Polysaccharides differ from each other in the identity of their recurring
monosaccharide units, in the length of their chains, in the types of bonds linking
the units, and in the degree of branching.
Homopolysaccharides contain only a single monomeric species. Some
homopolysaccharides serve as storage forms of monosaccharides that are used
as fuels; starch and glycogen are homopolysaccharides of this type. Other
homopolysaccharides (cellulose and chitin) serve as structural elements in plant
cell walls and animal exoskeleton.
Heteropolysaccharides contain two or more different kinds. In animal tissues,
the extracellular space is occupied by several types of heteropolysaccharides,
which form a matrix that holds individual cells together and provides protection,
shape, and support to cells, tissues, and organs.
Some Homopolysaccharides Are Stored Forms of Fuel:
The most important storage polysaccharides are starch in plant cells and
glycogen in animal cells. Both polysaccharides occur intracellularly as large
clusters or granules. Starch and glycogen are heavily hydrated, because they
have many exposed hydroxyl groups available to hydrogen-bond with water.
Most plant cells have the ability to form starch, and starch storage is especially
abundant in tubers (underground stems).
Starch contains two types of glucose polymer, amylose and amylopectin.
Amylose consists of long, unbranched chains of D-glucose residues connected by
(alpha 1-4) linkages. Such chains vary in molecular weight. Amylopectin also has
a high molecular weight but unlike amylose is highly branched. The glycosidic
linkages joining successive glucose residues in amylopectin chains are (alpha 1-
4); the branch points are (alpha 1-6) linkages.
Glycogen is the main storage polysaccharide of animal cells. Like amylopectin,
glycogen is a polymer of (alpha 1-4)-linked subunits of glucose, with (alpha 1-6)-
linked branched, but glycogen is more extensively branched and more compact
than starch. Glucose is especially abundant in the liver, and it is also present in
skeletal muscle. Because each branch in glycogen ends with a non-reducing
sugar unit, a glycogen molecule with n branches has n+1 non-reducing ends, but
only one reducing end. When glycogen is used as an energy source, glucose units
are removed one at a time from the non-reducing ends.
Cellulose is a fibrous, tough, water-soluble substance found in the cell walls of
plants. Like amylose, the cellulose molecule is linear, unbranched
homopolysaccharide, consisting of 10,000-50,000 D-glucose units. But there is a very important difference: in cellulose, the glucose