Cambridge A Level Biology 9700 — 2017 Oct/Nov Paper 3 · Variant 3

1 Plant cells contain enzymes that catalyse metabolic reactions. Some of these enzymes catalyse the release of oxygen from hydrogen peroxide. A plant extract solution can be produced which will contain these enzymes. When hydrogen peroxide and a plant extract solution are mixed oxygen is released. The oxygen forms bubbles that make a foam on the surface. You are required to investigate the effect of pH on the progress of the enzyme-catalysed reactions by: • changing the pH (independent variable) using buffers • measuring the height of the foam (dependent variable) produced by the release of oxygen. To follow the progress of this reaction you will need to measure the height of foam at different times for a total of 3 minutes. (a) (i) Decide how often you will take these measurements including the final height of foam at 3 minutes. State the times when you will measure the height of foam. .......................................................................................................................................[1] (ii) Fig. 1.1A shows how a test-tube will be set up at the start (0 minutes). Fig. 1.1B shows the test-tube after 3 minutes. A B plant extract + buffer + hydrogen peroxide at start after 3 minutes Fig. 1.1 The top of the foam may not form an even layer, so you will need to decide where to measure the layer of the foam. Draw a double-headed arrow on Fig. 1.1B to show where you will measure this layer of foam. [2] You are provided with: labelled contents hazard volume / cm3 E plant extract solution harmful 40 irritant H hydrogen peroxide harmful 70 solution irritant B4 buffer pH4 none 10 B5 buffer pH5 none 10 B7 buffer pH7 none 10 B9 buffer pH9 none 10 B10 buffer pH10 none 10 U unknown buffer none 10 D liquid detergent none 10 If any of these liquids come into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. Temperature affects the rate of an enzyme-catalysed reaction and may be a significant source of error. You are required to measure the temperature of the room before the start of the investigation and when all the measurements have been recorded. Proceed as follows: Read step 1 to step 9. 1. Measure the temperature of the room and record this measurement in (b)(iii) on page 5. 2. Put 1 cm3 of each buffer into separate test-tubes. 3. Put 2 cm3 of E into each test-tube. Gently shake to mix well. 4. Put 1 drop of D into each test-tube. Do not mix. The reaction will start as soon as H is added to the mixture of E and buffer. 5. Put 5 cm3 of H into one of the test-tubes. 6. Immediately start timing. 7. Using the strip of graph paper provided, measure the height of the foam as decided in (a)(ii), at each of your sampling times as stated in (a)(i), until you have recorded the final measurement at 3 minutes. Record your measurements in (b)(i). If the foam flows over the top of the test-tube, stop timing. Record the measurement of the height of the foam to the top of the test-tube and add an asterisk (*) to show that the foam has flowed over. 8. Repeat step 5 to step 7 with each of the other buffer solutions. 9. Measure the temperature of the room now that all the measurements have been recorded. Record the temperature in (b)(iii). (b) (i) Record your results for the height of foam (raw results) in an appropriate table. [4] (ii) Using your raw results for pH 5: • State the highest height of foam ........................ mm. • State the first time when this height was reached ........................ seconds. Using these measurements, calculate the rate of production of oxygen, as millimetres per second (mm s–1). You may lose marks if you do not show your working. rate of oxygen production = ................................................... mm s–1 [2] (iii) State the temperature of the room before the start of the investigation. ........................ °C State the temperature of the room after all the measurements have been recorded. ........................ °C State the difference between this temperature and the temperature of the room at the start of the investigation. difference = ......................... °C Explain whether temperature is a significant source of error in this investigation. ........................................................................................................................................... ........................................................................................................................................... [1] You are provided with a sample U which contains a buffer. You are now required to estimate the pH of the buffer in U using the same procedure. 10. Repeat step 2 to step 7 using U and record your results in (b)(iv). (iv) Record the maximum height of foam using U. ........................ mm [1] (v) Using the result in (b)(iv) and the results in (b)(i) estimate the pH of U. pH ........................ [1] (vi) This procedure investigated the effect of pH on the activity of the enzymes in a plant extract solution. To modify this procedure for investigating a different variable, the pH should be kept the same. Think about how you could modify this procedure to investigate the effect of substrate concentration, using 6% hydrogen peroxide solution, on the activity of enzymes in the plant extract solution. Describe how the independent variable, substrate concentration, would be changed in an investigation. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Describe how the dependent variable could be measured more accurately than measuring the height of foam. You may use a labelled diagram in the space provided. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... space for diagram [3] (c) A student investigated the effect of temperature on the rate of enzyme activity in a plant extract solution. All the other variables were standardised. The results are shown in Table 1.1. Table 1.1 temperature rate of enzyme activity / °C / arbitrary units (au) 5 6.5 16 39.0 27 31.0 38 9.5 49 1.0 Use a sharp pencil for drawing graphs. (i) Plot a graph of the data shown in Table 1.1. [4] (ii) Use your knowledge of enzymes and the data to explain the effect of temperature on the enzyme activity in the plant extract solution. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... .......................................................................................................................................[3]

Mark scheme: 1(a)(i) regular intervals + at least 5 times ; 1 1(a)(ii) top of arrow to appropriate height ; bottom of arrow to appropriate height ; 2 1(b)(i) 1 table drawn + heading pH ; 2 heading height of foam + mm ; 3 records results for at least three times + 5 pH values ; 4 height of foam recorded as whole millimetres ; 4 1(b)(ii) shows maximum height of foam for pH 5 divided by time ; answer to the appropriate degree of accuracy ; 2 1(b)(iii) decides whether temperature was a significant source of error ; 1 1(b)(iv) states the height of foam for U ; 1 Question Answer Marks 1(b)(v) estimates the correct pH using the result stated in (b)(iv) ; 1 1(b)(vi) states at least five concentrations of hydrogen peroxide ; concentrations of hydrogen peroxide prepared by proportional dilution or serial dilution ; describes counting the number of bubbles or collecting the volume of oxygen using displacement of water or alternative correct method ; 3 1(c)(i) 1 (x-axis) temperature °C + (y-axis) rate of enzyme activity / arbitrary units ; 2 (scale on x-axis) 10.0 to 2 cm, labelled at least each 2 cm + (scale on y-axis) 10.0 to 2 cm, labelled at least each 2cm ; 3 correct plotting of five points with a small cross or dot in circle ; 4 five plots, joined plot to plot + thin line drawn ; 4 1(c)(ii) correct reference to optimum temperature + 16°C ; as temperature increase more enzyme substrate complexes form or correct reference to kinetic energy ; after a specific temperature enzyme denatured ; 3

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2 J1 is a slide of a stained transverse section through a plant stem. You are not expected to be familiar with this specimen. Use a sharp pencil for drawing. (a) (i) The specimen on J1 has an irregular shape. Select a field of view so that you can observe a sector that includes: • a structure as shown by the shaded area and labelled R in Fig. 2.1 • part of the ring of vascular tissue. R Fig. 2.1 Draw one large plan diagram of the different tissues shown in the field of view to show: • the tissues making up R • the tissues in the part of the vascular ring below R • any other observable tissues. Use one ruled label line and label to identify the epidermis. You are expected to draw the correct shape and proportions of the different tissues. (ii) Observe the cells in the centre of the stem (pith) in the specimen on J1. These cells are not identical. Select one group of four adjacent (touching) cells which show some of the differences between these cells. Each cell of the group must touch at least two of the other cells. Make a large drawing of this group of four cells. Use one ruled label line and the label C to identify a structure made of cellulose. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section through a stem of a different type of plant. You are not expected to be familiar with this specimen. X magnification ×16 Fig. 2.2 (i) In Fig. 2.2 the line X is drawn across the length of an air space. Use the line X and the magnification to calculate the actual length of the air space, in µm. You may lose marks if you do not show your working. actual length = .................................................... µm [3]

Mark scheme: 2(a)(i) 1 plan diagram of appropriate size + no cells ; 2 correct section drawn ; 3 draws tissues in the part of the vascular ring below R ; 4 draws detail of tissues at the top of R correctly ; 5 draws tissues beneath the epidermis correctly ; 6 uses one label line + one label to identify the epidermis ; 6 2(a)(ii) 1 quality of line for outer wall of cells + cells of appropriate size ; 2 only four cells drawn, each cell of the group touching at least two of the other cells ; 3 cell walls drawn as two lines close together ; 4 a cell drawn with 5 sides or more ; 5 uses one label line + one label, C, to identify a structure made of cellulose ; 5 Question Answer Marks 2(b)(i) correct measurement of line X ; records correct units ; correct answer shown to appropriate degree of accuracy ; 3 2(b)(ii) states only observable differences ; any three observable differences of comparison ;;; e.g. on J1 the vascular tissue is positioned in the shape of a ring while the vascular bundles in Fig. 2.2. are distributed throughout the stem 4

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