Question 123 marks
Question
Enzyme E catalyses the hydrolysis of starch to maltose. The progress of this enzyme‑catalysed reaction can be followed by measuring the time taken for the substrate, starch, to disappear. You will investigate the effect of an inhibitor, C, on the hydrolysis of starch. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 risk E 1% amylase solution irritant 10 S 1% starch solution none 50 2% copper sulfate harmful C 20 solution irritant W distilled water none 100 iodine iodine solution irritant 25 unknown concentration of harmful U 10 copper sulfate solution irritant If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. (a) (i) Think about the hazards of using the materials in Table 1.1. Decide whether the risk of using E, S and C is low, medium or high. Complete Table 1.1, using the words low, medium or high, to state the risk of using E, S and C. You may use each word once, more than once or not at all. [1] You will need to make a serial dilution of 2.0% copper sulfate solution, C, which reduces the concentration by a factor of ten between each successive dilution. Fig. 1.1 shows you the first two beakers you will use to make your serial dilution. Decide which other concentrations of copper sulfate solution you will use to make your serial dilution. (ii) Complete Fig. 1.1 by drawing as many extra beakers as you need for your serial dilution. For each beaker: • state, under the beaker, the volume and concentration of copper sulfate solution available for use in the investigation • use one arrow with a label, above the beaker, to show the volume and concentration of copper sulfate solution added to prepare the concentration • use another arrow with a label, above the beaker, to show the volume of W added to prepare the concentration. 10 cm3 of 2.0% copper sulfate 0 cm3 …… solution, C of W 9 cm3 of 2.0% copper sulfate solution, C, to use Fig. 1.1 [3] Carry out step 1 to step 19. 1. Prepare the concentrations of copper sulfate solution as decided in (a)(ii) and shown in Fig. 1.1. Use a glass rod to mix the copper sulfate solutions and water. 2. Label the test‑tubes with the concentrations of copper sulfate solution prepared in step 1. 3. Label another test‑tube W. 4. Wipe the white tile clean with a damp paper towel and then dry the white tile. 5. Label the white tile, as shown in Fig. 1.2. The numbers indicate the sampling times in seconds. drop of 15 30 45 60 iodine 75 90 105 120 135 150 165 180 Fig. 1.2 6. Put one drop of iodine on the white tile at each sampling time, as shown in Fig. 1.2. 7. Put 1 cm3 of W into the test‑tube labelled W. 8. Put 3 cm3 of S into the same test‑tube. Mix well. 9. Put 0.5 cm3 of E into the same test‑tube. Use a glass rod to mix. 10. Start timing. 11. After 15 seconds, use the glass rod to transfer a drop of the mixture from the test‑tube onto the iodine that is labelled 15, on the white tile. 12. Clean the glass rod with a paper towel. 13. Repeat step 11 to step 12 at intervals of 15 seconds until the iodine does not change colour. 14. Record the time taken for the starch to disappear in an appropriate table in (a)(iii). If the starch has not disappeared at 180 seconds, record ‘more than 180’. 15. Repeat step 4 to step 14 replacing the 1 cm3 of W with 1 cm3 of the lowest concentration of copper sulfate solution. 16. Repeat step 15 with the other concentrations of copper sulfate solution. (iii) Record your results in an appropriate table. [5] 17. Label a test‑tube with the letter U. 18. Repeat step 4 to step 13 with U instead of W. 19. Record in (a)(iv) your result for U. (iv) State the result for U. result for U …… [1] (v) Using your results in (a)(iii) and (a)(iv), estimate the concentration of copper sulfate in U. …… [1] (vi) State one significant source of error in this investigation. Explain why this is a source of error. source of error …… explanation …… [1] (vii) Suggest how you could make one improvement to the independent variable so that a more accurate estimate of the concentration of copper sulfate in U can be obtained. …… [1] (viii) Suggest how you could make two improvements to the measuring of the dependent variable so that a more accurate estimate of the concentration of copper sulfate in U can be obtained. …… [2] Question 1 continues on page 10 (b) A student carried out an investigation into the effect of temperature on the activity of enzyme E. The results are shown in Table 1.2. Table 1.2 temperature rate of reaction / °C / arbitrary units 5.5 57.5 15.0 120.0 29.5 215.0 36.0 245.0 49.5 102.5 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil for drawing graphs. Fig. 1.3 [4] (ii) Use your graph in Fig. 1.3 to determine the rate of reaction when the temperature is 37.5 °C. rate of reaction = …… arbitrary units [1] (iii) Explain the shape of the graph: between 5.5 °C and 36.0 °C between 36.0 °C and 49.5 °C. [3] [Total: 23]
Mark scheme
1(a)(i) assesses the risk for E as medium or high, for S as low and for C as medium or high ; 1 1(a)(ii) 1 0.2%, 0.02%, 0.002%, 0.0002% (labels under correct sequence of beakers) ; 3 2 shows transfer of 1 (cm3) copper sulfate solution to each beaker from the previous beaker ; 3 shows 9 (cm3) of water added to each beaker ; max 2 if cm3 omitted from mp2 and mp3 1(a)(iii) 1 heading for independent variable: percentage concentration copper sulfate solution and before heading for dependent 5 variable and no units in body of table ; 2 heading dependent variable: time and seconds and no units in body of table ; 3 readings for all samples ; 4 time for iodine to remain yellow increases with increasing concentration ; 5 results recorded to nearest whole second ; 1(a)(iv) records result for U and seconds ; 1 1(a)(v) correctly estimates the concentration of copper sulfate in U ; 1 1(a)(vi) any one from: 1 1 difficulty in judging exact time of colour change ; 2 colour may change between time intervals ; 3 using glass rod causes drop sizes to be of unequal sizes ; 1(a)(vii) any one from : 1 more concentrations between the two concentrations that lie each side of the estimate ; AVP ; 1(a)(viii) any two from: 2 1 use colorimeter ; 2 standard colours to compare ; 3 more intermediate times ; 4 choose end point that is dark brown ; 5 use pipette for iodine or sample ; 1(b)(i) 1 x-axis: temperature / °C 4 and y-axis: rate of reaction / arbitrary units ; 2 scale on x-axis: 10 °C to 2 cm, labelled at least every 2 cm and scale on y-axis: 50 arbitrary units to 2 cm, labelled at least every 2 cm ; 3 correct plotting of all five points using small crosses or dots in circles ; 4 five plots joined with thin line passing through all points and line is either smooth curve or joined plot to plot ; 1(b)(ii) correct value from graph ; 1 1(b)(iii) any three from: 3 1 (between 5.5 °C and 36 °C) enzyme and substrate have more kinetic energy ; 2 more successful collisions or more enzyme substrate complexes formed ; 3 (between 36 °C and 49.5 °C) shape of the active site changes or enzyme dentures ; 4 fewer enzyme substrate complexes formed ;