Carbohydrates can be broken down in extrememly smaller molecules.
Each individual unit is called a monomer. These monomers join to make longer chains such as polymers.
In carbohydrates, the smallest indivdual monomer unit is called a monosaccharide.
Common monosaccharides include:-
The Benedicts Test
Using the Benedicts solution test is a good way to test for reducing sugars (e.g. glucose).
The steps of this test are:-
1) Put your monosaccharide solution into a testube.
2) Put a small amount of Benedict's solution into the testube.
3) Heat in a water bath for 3-4 minutes.
4) If a monosaccharide is present, it should turn a brick-red colour.
For any other disaccharide or no sugar, it should remain blue.
There are three main disaccharides:-
1) Maltose (glucose + glucose).
2) Sucrose (glucose + fructose).
3) Lactose (glucose + galactose)
Disaccharides have a glycosidic bond.
With the then exception of Maltose, they cannot be tested via Benedict's reagant.
Testing for Disaccharides
There is a method for the testing of disaccharides.
1) Using 2cm3 of the food solution, carry out a normal Benedict's test.
2) If there is no change, a reducing sugar is not present.
3) Therefore, add 2cm3 of a different sample of food solution to 2cm3 of hydrochloric acid and gently heat in a water bath for 5 minutes. The HCl will hydrolyse the disaccharide.
4) Then add some sodium hydrogencarbonate to the mixture in order to neutralize the acid (as Benedict's solution won't work unless neutral).
5) Re-test the solution by carrying out the Benedict's test.
6) If a non-reducing sugar was present in the original sample, the solution will now turn brick-red.
Formed by many monosaccharide bonds joined by glycosidic bonds.
They are insoluble. (large molecules.)
- Hydrolysing polysaccharides breaks them down into mono- and disaccharides.
Polysaccharides can be used for:-
1) Storage (e.g. Starch)
2) Structure. (support for plants)
Testing for Starch
You can test for starch using the iodine test:-
1) Place 2cm3 into test tube.
2) Add two drops of iodine and stir.
3) If starch is present, the solution will turn blue-black.