Cambridge A Level Biology 9700 — 2020 Oct/Nov Paper 3 · Variant 4

1 Amylase is an enzyme which hydrolyses starch into reducing sugars, as shown in Fig. 1.1. amylase starch reducing sugars Fig. 1.1 The progress of this reaction can be followed by measuring the concentration of starch remaining using iodine solution. You will need to: • prepare a serial dilution of a 1.0% starch solution, S • estimate the concentration of starch remaining after hydrolysis with amylase at two different pH values. You are provided with the materials shown in Table 1.1. Table 1.1 volume labelled contents hazard / cm3 S 1.0% starch solution none 50 iodine 0.01 mol dm−3 iodine solution none 20 W distilled water none 150 a sample of 1.0% starch solution which harmful X has been incubated with amylase and 20 irritant pH2 buffer a sample of 1.0% starch solution which harmful Y has been incubated with amylase and 20 irritant pH7 buffer 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) You need to carry out a serial dilution of the 1.0% starch solution, S, which reduces the concentration of the starch solution by a factor of 10 between each successive dilution. Fig. 1.2 shows you the first two beakers you will use to make your serial dilution. (i) Complete Fig. 1.2 by drawing as many extra beakers as you need for your serial dilution. For each beaker: • state, under the beaker, the volume and concentration of starch solution available for use in the investigation • use one arrow with a label, above the beaker, to show the volume and concentration of starch solution added to prepare the concentration • use another arrow with a label, above the beaker, to show the volume of W added to prepare the concentration. 0 cm3 of W ............................ 10 cm3 of 1.0% starch ............................ solution, S ............................ ............................ 9 cm3 of ............................ 1.0% starch ............................ solution, S, ............................ available for use ............................ ............................ ............................ ............................ ............................ Fig. 1.2 [3] Carry out step 1 to step 6. 1. Prepare the concentrations of starch solution as decided in (a)(i) and shown in Fig. 1.2. Use a glass rod to mix the starch solutions and water. 2. Label test-tubes with the concentrations of starch solution prepared in step 1. 3. Put 5 cm3 of each concentration of starch solution into an appropriately labelled test- tube. 4. Put 3 drops of iodine into each of the test-tubes. Shake gently to mix. 5. After step 4, observe the colour of the liquid in each test-tube. You may see the same colour in more than one test-tube. It may help to hold a piece of white card behind the test-tube to see the colour more clearly. Fig. 1.3 shows the key you need to use to record your results. Key colour symbol blue/black or purple +++++ blue ++++ dark brown +++ brown ++ yellow/orange + Fig. 1.3 6. Record your results in (a)(ii) using the symbols shown in the key in Fig. 1.3. (ii) Record your results in an appropriate table. [5] Carry out step 7 to step 10. 7. Label a test-tube X. 8. Put 5 cm3 of X into this test-tube. 9. Put 3 drops of iodine into this test-tube. Shake gently to mix. 10. Observe the colour of the liquid and record the result in (a)(iii) using the symbols shown in Fig. 1.3. 11. Repeat step 7 to step 10, using Y instead of X. (iii) Record your result for X and Y using the symbols shown in the key in Fig. 1.3. result for X ............................................................... result for Y ............................................................... [1] (iv) Using your results in (a)(ii) and (a)(iii), estimate the concentration of starch in X and Y. X = ............................................................... Y = ............................................................... [1] (v) A student investigated the hydrolysis of starch by amylase. The student decided to use a semi-quantitative method to measure the products. These products are reducing sugars. Suggest how the student would use a semi-quantitative method to measure the concentration of reducing sugars produced. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (b) One of the reducing sugars produced by the hydrolysis of starch is maltose. Maltose is a disaccharide that can be hydrolysed to glucose by the enzyme maltase. A student investigated the effect of maltose concentration on the initial rate of reaction of maltase. The results are shown in Table 1.2. Table 1.2 concentration initial rate of of maltose reaction of maltase / mol dm−3 / arbitrary units 0.00 0 0.60 90 1.30 175 1.85 225 2.45 255 3.55 260 3.95 260 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.4. Use a sharp pencil for drawing graphs. Fig. 1.4 [4] (ii) Use the graph in Fig. 1.4 to estimate the Michaelis–Menten constant, Km. Show your working on the graph and in the space below. Km = ......................................... mol dm–3 [2] (iii) A student investigated a different enzyme, Z, and found the Km value to be 0.50 mol dm–3. State which enzyme, Z or maltase, has a lower affinity for its substrate. Give a reason for your answer. enzyme .................................................. reason ............................................................................................................................... ........................................................................................................................................... [1] (iv) Using the graph in Fig. 1.4, explain the change in the rate of reaction between 0.00 mol dm−3 of maltose and 3.95 mol dm−3 of maltose. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 22]

Mark scheme: 1(a)(i) 1 labels under correct sequence of beakers: 1.0, 0.1, 0.01, 0.001, 0.0001 and % at least once ; 2 shows transfer of 1 (cm3) to each beakers from the previous beaker ; 3 shows 9 (cm3) of, W /water, added to each beaker ; 3 1(a)(ii) 1 heading for independent variable: percentage concentration of starch (before heading for dependent variable) and no units in body of table ; 2 heading for dependent variable: symbol ; 3 symbols for all samples ; 4 results for the highest concentration of starch more than for the lowest concentration of starch ; 5 results recorded for the highest concentration of starch more than for the next concentration of starch ; 5 1(a)(iii) 1 records results for X and Y ; 1 1(a)(iv) 1 estimates the correct concentrations of starch in X and Y ; 1 1(a)(v) 1 Benedict’s reagent ; 2 time to first colour change / prepare known colour standards ; 3 five reducing sugar concentrations ; 4 compare unknown, time / colour, with known concentrations, times / colours ; 3 1(b)(i) 1 x-axis: concentration of maltose / mol dm–3 and y-axis: initial rate of reaction of maltase / arbitrary units ; 2 scale on x-axis: 1 mol dm–3 to 2 cm, labelled at least every 2 cm and scale on y-axis: 50 arbitrary units to 2 cm, labelled at least every 2 cm ; 3 correct plotting of all points using small crosses or dots in circles ; 4 plots joined with thin line passing through all points ; 4 1(b)(ii) 1 correct calculation of ½ Vmax ; 2 correct value for Km from graph ; 2 1(b)(iii) maltase and has a higher Km ; 1 Question Answer Marks 1(b)(iv) any two from: as substrate concentration increases 1 more substrate molecules ; 2 more successful collisions or more enzyme substrate complexes formed ; levels off because 3 active sites are saturated ; 2

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2 L1 is a slide of a stained transverse section through a plant stem. You are not expected to be familiar with this specimen. (a) Select a field of view so that you can observe the different tissues as shown by the shaded area in Fig. 2.1. Fig. 2.1 Use a sharp pencil for drawing. You are expected to draw the correct shape and proportions of the different tissues. (i) Draw a large plan diagram of the region of the stem on L1 shown by the shaded area in Fig. 2.1. Use one ruled label line and label to identify the xylem. [5] (ii) Observe the cells in the central tissue (pith) of the stem section on L1. Select four adjacent, touching cells which show observable features of the central tissue. Each cell must touch at least two other cells. • Make a large drawing of this group of four touching 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 through a stem of a different type of plant. You are not expected to be familiar with this specimen. line B line C inner layer line A line A line C line B magnification ×32 Fig. 2.2 (i) The magnification of the stem section in Fig. 2.2 is ×32. Use line A, line B and line C to determine: • the mean actual diameter of the whole stem section • the mean actual diameter of the inner layer. Show all the steps in your working. mean actual diameter of whole stem section ............................................................... mean actual diameter of inner layer ............................................................... [4]

Mark scheme: 2(a)(i) 1 suitable size and no shading ; 2 draws only correct region of the stem and no cells drawn ; 3 draws correct proportions of vascular bundle compared to the bulge ; 4 draws flattened vascular bundle ; 5 label line and label to xylem ; 5 2(a)(ii) 1 suitable size and all lines sharp and continuous ; 2 draws only four whole cells and each cell touches at least two other cells ; 3 draws correct shape of cells ; 4 two lines around each cell and three lines where cells touch ; 5 label line and label to one cell wall ; 5 2(b)(i) 1 records measured total diameter of the stem and units ; 2 records measured diameter of the inner layer and units ; for total diameter or inner layer 3 shows addition of three measurements and shows division by 3 and division by 32 ; 4 (final answer) to the correct degree of accuracy and units ; 4 2(b)(ii) larger number to smaller number ; 1 Question Answer Marks 2(c) 1 records only observable differences ; 2, 3 and 4 any three from: feature Fig. 2.3 L1 number of vascular bundles many fewer ; shape of stem circular square ; air space absent present ; epidermis thick thin ; 3

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