Cambridge A Level Biology 9700 — 2025 Oct/Nov Paper 3 · Variant 8

1 Yeast cells contain enzymes that hydrolyse sucrose into reducing sugars, as shown in Fig. 1.1. enzymes in yeast cells sucrose reducing sugars Fig. 1.1 You will investigate the activity of the enzymes in yeast cells that are immobilised in sodium alginate beads and yeast cells that are in a suspension (‘free’ yeast cells). You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 Y yeast cell suspension low 20 A sodium alginate solution low 20 C calcium chloride solution low 20 S sucrose solution low 50 Benedict’s Benedict’s solution harmful irritant 20 W distilled water low 50 If any solution comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. You will need to: • immobilise some of the yeast cells in sodium alginate • put the immobilised yeast cells and some ‘free’ yeast cells into sucrose solution • test both solutions for reducing sugars • compare the activity of immobilised yeast cells and ‘free’ yeast cells. Investigating the activity of the enzymes in immobilised yeast cells Carry out step 1 to step 20. step 1 Put 10 cm3 of sodium alginate, A, into a beaker. step 2 Stir the yeast cell suspension, Y, and put 10 cm3 of Y into the beaker containing A. Mix well. step 3 Use a 1 cm3 syringe to collect 1 cm3 of the mixture of A and Y. Wipe the outside of the syringe. step 4 Hold this syringe over the beaker containing calcium chloride solution, C, as shown in Fig. 1.2. step 5 Slowly press down on the plunger so that a drop of the mixture is released into C. The drop will form a bead. step 6 Repeat step 5 until all of the mixture in the syringe has been formed into beads. slowly press down to release one drop at a time plunger barrel of syringe mixture of A and Y one drop released calcium chloride solution, C Fig. 1.2 step 7 Leave the beads in the beaker for at least 5 minutes. step 8 Set up a boiling water-bath ready for step 18. step 9 Label a beaker B. step 10 Put 20 cm3 of sucrose solution, S, into the beaker labelled B. step 11 Label 5 test-tubes 1, 2, 3, 4 and 5. step 12 Put 1 cm3 of Benedict’s into each of the test-tubes. step 13 After at least 5 minutes (step 7), separate the beads from solution C using the apparatus shown in Fig. 1.3. funnel beads filter paper Fig. 1.3 After step 14 you will remove a sample from B every minute for 5 minutes. step 14 Put all of the beads into the beaker labelled B. Stir and then start timing. step 15 At 1 minute, stir the contents of beaker B and use a syringe to transfer 1 cm3 of the solution surrounding the beads into the test-tube labelled 1. step 16 At 2 minutes, stir the contents of beaker B and use a syringe to transfer 1 cm3 of the solution surrounding the beads into the test-tube labelled 2. step 17 Continue taking samples at 3, 4 and 5 minutes using the test-tubes labelled 3, 4 and 5. step 18 Put test-tube 1 into the boiling water-bath and measure the time to the first colour change. Record your result in (a)(i). If there is no colour change after 60 seconds, stop timing and record the time as ‘more than 60’. step 19 Remove the test-tube from the boiling water-bath. step 20 Repeat step 18 and step 19 for the test-tubes labelled 2, 3, 4 and 5. Investigating the activity of the enzymes in ‘free’ yeast cells To investigate the activity of the enzymes in ‘free’ yeast cells, you will need to test the samples with Benedict’s solution immediately after the sample is taken. The water-bath should be boiling throughout this part of the investigation. Carry out step 21 to step 32. step 21 Label a beaker F. step 22 Put 20 cm3 of sucrose solution, S, into the beaker labelled F. step 23 Label 5 clean test-tubes 1, 2, 3, 4 and 5. step 24 Put 1 cm3 of Benedict’s into each of the test-tubes. step 25 Stir the yeast cell suspension, Y, and put 0.5 cm3 of Y into the beaker labelled F. Mix well and start timing. The timer should not be stopped until you have completed step 32 – keep the timer running continuously. step 26 At 1 minute, use a syringe to transfer 1 cm3 of the solution in F into the test-tube labelled 1. Do not stop the timer. step 27 Immediately put test-tube 1 into the boiling water-bath and measure the time to the first colour change. Record your result in (a)(i). If there is no colour change after 60 seconds, record the time as ‘more than 60’. step 28 Remove the test-tube from the boiling water-bath. step 29 At 2 minutes, use a syringe to transfer 1 cm3 of the solution in F into the test-tube labelled 2. Do not stop the timer. step 30 Immediately put test-tube 2 into the boiling water-bath and measure the time to the first colour change. Record your result in (a)(i). If there is no colour change after 60 seconds, record the time as ‘more than 60’. step 31 Remove the test-tube from the boiling water-bath. step 32 Repeat step 30 and step 31 at 3, 4 and 5 minutes using the test-tubes labelled 3, 4 and 5. (a) (i) Record your results in an appropriate table. [5] (ii) Describe and compare the trends in your results, with reference to the data in (a)(i). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (iii) Suggest an appropriate control for the experiment. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) The samples taken from B were tested for the presence of reducing sugars after all 5 samples had been taken. Suggest why each sample from F had to be tested for the presence of reducing sugars immediately. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Suspensions and solutions were stirred throughout the investigation. Explain two ways in which stirring increased the accuracy of the results. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... [2] (vi) State one possible source of error when determining the dependent variable for the ‘free’ yeast cells. Suggest one improvement to the procedure that would increase the accuracy of measuring the dependent variable. error ................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... improvement ..................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (b) A scientist carried out an investigation to determine the effect of pH on the activity of immobilised catalase enzyme and ‘free’ catalase enzyme. All other variables were kept constant. The results are shown in Table 1.2. Table 1.2 activity of catalase / arbitrary units (au) pH immobilised free 5 68 50 6 88 62 7 99 96 8 98 65 9 94 48 (i) Plot a line graph of the data in Table 1.2 on the grid in Fig. 1.4. Use a sharp pencil. Fig. 1.4 [4] (ii) State two conclusions about how pH affects the activity of immobilised catalase compared to ‘free’ catalase. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ......................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] [Total: 20]

Mark scheme: Question Answer Marks 1(a)(i) 1 (heading for independent variable) 5 (yeast) beads / B and free (yeast) to the left of the dependent variable ; 2 (heading for dependent variable) time to the first appearance of a colour change and s and no units in body of table ; 3 5 results for immobilised yeast cells and 5 results for ‘free’ yeast cells ; 4 correct trend for immobilised beads and ‘free’ yeast cells ; 5 results recorded to the nearest whole second ; 1(a)(ii) Any two from 1, 2 and 3: 3 1 (for beads) as time increases, the time to first colour change decreases ; 2 (for ‘free yeast’) as time increases, the time to first colour change decreases ; 3 correct comparison between the results for beads and for ‘free yeast’ / F ; 4 data quote to support 1, 2 or 3 ; 1(a)(iii) boiled enzyme / yeast / use water instead of yeast ; 1 1(a)(iv) ‘free’ yeast cells remain in the sample and continues to catalyse the reaction ; 1 1(a)(v) Any two from: 2 1 even concentration of yeast cells between B and F ; 2 even mixing in the substrate (sucrose) ; 3 even distribution of reducing sugars ; 1(a)(vi) 1 judging the time to first colour change while timing taking the sample in the sucrose / S ; 2 2 do each time in separate beakers one after each other / stop enzyme after taking each sample ; 1(b)(i) 1 x-axis: pH 4 and y-axis: activity of catalase / arbitrary units (au) and lines correctly labelled (immobilised / free) ; 2 scale on x-axis: 1.0 to 2 cm, labelled at least every 2 cm and y-axis: 20 to 2 cm, labelled at least every 2 cm ; 3 correct plotting of points using small dots in circles or crosses ; 4 plots joined with thin line passing through all points ; 1(b)(ii) 1 both have same optimum of pH7 ; 2 2 immobilised catalase has a higher activity over a wider range of pH ;

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