Cambridge A Level Biology 9700 — 2022 May/June Paper 3 · Variant 2

1 Plant tissues contain the enzyme catalase which catalyses the breakdown of hydrogen peroxide into oxygen gas and water. Ascorbic acid acts as an inhibitor of catalase. You will investigate the effect of changing ascorbic acid concentration on catalase inhibition. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 hydrogen peroxide H harmful irritant 50 solution W distilled water none 50 1 mol dm–3 ascorbic A irritant 10 acid solution If H or A comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You are also provided with five cylinders of potato tissue, labelled P. (a) You will need to carry out a serial dilution of the 1 mol dm–3 ascorbic acid, A, to reduce the concentration by a factor of ten between each successive dilution. You will need to prepare four concentrations of ascorbic acid in addition to the 1 mol dm–3 ascorbic acid solution, A. After the serial dilution is completed, you will need to have 9 cm3 of each concentration available to use. (i) Complete Fig. 1.1 to show how you will prepare your serial dilution. Fig. 1.1 shows the first two beakers you will use to make your serial dilution. You will need to draw three additional beakers. For each beaker, add labelled arrows to show: • the volume of ascorbic acid solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of ascorbic acid solution. 0 cm3 of W 10 cm3 of ...................1 mol dm–3 ascorbic acid ................... ................... 9 cm3 of 1 mol dm–3 ascorbic acid solution to use ................... ................... ................... ................... Fig. 1.1 [3] Carry out step 1 to step 13. step 1 Prepare the concentrations of ascorbic acid solution, as decided in (a)(i), in the beakers provided. step 2 Label test‑tubes with the ascorbic acid concentrations prepared in step 1. step 3 Label another test‑tube 0. step 4 On a white tile carefully cut the cylinders of potato tissue into thin discs that are approximately 1–2 mm thick. You will need to cut at least 70 discs. step 5 Place 10 potato discs into each labelled test‑tube. step 6 Add 1 cm3 distilled water, W, to the test‑tube labelled 0. step 7 Add 1 cm3 of each concentration of ascorbic acid to the appropriately labelled test‑tubes. step 8 Set up the apparatus as shown in Fig. 1.2 using the test‑tube labelled 0. The syringe barrel should be fully submerged in the beaker of water, B. bung delivery tube test-tube beaker of water, B 1 cm3 of 0 mol dm–3 ascorbic acid syringe barrel 10 potato discs Fig. 1.2 step 9 Add 5 cm3 of hydrogen peroxide solution H to the test‑tube labelled 0. Place the bung into the top of the test‑tube, making sure that the syringe barrel stays fully submerged. step 10 Record in (a)(ii) the initial volume of gas in the syringe barrel then start the stop‑clock. step 11 After 2 minutes record in (a)(ii) the final volume of gas in the syringe barrel. If the syringe barrel is full of gas, record as 10. step 12 Repeat step 9 to step 11 with each of the test‑tubes labelled in step 2. step 13 Calculate the total volume of gas produced at each concentration of ascorbic acid. Record these processed results in (a)(ii). (ii) Record your results in an appropriate table, including raw results and processed results. [6] (iii) Use your results in (a)(ii) to identify the greatest volume of gas produced in the reaction. greatest volume of gas produced = ............................................................... Use your answer to calculate the rate of gas production. Show your working. rate of gas production = ............................................... cm3 min–1 [2] (iv) Describe two improvements to the procedure that would make the measurements more accurate. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] Carry out step 14 to step 18. step 14 Label a test‑tube T. step 15 Put 5 cm3 of H into test‑tube T. step 16 Use a thermometer to measure the temperature of H in test‑tube T. Record this value, to the nearest 0.5 °C, in (b)(i). step 17 Add 10 discs of potato tissue to test‑tube T and start timing. step 18 After 2 minutes measure the temperature of the mixture in test‑tube T. Record this value, to the nearest 0.5 °C, in (b)(i). (b) (i) State the temperature of H before adding potato discs (step 16). .............................. °C State the temperature of H 2 minutes after adding potato discs (step 18). .............................. °C Calculate the change in temperature after 2 minutes. .............................. °C [2] (ii) State whether temperature is a significant source of error in this investigation. Explain your answer. ........................................................................................................................................... ..................................................................................................................................... [1] (c) A study was carried out in which volunteers were given different daily doses of ascorbic acid (vitamin C) in addition to their normal diet. The maximum ascorbic acid concentration in the blood plasma of each volunteer was measured. The results are shown in Table 1.2. Table 1.2 maximum ascorbic acid ascorbic acid daily dose concentration in blood plasma / mg / μmol dm–3 0 6 50 25 200 68 500 75 1000 75 (i) Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] (ii) Suggest an explanation for the results for a daily dose of 0 mg and the results for daily doses of between 500–1000 mg. 0 mg ................................................................................................................................... ........................................................................................................................................... 500–1000 mg ..................................................................................................................... ........................................................................................................................................... [2] [Total: 22]

Mark scheme: 1(a)(i) 1 correct concentrations (0.1, 0.01, 0.001, 0.0001) and mol dm–3 at least once ; 2 shows transfer of 1 (cm3) to each beaker from the previous beaker ; 3 shows 9 (cm3) of water added to each beaker ; 3 1(a)(ii) 1 heading for independent variable: ascorbic acid concentration / mol dm–3 (before heading for dependent variable) and no units in body of table ; 2 heading dependent variable: volume / cm3 and no units in body of table ; 3 results for all concentrations ; 4 volume of gas recorded for the lowest concentration of ascorbic acid greater than for the highest concentration of ascorbic acid ; 5 results recorded to an appropriate degree of accuracy ; 6 correct calculation of total volume ; 6 1(a)(iii) selects greatest volume from results in (a)(ii) ; calculates correct rate of gas production ; 2 1(a)(iv) any two from: 1 repeat, to identify anomalous results or calculate a mean ; 2 use a syringe with smaller divisions ; 3 method to standard the size of the potato discs ; 4 standardise the time between adding hydrogen peroxide and putting the bung on the test-tube ; 2 1(b)(i) temperatures recorded to 0.5 °C ; records the correct rise in temperature ; 2 1(b)(ii) yes: rate of reaction is affected by change in temperature and reason or no: rate of reaction is not affected by change in temperature and reason ; 1 Question Answer Marks 1(c)(i) 1 x-axis: ascorbic acid daily dose / mg and y-axis: maximum ascorbic acid concentration in blood plasma / mol dm–3 ; 2 scale on x-axis: 20 mg to 2 cm and labelled at least every 2 cm and scale on y-axis: 200 mol dm–3 to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all five points using small crosses or dots in circles ; 4 five plots joined with thin line passing through all points ; 4 1(c)(ii) 0 mg / day some ascorbic acid is already present in blood / obtained from other foods in the diet / produced by the body ; 500 to 1000 mg / day maximum rate of absorption (75 mol dm–3) is reached at a dose of 500 mg / day so the concentration does not increase with higher doses ; 2

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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 region in Fig. 2.1. Use a sharp pencil. Use one ruled label line and label to identify the epidermis. Fig. 2.1 [5] (ii) Observe one of the larger vascular bundles of the section on K1. Select a group of four adjacent xylem vessel elements. Each xylem vessel element must touch at least two other 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 shows a diagram of a stage micrometer scale that is being used to calibrate an eyepiece graticule. The length of one division on this stage micrometer is 1 mm. 0 10 20 30 40 50 60 70 80 90 100 Fig. 2.2 (i) Use Fig. 2.2 to calculate the actual length of one eyepiece graticule unit. Show your working. actual length = ............................................................... [3]

Mark scheme: 2(a)(i) 1 minimum size and no shading ; 2 correct section of the stem drawn and no cells drawn ; 3 draws a layer of vascular bundles under the epidermis ; 4 correct position of vascular tissue and air spaces ; 5 label line and label to epidermis ; 5 2(a)(ii) 1 minimum size and all lines sharp and continuous ; 2 draws only four whole xylem vessel elements and each xylem vessel element touches at least two other xylem vessel elements ; 3 two lines around each xylem vessel element and three lines where xylem vessel elements touch ; 4 correct shape of xylem vessel elements ; 5 label line and label to one xylem vessel element wall ; 5 2(b)(i) 1 shows division of 1 mm by 17 ; 2 answer with correct number of significant figures ; 3 appropriate units ; 3 Question Answer Marks 2(b)(ii) 74 eyepiece graticule units ; shows multiplication of answer from 2(b)(i) by eyepiece graticule units ; 2 2(b)(iii) three correct differences ; any three from, e.g.: feature K1 Fig. 2.3. shape triangular round ; vascular bundle distribution scattered and at edge at edge ; vascular bundle shape circular oval ; vascular bundle number more fewer ; 3

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