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CASE FILES: BIOCHEMISTRY
Figure 25-1b. The molecular structure of starch, indicating the repeating dis-
accharide unit, maltose, as well as the a-1,6-glycosidic bond present in the
branch points of amylopectin.
extended, rigid structure that is stabilized by interchain hydrogen bonds.
Starch, the plant storage polysaccharide, which is also a polymer of glucose,
differs in its structure in that the glucose monomer units are joined by a(1—4)
glycosidic bonds (Figure 25-1b). Starch is composed of two types of poly-
mers, amylose, which has a nonbranched helical structure, and amylopectin,
which is branched with a(1—6) glycosidic bonds joining the branches to the
main polymer chain. Although starch is easily digested by salivary and pan-
creatic amylase and the disaccharidases present on the brush border of intes-
tinal mucosal cells, cellulose cannot be hydrolyzed. The |3(1—>4) glycosidic
bonds of the cellulose chain cannot be cleaved by the amylases present in the
digestive tract.
Hemicelluloses are also polysaccharides that are structural components of
plant cell walls. However, unlike what their name implies, they are unrelated
to cellulose. They are polymers that are made up of a variety of sugar monomers
that include glucose, galactose, mannose, arabinose, and xylose, as well as
acidic forms of these monosaccharides. Xylose is the monosaccharide that is
most abundant. Hemicelluloses have a random, amorphous structure that is
suitable for their location in the plant cell wall matrix. Depending on their
molecular structure, hemicelluloses are partially digestible.
Lignins are formed by the irreversible dehydration of sugars that result in aro-
matic structures. The remaining alcohol or phenol OH groups can react with
each other and with aldehyde and ketone groups to form polymers. An example
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