Cambridge A Level Biology 9700 — 2019 Oct/Nov Paper 3 · Variant 6

1 Catalase is an enzyme that catalyses the break down of hydrogen peroxide, as shown in Fig. 1.1. catalase hydrogen peroxide water + oxygen Fig. 1.1 You will investigate the progress of this reaction by measuring the production of oxygen at different concentrations of catalase. Catalase can be found in plant extract. You will need to prepare different concentrations of plant extract, P, using proportional dilution. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume risk / cm3 H hydrogen peroxide harmful 60 solution irritant W distilled water none 80 P 100% plant extract harmful 50 solution irritant If H or P 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 H, W and P is low, medium or high. Complete Table 1.1, using the words low, medium or high, to state the risk of using H, W and P. You may use each word once, more than once or not at all. [1] You will need to prepare different concentrations of plant extract, P, using proportional dilution. You will need to prepare 10 cm3 of each concentration. Table 1.2 shows how to make up two of the concentrations of P you will use. Decide which other concentrations of P you will use. (ii) Complete Table 1.2 to show how you will prepare the concentrations of P you will use. Table 1.2 percentage concentration volume of P volume of W of P / cm3 / cm3 100 10.0 0.0 0 0.0 10.0 [2] When discs of filter paper that have been soaked in P are put into hydrogen peroxide solution, the discs rise to the surface as oxygen bubbles are produced. The higher the rate of oxygen production the faster the discs will rise to the surface. Carry out step 1 to step 8. 1. Prepare the concentrations of P as stated in Table 1.2. 2. Pick up one disc of filter paper using forceps. 3. Continue to hold the disc in the forceps and: • dip the disc in 100% P • remove excess P by briefly blotting the disc on a paper towel • put the disc at the bottom of the liquid in the beaker labelled H. 4. Immediately release the disc from the forceps and start timing. 5. Record in (a)(iii) the time taken for the disc to reach the surface of H and then remove the disc using the forceps. If the time taken for the disc to rise back to the surface is longer than 180 seconds then record ‘more than 180’. 6. Dip the forceps in the water in the beaker labelled For washing. Dry the forceps with a paper towel. 7. Repeat step 2 to step 6 two more times, using 100% P. 8. Repeat step 2 to step 7 using the other concentrations of P as stated in Table 1.2. (iii) Record your results in an appropriate table. [5] (iv) State the independent variable in this investigation. ..................................................................................................................................... [1] (v) Identify three sources of error in this investigation. Suggest an improvement to the procedure for each source of error that would improve the confidence in your results. 1. source of error ........................................................................................................... improvement .............................................................................................................. .................................................................................................................................... 2. source of error ........................................................................................................... improvement .............................................................................................................. .................................................................................................................................... 3. source of error ........................................................................................................... improvement .............................................................................................................. .................................................................................................................................... [3] (b) Another way of measuring the activity of catalase from plant tissue would be to measure the volume of oxygen produced when it reacts with hydrogen peroxide. A student decided to investigate the effect of changing the concentration of hydrogen peroxide on the volume of oxygen produced in 30 seconds. Table 1.3 shows the results of the student’s investigation. Table 1.3 volume of oxygen produced in 30 seconds / cm3percentage concentration of hydrogen peroxide test 1 test 2 test 3 test 4 test 5 mean 0.5 4.0 4.4 5.2 3.9 3.7 4.0 1.0 6.4 6.6 6.0 6.2 6.8 6.4 1.5 7.5 7.8 8.2 8.0 7.5 7.8 2.0 8.7 10.5 8.5 8.3 8.8 2.5 9.2 9.5 9.6 9.9 8.8 9.4 3.0 9.6 9.5 9.2 9.9 10.3 9.7 (i) Complete Table 1.3 by calculating the mean value at 2.0% hydrogen peroxide. Space for working. [2] (ii) Plot a graph of the mean values shown in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil for drawing graphs. Fig. 1.2 [4] (iii) Use your graph in Fig. 1.2 to determine the percentage concentration of hydrogen peroxide that produces 7.0 cm3 of oxygen in 30 seconds. percentage concentration = ......................................................% [1] (iv) Explain why the volume of oxygen production increases as the concentration of hydrogen peroxide increases. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3]

Mark scheme: 1(a)(i) H medium or high and W low and P medium or high ; 1 1(a)(ii) shows at least three more concentrations of P ; shows correct volumes of P and W to make these concentrations ; 2 1(a)(iii) 1. heading for independent variable: percentage concentration plant extract and before heading for dependent 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 disc to reach the surface increases with decreasing concentration ; 5. results recorded to nearest whole second ; 5 1(a)(iv) identifies the independent variable as concentration of plant extract ; 1 1(a)(v) any three from: 1. same hydrogen peroxide is used for each test and change hydrogen peroxide for each test ; 2. paper discs have varied amount of plant extract and standardise amount of extract ; 3. paper discs not blotted for same amount of time and standardise a way of blotting ; 4. judging when disc reaches surface and marking beaker to see disc reaching end-point ; 5. discs may stick to the side of beaker and use wider beaker ; 3 1(b)(i) 8.6 ; answer to correct degree of accuracy ; 2 Question Answer Marks 1(b)(ii) 1. x-axis: percentage concentration of hydrogen peroxide and y-axis: volume of oxygen produced in 30 seconds / cm3 ; 2. scale on x-axis: 0.5% to 2 cm, labelled at least every 2 cm and scale on y-axis: 2.0cm3 to 2 cm, labelled at least every 2 cm ; 3. correct plotting of five points using small crosses or dots in circles ; 4. six plots joined with thin line passing through all points and line is either smooth curve or joined plot to plot ; 4 1(b)(iii) correct value from graph ; 1 1(b)(iv) 1. more substrate molecules ; 2. more successful collisions ; 3. more enzyme substrate complexes formed ; 3

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2 N1 is a slide of a stained transverse section through a plant stem. You are not expected to be familiar with this specimen. Use a sharp pencil for drawing. You are expected to draw the correct shape and proportions of the different tissues. (a) (i) Draw a large plan diagram of the sector of the stem on N1, shown by the shaded area in Fig. 2.1. Use one ruled label line and label to identify the xylem. draw this sector Fig. 2.1 [5] (ii) Select four adjacent, touching xylem vessels. Each vessel must touch at least two other cells. Make a large drawing of this group of four touching xylem vessels. Use one ruled label line and label to identify the cell wall of one vessel. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section through the root of a different type of plant. You are not expected to be familiar with this specimen. An eyepiece graticule scale is shown on Fig. 2.2. The calibration of the eyepiece graticule scale is: 1 eyepiece graticule division = 24.6 μm 00 1010 2020 3030 XX YY Fig. 2.2

Mark scheme: 2(a)(i) 1. suitable size and no shading and no cells ; 2. draws correct sector and at least two vascular bundles ; 3. draws at least minimum number of tissue layers ; 4. correct proportion of vascular bundle to depth of stem ; 5. label line and label to xylem ; 5 2(a)(ii) 1. lines continuous, thin and sharp ; 2. draws only four whole xylem vessels and each vessel touches at least two other vessels ; 3. two lines around each vessel and three lines where vessels touch ; 4. at least one vessel with three or more angles ; 5. label line and label to the cell wall of one vessel ; 5 2(b)(i) 1. counts and records number of eyepiece graticule divisions for line X–Y ; 2. shows number of eyepiece graticule divisions units for line X–Y multiplied by 24.6 ; 3. correct answer ; 4. correct units ; 4 Question Answer Marks 2(b)(ii) 1. organises comparison into three columns with one column for features and collects only differences ; any three from: feature N1 Fig. 2.2 number of vascular bundles many one ; position of vascular bundles around edge in centre ; xylem tissue oval cross shaped ; central area hollow / cells absent not hollow / cells present ; epidermis smooth rough ; AVP described described ; 4

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