(1
e
6)-ramifications every 8
e
(a)
(b)
Nonreducing end
Branch point
Reducing end
FIGURE 3.24
(a) Molecular structure of glycogen (the polyglucose chain in the actual molecule are, of course, much longer); (b) Schematic
diagram showing the branched structure: note that while there is only one reducing end, there are multiple nonreducing ends from which
glucose units can be released.
around 10
15
kg of cellulose, half of
the carbon in the biosphere, is synthesised (and degraded) annually. In contrast to the storage polysaccharides, it is
a glucose polymer linked exclusively by
Cellulose represents the major structural component of plant cell walls
e
b
(1
e
4) glycosodic bonds, typically with up to 15,000 glucose residues. In
contrast (
Figure 3.25
)
to the extensively
4) structure of starch and glycogen, which have bent structures, more
accessible to hydration (as in the glycogen storage granules mentioned earlier), the
a
(1
e
4 linkages of cellulose
favour long straight chains. These can form fibrils of parallel chains, which interact with one another through
a hydrogen-bonding network. So, a simple change in the configuration of a glycosidic bond can produce spec-
tacular differences between a hydrated granular store of energy and a major component of vegetable cell walls,
which, for example in trees, must ensure a considerable role in load bearing. When we turn to the shell of
crustaceans like the lobster, which have a polymer composed of N-acetylglucosamine in a
b
-1
e
b
-1
e
4 linkage, the
changes in the properties are remarkable.
