H2 Bio 2024 YIJC Prelim P4 (A)

1 Suggested Answers 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 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.

2 (a) You will need to carry out a serial dilution of the 1moldm–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 1moldm–3 ascorbic acid solution, A. After the serial dilution is completed, you will need to have 9cm 3 of each concentration available to use. (i) Complete Table. 1.2 to show how you will prepare your serial dilution. Table 1.2 shows the first two beakers you will use to make your serial dilution. You will need to complete three additional beakers. For each beaker, state: • the concentration of the ascorbic acid • the concentration of the ascorbic acid transferred • the volume of ascorbic acid solution transferred • the volume of distilled water, W, added. Table 1.2 Concentration of ascorbic acid / moldm-3 Concentration of the ascorbic acid transferred / moldm-3 volume of ascorbic acid solution transferred / cm3 volume of distilled water, W, added / cm3 1.0 1.0 10.0 0.0 0.1 1.0 1.0 9.0 0.01 0.1 1.0 9.0 0.001 0.01 1.0 9.0 0.0001 0.001 1.0 9.0 Mark Scheme: 1 correct concentrations (0.01, 0.001, 0.0001); accept if student present in index form i.e. 1.0 x 10-2; 2 shows transfer of 1.0 (cm 3) to each beaker from the previous beaker; 3 shows 9.0 (cm3) of water added to each beaker; [3]

3 Carry out step 1 to step 13. 1 Prepare the concentrations of ascorbic acid solution, as decided in (a)(i), in the beakers provided. 2 Label test‐tubes with the ascorbic acid concentrations prepared in step 1. 3 Label another test‐tube 0. 4 On a white tile carefully cut the cylinders of potato tissue into thin discs that are approximately 1–2mm thick. You will need to cut at least 70 discs. 5 Place 10 potato discs into each labelled test‐tube. 6 Add 1cm3 distilled water, W, to the test‐tube labelled 0. 7 Add 1cm3 of each concentration of ascorbic acid to the appropriately labelled test‐tubes. 8 Set up the apparatus as shown in Fig. 1. 1 using the test ‐tube labelled 0. The syringe barrel should be fully submerged in the beaker of water, B. Fig 1.1 9 Add 5cm3 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. 10 Record in (a)(ii) the initial volume of gas in the syringe barrel then start the stop‐watch. 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. 12 Repeat step 9 to step 11 with each of the test‐tubes labelled in step 2. 13 Calculate the total volume of gas produced at each concentration of ascorbic acid. Record these processed results i

4 (ii) Record your results in an appropriate table, including raw results and processed results. Concentration of ascorbic acid / moldm-3 volume of gas in the syringe barrel / cm3 initial final total 1.0 0.0 1.3 1.3 0.1 0.0 4.4 4.4 0.01 0.0 5.2 5.2 0.001 0.0 6.0 6.0 0.0001 0.0 7.7 7.7 0 0.0 9.0 9.0 Mark scheme: 1 heading for independent variable : ascorbic acid concentration / moldm–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 trend: 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, i.e. 1 d.p. ; 6 correct calculation of total volume of gas produced; [6]

5 (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. Mark scheme: 1 selects greatest volume from results in (a)(ii); 2 calculates correct rate of gas production; rate of gas production = volume in cm3 / 2 mins rate of gas production = ............................................... cm3min–1 [2] (iv) Describe two improvements to the procedure that would make the measurements more accurate. 1 Repeat the experiment multiple times to identify any anomalous results and to calculate a more reliable mean value; 2 Use a potato slicer / mandolin to cut and standardise the size of the potato discs, ensuring they are all uniform in size to reduce variability in the results; 3 Standardise the time interval between adding hydrogen peroxide to the test tube and sealing it with a bung, ensuring consistent reaction conditions across all trials; 4 Use a syringe with smaller divisions to ensure more precise measurement of liquids; [2] Any 2 Carry out step 14 to step 18. 14 Label a test‐tube T. 15 Put 5cm3 of H into test‐tube T. 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). 17 Add 10 discs of potato tissue to test‐tube T and start timing.

6 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). 30.0 .............................. °C State the temperature of H 2 minutes after adding potato discs (step 18). 31.0 .............................. °C Calculate the change in temperature after 2 minutes. 1.0 .............................. °C 1 temperatures recorded to 0.5 °C ; 2 records the correct rise / fall in temperature ; [2] (ii) State whether temperature is a significant source of error in this investigation. Explain your answer. yes, rate of reaction is affected by change in temperature because for every 10°C increase in temperature, rate of catalase activity doubles; OR no, rate of reaction is not affected by change in temperature because there is no change in temperature / small change / quote fig from b(i) after 2 mins suggesting there reaction is neither exothermic or endothermic; [1]

7 (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 1.3. Table 1.3 (i) Plot a graph of the data in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil. Fig. 1.3 0 200 400 600 800 1000 0 20 40 60 80 Maximum ascorbic acid concentration in blood plasma / µmol dm-3 ascorbic acid daily dose / mg

8 Marking points: 1 1 x-axis: ascorbic acid daily dose / mg and y-axis: maximum ascorbic acid concentration in blood plasma / µmol dm–3; 2 scale on x-axis: 200 mg to 2 cm and labelled at least every 2 cm and scale on y-axis: 20 µmol 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 4 five plots joined with thin line passing through all points; [4] (ii) Suggest an e