Cambridge A Level Biology 9700 — 2021 May/June Paper 3 · Variant 5

1 A farmer who grows grapes needs to pick them when they have the highest reducing sugar content. You will be using the Benedict’s test to determine the concentration of reducing sugars in different samples of grape juice, U1, U2 and U3. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 U1 unknown concentration of reducing none 20 sugars U2 unknown concentration of reducing none 20 sugars U3 unknown concentration of reducing none 20 sugars W distilled water none 100 B Benedict’s solution harmful 50 irritant R 2% reducing sugar solution none 50 If any solution comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. (a) The Benedict’s test requires you to heat test-tubes containing some of the above solutions in a boiling water-bath. Read step 1 to step 5 on page 3 and then assess the risk of using a boiling water-bath in these steps as low, medium or high. State the level of risk. ................................................................................................................ (i) State one reason for your answer. ........................................................................................................................................... ..................................................................................................................................... [1] Carry out step 1 to step 9. 1. Set up a water-bath and heat to boiling ready for step 5, step 15 and step 22. 2. Label 3 test-tubes U1, U2 and U3. 3. Put 2 cm3 of U1 in the test-tube labelled U1. 4. Put 2 cm3 of B into the test-tube containing U1. Shake gently to mix. 5. Put this test-tube into the boiling water-bath. Start timing. 6. Measure the time taken for the first appearance of a colour change in the test-tube. If there is no colour change after 120 seconds, stop timing. 7. Record the result in (a)(ii). If there is no colour change after 120 seconds, record as ‘more than 120’. 8. Remove the test-tube from the water-bath and put it in the test-tube rack. 9. Repeat step 3 to step 8 for U2 and U3. (ii) Record your results for U1, U2 and U3 in an appropriate table. [3] (iii) Using your results in (a)(ii), state which sample, U1, U2 or U3, has the highest concentration of reducing sugars. ..................................................................................................................................... [1] You will need to carry out a serial dilution of the 2% reducing sugar solution, R, to reduce the concentration by half between each successive dilution. Fig. 1.1 shows the first 2 beakers you will use to make your serial dilution. (iv) Complete Fig. 1.1 by drawing as many extra beakers as you need for your serial dilution of reducing sugar solution. For each beaker add labelled arrows to show: • The volume of reducing sugar solution transferred • The volume of water added. Under each beaker, state the concentration of reducing sugar solution. 0 cm3 of W 20 cm3 of 2.0% reducing sugar, R 10 cm3 2.0% of reducing sugar, R, to use Fig. 1.1 [3] Carry out step 10 to step 19. 10. Prepare the concentrations of reducing sugar solution as shown in Fig. 1.1. 11. Label test-tubes with the concentrations you prepared in step 10. 12. Put 2 cm3 of 2% reducing sugar solution into the appropriately labelled test-tube. 13. Put 2 cm3 of B into the same test-tube. Shake gently to mix. 14. Repeat step 12 to step 13 with the other concentrations you prepared in step 10. 15. Put all the test-tubes into the boiling water-bath. Start timing. 16. Leave the test-tubes in the boiling water-bath for 1 minute. 17. Remove the test-tubes with a test-tube holder and put them into a test-tube rack. 18. Observe the colour in each test-tube. You may see the same colour in more than one test-tube. 19. Record the colours in (a)(v) using the words: red orange brown green blue (v) Record your results in an appropriate table. [4] In (a)(iii) you identified the solution with the highest concentration of reducing sugars as either U1, U2 or U3. 20. Put 2 cm3 of the solution you identified in (a)(iii) into a test-tube. 21. Put 2 cm3 of B into the same test-tube. Shake gently to mix. 22. Put the test-tube into the boiling water-bath. Start timing. 23. Leave the test-tube in the boiling water-bath for 1 minute. 24. Remove the test-tube with a test-tube holder and put it into a test-tube rack. 25. Observe the colour in the test-tube. Record the colour in (a)(vi), using one of the words: red orange brown green blue (vi) State the colour observed in step 25 ........................................................................ Using your results in (a)(v), estimate the concentration of reducing sugars in this sample. ..................................................................................................................................... [1] A student carries out the same procedure and finds that one of the unknown samples gives a negative result for reducing sugars. The student thinks that the sample may contain non-reducing sugars. (vii) Describe how you would modify the procedure to test for non-reducing sugars. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (b) As grapes ripen, the concentrations of the different reducing sugars change. A scientist measured the concentration of glucose and fructose at different stages of ripening. The results are shown in Table 1.2. Table 1.2 stage of ripening percentage concentration of reducing sugar glucose fructose unripe 9 1 ripening 7.5 9.5 ripe 3 22 Draw a bar chart of the data in Table 1.2 on the grid in Fig. 1.2. Use a sharp pencil for drawing bar charts. Fig. 1.2 [4] [Total: 20] Question 2 starts on page 10.

Mark scheme: 1(a)(i) medium or high and reason, e.g. idea of damaging skin ; 1 1(a)(ii) 1 heading for independent variable: reducing sugar solution and heading for dependent variable: time / s ; 2 records times as whole seconds; ; 3 records U2 as the shortest time ; 3 1(a)(iii) correctly identifies the sample with the highest concentration according to candidate’s result for U ; 1 1(a)(iv) 1 1.00, 0.5, 0.25 and 0.125 (labels under correct sequence of beakers) and,% / percentage, once ; 2 shows transfer of 10 (cm3) from first beaker to second beaker and 1 (cm3) from second beaker to third beaker and 1 (cm3) from third beaker to fourth beaker ; 3 shows 10 (cm3) of W added to second, third and fourth beakers ; 3 1(a)(v) 1 heading for independent variable: percentage concentration of reducing sugar ; 2 heading for dependent variable: colour or observation ; 3 records colours for five concentrations using the colours stated ; 4 records expected pattern of results ; 4 1(a)(vi) correct estimate of reducing sugar concentration according to candidate’s results ; 1 1(a)(vii) 1 add hydrochloric acid ; 2 heat ; 3 add sodium hydrogencarbonate (then carry out Benedict’s test) ; 3 1(b) 1 x-axis: stage of ripening and the labels: glucose, fructose, unripe, ripening, ripe and y-axis: percentage concentration of reducing sugar ; 2 x-axis: bars the same width, evenly spaced and scale on y-axis: 5 to 2 cm, labelled every 2 cm ; 3 correct plotting of six bars ; 4 grouping of bars, with ruled horizontal lines and labelled correctly ; 4

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2 N1 is a slide of a stained transverse section through a plant organ. (a) Set up the microscope so that you can observe the section on N1. Observe the different tissues in the area on N1 shown by the shaded region in Fig. 2.1. shaded region Fig. 2.1 Use a sharp pencil for drawing. (i) Draw a large plan diagram of the area of the section on N1 as shown by the shaded region in Fig. 2.1. Your drawing should show the correct shapes and proportions of the different tissues. Use one ruled label line and label to identify xylem tissue. [5] (ii) Observe the cells in the central tissue of the section on N1. 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. 2.2 is a photomicrograph of a stained transverse section of a different type of plant. X Y scale bar 240 μm Fig. 2.2 (i) Use the scale bar and line X–Y on Fig. 2.2 to calculate the actual length of the vascular bundle. Show your working and use appropriate units. .......................................................... [3]

Mark scheme: 2(a)(i) 1 uses most of the available space and at least two layers of tissue and no shading ; 2 draws part of plant organ and no cells ; 3 draws correct shape of plant organ ; 4 shows subdivision of the vascular tissue ; 5 label line and label to identify the xylem tissue ; 5 2(a)(ii) 1 uses most of the available space and lines thin and continuous ; 2 draws only four cells and each cell touching at least two other cells ; 3 two lines drawn around each cell, three lines where cells touch ; 4 correct shape of cells ; 5 label line and label to identify the cell wall ; 5 2(b)(i) 1 length of scale bar in μm divided by 240 μm to calculate magnification ; 2 length of the vascular bundle divided by value for magnification ; 3 answer stated to the appropriate degree of accuracy ; 3 2(b)(ii) 1 identifies organ as a stem ; 2 vascular bundles in a ring around the edge ; 3 central pith ; 3 2(b)(iii) only observable features stated ; records the observable differences between N1 and Fig.2.2 ; ; ; e.g. N1 rectangular shape and the shape of Fig.2.2 is circular N1 has bulges in the corners and Fig.2.2 has no bulges N1 has fewer vascular bundles and Fig.2.2 has more vascular bundles 4

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