Disaccharides and Polysaccharides
Another important reaction of monosaccharides is that because of their OH groups, the
rings can be joined together to form disaccharides. The reaction is a dehydration reaction
between molecules, thus an intermolecular dehydration reaction.
|If we start with a-D-glucose and join two
molecules together, we get maltose. The maltose, of course, can be
hydrolyzed to form glucose.
|The disaccharides, in turn, can be further dehydrated to join more rings
together and form polysaccharides. If we start with a-D-glucose,
the polysaccharides that can be formed include starch and glycogen.
There are differences between starch and glycogen which are not shown in this segment of
the polymer. These polymers can also be hydrolyzed back to form the disaccharides and the
It's through reactions like these that glucose molecules are stored for future use when
energy will be needed. Plants convert the glucose into starch and animals
convert the glucose into glycogen. Then these polysaccharides are
hydrolyzed as needed in order to have some glucose available to provide energy.
|If b-D-glucose rings are allowed to react with
one another by dehydration, then the polymer that is formed is cellulose
rather than starch or glycogen. Cellulose can also be broken back down into the original
monosaccharides by hydrolysis reactions.
The enzymes that break down polysaccharides are specific to the type of linkage in the
polysaccharide. The enzymes that hydrolyze the beta (b) linkages
in cellulose are different from the enzymes that hydrolyze alpha (a)
linkages. The beta (b) linkages are not broken down by the
enzymes that people have and consequently, cellulose does not provide glucose in our
diets. Cellulose is one form of carbohydrate that plants use as a building material to
provide structural strength rather than for storing glucose for future use.
|The glycosidic bonds in starch are also a-1,4 linkages.
It turns out that there are different
kinds of starch, one is amylose and another is amylopectin.
|Amylose consists of glucose rings hooked together using a-1,4 linkages. An amylose molecule will have hundreds of
glucose rings hooked together in this way.
|Amylopectin has chains that are branched in this way. It
has a-1,4 linkages, but it also has a-1,6 linkages where the a-1-OH
of one ring links to the #6 OH in another ring. Amylopectin molecules can contain
thousands of glucose rings hooked together in this way.
|Starch, whether it's in the form of amylose or amylopectin, is not
a reducing sugar. The first ring cannot open up because there's no hydrogen on
the circled oxygen to allow for ring opening. Similarly the next ring, and the next ring,
et cetera, cannot open up. So polysaccharides, such as starch, are not reducing sugars.
They need to be hydrolyzed and broken down into smaller units, such glucose or perhaps
maltose, before the rings can open up.
|The glycosidic bonds in cellulose are not a-1,4 linkages. That is because the oxygen that's attached to the #1
carbon is in the beta position, but it's still hooked up to the #4 carbon
of the second ring. Consequently, the glycosidic bonds in cellulose are referred to as
being b-1,4 linkages. Cellulose is also not a
reducing sugar. It can be hydrolyzed to form glucose, but not by the digestive enzymes in
|Glycogen is the polysaccharide that is used by animals
for storing glucose for future use. Like amylopectin, it also consists of a-1,4 linkages and a-1,6
linkages. One of the functions of having many branches in the molecule is that it
gives a lot more ends for enzymes to work on to hydrolyze the glycogen to form glucose.
Thus, glucose can be released much more quickly than if there was just one end for the
enzymes to work on.
The diagram in your workbook titled "The
Life Cycle of Glucose" (Example 11) incorporates some of these changes that can happen
to glucose into the energy cycle diagram that you looked at before. The
orientation is a little bit different, but trace through that diagram to see how
glucose is formed, what can happen to it, and then how it can be used to provide
energy for living things. Please take a moment to trace through that diagram and
then we will continue.
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