Cambridge A Level Biology 9700 — 2025 May/June Paper 3 · Variant 8

1 Catalase is an enzyme found in yeast cells. It catalyses the breakdown of hydrogen peroxide to produce water and oxygen, as shown in Fig. 1.1. catalase hydrogen peroxide water + oxygen Fig. 1.1 You will investigate the effect of copper sulfate on the progress of this reaction. You will do this by stopping the reaction after 5 minutes and measuring the concentration of hydrogen peroxide remaining. Potassium manganate(VII) is used to measure the concentration of hydrogen peroxide. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 Y yeast suspension none 20 H hydrogen peroxide solution irritant 20 C 1.0% copper sulfate solution irritant 30 A dilute sulfuric acid irritant 30 P potassium manganate(VII) solution harmful 30 W distilled water none 150 If any solution comes into contact with your skin, wash off immediately under cold water. You should wear suitable eye protection. It is recommended that you wear gloves when using A and P. You will need to carry out a serial dilution of the 1.0% copper sulfate solution, C, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of copper sulfate solution in addition to the 1.0% copper sulfate solution, C. After the serial dilution is completed, you need to have 10 cm3 of each concentration available to use. (a) (i) Complete Fig. 1.2 to show how you will prepare your serial dilution. Each beaker should have: • a labelled arrow to show the volume of copper sulfate solution transferred • a labelled arrow to show the volume of distilled water, W, added • a label under each beaker to show the concentration of the copper sulfate solution. 0 cm3 of distilled water W 20 cm3 of 1.0% copper sulfate solution, C 1.0% copper sulfate solution Fig. 1.2 [3] Carry out step 1 to step 7. step 1 Prepare the concentrations of copper sulfate solution as shown in Fig. 1.2. step 2 Label test‑tubes with the concentrations prepared in step 1. step 3 Put 1 cm3 of the 1.0% copper sulfate solution into the appropriately labelled test‑tube. step 4 Put 1 cm3 of each of the other concentrations of copper sulfate solution, as prepared in step 1, into the appropriately labelled test‑tube. step 5 Stir the yeast suspension, Y, and put 1 cm3 of Y into each test‑tube. Shake the test‑tubes gently to mix. Wait for 2 minutes. step 6 Put 1 cm3 of hydrogen peroxide solution, H, into each test‑tube. Shake gently to mix. Wait for 5 minutes. step 7 After 5 minutes, put 2 cm3 of sulfuric acid, A, into each test‑tube. Shake gently to mix. The addition of sulfuric acid stops the breakdown of hydrogen peroxide. You will now compare the concentration of hydrogen peroxide remaining in each test‑tube using potassium manganate(VII) solution, P. • When a drop of P is added to hydrogen peroxide solution, you will see a pink colour that quickly turns colourless as P reacts with the hydrogen peroxide. • You will continue adding P, one drop at a time, until the end‑point is reached. • The end‑point is when the pink colour stays for at least 5 seconds. • You will count the number of drops to reach the end‑point. • The greater the concentration of hydrogen peroxide, the more drops of P are needed to reach the end‑point. Carry out step 8 to step 14. step 8 Fill the syringe labelled P with solution P. step 9 Wipe the outside of the syringe with a paper towel. step 10 Hold the syringe labelled P over the test‑tube containing the lowest concentration of copper sulfate solution. Release one drop of P into the test‑tube. step 11 Shake the test‑tube to mix. Observe the colour to see if the end‑point is reached. The end‑point is when the pink colour stays for at least 5 seconds. step 12 Repeat step 10 and step 11, counting the total number of drops released until the end‑point is reached. You may need to refill the syringe with P. step 13 Record in (a)(ii) the number of drops of P added. If the end‑point has not been reached with 30 drops, record the result as ‘more than 30’. step 14 Repeat step 8 to step 13 with each of the other concentrations of copper sulfate solution prepared in step 1. (ii) Record your results in an appropriate table. [5] (iii) Describe the effect of changing the concentration of copper sulfate solution on the concentration of hydrogen peroxide remaining in the test‑tubes. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Describe one source of error in step 10 to step 12. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] River water can sometimes be contaminated with copper sulfate from factories. You will use the procedure described in step 5 to step 12 to estimate the concentration of copper sulfate in a sample of river water, R. You are provided with the materials shown in Table 1.2. Table 1.2 labelled contents hazard volume / cm3 R sample of river water with unknown irritant 20 concentration of copper sulfate step 15 Label a test‑tube R. Put 1 cm3 of R into the test‑tube. step 16 Repeat step 5 to step 12. Record the number of drops of P needed to reach the end‑point in (a)(v). (v) State the number of drops needed to reach the end‑point for sample R. .......................................................... [1] (vi) Use your results in (a)(ii) and (a)(v) to estimate the concentration of copper sulfate in the sample of river water, R. .......................................................... [1] Question 1 continues on page 10. (b) Some scientists investigated a possible treatment for controlling blood sugar levels in humans. The scientists measured the effect of an inhibitor found in green tea on the activity of the enzyme sucrase. This enzyme hydrolyses sucrose into glucose and fructose. The results are shown in Table 1.3. Table 1.3 concentration of inhibitor / mg cm–3 percentage inhibition of sucrase 0.50 8.0 1.00 27.5 1.50 44.5 2.00 51.0 2.50 52.5 (i) Plot a graph of the data in Table 1.3 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] (ii) Draw two lines on your graph in Fig. 1.3 to show the concentration of inhibitor that causes 24% inhibition of sucrase. [1] (iii) Suggest how the inhibitor in green tea reduces the activity of the enzyme sucrase. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iv) The scientists calculated the percentage inhibition of sucrase by measuring the concentration of reducing sugars in the solution after 5 minutes. Describe how the scientists could determine the concentration of reducing sugars in the solution. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] [Total: 22]

Mark scheme: Question Answer Marks 1(a)(i) 1 concentrations labelled under correct sequence of beakers: 0.5%, 0.25%, 0.125%, 0.0625% ; 3 2 shows transfer of 10 cm3 of copper sulfate to each beaker from previous beaker ; 3 shows 10 cm3 of W added to each beaker ; 1(a)(ii) 1 heading for independent variable: percent(age) / %, concentration of, copper sulfate / C 5 and to the left of the dependent variable ; 2 heading for dependent variable: number of drops of, P / potassium manganate(VII) ; 3 records the number of drops for each of five concentrations of, copper sulfate / C ; 4 expected trend (the higher the concentration of copper sulfate the more drops); 5 results recorded in whole drops ; 1(a)(iii) the higher the concentration of copper sulfate, the higher the concentration of hydrogen peroxide ; 1 1(a)(iv) any one from: 1 1 size of drops varies ; 2 difficulty of releasing P drop by drop / AW ; 3 difficulty of observing the pink colour / AW ; 1(a)(v) records number of drops as whole number ; 1 1(a)(vi) estimates the concentration R using the candidate’s results ; 1 1(b)(i) 1 label on x-axis: concentration of inhibitor / mg cm-3 4 and label on y-axis: percentage inhibition of sucrase ; 2 scale on x-axis: 0.5 to 2 cm, labelled at least every 2 cm and y-axis: 10.0 to 2 cm, labelled at least every 2 cm ; 3 correct plotting of all five points using small dots in circles or crosses ; 4 five plots joined with thin line passing through all points ; 1(b)(ii) shows both intercepts (one line drawn from the y-axis across to the line on the graph and one line drawn down from the 1 graph line to the x-axis) ; 1(b)(iii) any one from: 2 1 binds to the enzyme’s active site / allosteric site ; 2 changes active site shape ; any one from: 3 prevents substrate from binding / formation of enzyme substrate complexes ; 4 reduces the number of enzyme substrate complexes ; 1(b)(iv) 1 prepare known concentrations of reducing sugars ; 3 2 (test known concentrations with) Benedict’s solution and heat to at least 80 °C ; 3 test sample with Benedict’s and compare, time taken / final colour, to, standard / known, concentrations ;

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2 M1 is a slide of a stained transverse section through a 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 lower epidermis. [5] (ii) Observe the xylem vessel elements in the leaf on M1. Select a line of four adjacent xylem vessel elements. Each xylem vessel element must touch at least one other xylem vessel element. • Make a large drawing of this line 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 shows a photomicrograph of a transverse section through a different leaf from that on M1. Fig. 2.2 Identify three observable differences, other than colour, between the section on M1 and the section in Fig. 2.2. Record these three observable differences in Table 2.1. Table 2.1 feature M1 Fig. 2.2

Mark scheme: 2(a)(i) 1 uses most of the available space 5 and draws the upper epidermis as 3 lines and the lower epidermis as 2 lines ; 2 correct region of leaf section and no cells ; 3 draws correct proportions of the vascular tissue ; 4 draws subdivisions of the vascular tissue ; 5 label line to lower epidermis and label to identify the lower epidermis ; 2(a)(ii) 1 lines are continuous, thin and sharp and no shading ; 5 2 draws a line of four xylem vessel elements and each touches at least one other xylem vessel element ; 3 cell wall drawn as two lines and drawn around each cell ; 4 correct shape of cells ; 5 label line and label to identify the wall of one xylem vessel element ; 2(b) states differences that are (only) observable features ; 4 any three from : feature M1 Fig. 2.2 number of vascular bundles more fewer ; sizes of the vascular bundles larger smaller ; presence of trichomes absent present ; shape of leaf straight curled ; 2(c) 1 states the correct length of the scale bar and appropriate units ; 4 2 states the correct length of the gap between the ends of the leaf (D–E) and appropriate units ; 3 shows the length for D–E divided by the length of the scale bar and multiplied by 145 m ; 4 states correct answer ;

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