WJEC Biology for AS Level Student Book: 2nd Edition

Apparatus ▪ Starch solution (2 g / 100 cm 3 ) made up in buffer pH 7 ▪ Amylase solution (2 g / 100 cm 3 ) made up in buffer pH 7 ▪ Copper sulphate solutions, 0.025, 0.050, 0.075 and 0.100 mol dm –3 , made up in buffer pH 7 ▪ Water bath at 40ºC ▪ Test tubes ▪ Syringes ▪ Dropping tiles ▪ Dropping pipettes ▪ Iodine solution in potassium iodide Method 1. Place 4 cm 3 starch solution, 4 cm 3 amylase solution and 2 cm 3 copper sulphate solution in separate test tubes, in the 40ºC water bath for 10 minutes, to equilibrate to temperature. 2. Add the copper sulphate and then the amylase to the starch solution and invert the test tube once to mix the solutions. 3. Replace the mixture in the 40ºC water bath. 4. Immediately and at 1 minute intervals, place a 3-drop sample of the mixture in a well of the dropping tile and add 1 drop of iodine solution. 5. Note the time taken until the iodine solution does not change colour when added to the starch–amylase mixture. 6. Repeat steps 1–5 for all concentrations of copper sulphate. Results Concentration of copper sulphate added / mol dm –3 Final concentration of copper sulphate / mol dm –3 Time until iodine did not change colour / minutes time 1 / minutes –1 1 2 3 1 2 3 Mean 0 0 12 15 18 0.083 0.067 0.056 0.069 0.025 0.05 27 24 24 0.050 0.10 36 45 48 0.075 0.15 92 71 71 0.100 0.20 ∞ ∞ ∞ 0 0 0 0 1. Complete the table, using the correct number of decimal places. 2. Plot a graph time 1 / minutes –1 against the concentration of copper sulphate. 3. Describe the trend of the graph. 4. Relate the trend to the effect of copper ions acting as a non-competitive inhibitor of amylase. Further work ▪ The same experimental design may be used to test the effect of other non-competitive inhibitors on amylase, such as lead nitrate solution. ▪ The effect of different ratios of starch:maltose could be tested in an equivalent experiment to determine if maltose is a competitive inhibitor of amylase. Study point Smaller graduations on apparatus for measuring volume allow a more accurate reading than larger graduations, e.g. a 1 cm 3 syringe graduated to 0.01 cm 3 is more accurate than a 10 cm 3 syringe, graduated in cm 3 . Study point The explanation must be about the results given so it is important to refer to the enzyme and substrate by name.  1.4 Enzymes and biological reactions 91