Cambridge A Level Biology 9700 — 2022 Oct/Nov Paper 3 · Variant 3

1 You are provided with a solution labelled E containing an enzyme which coagulates (clots) milk. Enzyme E hydrolyses (breaks) peptide bonds between certain amino acids in a protein found in milk and this results in the coagulation of the milk. Calcium ions are needed for this coagulation. When a mixture of milk, calcium chloride solution and E is gently turned in a test-tube, the milk will coagulate, producing lumps of a white solid in a liquid. The end-point of the enzyme-catalysed coagulation is when all the milk coagulates, as shown in Fig. 1.1. liquid white solid Fig. 1.1 You will investigate the effect of temperature on the time taken to reach the end-point. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 M 100% milk none 150 E enzyme solution harmful irritant 20 C calcium chloride solution harmful irritant 20 If C or E comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You will test the activity of enzyme E at 30 °C and other temperatures up to a maximum of 50 °C. (a) (i) Complete Table 1.2 to show three other temperatures that you will use to show the effect of temperature on the activity of enzyme E. Table 1.2 temperature / °C 30 50 ................... ................... ................... [1] Carry out step 1 to step 14. step 1 Set up a water-bath ready for step 6. The starting temperature of the water-bath should be 30 °C, as shown in Table 1.2. step 2 Put 10 cm3 of M into a test-tube. step 3 Repeat step 2 so that you have two test-tubes containing M. step 4 Put 1 cm3 of C into each test-tube. step 5 Gently shake the test-tubes to mix M and C. step 6 Put the test-tubes into the water-bath and leave for 3 minutes. (ii) Explain why the test-tubes are left in the water-bath for 3 minutes in step 6. ........................................................................................................................................... ..................................................................................................................................... [1] step 7 Remove one of the test-tubes from the water-bath. The process of coagulation will start when E is added to the test-tube. step 8 Put 1 cm3 of E into the test-tube, so that it runs down the side of the test-tube and forms a layer on the surface of the mixture, as shown in Fig. 1.2. push gently syringe E in syringe test-tube held at an angle E running down layer of E forming inside of test-tube on top of mixture mixture Fig. 1.2 step 9 Gently shake the test-tube to mix the solutions and start timing. step 10 Rotate the test-tube and continue to rotate it while observing the mixture until the end-point is reached. Stop timing when the end-point is reached. If the end-point has not been reached by 180 seconds, stop timing and record this as ‘more than 180’. step 11 Record in (a)(iii) the time to reach the end-point. step 12 Repeat step 7 to step 11 with the other test-tube in the water-bath. step 13 Set up the water-bath at the next temperature after 30 °C stated in Table 1.2. step 14 Repeat step 2 to step 13 for each temperature stated in Table 1.2. (iii) Record your results in an appropriate table. [5] (iv) State the independent variable in this experiment. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Suggest a suitable control for this investigation to show that it is the enzyme E that coagulates the milk. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vi) The procedure described by step 1 to step 14 investigated the effect of temperature on the activity of enzyme E, using the time taken to reach the end-point. Describe how you would modify the procedure to investigate the effect of changing the concentration of milk on the time taken to reach the end-point. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (b) A scientist carried out an investigation into the effect of enzyme concentration on the coagulation of milk. The scientist calculated the activity of the enzyme for each concentration of enzyme. The results are shown in Table 1.3. Table 1.3 % enzyme activity of enzyme concentration / arbitrary units (au) 0.05 19 0.10 34 0.15 50 0.20 65 0.30 96 (i) Plot a graph of the data in Table 1.3 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] (ii) Use your graph to determine the activity of the enzyme when the enzyme concentration is 0.25%. Show on your graph how you determined this value. activity of the enzyme = .......................................................... au [2] (iii) Describe the trend in the results shown by your graph. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Suggest an explanation for the trend you described in (b)(iii). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 21]

Mark scheme: Question Answer Marks 1(a)(i) 1 states 35, 40, 45 ; 1 1(a)(ii) 1 contents of the test-tubes reaching the temperature of the water in the water-bath ; 1 1(a)(iii) 1 heading for independent variable: temperature / °C and before heading for dependent variable and no units in body of 5 table and heading for dependent variable: time / seconds ; 2 records times for all temperatures ; 3 for the highest temperature the fastest time ; 4 records repeats ; 5 records to the nearest whole second ; 1(a)(iv) 1 states independent variable as temperature ; 1 1(a)(v) 1 uses boiled and cooled enzyme or replaces enzyme with water ; 1 1(a)(vi) 1 standardise temperature by using a stated temperature ; 3 2 uses at least five concentrations of milk ; 3 uses proportional or serial dilution (as appropriate) ; 1(b)(i) 1 1. x-axis: % enzyme concentration 4 and y-axis: activity of enzyme (/) au ; 2 scale on x-axis: 0.05 to 2 cm, labelled at least every 2 cm and scale on y-axis: 20 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) 1 correct estimate based on candidate’s graph ; 2 2 shows intercepts on the graph ; 1(b)(iii) 1 as enzyme concentration increases the enzyme activity increases ; 1 1(b)(iv) 1 more enzyme present, more active sites ; 2 2 more enzyme substrate complexes form / more ESCs ;

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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 xylem. draw this region Fig. 2.1 [5] (ii) Observe the epidermis of the stem on K1 and the layer of cells beneath. Select a group of four adjacent cells. This group must include two cells from the epidermis and two cells from below the epidermis. 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] Fig. 2.2 is a photomicrograph of a stained transverse section of a stem from a different type of plant. Fig. 2.2 (b) Identify three observable differences, other than size and colour, between the stem in Fig. 2.2 and the stem on K1. Record these three observable differences in an appropriate table. [4]

Mark scheme: 2(a)(i) 1 minimum size and number of tissue layers ; 5 2 draws a quarter of the stem and no cells ; 3 draws minimum number of vascular bundles ; 4 vascular bundle subdivided ; 5 label line and label to xylem ; 2(a)(ii) 1 lines continuous, thin and sharp and no shading ; 5 2 draws only four cells each one touching two others ; 3 two lines around each cell and three lines where cells touch ; 4 both cells beneath the epidermis deeper than the cells in the epidermis ; 5 label line and label to cell wall ; 2(b) 1 organises comparison into 2 / 3 columns ; 4 2 three correct differences ;;; any three from: feature K1 Fig. 2.2 number of vascular bundles fewer more ; location of vascular bundles in a ring towards periphery scattered ; arrangement of xylem / phloem inner xylem / outer phloem random ; pith present absent ; trichomes present absent ; 2(c)(i) 1 shows counting on photomicrograph ; 4 2 correct number of vascular bundles counted ; 3 shows number of vascular bundles in quarter multiplied by 4 ; 4 correct answer ; 2(c)(ii) 1 helps to transport water etc. or helps support stem ; `1

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