PAG 9.1

Question 1

Equipment

Answer 1

• Distilled water
• Protein suspension
• 2 test tubes
• 5 cm3 syringe
• Pipette
• Sodium hydroxide solution
• Copper sulfate solution

Question 2

Method

Answer 2

-eye protection must be worn.1. Use the syringe to add 2 cm3 distilled water to one of the test tubes.2. Add 2 cm3 sodium hydroxide to the water.3. Shake gently to mix.4. Using the pipette and holding the tube at a 45° angle, carefully allow a small volume of copper sulfate to slide down the side of the tube so it forms a layer at the top of the mixed solution.5. Look for any colour change at the top of the solution, where the copper sulphate is sitting, and record your observations.6. Shake gently to mix.7. Observe and record the colour and appearance of the resulting solution.8. Carefully set the tube aside.9. Repeat the procedure using the protein suspension instead of the distilled water in a clean test tube-

Question 3

1. What colour did the test turn when performed on distilled water?

Answer 3

1. Blue ring sits at top of solution, light blue solution formed when mixed.

Question 4

2. What colour did the test turn when performed on the protein suspension?

Answer 4

2. Lilac/violet/purple ring sits at top of solution, Lilac/violet/purple solution formed when mixed.

Question 5

3. What is the biochemical explanation for the positive result with the biuret test?

Answer 5

3. Detects the presence of peptide bonds. Relies on the reduction of copper(II) ions to copper(I) – the latter form a complex with the nitrogen component of the peptide bond in an alkaline solution. The violet colour indicates the presence of protein.