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.