Cambridge A Level Biology 9700 — 2023 Oct/Nov Paper 3 · Variant 4

1 The progress of some enzyme-catalysed reactions can be followed by measuring the time taken for the substrate to be hydrolysed. The enzyme amylase catalyses the hydrolysis of starch to maltose. You will investigate the effect of different concentrations of amylase on the hydrolysis of starch. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 risk E 2.0% amylase solution irritant 50 ............................ S 1.0% starch solution none 50 ............................ W distilled water none 100 iodine iodine solution irritant 25 ............................ unknown concentration of U irritant 10 amylase solution If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. (a) (i) Think about the hazards of using the materials in Table 1.1 and decide whether the risk of using E, S and iodine is low, medium or high. Complete Table 1.1, using the words low, medium or high, to state the risk of using E, S and iodine. You may use each word once, more than once or not at all. [1] You will need to: • prepare different concentrations of amylase solution • record the time taken for the amylase to hydrolyse starch • use your results to estimate the concentration of an unknown solution of amylase, U. You will need to use proportional dilution to make different concentrations of amylase solution, E. You will need to prepare 10 cm3 of each concentration, using E and W. Table 1.2 shows how to prepare two of the concentrations you will use. Decide which other concentrations of amylase solution you will use. (ii) Complete Table 1.2 to show how you will prepare the other concentrations. Table 1.2 percentage concentration volume of E / cm3 volume of W / cm3 of amylase solution 2.0 10.0 0.0 0.4 2.0 8.0 [2] During the investigation you will be sampling at intervals and using iodine solution to test for the presence of starch. The end-point is reached when the iodine solution is a dark yellow-brown. There may also be some specks of blue-black present in the iodine solution. These specks can be ignored. Carry out step 1 to step 14. step 1 Prepare the concentrations of amylase solution as shown in Table 1.2, in the beakers provided. Mix well. step 2 Label test-tubes with the concentrations of amylase solution prepared in step 1. step 3 Label the white tile with the numbers shown in Fig. 1.1. The numbers indicate the sampling times in seconds. step 4 Put one drop of iodine on the white tile at each sampling time, as shown in Fig. 1.1. drop of 15 30 45 60 iodine 75 90 105 120 135 150 165 180 Fig. 1.1 step 5 Put 3 cm3 of S into each of the test-tubes labelled in step 2. step 6 Put 1 cm3 of the 2.0% amylase solution, E, into the test-tube labelled 2.0%. Use a glass rod to mix. step 7 Start timing. step 8 After 15 seconds, use the glass rod to transfer a drop of the mixture from the test-tube onto the drop of iodine that is labelled 15, on the white tile. step 9 Clean the glass rod with a paper towel. step 10 Repeat step 8 to step 9 at 15-second intervals until the end-point is reached. step 11 Record in (a)(iii) the time when the end-point is first observed. If the end-point is not reached at 180 seconds, record as ‘more than 180’. step 12 Clean the white tile with a damp paper towel and then dry the white tile. Make sure all the numbers are still visible. step 13 Put one drop of iodine on the white tile at each sampling time, as shown in Fig. 1.1. step 14 Repeat step 6 to step 13 using the other concentrations of amylase solution you prepared in step 1. (iii) Record your results in an appropriate table. [5] (iv) State the independent variable in this investigation. ..................................................................................................................................... [1] Carry out step 15 to step 23. step 15 Put one drop of iodine on the white tile at each sampling time, as shown in Fig. 1.1. step 16 Label a test-tube with the letter U. step 17 Put 3 cm3 of S into this test-tube. step 18 Put 1 cm3 of solution U into the same test-tube. Use a glass rod to mix. step 19 Start timing. step 20 After 15 seconds, use the glass rod to transfer a drop of the mixture from the test-tube onto the drop of iodine that is labelled 15, on the white tile. step 21 Clean the glass rod with a paper towel. step 22 Repeat step 20 to step 21 at 15-second intervals until the end-point is reached. step 23 Record, in (a)(v), the time when the end-point is first observed. If the end-point is not reached at 180 seconds, record as ‘more than 180’. (v) State the result for U. result for U ......................................................... [1] (vi) Using your results in (a)(iii) and (a)(v), estimate the concentration of amylase in U. ..................................................................................................................................... [1] (vii) Suggest one source of error in the procedure described in step 10 of this investigation. ..................................................................................................................................... [1] (viii) Suggest how you could make one improvement to reduce the source of error stated in (a)(vii). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ix) Describe how you would modify the procedure to investigate the effect of pH on the time taken for amylase to hydrolyse starch. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) A student investigated the effect of different substrate concentrations on the activity of an enzyme in the presence of an inhibitor. The concentration of the inhibitor was standardised. The results are shown in Table 1.3. Table 1.3 concentration of substrate rate of reaction / mol dm–3 / arbitrary units 0.0 0.000 0.2 0.750 0.4 1.375 0.6 1.800 0.8 2.175 1.0 2.250 (i) Plot a graph of the data in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil. Fig. 1.2 [4] (ii) Use your graph in Fig. 1.2 to determine the rate of reaction when the concentration of substrate is 0.45 mol dm–3. rate of reaction = .................................. arbitrary units [1] (iii) The Vmax of this enzyme is 2.250 arbitrary units. Using this information and the graph in Fig. 1.2, state whether the inhibitor is a competitive inhibitor or a non-competitive inhibitor. ........................................................................................................................................... Explain your answer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 22]

Mark scheme: Question Answer Marks 1(a)(i) all three correct: 1 E: medium S: low iodine: medium ; 1(a)(ii) 1 (percentage concentration of amylase) 1.6, 1.2, 0.8 ; 2 2 correct volumes of E and W to make 10 cm3 ; 1(a)(iii) 1 heading for independent variable: 5 percentage concentration of amylase and before heading for dependent variable ; 2 heading for dependent variable: time (/) seconds / s and no units in body of table ; 3 records a time for all concentrations ; 4 time to end-point for the lowest concentration of amylase is greater than for the highest concentration of amylase ; 5 records only times in Fig. 1.1 ; 1(a)(iv) concentration of amylase ; 1 1(a)(v) records a time for U ; 1 1(a)(vi) estimates the correct concentration of U ; 1 1(a)(vii) any one error from: 1 1 sample times are too far apart ; 2 difficult to judge the end-point ; 3 drop size of sample varies ; 1(a)(viii) one improvement to match the error stated in (a)(vii): 1 1 use closer times ; 2 use colour standards ; 3 use a pipette ; 1(a)(ix) 1 use same concentration of amylase ; 2 2 use five different pH ; 1(b)(i) 1 x-axis: concentration of substrate / mol dm–3 4 and y-axis: rate of reaction / arbitrary units (au) ; 2 scale on x-axis: 0.2 (mol dm–3)to 2 cm and labelled at least every 2 cm and scale on y-axis: 0.5 (au) to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all six points using small crosses or dots in circles ; 4 six plots joined with thin line passing through all points ; 1(b)(ii) (from graph) correct rate of reaction at 0.45 mol dm–3 ; 1 1(b)(iii) competitive inhibitor 2 any two from: 1 reaches the same Vmax ; 2 inhibitor competes with substrate for the active site ; 3 increasing the substrate concentration decreases the effect of the inhibitor ;

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2 M1 is a slide of a stained transverse section through a plant leaf. (a) (i) Draw a large plan diagram of the region of the leaf on M1 indicated by the shaded area in Fig. 2.1. Use a sharp pencil. draw this region Fig. 2.1 Use one ruled label line and label to identify the palisade tissue. [5] (ii) Observe the cells in the upper epidermis on the section of the leaf on M1. Select a line of four adjacent epidermal cells. Each cell must touch at least one of the other epidermal cells. • Make a large drawing of this line of four epidermal cells. • Use one ruled label line and label to identify the cell wall of one epidermal cell. [4] (b) Fig. 2.2 is a photomicrograph of a stained transverse section of a different leaf from M1. You are not expected to be familiar with this specimen. Fig. 2.2 Identify three observable differences, other than colour, between the leaf section in Fig. 2.2 and the leaf section on M1. Record these three observable differences in Table 2.1. Table 2.1 feature Fig. 2.2 M1

Mark scheme: 2(a)(i) 1 suitable size using most of the available space ; 5 2 correct section of the leaf drawn and no cells drawn ; 3 draws an area of tissue above the vascular bundle in the midrib ; 4 draws a flattened, subdivided vascular bundle in the midrib ; 5 label line and label to the palisade layer ; 2(a)(ii) 1 suitable size and all lines sharp and continuous and no shading ; 4 2 draws only four whole cells and each cell touches at least one other cell and two lines around each cell and three lines where cells touch ; 3 draws correct shape of cells ; 4 label line and label to one cell wall ; 2(b) 1 only observable differences and more than one observable difference ; 4 2, 3, 4 three correct differences: any three from: feature L1 Fig. 2.2 trichomes present absent ; shape of vascular bundle round crescent ; number of vascular bundles more less ; palisade tissue thicker thinner ; AVP ; 2(c)(i) 1 divides 1 by 40 ; 3 2 shows multiplication by 1000 or 103 ; 3 correct answer ; 2(c)(ii) correct number of eyepiece graticule units and shows multiplication of eyepiece graticule units by the answer to (c)(i) ; 1 2(c)(iii) any one from: 1 1 traps water vapour ; 2 reduces transpiration ; 3 idea of defence ;

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