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. (This diagram is also shown in Example 19 in your workbook.)
|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
monosaccharides. (Also shown in Example 20 in your workbook.)
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. (This diagram is also shown in Example 21 in your
The enzymes that break down poysaccharides 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.
|Let me use the disaccharide, maltose to
point out some additional terminology that you may run into. (Use Example 22 in your
workbook for notes.)
|The linkage or bond in the center that links the two glucose units is
referred to as a glycosidic bond or glycosidic linkage.
compound that contains a carbon bonded to two oxygens, both of which are bonded to
additional oxygens are sometimes referred to as an acetal. That's not a
term that I use much, but you may see it or run into it.
|This glycosidic bond links C#1 of the left-hand glucose molecule to C#4 of
the right-hand glucose molecule. (The Hs bonded to these Cs are left out for clarity.)
This C#1 has it's O, from what used to be it's OH, in the alpha(a)
position, so this is sometimes referred to as an a(1®4) linkage. Sometimes the notation is written as a-1,4 or sometimes as a-1-4.
/ \ / \
O C3 ¾
Whichever way the notation is written, it's trying to denote the same thing. The link
that holds the two glucose molecules together connects C#1 of one glucose to C#4 of the
other glucose, and that the oxygen is in the alpha position on C#1. This linkage was
formed by a dehydration reaction that used the OH that was in the alpha position on C#1 of
the first glucose ring and the OH on C#4 of the second glucose ring.
|Note that the a-1,4 linkage will prevent the
left-hand ring from opening up. In order to open up, the red C-O
bond must break, which requires that the hydrogen that was on the green oxygen goes back to the red oxygen,
but it's not there anymore.
/ \ / \
|Let's use maltose as a specific example. The left-hand part of the
molecule is stuck in the ring position. The right-hand ring, on the other hand, still can
open because the hydrogen on the #1 OH can and will move back to the oxygen in the ring,
thus, opening the ring and forming the double bond of the aldehydo group.
So, one ring can continue to open and this disaccharide can continue to act as a
reducing sugar. However, one ring is stuck in the closed position because the hydrogen
that originally came from the #5 OH and moved to the #1 OH when the ring closed was lost
in the dehydration reaction and and is not available to move back and open the ring. Note
that only half of the glucose rings in maltose can open and close and form the double bond
that allows for the reducing reaction. Consequently, maltose and other similar
disaccharides will only reduce half as quickly and half as much as an equal weight of
other similar monosaccharides. That's something to keep in mind when you do your
experiment for this lesson.
|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
as 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.
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