What is the basic monomer unit of a carbohydrate?
A sugar, known as a saccharide.
How are carbohydrates built?
Many monosaccharides are paired up by condensation reaction (removal of water) to form a disaccharide, joined by a glycosidic bond.
What are monosaccharides?
Sweet tasting, soluble substances with the general formula (CH2O)n, with n being between 3-7.
Test for reducing sugars
- Add sample to a test tube, in liquid form or ground up in water.
- Add equal volume of Benedict’s Reagent.
- Gently heat in boiling water bath for 5 mins.
- If reducing sugar is present, solution turns orange-brown.
What are reducing sugars?
All mono and some disaccharides. Reducing sugar is able to donate electrons (reduce) another chemical.
What is Benedict’s Reagent?
Alkaline solution of Copper II Sulphate. Forms an insoluble red precipitate of Copper I Oxide when heated with reducing sugar.
What happens when water is added to a disaccharide?
Glycosidic bond breaks, releasing monosaccharides, in hydrolysis reaction.
What are disaccharides?
Pairs of monomers joined.
What happens with non-reducing sugars, e.g. sucrose?
They do not change colour of Benedict’s Reagent when they are heated with it, so must be hydrolysed into monosaccharides.
Test for non-reducing sugars
- Solution remains blue after initial test.
- Add another equal amount of dilute HCl and Sample to test tube and heat it.
- Slowly add sodium hydrogencarbonate solution to neutralise HCl (test with pH paper to check alkaline).
- Repeat Benedict’s test, solution should now turn orange/brown.
Why does the solution at the end of the test for non-reducing sugars go brown?
Reducing sugars were produced from the hydrolysis of the non-reducing sugar when HCl was added.
What are polysaccharides?
Polymers formed by combining many monosaccharide molecules by glycosidic bonds from condensation reactions.
Why is it advantageous for polysaccharides to be large molecules?
They are suitable for storage, e.g. starch and glycogen.
Test for Starch
- Add sample to a test tube or spotting tile.
- Add 2 drops of iodine and shake or stir.
- Presence of starch is indicated by a blue-black coloration.
