CH2OH
Glucose in ring form -
Active groups
All sugars contain one of two active groups an aldehyde group – C=O or CHO or a ketone group - C
H C=O
C
This group affects the sugars’ properties very drastically and is the part of the sugar which is needed to react with any other substance, this is why sucrose is unreactive, because its’ active group is tied up in the bond between its’ two monomers.
Classifying Saccharides
The general formula for most Saccharides is Cx(H20)y. All Saccharides are separated into either Aldehydes or Ketones, depending on the composition of the saccharides’ active group. A saccharide can be classed as a monosaccharide, a disaccharide or a polysaccharide, depending on how many of a certain basic sugar molecule are combined to make it.
1 monosaccharide + another = a disaccharide
Monosaccharides
The general formula for Monosaccharides is (CH2O)n. For example Glucose - C6H12O6 and Fructose – C5H10O5. Both of these conform to the formula (CH2O)n. Monosaccharides can be classed as trioses, tetroses, pentoses, and heptoses depending on the number of carbon atoms there are in the molecule. Their specific molecular formulas changes depending on the no. of carbon atoms they contain, but they still conform to the general formula (CH2O)n. All monosaccharides are sweet to taste and are soluble in water.
Uses of different Monosaccharides
Trioses : Are involved mainly in carbohydrate metabolism
Pentoses : Are used in the synthesis of nucleic (DNA/RNA) acids, Synthesis of ATP (energy molecules),are CO2 acceptors in photosynthesis.
Hexoses: Used as respiratory substrates (e.g. glucose), used in the synthesis of di./poly. saccharides
Condensation Polymerization
Monomers can be joined together in chains to form larger polysaccharides this can be done by condensation polymerization. Condensation polymerization is a process during which two simple sugars bond by losing 2 hydrogen atoms and one oxygen atom (water) and use the leftover bonds on the remaining oxygen atom to bond with each other.
It is called condensation polymerization because water is produced during the reaction.
Disaccharides
Some examples of disaccharides are maltose, lactose and sucrose. They are used mainly as respiratory substrates. They are formed by condensation polymerization.
Maltose is found mainly in plants, lactose in milk and sucrose (cane sugar) in plants.
Polysaccharides
Polysaccharides are complex sugars and are made up of three or more simple sugars joined together to make a larger molecule. An example of a polysaccharide is starch, which is made up of many thousands of alpha glucose molecules; these glucose molecules are joined by 1-4 glycosidic bonds, through condensation polymerization, which are basically bonds between 2 simple sugars using an oxygen atom as a link. Polysaccharides are generally not sweet or soluble. The most common forms of polysaccharides we know are starch and glycogen, both almost identical to each other. Starch is used as an energy store molecule in plants and glycogen in animals is used for the same purpose. Polysaccharides are used as storage molecules because of the fact that they are so large they are insoluble and they can coil/fold into compact shapes, they are also used as structural molecules (cellulose).
Tests for Saccharides
The most common test for reducing sugars (all mono/disaccharides except sucrose) is The Benedicts test. This is done by adding copper sulphate to a solution of the sugar and water and then heating it gently. If the sugar is a reducing sugar it will reduce the copper sulphate and make a residue of brick red copper oxide in the solution.
A common test for starch/ glycogen (polysaccharides) is to add iodine to it, suspended in water. The suspension should turn from brown to blue black.
References
Biological science 1,2 – by D.J. Taylor, N.P.O. Green and G.W. Stout
Biological science 2 – by D.J. Taylor, N.P.O. Green and G.W. Stout
