Biology 2024 · Oct/Nov

Plant cells produce the enzyme catalase that catalyses the hydrolysis (breakdown) of hydrogen peroxide into water and oxygen, as shown in Fig. 1.1. hydrogen peroxide water + oxygen catalase A cylinder of potato tissue will have catalase molecules on its surface. When potato tissue is put into hydrogen peroxide solution, oxygen bubbles are released and a foam forms at the surface of the hydrogen peroxide solution. You will investigate the effect of ethanol concentration on the activity of catalase by measuring the height of the foam. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm H hydrogen peroxide solution harmful irritant W water none 200 E 100% ethanol flammable 80 P 5 potato cylinders none - If any of H comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. Carry out step 1 to step 7. step 1 Put the five potato cylinders onto a white tile or cutting surface and cut each cylinder to a length of 4 cm. step 2 Label a beaker 0 and put 30 cm of W into this beaker. step 3 Put one potato cylinder into the beaker labelled 0. Start timing and leave for 5 minutes. step 4 Label one test-tube 0. step 5 After 5 minutes (step 3), remove the potato cylinder from the beaker labelled 0 and put it into the test-tube labelled 0. step 6 Put 5 cm of hydrogen peroxide solution into this test-tube. Immediately start timing. step 7 Measure and record the height of the foam at 1 minute and at 2 minutes. height of foam at 1 minute height of foam at 2 minutes

(i) The length of the potato cylinders may not be precisely 4 cm. Identify one other source of error in the procedure you have carried out. You will investigate the effect of different concentrations of ethanol on catalase activity. You will need to carry out a serial dilution of the 100% ethanol, E, to reduce the concentration by half between each successive dilution. You will need to prepare three concentrations of ethanol in addition to the 100% ethanol, E. After the serial dilution is completed, you will need to have 30 cm of each concentration available to use.

Complete Fig. 1.2 to show how you will prepare your serial dilution. Each beaker should have: 100% ethanol 60 cm 100% ethanol, E 0 cm of water, W [3] Carry out step 8 to step 14. step 8 Prepare the concentrations of ethanol as shown in Fig. 1.2. step 9 Put one potato cylinder into each beaker. Start timing and leave for 5 minutes. step 10 Label test-tubes with the ethanol concentrations prepared in step 8. step 11 After 5 minutes (step 9), remove the potato cylinders from the beakers and put them into the appropriately labelled test-tubes. step 12 Put 5 cm of hydrogen peroxide solution into the test-tube labelled 100. Immediately start timing. step 13 Measure the height of the foam after 1 minute and after 2 minutes. Record the results in (a)(iii). step 14 Repeat step 12 and step 13 with the potato cylinders in each of the other test-tubes.

Record your results in an appropriate table. Include the results recorded in step 7.

Calculate the rate at which the foam was produced at 1 minute and at 2 minutes for 100% ethanol, E. Show your working. Include the unit in your answer. rate at 1 minute = rate at 2 minutes =

Use your results in (a)(iv) to describe how the rate changes with time.

With reference to the activity of catalase, explain why the results of 0 (step 7) were important in this investigation.

State the dependent variable in this investigation.

Suggest two improvements to the investigation you have carried out to increase the confidence in your results. 1 2

Some fruits turn brown when they are cut into slices for eating. This browning can make the fruit less appealing to eat and difficult to sell. The enzyme polyphenol oxidase (PPO) catalyses an oxidation reaction when fruit is cut, causing the fruit tissue to turn brown. Scientists carried out an investigation to see how exposing wampee fruits to different concentrations of ethanol affected the activity of PPO in the fruit tissue. A large sample of wampee fruits was divided into five equal groups. Each group of fruit was sealed in a plastic container and treated with a different concentration of ethanol. After a short time in storage, the activity of the enzyme PPO in the wampee fruit tissue was recorded. The results are shown in Table 1.2. Table 1.2 concentration of ethanol PPO activity / arbitrary units 0 2.20 100 1.40 200 0.95 400 0.70 500 0.60

Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil.

Suggest an explanation for the pattern shown in the data. [Total: 21]

K1 is a slide of a stained transverse section through a plant stem.

(i) Draw a large plan diagram of the region of the stem on K1 indicated by the shaded area in Fig. 2.1. Use a sharp pencil. Use one ruled label line and label to identify the epidermis. draw this region

Observe the cells in the centre of the stem on K1. Select a group of four adjacent cells. Each cell must touch at least two of the other cells.

Fig. 2.2 is a photomicrograph of a stained transverse section of a stem from a different plant. magnification ×12

Measure the length of the vascular bundles using the lines M1, M2, M3, M4 and M5 in Fig. 2.2 and calculate the mean actual length of the vascular bundles. Show your working. M1 = M2 = M3 = M4 = M5 = mean actual length

A student suggested that the mean actual length of the vascular bundles calculated in

(i) was not accurate for the whole plant. Describe two modifications to the method used in (b)(i) that would allow a more accurate mean length of the vascular bundles for the whole plant to be calculated. 1 2

Fig. 2.3 is the same photomicrograph as that shown in Fig. 2.2. Identify three observable differences, other than colour, between the stem section on K1 and the stem section in Fig. 2.3. Record these three observable differences in an appropriate table. [4] [Total: 19] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. Local Examinations Syndicate (UCLES), which is a department of the University of Cambridge.