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

1 Some biological molecules produce a colour change when tested with a specific reagent. You will investigate which biological molecules are present in four solutions, S1, S2, S3, and S4. Each solution contains one or none of the biological molecules: protein, starch, reducing sugar. You will also investigate the contents of solution U which contains a mixture of biological molecules. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 C 0.15% copper sulfate none 10 solution P 5% potassium hydroxide harmful 10 solution irritant iodine iodine solution irritant 5 B Benedict’s solution harmful 20 irritant S1 unknown solution none 10 S2 unknown solution none 10 S3 unknown solution none 10 S4 unknown solution none 10 U unknown mixture of none 10 biological molecules 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 the test for protein on S1, S2, S3, and S4, using step 1 to step 9. 1. Label four test-tubes, S1, S2, S3, and S4. 2. Put 1 cm3 of S1 into the test-tube labelled S1. 3. Repeat step 2 for S2, S3, and S4. 4. Put 1 cm3 of P into each test-tube. Shake gently to mix. 5. Put 1 cm3 of C into each test-tube. Shake gently to mix. 6. Leave the test-tubes for at least 1 minute. Shake gently to mix. 7. Observe the colour in each test-tube and record your observations in (a)(iii). 8. Identify the solution or solutions (S1, S2, S3, S4) that contain protein and record this in (a)(iv). 9. Keep any test-tubes from step 8 that contain protein. Empty the test-tubes that do not contain protein into the container labelled ‘For waste’. Rinse the empty test-tubes for use in step 10. (a) (i) State the reagent that is used to test for reducing sugar and the colour or colours produced if a reducing sugar is present. reagent .............................................. colour or colours ................................................................................................................ Describe how you will use the reagent to carry out the test for reducing sugar. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3] 10. Carry out the test for reducing sugar on each of the solutions using the labelled test-tubes from step 9. You do not need to test any solutions identified as containing protein in step 8. 11. Observe the colour in each test-tube and record your observations in (a)(iii). 12. Identify the solution or solutions (S1, S2, S3, S4) that contain reducing sugar and record this in (a)(iv). 13. Keep any test-tubes from step 12 that contain reducing sugar. Empty the test-tubes that do not contain reducing sugar into the container labelled ‘For waste’. Rinse the empty test-tubes for use in step 14. (ii) State the reagent that is used to test for starch and the colour that is produced if starch is present. reagent .......................................................... colour ............................................................ [1] 14. Carry out the test for starch on each of the solutions using the labelled test-tubes from step 13. You do not need to test any solutions identified as containing protein or reducing sugar in step 8 and step 12. 15. Observe the colour in each test-tube and record your observations in (a)(iii). 16. Identify the solution or solutions (S1, S2, S3, S4) that contain starch and record this in (a)(iv). 17. Identify the solution or solutions, if any, that do not contain protein, reducing sugar or starch. Record this as ‘none’ in (a)(iv). (iii) Record your observations in an appropriate table. [4] (iv) Identify the biological molecules present in solutions S1, S2, S3 and S4. If any solution does not contain protein, reducing sugar or starch write the answer as 'none'. S1 ...................................................................................................................................... S2 ...................................................................................................................................... S3 ...................................................................................................................................... S4 ...................................................................................................................................... [3] 18. Carry out the tests for protein, reducing sugar and starch using solution U. 19. Observe the colour from each test and record your observations in (a)(v). (v) Record the colours observed for each test for U. protein test ............................................................ reducing sugar test ................................................ starch test .............................................................. [1] (vi) Identify the biological molecules present in U. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [1] (b) A student carries out the same tests for protein, reducing sugar and starch on two unknown solutions, Y and Z. The student also carries out the test for the presence of lipid. (i) Describe the test for lipid and the expected observation if a lipid is present. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... observation if a lipid is present .......................................................................................... [2] From the observations for each test, the student concludes: • solution Y contains protein and reducing sugar • solution Z contains protein and lipid. The student also carries out the test for non-reducing sugar on solution Y and on solution Z. The student observes a positive result for the presence of a non-reducing sugar in solution Y and in solution Z. The student concludes that a non-reducing sugar is present in solution Y and in solution Z. (ii) Suggest why the student’s conclusion may not be correct for solution Y. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [1] (iii) The student thought that the protein in solution Z could be the enzyme amylase. Describe how the student could determine whether the protein in solution Z is amylase. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3] [Total: 19]

Mark scheme: 1(a)(i) 1 Benedict’s solution and colour(s) stated, e.g. red, brown, green, yellow ; 2 equal volumes or greater volume of Benedict’s solution than sample ; 3 boil or heat to a temperature of 80°C or higher ; 3 1(a)(ii) iodine solution and blue-black colour ; 1 1(a)(iii) 1 heading for independent variable: sample ; 2 heading for dependent variable: colour or observation ; 3 records four (colours) for protein test, three (colours) for reducing sugar test, two (colours) for starch test ; 4 records, colours / observations, for all the tests ; 4 1(a)(iv) identifies correct biological molecule for each solution ; ; ; S1 none S2 reducing sugar S3 protein S4 starch 3 1(a)(v) protein test – violet / AW reducing sugar test – blue starch test – blue-black ; 1 1(a)(vi) protein and starch ; 1 1(b)(i) add ethanol and water to sample (and shake) ; observation: white layer of emulsion ; 2 1(b)(ii) no conclusion possible as result for reducing sugar test same as for non-reducing sugar test ; 1 1(b)(iii) 1 mix solution Z with starch ; 2 leave for a (stated) time ; 3 test for starch / described ; 3

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2 L1 is a slide of a stained transverse section through a plant leaf. (a) Set up the microscope so that you can observe the section on L1. Use a sharp pencil for drawing. (i) Draw a large plan diagram of the whole section of the leaf on L1. Your drawing should show the correct shapes and proportions of the different tissues. Use one ruled label line and label to identify the palisade layer. [5] (ii) Observe the cells in the layer just below the epidermis of the section on L1. Select a line of four adjacent cells that make up this tissue. Each cell you draw must touch at least one of the other cells. • Make a large drawing of this line of four cells. • Use one ruled label line and label to identify the cell wall of one cell. [5] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through a leaf of a different type of plant. Fig. 2.1 The leaf sections on L1 and in Fig. 2.1 are from plants that are adapted to grow in dry environments (xerophytic plants). (i) Observe the leaf sections on L1 and in Fig. 2.1. • State one xerophytic feature that is visible on L1. ........................................................................................................................................... • State one xerophytic feature visible in Fig. 2.1 that is a different feature from the one you have identified for L1. ........................................................................................................................................... [2]

Mark scheme: 2(a)(i) 1 uses most of the available space and at least three layers of tissue ; 2 draws whole leaf and no cells ; 3 correct shape of leaf ; 4 correct proportion of vascular tissue to width of leaf ; 5 label line and label to identify the palisade layer ; 5 2(a)(ii) 1 uses most of the available space and lines thin and continuous ; 2 draws only four cells in a line and each cell touching at least one other cell ; 3 two lines drawn around each cell and three lines where cells touch ; 4 cells with thick walls ; 5 label line and label to identify the cell wall ; 5 2(b)(i) (L1) thick cuticle or thick epidermal cell wall ; (Fig.2.1) trichomes or sunken stomata ; 2 2(b)(ii) any one from: feature L1 Fig.2.1 vascular tissue one central in location circular in shape many ; in mid-rib and in a line ; V-shaped ; 1 2(c)(i) 1 x-axis: type of tree and y-axis: percentage of total trees in the forest ; 2 x-axis has bars of the same width, evenly spaced and scale on y-axis: 10 to 2 cm, labelled at least every 2 cm ; 3 correct plotting of five bars ; 4 five separate bars, with ruled horizontal lines and labelled correctly ; 4 2(c)(ii) 1 uses 13.5 (10.5 + 3.0) ; 2 shows division by 100 ; 3 shows multiplication by 2344 ; 4 correct answer to whole number ; 4

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