2: Test for Sugars

Lesson Objectives:

• Identify the enzyme name based on the substrate, give its site of production and products. (AO1/2: E/D)

• Give reasons for heating sample solution when testing for carbohydrates. (AO2: C)• Apply your knowledge of sugar tests to conduct the practical independently and answer

the practical questions (AO2 and AO3: B/A)

Homework: Suggest how the cross-linkages in the amino acids increase the strength and stability of a collagen fibre.

Keywords: bonds, reduce/reducing, heat

About 100 Amino Acids, 20 of which occur in proteins. Every amino acid has:•Amino group (NH2)•Carboxyl group (COOH) an acidic group•Hydrogen group (--H)•R group, variety of different chemical group. Each amino acid has a different R group.

Structure of Proteins AO1

All mono and disaccharides are reducing sugars.

•What is a reduction reaction?Benedict’s reagent is an alkaline solution of copper (II) sulphate. When a reducing sugar is heated with Benedict's reagent it donates the electrons changing the colour of the solution.

Test for Reducing sugars AO1

TEST FOR SUGARS:•Grind the food sample in water.•Add an equal amount of Benedict’s reagent to the food sample.•Heat the mixture in a water bath. (Why is the sample heated?)•Look at the different shades to determine whether the amount of sugar present is high or low.

Test for Reducing sugars AO1/AO2

Sucrose is not a reducing sugar. •Add HCl to the ground food sample. ( HCl will hydrolyse the disaccharide into monosaccharides).•Place the test tube in the water bath and heat gently.•Add hydrogen carbonate to neutralise the acid (Benedict’s reagent doesn’t work in acidic condition). Using a pH indicator check if the solution is alkaline.•Re-test this solution repeating the same procedure for reducing sugars.

Test for Non-Reducing sugars AO1

•Place the sample on a spotting tile.•Add two drops of Iodine solution.•Blue-black colouration confirms the presence of starch.

Test for Starch AO1

•Place the sample solution in a test tube and add equal amount of sodium hydroxide.•Add a few drops of dilute copper (II) sulphate and mix gently.•A purple colouration shows the presence of peptide bonds.

Test for Proteins AO1

Things to think about during the practical

1. What is a reduction reaction?2. Why is copper sulphate added in equal amounts to

the solution?3. When Benedict’s reagent is added to fructose,

glucose and sucrose, what do you observe and why?4. Why is the solution heated in a water bath?5. Explain why it is not possible to distinguish between

very concentrated samples even though their concentrations are high.

6. Which real life setting make use of Benedict’s test?