Condensation Reactions
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Condensation Reactions

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. 

Equation showing the formation of maltose. [67051.jpg]


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. 

Equation showing the formation of a polysaccharide. [67053.jpg]

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. 

Equations showing th formation of cellulose. [67054.jpg]

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.

Structure of starch. [67058.jpg]

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.

Structure of amylose. [67059.jpg]

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.

Structure of amylopectin. [67060.jpg]

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.

67058.jpg (4198 bytes)


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 humans.

Structure of cellulose. [67061.jpg]


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.

Structure of glycogen. [67063.jpg]

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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E-mail instructor: Sue Eggling

Clackamas Community College
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