Cambridge A Level Biology 9700 — 2023 May/June Paper 3 · Variant 3

1 When milk is dropped into a test-tube containing copper sulfate solution, the drop of milk sinks to the bottom of the test-tube. The speed at which the drop of milk sinks will depend on the amount of protein, fat and other solids that are present in the milk. You will investigate the effect of different concentrations of milk on the time taken for the drop to sink to the bottom of the test-tube. You are provided with the materials shown in Table 1.1. Table 1.1 volume labelled contents hazard / cm3 C copper sulfate solution irritant 200 M 100% milk none 50 W distilled water none 30 If any solution 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 3 to practise releasing one drop of M from a pipette. step 1 Put some of M into a pipette. step 2 Hold the pipette containing M over an empty test-tube, as shown in Fig. 1.1. Squeeze the pipette gently to release one drop. It is important that the drop does not contain air bubbles. step 3 Repeat step 2 until you can release one drop at a time. one drop released squeeze gently pipette containing milk, M test-tube Fig. 1.1 You will investigate the effect of different concentrations of milk on the time taken for one drop of milk to sink through copper sulfate solution. You will need to: • prepare different concentrations of milk • record the time taken for one drop of each concentration of milk to sink through copper sulfate solution. (a) You will need to use proportional dilution to make different concentrations of milk, M, between 95% and 55%. You will need to prepare 10 cm3 of each concentration, using M and W. Table 1.2 shows two of the concentrations of milk you will use and how to prepare one of the concentrations. Decide which other concentrations of milk you will use. (i) Complete Table 1.2 to show how you will prepare the concentrations of milk you will use. Table 1.2 percentage volume of M volume of W concentration of milk / cm3 / cm3 95.0 55.0 5.5 4.5 [2] Carry out step 4 to step 14. step 4 Prepare the concentrations of milk, shown in Table 1.2, in the beakers provided. step 5 Draw a line 2 cm from the top of a clean, dry test-tube, as shown in Fig. 1.2. You will start timing from this line in your investigation. test-tube filled to the top with C 2 cm line drawn 2 cm from top of test-tube Fig. 1.2 step 6 Fill this test-tube with C, up to the top, as shown in Fig. 1.2. step 7 Stir the 95% concentration of milk. step 8 Put some of the 95% milk into a pipette. step 9 Put one drop of 95% milk into the test-tube containing C, as shown in Fig. 1.3. Start timing when the drop passes the line you have drawn. one drop released squeeze gently pipette C filled to top of test-tube containing milk, M line drawn 2 cm from top of test-tube test-tube Fig. 1.3 step 10 Stop timing when the drop reaches the bottom of the test-tube. If the drop has not reached the bottom of the test-tube after 120 seconds, stop timing and record ‘more than 120’. step 11 Record your result in (a)(ii). step 12 Carry out step 7 to step 10 as many times as you need, so that you are confident in your results for 95% milk. Remove any milk remaining at the surface of C with a pipette between each trial and add fresh C up to the top of the test-tube. Record all your results in (a)(ii). step 13 When you are confident in your results for 95% milk, empty the contents of the test-tube into the container labelled For waste and rinse out the test-tube. step 14 Repeat step 6 to step 13 for the other concentrations of milk you prepared in step 4. (ii) Record your results in an appropriate table. [5] (iii) Describe the trend in your results in (a)(ii). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Using Table 1.1 and step 4 to step 13, assess the risk of this procedure as low, medium or high. Give one reason for your answer. risk ..................................................................................................................................... reason ............................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [1] (v) Complete Table 1.3 to identify two possible sources of error in this investigation and describe how you could improve the procedure to reduce the effect of each source of error. Table 1.3 error improvement [4] Turn over for Question 1(b). (b) A student investigated the effect of temperature on the density of a milk sample. The results from the investigation are shown in Table 1.4. The density of milk is shown in arbitrary units (au). Table 1.4 temperature / °C density of milk / au 5 34 10 33 20 31 25 26 30 24 35 21 (i) Plot a graph of the data shown in Table 1.4 on the grid in Fig. 1.4. Use a sharp pencil. Fig. 1.4 [4] (ii) The student measured the temperature of the laboratory as 23 °C. Use your graph to determine the density of milk at 23 °C. density of milk = .................................................... au [1] (iii) A student suggested the hypothesis: As the temperature increases by 10 °C the density of the milk decreases by 2 au. Explain whether or not the data provides evidence to support the hypothesis. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 20]

Mark scheme: 1(a)(i) 1 at least 3 additional concentrations ; 2 volumes of M and W add up to 10 ; 2 1(a)(ii) 1 heading for percentage concentration of milk ; 2 heading for time and seconds ; 3 records result for each concentration prepared ; 4 records at least two results for each concentration prepared ; 5 results recorded as whole seconds ; 5 1(a)(iii) describes the trend correctly ; 1 1(a)(iv) medium risk and copper sulfate is an irritant ; 1 1(a)(v) error improvement drops sometimes stick to the side of test-tube ; use a wider vessel ; drops of milk remain in the milk ; use fresh copper sulfate each time ; 4 1(b)(i) 1 x-axis: temperature / °C and y-axis density of milk / arbitrary units ; 2 x-axis: scale 5 to 2 cm, labelled at least every 2 cm and y-axis: scale 4 or 5 to 2 cm, labelled at least every 2 cm, 20 at origin ; 3 correct plotting of 6 points ; 4 6 plots joined with thin line passing through all points, line is either a smooth curve or joined point to point with a straight line ; 4 1(b)(ii) correct answer for the density of milk at 23 °C ; 1 1(b)(iii) 1 between 10 °C and 20 °C the density of milk decreases by 2au (supports hypothesis) ; 2 between 20 °C and 30 °C the density of milk does not decrease by 2au (does not support hypothesis) ; 2

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2 L1 is a slide of a stained transverse section through a plant root. (a) (i) Draw a large plan diagram of the whole section on L1. Use a sharp pencil. Use one ruled label line and label to identify the cortex. [5] (ii) Observe the xylem on the section of the root on L1. Select a group of four adjacent xylem vessel elements. Each xylem vessel element must touch at least two other xylem vessel elements. • Make a large drawing of this group of four xylem vessel elements. • Use one ruled label line and label to identify the cell wall of one xylem vessel element. [5] (iii) Describe one observable feature that you used to identify the cells on L1 as xylem vessel elements. ..................................................................................................................................... [1] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through a different plant and a different organ. Fig. 2.1

Mark scheme: 2(a)(i) 1 uses most of the available space and draws minimum number of tissues ; 2 draws the whole root and no cells ; 3 shows two lines around stele ; 4 shows subdivision of stele ; 5 label line and label to the cortex ; 5 2(a)(ii) 1 uses most of the available space and lines continuous, thin and sharp ; 2 each xylem vessel element touching at least two other vessel elements ; 3 draws one large xylem vessel element and three smaller vessel elements ; 4 two lines around each cell ; 5 label line and label to the cell wall ; 5 2(a)(iii) thick walls or large lumen ; 1 2(b)(i) uses only observable differences ; any three from : feature L1 Fig. 2.1 overall shape circular angular ; trichomes present absent ; palisade mesophyll absent present ; stomata absent present ; 4 2(b)(ii) vascular bundle in centre ; 1 2(c) 1 correct measurement of scale bar and line P – Q ; 2 shows length of scale bar divided by 150 ; 3 shows length of line P – Q divided by answer to mp 2 ; 4 correct answer and appropriate units ; 4

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