Elements: C, H, O in the general ratio 1:2:1

There are two common isomers of the glucose monomer, the a and the b forms. These have the same formula C₆H₁₂O₆ but have the OH group on the number 1 carbon at different angles.

The types of monosaccharide are classified by the number of carbons in them as follows:

Triose 3, Tetrose 4, Pentose 5, Hexose 6, Septose 7. Some are linear molecules, for example fructose, while others are ring molecules, for example glucose.

Two molecules of a-glucose can be joined by condensation of the two adjacent OH groups. This type of bond is called glycosidic.

The production of a long chain polymer from a-glucose gives a spiral shaped macromolecule because of the oxygen bonds being greater than 90^o. The molecule formed is plant starch (amylopectin), or glycogen.

When b-glucose is polymerised each adjacent glucose unit must be rotated through 180^o to achieve OH group alignment and this alters the final shape and therefore the use of the polymer, and we get cellulose. This a straight chain molecule because the bonds alternate like this: \O/ then /O\, so each alternate bond compensates for the previous misalignment of the polymer!

Sugar Tests:

Some sugar molecules are able to react with Benedict’s Solution when heated and reduce the copper compound in the solution to give a red precipitate. We refer to these types as reducing sugars (glucose, maltose).

Some disaccharides and polysaccharides do not react with Benedict’s solution until they have their glycosidic bonds broken by treatment with hot dilute acid. Once this is done the acid is neutralised and the Benedict’s test gives a red precipitate. Sugars like this are therefore called non-reducing sugars (sucrose, cellulose).

To test for starch, Iodine in KI solution is added to the food and a blue black colour forms as the starch and iodine form a ‘Starch-iodine complex’.