Cambridge A Level Biology 9700 — 2020 May/June Paper 3 · Variant 1

1 Invertase is an enzyme found in honey. Invertase breaks down sucrose into reducing sugars, as shown in Fig. 1.1. invertase sucrose reducing sugars Fig. 1.1 You will carry out an investigation to determine the concentration of invertase in a sample of honey. The presence of the reducing sugars can be detected by using Benedict’s solution. The time taken for the Benedict’s solution to first show a colour change can be used to estimate the concentration of invertase in the honey extract, H. You will need to prepare a serial dilution of 1.0% invertase solution, E. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 H honey extract none 10 harmful E 1.0% invertase solution 10 irritant harmful B Benedict’s solution 30 irritant S 5% sucrose solution none 25 W distilled water none 150 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 invertase solution, E, as shown in Table 1.1. State whether the risk of using the invertase solution, E, is low, medium or high. Give a reason for your answer. risk ......................................................... reason ............................................................................................................................... ........................................................................................................................................... [1] You need to carry out a serial dilution of the invertase solution, E, to reduce the concentration of invertase by a factor of 10 between each successive dilution. Fig. 1.2 shows how to prepare the 1.0% and 0.1% concentrations of invertase solution. (ii) Complete Fig. 1.2 by drawing as many extra beakers as you need for your serial dilution of invertase solution. For each beaker: • state, under the beaker, the volume and concentration of invertase solution available for use in the investigation • use one arrow with a label, above the beaker, to show the volume and concentration of invertase 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. 9.0 cm3 of distilled water, W10.0 cm3 of 1.0% invertase 1.0 cm3 of 1.0% solution, E invertase solution, E 9.0 cm3 of 1.0% invertase solution, E, to use 9.0 cm3 of 0.1% invertase solution, to use Fig. 1.2 [3] Carry out step 1 to step 14 to determine the concentration of invertase in the honey extract, H. 1. Prepare the concentrations of invertase solution as you decided in (a)(ii) and shown in Fig. 1.2. Use a glass rod to mix the invertase solutions and water. 2. Set up a water-bath and heat the water to approximately 30 °C. 3. Label test-tubes with the concentrations of invertase solution prepared in step 1. 4. Put 2 cm3 of each concentration of invertase solution into an appropriately labelled test-tube. 5. Label another test-tube H. 6. Put 2 cm3 of honey extract, H, into the test-tube labelled H. 7. Put 2 cm3 of S into each labelled test-tube, including the test-tube labelled H. 8. Put all of the test-tubes into the water-bath at approximately 30 °C. Leave the test-tubes in the water-bath for 10 minutes. You do not need to keep the water-bath at 30 °C. While you are waiting, use your time to continue with question 1. 9. After the 10 minutes remove the test-tubes from the water-bath and put them in a test-tube rack. 10. Put 4 cm3 of Benedict’s solution, B, into each of the test-tubes. 11. Heat the water-bath to boiling and then stop heating. 12. Put the test-tube labelled H into the water-bath. Start timing. 13. Measure the time taken to the first colour change. Record the result in (a)(iii). If there is no colour change after 180 seconds, record as ‘more than 180’. 14. Repeat step 12 and step 13 using each of the concentrations of invertase solution you prepared in step 1, instead of H. Record your results in (a)(iv). (iii) State the result for H. ........................................................................ seconds [1] (iv) Record your results in an appropriate table for the concentrations of invertase solution you prepared in step 1. [5] (v) Use your results from (a)(iii) and (a)(iv) to estimate the concentration of invertase in H. concentration of invertase in H = ..................................................... % [1] (vi) Suggest how you would make improvements to this investigation in order to obtain a more accurate estimate of the invertase concentration in H. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (b) Honey contains a mixture of sugars. The percentage concentration of these sugars changes over time. The percentage concentration of sucrose in honey was measured for a period of 25 weeks. The results are shown in Table 1.2. Table 1.2 storage time percentage concentration of / weeks sucrose 0 2.3 3 1.8 6 1.3 12 0.9 25 0.5 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil for drawing graphs. Fig. 1.3 [4] (ii) Describe the change in percentage concentration of sucrose shown by your graph in Fig. 1.3. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) State which sugars, other than sucrose, will be present in the honey after 25 weeks. Explain your answer. sugars present .................................................................................................................. explanation ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] [Total: 22]

Mark scheme: 1(a)(i) risk: medium or high and states reason with correct reference to hazard ; 1 1(a)(ii) 1 labels under correct sequence of beakers: 0.01, 0.001, 0.0001 and % once ; 2 shows transfer of 1 (cm3) from second beaker to third beaker and 1(cm3) from third beaker to fourth beaker and 1 (cm3) from fourth beaker to fifth beaker and cm3 once ; 3 shows 9 (cm3) of W added to each beaker and cm3 once ; 3 1(a)(iii) time for H recorded ; 1 1(a)(iv) 1 heading for independent variable: invertase / E and concentration and % ; 2 heading for dependent variable: time taken for first colour change and seconds / s ; 3 readings for at least four concentrations ; 4 shortest time is for 1%; 5 records results to nearest second ; 5 1(a)(v) correct interpretation from table ; 1 1(a)(vi) any three from: 1 use concentrations of invertase solution within a narrower range either side of estimated concentration of H ; 2 repeat and calculate a mean for each concentration ; 3 ref. to staggered start / ref. to complete experiment for each concentration before moving on to next ; 4 maintain water-bath at 30°C; 5 ref. to considering that honey extract may contain sucrose / reducing sugar / glucose ; 3 1(b)(i) 1 x-axis: storage time / weeks and y-axis: percentage concentration of sucrose ; 2 scale on x-axis: 5 (weeks) to 2 cm and scale on y-axis: 0.5(%) to 2 cm ; 3 correct plotting of all points using small crosses or dots in circle ; 4 points joined with a thin line passing through all points as either a line of best fit or straight lines joining each point to the next ; 4 Question Answer Marks 1(b)(ii) any two from: 1 decrease over time ; 2 ref. to levelling off / reduced gradient over time ; 3 use of data to support ; 2 1(b)(iii) 1 glucose and fructose ; 2 invertase in honey breaks down the sucrose ; 2

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2 J1 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 in the sector shown by the shaded area in Fig. 2.1. draw this sector 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 sector of the stem on J1 shown by the shaded area in Fig. 2.1. Use one ruled label line and label to identify the epidermis. [5] (ii) Observe one vascular bundle in the stem on J1. The xylem within the vascular bundle is made up of rows of xylem vessel elements. Select a row of at least three adjacent, touching, xylem vessel elements. Make a large drawing of three adjacent, touching xylem vessel elements in this line. On your drawing use a ruled label line and label to identify one lumen. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section through a stem of a different type of plant that grows in water. You are not expected to be familiar with this specimen. X Y scale bar 1.5 mm Fig. 2.2 (i) Use the scale bar on Fig. 2.2 to calculate the actual diameter of the stem indicated by line X–Y. Show all the steps in your working and use appropriate units.

Mark scheme: 2(a)(i) Slide J1 Helianthus stem 1 suitable size and no shading ; 2 draws only sector and no cells ; 3 correct proportions of tissues ; 4 correct shapes and number of tissues / layers ; 5 label line and label to epidermis ; 5 2(a)(ii) 1 suitable size and lines continuous, thin and sharp ; 2 draws only three whole cells ; 3 draws (xylem) cells with suitable wall thickness ; 4 draws two lines around each cell and three lines where two cells touch ; 5 label line and label to lumen in one vessel element only ; 5 2(b)(i) 1 records measured length of line X–Y and measured length of scale bar ; A suitable range of measurements based on printed diagram 2 divides measured length of line X–Y by measured length of scale bar ; ecf from mp1 3 multiplies answer by 1.5 (length represented by scale bar) ; ecf from mp2 4 correct units for final answer (mm) ; or 1 records measured length of line X–Y and measured length of scale bar ; A suitable range of measurements based on printed diagram 2 divides 1.5 (length represented by scale bar) by measured length of scale bar ; ecf from mp1 3 multiplies answer by measured length of line X–Y ; ecf from mp2 4 correct units for final answer (mm) ; 4 Question Answer Marks 2(b)(ii) any three from: 1 position of vascular bundles (J1 around the edge, Fig. 2.2 in central region) ; 2 number of vascular bundles (more present in J1 than in Fig. 2.2) ; 3 size of air spaces (large / larger in Fig. 2.2) ; 4 number of air spaces (J1 has no / few air spaces, Fig. 2.2 has many) ; 5 thickness of epidermis (Fig. 2.2 has a thinner epidermis compared with J1) ; 3 2(b)(iii) any one from: 1 numerous / large air spaces (for buoyancy) ; 2 central / single vascular bundle (for flexibility) / minimal support needed in water ; 3 thin / absent cuticle (for gas exchange) ; 1

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