Cambridge A Level Biology 9700 — 2021 Oct/Nov Paper 3 · Variant 5

1 Plant cells contain the enzyme catalase which catalyses the breakdown of hydrogen peroxide, releasing oxygen. When a mixture of hydrogen peroxide and plant extract is put into a syringe, bubbles of oxygen are released. You are going to investigate the effect of substrate concentration on the activity of catalase. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 H 3.0% hydrogen peroxide solution irritant 30 P plant extract solution none 40 U unknown concentration of hydrogen peroxide solution irritant 10 W distilled water none 100 If any of the solutions come into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection and wear gloves to protect your hands when using hydrogen peroxide, H and U. (a) You will need to carry out a serial dilution of the 3.0% hydrogen peroxide solution, H, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of hydrogen peroxide solution in addition to the 3.0% hydrogen peroxide solution, H. After the serial dilution is completed, you will need to have 10 cm3 of each concentration available to use. (i) Complete Fig. 1.1 to show how you will prepare your serial dilution. Fig. 1.1 shows the first two beakers you will use to make your serial dilution. You will need to draw three additional beakers. For each beaker add labelled arrows to show: • the volume of hydrogen peroxide solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of hydrogen peroxide solution. 0 cm3 of W ............................. 20 cm3 of ............................. 3.0% .............................hydrogen peroxide ............................. solution, H 10 cm3 of 3.0% hydrogen peroxide solution to use ............................. ............................. ............................. ............................. Fig. 1.1 [3] Carry out step 1 to step 24. 1. Prepare the concentrations of hydrogen peroxide solution, as decided in (a)(i), in the beakers provided. 2. Put W into the test-tube so that it is approximately half-full. 3. Use the glass rod to stir the plant extract solution, P. 4. Put the nozzle of a clean syringe into the beaker containing P. 5. Pull the plunger out to the 1 cm3 mark so that P enters the syringe, as shown in Fig. 1.2. 10 9 8 7 6 5 4 3 2 1 P beaker containing P Fig. 1.2 6. Remove the syringe from the beaker containing P and wipe the nozzle with a paper towel. 7. Put the nozzle of the same syringe into the beaker containing 3.0% hydrogen peroxide solution, H. 8. Pull the plunger out to the 2 cm3 mark so that 1 cm3 of the 3.0% hydrogen peroxide solution enters the syringe, as shown in Fig. 1.3. 10 9 8 7 6 5 4 mixture of 1 cm3 of P 3 2 and 1 cm3 of 3.0% hydrogen 1 peroxide solution beaker containing 3.0% hydrogen peroxide solution, H Fig. 1.3 9. Carefully wipe the nozzle with a paper towel to remove excess hydrogen peroxide solution. 10. Put the tubing onto the syringe nozzle. 11. Put the syringe into a beaker as shown in Fig. 1.4. syringe nozzle mixture of 1 cm3 of P and 1 cm3 of delivery 1 hydrogen peroxide solution tube 2 3 4 5 6 7 8 9 test-tube 10 syringe water plunger beaker Fig. 1.4 12. Put the end of the delivery tube into the test-tube as shown in Fig. 1.4. 13. Start timing when the first bubble is observed in the water in the test-tube. 14. Count the number of bubbles produced in 120 seconds. Record the results in (a)(ii). 15. Hold the syringe with the nozzle up and remove the tubing from the syringe. Put the tubing onto a paper towel. 16. Hold the syringe with the nozzle pointing downwards over the container labelled ‘For waste’ and empty the syringe. 17. Fill the syringe with water from the container labelled ‘For washing’. 18. Put the tubing back onto the syringe nozzle. 19. Empty the syringe through the tubing into the container labelled ‘For waste’. 20. Repeat step 15 to step 19 twice more to wash the syringe and the tubing. 21. Remove the tubing from the syringe. Put the tubing onto a paper towel. 22. Repeat step 3 to step 21 with each of the other concentrations of hydrogen peroxide solution prepared in step 1. (ii) Record your results in an appropriate table. [5] 23. Repeat step 3 to step 13 with the unknown concentration of hydrogen peroxide solution, U. 24. Count the number of bubbles produced in 120 seconds. Record the result in (a)(iii). (iii) State the number of bubbles produced in 120 seconds for U. ...................................... [1] (iv) Using your results from (a)(ii) and (a)(iii) estimate the concentration of U. ...................................................... % [1] (v) The volume of oxygen was measured by counting the number of bubbles. Suggest why this is a source of error. ........................................................................................................................................... ..................................................................................................................................... [1] (vi) Suggest an improvement to the procedure to provide a more accurate measurement of the volume of oxygen produced. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vii) In the procedure described in step 1 to step 14, the effect of the concentration of hydrogen peroxide on catalase activity was investigated. Describe how you would modify this procedure to investigate the effect of changing pH on the number of bubbles of oxygen produced. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) A student carried out another investigation into the effect of temperature on the activity of catalase. Test-tubes containing plant extract and hydrogen peroxide were placed in thermostatically controlled water-baths at five different temperatures for 120 seconds. The results are shown in Table 1.2. The activity of the enzyme is shown in arbitrary units (au). Table 1.2 temperature / °C activity of enzyme / au 15.5 3.4 26.0 6.5 37.5 10.1 45.5 8.2 55.0 1.2 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.5. Use a sharp pencil for drawing graphs. Fig. 1.5 [4] (ii) Use your graph to find the activity of the enzyme when the temperature was 30.1 °C. activity of the enzyme = .................................................... au [1] (iii) Suggest an explanation for the difference in activity of the enzyme between 37.5 °C and 55.0 °C. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] [Total: 22]

Mark scheme: 1(a)(i) 1 correct concentrations (1.5, 0.75, 0.375, 0.1875) and % ; 2 shows transfer of 10 cm3 to each beaker from the previous beaker ; 3 shows addition of 10 cm3 water to each beaker ; 3 1(a)(ii) 1 heading for independent variable: percentage concentration of hydrogen peroxide (before heading for dependent variable) and no units in body of table ; 2 heading for dependent variable: number of bubbles ; 3 results for all samples ; 4 number of bubbles for the highest concentration of hydrogen peroxide more than for the lowest concentration of hydrogen peroxide ; 5 results recorded in whole numbers ; 5 1(a)(iii) number is less than for 3.0% ; 1 1(a)(iv) correct estimate based on candidate’s results ; 1 1(a)(v) identifies one error e.g. size of bubble varies ; 1 1(a)(vi) any one from: use a gas syringe ; use an up-turned measuring cylinder over water ; 1 1(a)(vii) same concentration of hydrogen peroxide ; five different pH buffers ; 2 1(b)(i) 1 x-axis: temperature / °C and y-axis: activity of enzyme / au ; 2 scale on x-axis: 10 °C to 2 cm, labelled at least every 2 cm and scale on y-axis: 2 au 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 ; 4 1(b)(ii) correct estimate based on candidate’s graph ; 1 Question Answer Marks 1(b)(iii) any three from: 1 at 37.5 °C more enzyme substrate complexes are formed than at 55.0 °C ; ora above 37.5 °C 2 enzyme is denatured ; 3 active site changes shape ; 4 substrate and active site are no longer complementary ; 3

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2 M1 is a slide of a stained transverse section through a plant stem. (a) Set up the microscope so that you can observe the section on M1. Use a sharp pencil for drawing. (i) Draw a large plan diagram of the whole section on M1. Your drawing should show the correct shapes and proportions of the different tissues. Use one ruled label line and label to identify the epidermis. [5] (ii) Observe the cells in the central tissue of the section on M1. Select four adjacent, touching cells of the central tissue. Each cell must touch at least two of the other cells. • Make a large drawing of this group of four cells. • Use one ruled label line and label to identify the cell wall of one cell. [5] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through a stem of a different type of plant. Fig. 2.1

Mark scheme: 2(a)(i) 1 minimum size and no shading ; 2 no cells drawn ; 3 correct position of vascular tissue ; 4 draws correct shape of vascular tissue ; 5 label line and label to epidermis ; 5 2(a)(ii) 1 minimum size and all lines sharp and continuous ; 2 draws only four whole cells and each cell touches at least two other cells ; 3 two lines drawn around each cell and three lines where cells touch ; 4 draws correct shape of cells and at least one air space ; 5 label line and label to one cell wall ; 5 2(b) any three differences e.g. feature K1 Fig. 2.2 vascular bundles joined separate ; shape of stem wavy round ; number of layers less more ; ring of cells below epidermis absent present ; 3 Question Answer Marks 2(c)(i) count squares that are half full and more than half full and full as 1 ; 1 2(c)(ii) counts and records the size of the vascular bundle A ; counts and records the size of the vascular bundle B ; 2 2(c)(iii) shows area of vascular bundle A divided by vascular bundle B ; shows multiplication by 100 ; 2

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