Cambridge A Level Biology 9700 — 2019 May/June Paper 3 · Variant 1

1 Catalase breaks down hydrogen peroxide as shown in Fig. 1.1. catalase hydrogen peroxide water + oxygen Fig. 1.1 The decrease in the concentration of hydrogen peroxide can be measured to investigate the progress of this enzyme-catalysed reaction. Catalase can be found in plant extract. Potassium manganate(VII) solution can be used to estimate the concentration of hydrogen peroxide. The greater the concentration of hydrogen peroxide, the higher the volume of potassium manganate(VII) solution needed to reach the end-point. The end-point is reached when the first colour change appears and stays for at least 5 seconds. Potassium manganate(VII) solution will be added one drop at a time using a syringe. Carry out step 1 to step 3 to practise releasing drops from a syringe. 1. Fill a syringe with 1 cm3 of distilled water, W. 2. Hold the syringe over an empty beaker as shown in Fig. 1.2. Push the plunger slowly to release one drop. 3. Repeat this until you can release one drop at a time. push plunger slowly syringe one drop released Fig. 1.2 You will need to investigate the effect of changing pH on the activity of the enzyme catalase by: • changing the pH using different pH buffers • measuring the volume of P needed to reach the end-point for each pH. The lower the concentration of hydrogen peroxide remaining after 5 minutes, the lower the volume of P needed to reach the end-point. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 A dilute sulfuric acid irritant 25 H 1% hydrogen peroxide solution irritant 25 1% potassium manganate(VII) oxidising P 50 solution harmful harmful E plant extract solution 25 irritant B3 buffer pH3 none 10 B4 buffer pH4 none 10 B5 buffer pH5 none 10 B6 buffer pH6 none 10 B7 buffer pH7 none 10 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. State whether you think the risk of using the materials is low, medium or high. Give a reason for your answer. risk ...................................................................................................................................... reason ................................................................................................................................ ..................................................................................................................................... [1] Carry out step 4 to step 18 to investigate the effect of pH on the activity of catalase in the plant extract E. 4. Label 5 beakers pH3, pH4, pH5, pH6 and pH7. 5. Put 1 cm3 of each buffer into the appropriately labelled beaker. 6. Put 1 cm3 of E into each beaker. Stir gently to mix. 7. Leave the beakers for at least 1 minute. Stir gently to mix. The reaction will start as soon as H is added to the mixture of E and buffer in step 8. 8. Put 1 cm3 of H into each beaker. Stir gently to mix. 9. Leave for 5 minutes. While you are waiting, use your time to continue with Question 1. 10. After 5 minutes, put 1 cm3 of A into each beaker. Stir gently to mix. You will now test each solution to find the volume of P needed to reach the end-point. 11. Put the beaker labelled pH3 onto a white tile. 12. Fill the syringe labelled P with 2.0 cm3 of P. 13. Wipe off any P from the outside of the syringe with a paper towel. 14. Put one drop of P, as shown in Fig. 1.2, into the beaker labelled pH3. 15. Stir gently to mix. 16. Repeat step 14 and step 15 until the end-point is reached. You may need to refill the syringe with P. 17. Record in (a)(ii) the volume of P added. 18. Repeat step 11 to step 17 with each of the beakers labelled pH4, pH5, pH6 and pH7. (ii) Record your results in an appropriate table. [6] (iii) State the independent variable in this investigation. ..................................................................................................................................... [1] (iv) Use your results in (a)(ii) to suggest an optimum pH for catalase in this plant extract. Give a reason for your answer. optimum pH ................................... reason ................................................................................................................................ ..................................................................................................................................... [1] (v) Identify one source of error in this investigation. Explain why this is a source of error. source of error .................................................................................................................... explanation ......................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vi) Suggest how you would make improvements to this investigation in order to obtain a more accurate estimate of the optimum pH for catalase in this plant. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (b) A student carried out another investigation into the effect of temperature on the release of oxygen from hydrogen peroxide, using a plant extract containing the enzyme catalase. Test-tubes containing plant extract and hydrogen peroxide were placed in thermostatically controlled water-baths, at five different temperatures for 180 seconds. The volume of oxygen produced was measured. All other variables were standardised. The results are shown in Table 1.2. Table 1.2 volume of oxygen produced temperature / °C in 180 seconds / cm3 17.5 3.7 25.0 6.4 38.5 9.8 47.5 4.3 60.0 0.3 (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) Use your graph in Fig. 1.3 to determine the volume of oxygen produced when the temperature is 29.5 °C. Show on your graph how you determined your answer. volume of oxygen = .......................................................... [1] (iii) Suggest an explanation for the results between 20.0 °C and 60.0 °C. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (iv) Suggest an appropriate control the student could have used for this investigation. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 22]

Mark scheme: 1(a)(i) risk high + reason ; 1 1(a)(ii) 1. (heading) pH ; 2. (heading for dependent) volume of potassium manganate(VII) / P + units ; 3. collects results for all pHs ; 4. correct trend ; 5. records volumes up to 2 decimal places or whole numbers if drops given ; 6. records results to same precision ; 6 1(a)(iii) pH ; 1 1(a)(iv) estimates optimum pH from own results + least volume of P ; 1 1(a)(v) any 1 from: 1. idea of difficulty in reading the volume on syringe because of the dark colour; 2. drops stick to the side of the tube so the volume is not accurate ; 3. difficulty of judgement / identification of appearance colour ; 4. drop size different / different pressure on syringe / syringe sticking ; 5. water in named solutions evaporate / hydrogen peroxide decomposes over time ; 6. idea that the time the mixture was left between step 10 and step 11 differs for each pH tested ; 1 Question Answer Marks 1(a)(vi) any 3 from: 1. more / wider / narrower / different / examples / range of, pH (buffers) ; 2. given range of pH buffers either side of the candidates suggested optimum ; 3. use a colorimeter or have a colour standard to compare ; 4. replicate / repeat, to calculate a mean / identify anomalous results ; 5. use a burette / syringe / pipette with smaller divisions ; 6. do each pH separately / repeat / take more readings (each pH) ; 3 1(b)(i) 1. x-axis labelled as temperature / °C + y-axis labelled as volume of oxygen produced in 180 seconds (/) cm-3 ; 2. scale on x-axis 10 to 2 cm + labelled each 2 cm + scale on y-axis is 2 to 2 cm + labelled each 2 cm; 3. correct plotting of 5 points with a small cross or dot in circle ; 4. smooth, thin line, joined point to point, through 5 points ; 4 1(b)(ii) correctly reads from graph volume of oxygen + units cm3 ; 1 1(b)(iii) any 3 from: 1. increasing kinetic energy as temperature increases ; 2. up to 38.5 °C there are more successful collisions / more substrate binds to active site / more enzyme substrate complexes formed ; 3. above 38.5 °C active site of enzyme becomes denatured ; 4. above 38.5 °C active site shape is changed ; 5. above 38.5 °C so less enzyme substrate complexes formed / substrate cannot bind ; 3 Question Answer Marks 1(b)(iv) use of boiled plant extract / enzyme or distilled water to replace the enzyme / plant extract ; 1

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2 J1 is a slide of a stained transverse section through a plant root. You are not expected to be familiar with this specimen. Use a sharp pencil for drawings. You are expected to draw the correct shape and proportions of the different tissues. (a) (i) Draw a large plan diagram of the whole root. Use one ruled label line and label to identify the epidermis. [5] (ii) Observe the cells in the cortex of the root on J1. Select four adjacent, touching cells which show observable features of the cortex 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. 2.1 is a photomicrograph of a stained transverse section through a root of a different type of plant. You are not expected to be familiar with this specimen. Z 273 μm scale bar Fig. 2.1 (i) Annotate Fig. 2.1 to describe three observable differences between the root in Fig. 2.1 and the root on J1. • Draw label lines to three features of the root in Fig. 2.1 that are different from the root on J1. • Label one line Q, label one line R and label one line S. • Next to each letter, describe how each feature of the root in Fig. 2.1 differs from the root on J1.

Mark scheme: 2(a)(i) J1 is TS Maize root 1. minimum size + no shading + no cells ; 2. whole root drawn + at least 3 lines ; 3. correct root shape + at least 5 lines across width of section from the centre of drawing; 4. correct proportion of the diameter of the central region to the total diameter of the root ; 5. label line and label to epidermis ; 5 2(a)(ii) 1. lines continuous, thin and sharp + minimum size ; 2. draws only 4 whole cells + each cell touching at least two of the other cells ; 3. cell wall drawn as two lines + 3 lines where two cells touch ; 4. shows an air space between cells ; 5. label line and label to cell wall ; 5 2(b)(i) annotates 3 correct features on Fig 2.1 using labels Q, R and S ; ; ; e.g. Fig 2.1 xylem tissue arranged in a cross shape, J1 arranged in a ring 3 Question Answer Marks 2(b)(ii) 1. correct measurement of the scale bar + units ; 2. correct measurement of length of line Z + units ; method A method B method C mp 3 scale bar measurement multiplied by 1000 + divided by 273 ; mp 3 length of line Z divided by length of the scale bar ; if line Z or the scale bar has been converted it must be shown mp 3 (1 mm =) 273 divided by scale bar length ; if a conversion of units is done it must be shown mp 4 displays measurement of line Z + divided by magnification ; mp 4 answer to mp3 multiplied by 273 ; mp 4 answer to mp3 multiplied by length of Z ; 5. correct answer + units ; 5

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