Required Practical: Enzymes (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Enzyme rate required practical

• Aim: To investigate the effect of pH on the rate of reaction of amylase

• Process:

  • Use the enzyme amylase to break down starch at a range of pH values, using iodine solution as an indicator for the reaction occurring

  • Use a continuous sampling technique to monitor the progress of the reaction

• Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose)

• Starch can be tested for easily using iodine solution

Testing for amylase activity

Method

1. Place single drops of iodine solution in rows on the tile

2. Label a test tube with the pH to be tested

3. Use the syringe to place 2 cm³ of amylase in the test tube

4. Add 1 cm³ of pH buffer solution to the test tube using a syringe

5. Use another test tube to add 2 cm³ of starch solution to the amylase and buffer solution, start the stopwatch whilst mixing using a pipette

6. After 30 seconds, use a pipette to place one drop of the mixture on the first drop of iodine solution, which should turn blue-black

7. Wait another 30 seconds and place another drop of the mixture on the second drop of iodine

8. Repeat every 30 seconds until iodine solution remains orange-brown

9. Repeat experiment at different pH values – the less time the iodine solution takes to remain orange-brown, the quicker all the starch has been digested and so the better the enzyme works at that pH

10. Calculate the rate of reaction at each pH as 1 ÷ time taken (e.g. 1 ÷ 120 s = 0.0083 s⁻¹). The shortest time gives the fastest rate. When you write a conclusion, describe the effect on the rate or enzyme activity, not the time

Use this image

Adjustments to the method

• Temperature must be controlled (it is a control variable) by using a water bath, e.g. at 35 °C

  • All solutions that need to be used (starch, amylase, pH buffers) should be placed in a water bath and allowed to reach the temperature (using a thermometer to check) before being used

• Other control variables: the volume and concentration of the amylase, starch and buffer solutions, and the volume of mixture and of iodine solution used in each test