Cambridge A Level Biology 9700 — 2024 Oct/Nov Paper 3 · Variant 3

1 Plant cells produce the enzyme catalase that catalyses the hydrolysis (breakdown) of hydrogen peroxide into water and oxygen, as shown in Fig. 1.1. catalase hydrogen peroxide water + oxygen Fig. 1.1 A cylinder of potato tissue will have catalase molecules on its surface. When potato tissue is put into hydrogen peroxide solution, oxygen bubbles are released and a foam forms at the surface of the hydrogen peroxide solution. You will investigate the effect of ethanol concentration on the activity of catalase by measuring the height of the foam. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 harmful H hydrogen peroxide solution 40 irritant W water none 200 E 100% ethanol flammable 80 P 5 potato cylinders none – If any of H comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. Carry out step 1 to step 7. step 1 Put the five potato cylinders onto a white tile or cutting surface and cut each cylinder to a length of 4 cm. step 2 Label a beaker 0 and put 30 cm3 of W into this beaker. step 3 Put one potato cylinder into the beaker labelled 0. Start timing and leave for 5 minutes. step 4 Label one test-tube 0. step 5 After 5 minutes (step 3), remove the potato cylinder from the beaker labelled 0 and put it into the test-tube labelled 0. step 6 Put 5 cm3 of hydrogen peroxide solution into this test-tube. Immediately start timing. step 7 Measure and record the height of the foam at 1 minute and at 2 minutes. height of foam at 1 minute .................. height of foam at 2 minutes .................. (a) (i) The length of the potato cylinders may not be precisely 4 cm. Identify one other source of error in the procedure you have carried out. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] You will investigate the effect of different concentrations of ethanol on catalase activity. You will need to carry out a serial dilution of the 100% ethanol, E, to reduce the concentration by half between each successive dilution. You will need to prepare three concentrations of ethanol in addition to the 100% ethanol, E. After the serial dilution is completed, you will need to have 30 cm3 of each concentration available to use. (ii) Complete Fig. 1.2 to show how you will prepare your serial dilution. Each beaker should have: • a labelled arrow to show the volume of ethanol transferred • a labelled arrow to show the volume of distilled water, W, added • a label under the beaker to show the concentration of ethanol. 0 cm3 of water, W 60 cm3 of 100% ethanol, E 100% ethanol Fig. 1.2 [3] Carry out step 8 to step 14. step 8 Prepare the concentrations of ethanol as shown in Fig. 1.2. step 9 Put one potato cylinder into each beaker. Start timing and leave for 5 minutes. step 10 Label test-tubes with the ethanol concentrations prepared in step 8. step 11 After 5 minutes (step 9), remove the potato cylinders from the beakers and put them into the appropriately labelled test-tubes. step 12 Put 5 cm3 of hydrogen peroxide solution into the test-tube labelled 100. Immediately start timing. step 13 Measure the height of the foam after 1 minute and after 2 minutes. Record the results in (a)(iii). step 14 Repeat step 12 and step 13 with the potato cylinders in each of the other test-tubes. (iii) Record your results in an appropriate table. Include the results recorded in step 7. [5] (iv) Calculate the rate at which the foam was produced at 1 minute and at 2 minutes for 100% ethanol, E. Show your working. Include the unit in your answer. rate at 1 minute = ............................................................... rate at 2 minutes = ............................................................... [1] (v) Use your results in (a)(iv) to describe how the rate changes with time. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vi) With reference to the activity of catalase, explain why the results of 0 (step 7) were important in this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vii) State the dependent variable in this investigation. ..................................................................................................................................... [1] (viii) Suggest two improvements to the investigation you have carried out to increase the confidence in your results. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... [2] (b) Some fruits turn brown when they are cut into slices for eating. This browning can make the fruit less appealing to eat and difficult to sell. The enzyme polyphenol oxidase (PPO) catalyses an oxidation reaction when fruit is cut, causing the fruit tissue to turn brown. Scientists carried out an investigation to see how exposing wampee fruits to different concentrations of ethanol affected the activity of PPO in the fruit tissue. A large sample of wampee fruits was divided into five equal groups. Each group of fruit was sealed in a plastic container and treated with a different concentration of ethanol. After a short time in storage, the activity of the enzyme PPO in the wampee fruit tissue was recorded. The results are shown in Table 1.2. Table 1.2 concentration of ethanol PPO activity / μdm3 dm–3 / arbitrary units 0 2.20 100 1.40 200 0.95 400 0.70 500 0.60 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] (ii) Suggest an explanation for the pattern shown in the data. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 21]

Mark scheme: Question Answer Marks 1(a)(i) measuring the height of the foam; 1 1(a)(ii) 1 states 50%, 25%, 12.5% ; 3 2 shows transfer of 30 cm3 to each beaker from the previous beaker ; 3 shows 30 cm3 of W added to each beaker ; 1(a)(iii) 1 heading for independent variable: % concentration of ethanol ; 5 2 heading for dependent variable: height of foam / mm ; 3 states results for 0 % concentration ethanol and results at 1 minute and at 2 minutes for all concentrations ; 4 correct trend for results (the height of foam at the highest concentration is lower than at the lowest concentration of ethanol ; 5 records results to the nearest whole millimetre ; 1(a)(iv) shows how both rates are calculated and units for both rates included ; 1 1(a)(v) states how the rate changes with time for results collected ; 1 1(a)(vi) shows the activity of catalase without ethanol ; 1 1(a)(vii) height of the foam ; 1 1(a)(viii) any two from: 2 1 more concentrations of ethanol ; 2 carry out repeats and calculate a mean ; 3 use a gas syringe to measure volume of gas 1(b)(i) 1 x–axis: labelled ‘concentration of ethanol / ldm–3 and 4 y-axis: labelled ‘PPO activity arbitrary units / au’ ; 2 scale on x–axis: 100 to 2 cm and scale on y–axis: 0.5 to 2 cm, labelled at least every 2 cm ; 3 correct plotting of 5 points with cross or dot in circle ; 4 smooth, thin line, joined point to point, through all the points ; 1(b)(ii) any two from: 2 1 as concentration of ethanol increases PPO activity decreases ; 2 ethanol acts as an inhibitor ; 3 as concentration of ethanol increases less enzyme-substrate complexes ; 4 at higher concentrations of ethanol PPO enzyme starts to denature ;

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2 K1 is a slide of a stained transverse section through a plant stem. (a) (i) Draw a large plan diagram of the region of the stem on K1 indicated by the shaded area in Fig. 2.1. Use a sharp pencil. Use one ruled label line and label to identify the epidermis. draw this region Fig. 2.1 [5] (ii) Observe the cells in the centre of the stem on K1. Select a group of four adjacent cells. 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. 2.2 is a photomicrograph of a stained transverse section of a stem from a different plant. M2 M3 M1 M4 M5 magnification ×12 Fig. 2.2 (i) Measure the length of the vascular bundles using the lines M1, M2, M3, M4 and M5 in Fig. 2.2 and calculate the mean actual length of the vascular bundles. Show your working. M1 = M2 = M3 = M4 = M5 = mean actual length ......................................................... [4]

Mark scheme: 2(a)(i) 1 uses most of the available space and no shading ; 5 2 draws the correct region and no cells included ; 3 draws the layer of tissue beneath the epidermis ; 4 draws at least one vascular bundle ; 5 label line and label to a epidermis ; 2(a)(ii) 1 uses most of the available space and lines are continuous, thin and sharp ; 5 2 draws only four cells and each cell touches at least two other cells ; 3 two lines around each cell and three lines where cells touch ; 4 draws cells with 5 or 6 sides ; 5 label line and label to cell wall of one cell ; 2(b)(i) 1 states five measurements for length of vascular bundles and units ; 4 2 shows five measurements added together and divided by 5 (to calculate a mean) ; 3 shows value divided by 12 ; 4 answer given to nearest 0.5 mm ; 2(b)(ii) any two for one mark from: 1 1 measure all the vascular bundles 2 larger magnification to make it easier to measure 3 take more sections 2(c) only observable differences ; 4 differences between K1 and Fig. 2.3: any three from: (feature) K1 Fig. 2.1 thickness of epidermis thin Thick ; shape of stem circular Wavy ; number of vascular bundles more Fewer ; position of vascular bundles scattered arranged in rings ;

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