Cambridge A Level Biology 9700 — 2022 Oct/Nov Paper 3 · Variant 5

1 Yeast cells contain the enzyme catalase which catalyses the breakdown of hydrogen peroxide, releasing oxygen. You will investigate the effect of substrate concentration on the activity of catalase in yeast. You will need to immobilise the yeast in sodium alginate beads. When a bead containing yeast is dropped into hydrogen peroxide solution the bead will sink. As oxygen is released the bead will rise. The faster the oxygen is 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 7.0% yeast cell suspension none 15 H 6.0% hydrogen peroxide solution harmful irritant 30 S 2.0% sodium alginate solution none 30 W distilled water none 100 C 1.5% calcium chloride solution none 30 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 7 to immobilise the yeast. 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 mixture of S and Y calcium chloride solution, 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. You will use these beads in step 11. You will need to carry out a serial dilution of the 6.0% hydrogen peroxide solution, H, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of solution in addition to the 6.0% hydrogen peroxide solution, H. After the serial dilution is completed, you will need to have 10 cm3 of each concentration available to use. (a) (i) Complete Fig. 1.2 to show how you will prepare your serial dilution. Fig. 1.2 shows the first two beakers you will use to make your serial dilution. You will need to draw three additional beakers. For each beaker add labelled arrows to show: • the volume of hydrogen peroxide solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of hydrogen peroxide solution. 0 cm3 of W 10 cm3 of 20 cm3 of 6.0% hydrogen 6.0% peroxide solution hydrogen peroxide solution, H 10 cm3 of 6.0% hydrogen peroxide solution to use Fig. 1.2 [3] Carry out step 8 to step 16. step 8 Prepare the concentrations of hydrogen peroxide solution, as decided in (a)(i), in the beakers provided. step 9 Label the small test-tubes with the concentrations you prepared in step 8. step 10 Put 10 cm3 of each hydrogen peroxide concentration into the appropriately labelled test-tube. Leave these test-tubes in a test-tube rack. step 11 Tip the contents of the large test-tube from step 7 into a Petri dish. step 12 Pick up a bead using blunt forceps. step 13 Drop the bead into the test-tube containing 6.0% hydrogen peroxide solution, H. Start timing when the bead reaches the bottom of the test-tube. If the bead does not sink to the bottom of the test-tube, record the time as zero. step 14 Time how long it takes for the bead to reach the surface of the hydrogen peroxide solution. If the bead does not reach the surface after 180 seconds, stop timing and record as ‘more than 180’. step 15 Record the result from step 14 in (a)(ii). step 16 Repeat step 12 to step 15 with the remaining concentrations of hydrogen peroxide solution. (ii) Record your results in an appropriate table. [5] (iii) State one significant source of error in this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Suggest how you could make an improvement to this investigation to reduce the error stated in (a)(iii). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] You will need to estimate the concentration of hydrogen peroxide in U. You are provided with U, as shown in Table 1.2. Table 1.2 labelled contents hazard volume / cm3 unknown concentration of U harmful irritant 30 hydrogen peroxide solution 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 17 to step 22. step 17 Label a clean test-tube U. step 18 Put 10 cm3 of U into the test-tube labelled U. step 19 Pick up a bead using blunt forceps. step 20 Drop the bead into the test-tube containing U. Start timing when the bead reaches the bottom of the test-tube. step 21 Time how long it takes for the bead to reach the surface of solution U. If the bead does not reach the surface after 180 seconds, stop timing and record as ‘more than 180’. step 22 Record the result from step 21 in (a)(v). (v) State the result for U. result for U ......................................................... [1] (vi) Using your results from (a)(ii) and (a)(v), estimate the concentration of hydrogen peroxide in U. concentration of hydrogen peroxide in U ..................................................... % [1] (vii) In the procedure described in step 1 to step 16, the effect of the concentration of hydrogen peroxide on catalase activity was investigated. Describe how you would modify this procedure to investigate the effect of temperature on the time taken for the beads to rise. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) Immobilised enzymes are often used in industry, for example in the production of lactose-free milk. This can increase productivity and reduce costs, as the enzyme is easy to reuse and the product is not contaminated by the enzyme. A student investigated the effect of bead diameter on the hydrolysis of lactose. The beads contained the enzyme lactase. Lactase catalyses the hydrolysis of lactose into glucose and galactose. The student: • put beads with a diameter of 2 mm into a syringe, up to the 5 cm3 line • put 5 cm3 of milk containing lactose into this syringe • left the syringe for 5 minutes • measured the concentration of lactose in the milk after 5 minutes. The student used this method with the bead diameters shown in Fig. 1.3. bead diameter 2 mm 4 mm 6 mm 8 mm 10 mm Fig. 1.3 Table 1.3 shows the results of this investigation. Table 1.3 percentage concentration of bead diameter / mm lactose after 5 minutes 2 20.5 4 21.0 6 29.5 8 40.5 10 69.0 (i) Plot a graph of the data shown in Table 1.3 on the grid in Fig. 1.4. Use a sharp pencil. 20 0 Fig. 1.4 [4] (ii) Use your graph to find the concentration of lactose in the milk after 5 minutes, when the bead diameter was 5 mm. concentration of lactose = ..................................................... % [1] (iii) Explain why the percentage concentration of lactose in the milk after 5 minutes increases as the bead diameter increases. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] [Total: 22] Question 2 starts on page 12.

Mark scheme: Question Answer Marks 1(a)(i) 1 correct concentrations (3.0, 1.5, 0.75, 0.375) and % at least once ; 3 2 shows transfer of 10 (cm3) to each beaker from the previous beaker ; 3 shows 10 (cm3) added to each beaker ; 1(a)(ii) 1 heading for independent variable: percentage concentration of hydrogen peroxide (before heading for dependent 5 variable) and no units in body of table ; 2 heading for dependent variable: time and seconds and no units in body of table ; 3 results for all concentrations ; 4 time for the highest concentration of hydrogen peroxide less than for the lowest concentration of hydrogen peroxide ; 5 results recorded in whole seconds ; 1(a)(iii) 1 identifies one error e.g. difficult to judge when the bead reaches the top, size of bead varies, only one bead was used 1 for each concentration ; 1(a)(iv) improvement to the error identified in (a)(iii): 1 1 draw a line on the test-tube to show the surface more clearly / use a black card ; 2 measure the size of each bead and use the same size beads ; 3 repeat and calculate the mean ; 1(a)(v) 1 records a time for U between that recorded for 3.0% and 0.75% and seconds ; 1 1(a)(vi) 1 correct estimate for U ; 1 1(a)(vii) any two from: 2 1 same concentration of hydrogen peroxide ; 2 5 different temperatures ; 3 thermostatically controlled water-bath ; 1(b)(i) 1 x-axis: bead diameter / mm 4 and y-axis: percentage concentration of lactose after 5 minutes ; 2 scale on x-axis: 2 mm to 2 cm, labelled at least every 2 cm and scale on y-axis: 10% to 2 cm, labelled at least every 2 cm ; 3 correct plotting of all five points using small crosses or dots in circles ; 4 five plots joined with thin line passing through all points ; 1(b)(ii) 1 correct concentration based on candidate’s graph ; 1 1(b)(iii) 1 larger beads have a smaller total surface area ; ORA 3 plus any two explanations (ORA): 2 less enzymes in contact with lactose ; 3 less active sites for the substrate to bind to ; 4 fewer enzyme substrate complexes / ESCs ; 5 decreased rate of lactose hydrolysis ;

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2 M1 is a slide of a stained transverse section through a plant stem. (a) (i) Draw a large plan diagram of the region of the stem on M1, indicated by the shaded area in Fig. 2.1. Use a sharp pencil. Your drawing should include three vascular bundles. draw this region Fig. 2.1 Use one ruled label line and label to identify the xylem. [5] (ii) Observe the central tissue on the section of the stem on M1. Select a group of four adjacent cells which show observable features of the central tissue. Each cell must touch at least two other cells. • Make a large drawing of this group of four cells. • Use one ruled label line and label to identify the cell wall of one cell. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section of a different stem. Fig. 2.2 Identify three observable differences between the stem in Fig. 2.2 and the stem on M1. Record the three observable differences in Table 2.1. Table 2.1 feature Fig. 2.2 M1

Mark scheme: 2(a)(i) 1 minimum size ; 5 2 draws correct section of the stem drawn and no cells drawn ; 3 draws three whole vascular bundles ; 4 draws vascular bundles with three layers ; 5 label line and label to xylem ; 2(a)(ii) 1 minimum size and all lines continuous, thin and sharp and no shading ; 5 2 draws only four whole cells and each cell touches at least two other cells ; 3 two lines around each cell and three lines where cells touch ; 4 correct shape of cells ; 5 label line and label to one cell wall ; 2(b) 1 only observable differences ; 4 2 three correct differences ;;; any three from e.g.: feature Fig. 2.2 M1 vascular bundles joined separate ; shape of stem square jagged ; trichomes present absent ; central region hollow filled with cells ; 2(c) 1 correct measurement of scale bar and X–Y and units ; 4 2 shows length of scale bar divided by 253.5 ; 3 shows length of line X–Y divided by answer to MP2 ; 4 correct answer and units ;

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