Plants store glucose as the polysaccharide starch. The cereal
grains (wheat, rice, corn, oats, barley) as well as tubers such
as potatoes are rich in starch.
Starch can be separated into two fractions--amylose and
amylopectin. Natural starches are mixtures of amylose (10-20%)
and amylopectin (80-90%).
Amylose forms a colloidal dispersion in hot water whereas
amylopectin is completely insoluble. The structure of amylose
consists of long polymer chains of glucose units connected by
an alpha acetal linkage. The graphic on the left shows
a very small portion of an amylose chain. All of the monomer
units are alpha -D-glucose, and all the alpha acetal links connect
C # 1 of one glucose to C # 4 of the next glucose.
Starch - Amylose - Chime
in new window
Acetal Functional Group:
Carbon # 1 is called the anomeric carbon and is the
center of an acetal functional group. A carbon that has two ether
oxygens attached is an acetal.
The Alpha position is defined as the ether oxygen being
on the opposite side of the ring as the C # 6. In the chair structure
this results in a downward projection. This is the same
definition as the -OH in a hemiacetal.
graphic of hemiacetal in a new window