Cambridge A Level Biology 9700 — 2025 May/June Paper 3 · Variant 2

1 Invertase is an enzyme that catalyses the breakdown of sucrose into glucose and fructose. Invertase can be extracted from yeast cells. You will investigate the effect of an invertase extract on a sucrose solution and estimate the concentration of reducing sugars produced. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 E invertase extract irritant 20 R 0.5% reducing sugar solution none 40 W distilled water none 100 S 0.2% sucrose solution none 20 Benedict’s Benedict’s solution harmful irritant 20 If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You will need to make the different concentrations of reducing sugar solution using the 0.5% reducing sugar solution, R. You will need to prepare 20 cm3 of each concentration, using R and W. Table 1.2 shows the concentrations of reducing sugar you will use. Decide which volumes of R and W you will use. (a) (i) Complete Table 1.2 to show how you will prepare the concentrations of reducing sugar using R and W. Table 1.2 percentage concentration volume of R volume of W of reducing sugar / cm3 / cm3 0.5 20.0 0.0 0.1 0.05 0.01 0.4 0 0.0 20.0 [2] Preparing reducing sugar standards. Carry out step 1 to step 8. step 1 Set up a water-bath and heat it to boiling, ready for step 6 and step 15. step 2 In the beakers provided, prepare the concentrations of reducing sugar shown in Table 1.2. step 3 Label test-tubes with the concentrations of reducing sugar stated in Table 1.2. step 4 Put 2 cm3 of Benedict’s solution into each labelled test-tube. step 5 Put 2 cm3 of the 0.5% reducing sugar solution, R, into the appropriately labelled test-tube. step 6 Put the test-tube containing R into the water-bath and start timing. step 7 Record in (a)(ii) the time taken to the first appearance of a colour change. If there is no colour change after 120 seconds, stop timing and record the results as ‘more than 120’. step 8 Repeat step 5 to step 7 with the other concentrations of reducing sugar. (ii) Record your results in an appropriate table. [5] Investigating invertase. Carry out step 9 to step 16. step 9 Label one test-tube W and label one test-tube E. step 10 Put 1.0 cm3 of 0.2% sucrose solution, S, into these test-tubes. step 11 Add 1.0 cm3 of distilled water, W, to test-tube W and mix well. step 12 Add 1.0 cm3 of invertase extract, E, to test-tube E and mix well. step 13 Leave the test-tubes for 5 minutes. step 14 After the 5 minutes, put 2 cm3 of Benedict’s solution into each test-tube. step 15 Put the test-tubes in the water-bath prepared in step 1. step 16 Record in (a)(iii) the time taken to the first appearance of a colour change. If there is no colour change after 120 seconds, stop timing and record the results as ‘more than 120’. (iii) Record the time taken to the first appearance of a colour change in test-tube W and test-tube E. result for W ............................................................ s result for E ............................................................ s [1] (iv) Use your results in (a)(ii) and (a)(iii) to estimate the concentration of reducing sugar in test-tube W and test-tube E. concentration in test-tube W = ........................................................... % concentration in test-tube E = ........................................................... % [1] (v) With reference to the invertase extract, distilled water and sucrose solution, explain the results in (iv). test-tube W ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... test-tube E ......................................................................................................................... ........................................................................................................................................... [3] (vi) Suggest two improvements to the procedure that would give you a more accurate value for your estimated concentration of reducing sugar in test-tube E. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (b) Yeast cells also produce the enzyme catalase. Catalase breaks down hydrogen peroxide into oxygen gas and water. A student added different concentrations of catalase enzyme to hydrogen peroxide and counted the number of oxygen bubbles produced in 5 minutes. Table 1.3 shows the results of the investigation. Table 1.3 percentage concentration number of bubbles of of catalase oxygen in 5 minutes 0.0 1 2.0 22 4.0 44 6.0 50 8.0 94 10.0 118 (i) Plot a graph of the data shown in Table 1.3 on the grid in Fig. 1.1. Use a sharp pencil. Fig. 1.1 [4] (ii) State the percentage concentration of catalase that gave an anomalous result. ....................... percentage concentration [1] (iii) Describe the trend shown by the results. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Suggest an explanation for the result at 0% concentration of catalase. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (v) The student observed that the size of the bubbles varied. Suggest a more accurate method of measuring the oxygen produced. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 22]

Mark scheme: Question Answer Marks 1(a)(i) 1 correct volumes of R (4.0, 2.0) ; 2 2 correct volumes of W to make 20 cm3 ; ecf 1(a)(ii) 1 heading for independent variable: percentage concentration of reducing sugar (before heading for dependent variable) 5 and no units in body of table ; 2 heading for dependent variable: time / seconds and no units in body of table ; 3 a time for each concentration ; 4 time to reach the end-point for the highest concentration of reducing sugar is shorter than for the lowest concentration of reducing sugar ; 5 records time in whole seconds ; 1(a)(iii) records the time for W as ‘more than 120’ and records the time for E as a value less than 120 ; 1 1(a)(iv) correct estimate for W and E based on candidate’s results ; 1 1(a)(v) 1 sucrose is not a reducing sugar ; 3 2 (for W) there is no invertase (so) sucrose is not broken down / hydrolysed ; 3 (for E) invertase has, broken down / hydrolysed, the sucrose into reducing sugars ; 1(a)(vi) any two from: 2 1 use concentrations with narrower intervals ; 2 use stated concentrations that are either side of the estimate stated in 1(a)(iv) ; 3 repeat (the procedure) and calculate a mean ; 1(b)(i) 1 x-axis: percentage concentration of catalase 4 and y-axis: number of bubbles of oxygen in 5 minutes ; 2 scale on x-axis: 2% to 2 cm and labelled at least every 2 cm and scale on y-axis: 20 to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all six points using dots in circles or small crosses ; 4 plots are joined with a thin line passing through all points ; 1(b)(ii) 6.0 ; 1 1(b)(iii) as the concentration of catalase increases the number of bubbles of oxygen increases ; 1 1(b)(iv) hydrogen peroxide, breaks down / forms oxygen, with no enzyme ; 1 1(b)(v) measuring the volume of oxygen or description e.g. using a gas syringe, water displacement method ; 1

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2 K1 is a slide of a stained transverse section through a plant stem. (a) (i) Draw a large plan diagram of the region of the stem on K1 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 a vascular bundle. [5] (ii) Observe one vascular bundle of the section on K1. Select one large xylem vessel element and a group of three adjacent smaller xylem vessel elements. • Make a large drawing of this group of four xylem vessel elements. • Use one ruled label line and label to identify the wall of one xylem vessel element. [5] (b) Fig. 2.2 is a photomicrograph of a vascular bundle from the root of the same plant species as the stem on K1. X PP QQ magnification ×350 Fig. 2.2 (i) Line P–Q represents the width of the vascular bundle. Use the magnification and the line P–Q to calculate the actual width of the vascular bundle. Show your working and give your answer in micrometres (μm). actual width = ......................................................... μm [3]

Mark scheme: 2(a)(i) 1 appropriate size and no shading ; 5 2 draws quarter of the stem section; 3 draws the correct pattern and size of the vascular bundles and no cells; 4 draws an enclosed area in the top of each vascular bundle ; 5 label line and label to vascular bundle ; 2(a)(ii) 1 approprate size and all lines sharp and continuous ; 5 2 draws one large xylem vessel element and three small xylem vessel elements ; 3 two lines around each xylem vessel element and three lines where xylem vessel elements touch ; 4 draws correct shape of xylem vessel elements ; 5 label line and label to the wall of one xylem vessel element ; 2(b)(i) 1 correct measurement of the width of the vascular bundle (P–Q) and units ; 3 2 shows division of the measured width of the vascular bundle by 350 ; 3 correct answer ; 2(b)(ii) records only observable features ; 4 records one similarity ; e.g. (both) have phloem and xylem (both) have thickening to cell walls (in both) xylem is larger than phloem any two correct differences ; ; e.g. feature K1 Fig. 2.2 shape oval circular xylem circular star shape endodermis absent present vascular cap present absent 2(b)(iii) iodine / iodine in potassium iodide ; 1

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