A carbohydrate that contains three monosaccharides is called a trisaccharide,and so
on. Carbohydrates that contain from four to ten monosaccharides are usually called by
the general term oligosaccharides.Carbohydrates that contain larger numbers of mono-
saccharide units are called polysaccharides.
Amylose, a type of starch found in plants, is a polysaccharide. It consists of a linear
combination of several thousand glucose units joined by -1,4-linkages. In our bodies we
store glucose in a form known as amylopectin, or glycogen. Glycogen is similar to amylose;
it typically contains about 30,000 glucose monomers with the majority linked via an
-1,4-linkage but, unlike amylose, it contains occasional branches via -1,6-linkages. In
Figure 28-13 each sphere represents a glucose unit. All links are -1,4 except where
branching takes place; branching utilizes an -1,6-linkage. Our digestive systems contain
enzymes (see Sections 16-9, 28-9) that can break this biopolymer, glycogen, into its
monomeric glucose units as we need energy to drive other biological processes.
Cellulose, the structural biopolymer found in plants, contains the -1,4-linkage of
glucose units. Our digestive systems are incapable of breaking the -1,4-linkage, so we
cannot utilize cellulose as a source of stored glucose. Some animals such as termites and
cattle are able to digest cellulose.
Figure 28-13 Amylopectin, or glycogen.28-8 Carbohydrates 1129Bacteria in the digestive tracts of termites
and cattle produce an enzyme that allows
them to digest the cellulose in wood and
grass.