Cambridge A Level Biology 9700 — 2023 May/June Paper 3 · Variant 1

1 Yeast cells contain the enzyme catalase which catalyses the breakdown of hydrogen peroxide, releasing oxygen. You will investigate the effect of pH on the activity of catalase in an extract from yeast cells. You will need to immobilise the yeast cells in sodium alginate beads. When a bead containing yeast cells is dropped into hydrogen peroxide solution the bead will sink. As oxygen is released the bead will rise. The more oxygen released, the faster the bead will rise. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 Y yeast cell suspension none 15 H 3.0% hydrogen peroxide solution harmful 30 irritant S sodium alginate solution none 30 C calcium chloride solution none 30 B3 buffer pH 3 none 10 B4 buffer pH 4 none 10 B6 buffer pH 6 none 10 B7 buffer pH 7 none 10 B8 buffer pH 8 none 10 If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. Carry out step 1 to step 19. step 1 Put 10 cm3 of C into a large test-tube. step 2 Put 5 cm3 of S into a small beaker. step 3 Stir Y and put 3 cm3 of Y into the beaker used in step 2. Mix well. step 4 Use a 5 cm3 syringe to collect 2 cm3 of the mixture of S and Y (prepared in step 3). step 5 Position the 5 cm3 syringe over the large test-tube containing C as shown in Fig. 1.1. gently press down with thumb to release one drop at a time 5 cm3 syringe barrel resting on the top of a large test-tube 2 cm3 of a mixture of S and Y 10 cm3 of C large test-tube Fig. 1.1 step 6 Gently press down on the plunger of the 5 cm3 syringe with your thumb to release one drop into solution C. The drop should form a bead. step 7 Repeat step 6 until you have used all 2 cm3 of the mixture. Leave the beads in the solution C for 1 minute. step 8 Tip the contents of the large test-tube from step 7 into a Petri dish. step 9 Put two beads into each of the beakers containing pH buffers B3, B4, B6, B7 and B8. step 10 Label a small test-tube B3. step 11 Put 3 cm3 of the pH buffer B3 into the test-tube labelled B3. step 12 Put 3 cm3 of hydrogen peroxide solution, H, into this test-tube and shake to mix. Leave this test-tube in a test-tube rack. step 13 Pick up a bead from the pH buffer B3 using blunt forceps. step 14 Drop the bead into the test-tube from step 12. Start timing when the bead reaches the bottom of the test-tube. step 15 Time how long it takes for the bead to reach the surface of the liquid. If the bead does not reach the surface after 60 seconds, stop timing and record as ‘more than 60’. step 16 Record the result from step 15 in (a)(i). step 17 Pick up the second bead from the pH buffer B3 using blunt forceps. step 18 Repeat step 14 to step 16. step 19 Repeat step 10 to step 18 with the remaining pH buffers instead of B3. (a) (i) Record your results in an appropriate table. [5] (ii) State the independent variable in this investigation. ..................................................................................................................................... [1] (iii) State one significant source of error in this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] You will need to estimate the pH of the solution, U. You are provided with U, as shown in Table 1.2. Table 1.2 labelled contents hazard volume / cm3 U solution of unknown pH none 10 If U comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. Carry out step 20 to step 27. step 20 Put one bead into the beaker containing solution U. step 21 Label a clean test-tube U. step 22 Put 3 cm3 of solution U into the test-tube labelled in step 21. step 23 Put 3 cm3 of hydrogen peroxide solution into this test-tube. Leave this test-tube in a test-tube rack. step 24 Pick up the bead from the beaker containing solution U, using blunt forceps. step 25 Drop the bead into the test-tube from step 23. Start timing when the bead reaches the bottom of the test-tube. step 26 Time how long it takes for the bead to reach the surface of the liquid. If the bead does not reach the surface after 60 seconds, stop timing and record as ‘more than 60’. step 27 Record the result from step 26 in (a)(iv). (iv) State the result for solution U. result for solution U ......................................................... [1] (v) Using your results from (a)(i) and (a)(iv), estimate the pH of solution U. pH of solution U ......................................................... [1] (vi) In the procedure described in step 1 to step 19, the effect of pH on catalase activity was investigated. Describe how you would modify this procedure to investigate the effect of concentration of substrate on the time taken for the beads to rise. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) Immobilised enzymes are often used in industry. For example the enzyme lactase is used to produce lactose-free milk. A student measured the initial rate of reaction of human lactase at different concentrations of lactose and plotted a graph, as shown in Fig. 1.2. 10 8 6 initial rate of reaction / au 4 2 0 20 40 60 80 100 120 140 concentration of lactose / mmol dm–3 Fig. 1.2 (i) Explain the change in the initial rate of reaction between: 20 mmol dm–3 and 40 mmol dm–3 of lactose ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... 60 mmol dm–3 and 140 mmol dm–3 of lactose. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (ii) Use the graph in Fig. 1.2 to estimate the Michaelis-Menten constant (Km) of lactase. Show your working on the graph in Fig. 1.2. Km = ....................................... mmol dm–3 [3] (c) Lactose is found in the milk of many mammals. A scientist investigated the concentration of lactose in the milk of different mammals. Table 1.3 shows the results of this investigation. Table 1.3 type of mammal concentration of lactose / mmol dm–3 rabbit (RA) 60.0 seal (SE) 2.5 goat (GO) 137.5 sheep (SH) 150.0 horse (HO) 222.5 Plot a bar chart of the data shown in Table 1.3 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] [Total: 20]

Mark scheme: 1(a)(i) 1 heading for independent variable: pH (before heading for dependent variable) ; 2 heading dependent variable: time for beads to rise / seconds and no units in body of table ; 3 two results for each pH ; 4 time taken for pH3 is longer than the time taken for pH7 ; 5 results recorded in whole seconds ; 5 1(a)(ii) pH ; 1 1(a)(iii) identifies one error e.g. difficult to judge when the bead reaches the top, size of bead varies ; 1 1(a)(iv) records a time for U between that recorded for pH4 and pH7 and seconds ; 1 1(a)(v) correct estimate for U ; 1 1(a)(vi) any two from: 1 same pH ; 2 5 different concentrations of hydrogen peroxide ; 3 serial or proportional dilution ; 2 1(b)(i) 20 mmol dm–3 and 40 mmol dm–3 1 lactose concentration increases so more enzyme substrate complexes ; 60 mmol dm-3 and 140 mmol dm–3 2 all active sites are saturated, so maximum rate of reaction is reached ; 2 1(b)(ii) 1 shows Vmax on graph ; 2 shows ½ Vmax on graph ; 3 18 ; 3 Question Answer Marks 1(c) 1 x-axis: type of mammal and bars labelled RA, SE, GO, SH and HO and y-axis: concentration of lactose / mmol dm–3 ; 2 scale on x-axis: even width of bars and scale on y-axis: 50 mmol dm–3 to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all five bars ; 4 five separate bars drawn and with horizontal and vertical lines joined precisely ; 4

More questions on Factors that affect enzyme action

2 J1 is a slide of a stained transverse section through a plant leaf. (a) (i) Draw a large plan diagram of the region of the leaf on J1, indicated by the shaded area in Fig. 2.1. Use a sharp pencil. draw this region Fig. 2.1 Use one ruled label line and label to identify the lower epidermis. [5] (ii) Observe the vascular tissue in the midrib on the section of the leaf on J1. Select one large xylem vessel element and three adjacent smaller 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 lumen of one xylem vessel element. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section of a different leaf. Fig. 2.2

Mark scheme: 2(a)(i) 1 minimum size ; 2 correct section of the leaf drawn and no cells drawn ; 3 draws correct shape of the midrib ; 4 draws one large vascular bundle and large layer above it in the midrib ; 5 label line and label to lower epidermis ; 5 2(a)(ii) 1 minimum size and all lines sharp and continuous and no shading ; 2 one large and three small 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 the lumen of one xylem vessel element ; 5 Question Answer Marks 2(b)(i) only observable differences ; any three from e.g.: feature Fig. 2.2 J1 trichomes present absent ; large central cells absent present ; shape curved straight ; cuticle thick thin ; 4 2(b)(ii) any two from: 1 trichomes and traps water vapour inside the leaf ; 2 curled leaf and increases humidity to reduce the water potential gradient ; 3 sunken stomata and exposed to the humid atmosphere with a low water potential gradient ; 4 thick cuticle and reduces water loss by transpiration ; 2 2(c) 1 correct measurement of scale bar and structure X ; 2 shows length of scale bar divided by 430 ; 3 shows length of structure X divided by the answer to MP2 ; 4 correct answer and units ; 4

More questions on The microscope in cell studies