Cambridge IGCSE Biology (9-1) 0970 — 2021 May/June Paper 6 · Variant 1

1 A student investigated the effect of the concentration of sugar solutions on osmosis in potato cells. Step 1 The student labelled four test-tubes A, B, C and D. Step 2 The volumes of 1 mol per dm3 sugar solution and distilled water shown in Table 1.1 were used to make solutions containing different concentrations of sugar in each test-tube. (a) (i) Complete Table 1.1 by writing in the concentration of the sugar solution in test-tube C. Table 1.1 volume of 1 mol per dm3 sugar volume of distilled water concentration of sugar test-tube solution / cm3 / cm3 solution / mol per dm3 A 20 0 1.0 B 12 8 0.6 C 8 12 D 0 20 0.0 [1] Step 3 The student was given four potato cylinders which had all been cut from one potato. The diameters of the potato cylinders were all the same but the lengths of the potato cylinders varied. Step 4 The student cut all four potato cylinders to exactly 40 mm in length. Step 5 One potato cylinder was put into each labelled test-tube. The potato cylinders were left in the sugar solutions for 20 minutes. Step 6 After 20 minutes the potato cylinders were removed from the test-tubes. Fig. 1.1 is a diagram showing the actual size of the potato cylinders from test-tubes A, B, C and D at the end of step 6. A B C D Fig. 1.1 (ii) Prepare a table to record the results in the space provided. Measure the length of each of the potato cylinders in Fig. 1.1 and record these measurements in your table. [3] (iii) Explain why it was important that the potato cylinders were all cut to the same length in step 4. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Identify the variable that the student changed in this investigation (independent variable). ..................................................................................................................................... [1] (v) Suggest two improvements that you could make to the method used in this investigation. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (vi) Describe one safety precaution that should be taken while preparing the potato cylinders in step 4. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (b) Plan an investigation to find out the effect of temperature on osmosis in plant tissue. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6] (c) Potato cells contain starch grains. (i) State the solution that would be used to test for the presence of starch and give the result of a positive test. solution .............................................................................................................................. positive test result ............................................................................................................. ........................................................................................................................................... [2] Fig. 1.2 is a photomicrograph of some plant cells that contain starch grains. starch grain cell X A B magnification ×800 Fig. 1.2 (ii) Draw a large diagram of cell X in Fig. 1.2. Label one starch grain on your drawing. [5] (iii) Line AB represents the diameter of the starch grain. Measure the length of line AB on Fig. 1.2. length of line AB ................................................ mm Calculate the actual diameter of the starch grain using your measurement for line AB, the information in Fig. 1.2 and the formula: length of line AB on Fig. 1.2 magnification = actual diameter of the starch grain Give your answer to two significant figures. ................................................................ [3] [Total: 25]

Mark scheme: 1(a)(i) 0.4 ; 1 1(a)(ii) table drawn with a minimum of two, columns / row, and a header line ; headers with units ; four correct lengths ±1 mm recorded ; 3 MP2 concentration (of sugar) / mol per dm3 or test tube / AW and length (of potato after 20 minutes) / mm MP3 A=35 B=37 C= 39 D=42 1(a)(iii) so that the results were comparable / so sugar concentration is the only variable that is changed / to ensure the surface area was the same / so surface area does not affect the rate of osmosis / AW ; 1 1(a)(iv) concentration of the sugar solution ; 1 1(a)(v) any two from: measure mass of cylinders (instead of length) ; more intermediate concentrations of sugar solution ; extend the range of concentrations used ; repeat at each concentration / replicate the investigation ; soak for longer than 20 minutes ; use warmer sugar solutions (to speed up diffusion) ; keep all at the same temperature ; dry cylinders before measuring ; 2 1(a)(vi) cut away from body / use wooden board or tile / cut on flat surface / keep blade away from body parts / dry before cutting ; 1 Question Answer Marks Guidance 1(b) independent variable: 1 ref. to using at least 2 different temperatures ; method: 2 method of maintaining a temperature described ; 3 use of a tissue e.g. potato / leaves / root / stem, immersed in water / sugar / salt solution ; 4 method measuring dependent variable described, i.e. length / mass / volume / surface area of (cylinders / tissue / leaves) before and after (immersion) or measuring volume of immersion liquid before and after ; 5 + 6 variables kept constant ;; max two from: • same concentration (of solution) / same volume (of solution / water) • same (initial) length / volume / mass / surface area, of potato or tissue • same type / age /variety / AW, of (potato, tissue) / same potato / same plant • same pH / use a buffer • same immersion time 7 equilibration of temperature for tissue / solution ; 8 drying before reweighing (if mass measured) ; 9 at least two repeats (of each temperature) / total of three trials ; 10 AVP ; 6 1(c)(i) iodine solution ; blue-black ; 2 Question Answer Marks Guidance 1(c)(ii) lines: clear, single, unbroken with no shading ; size: central cell has a minimum diameter of 56 mm ; detail one: double line (minimum) for entire cell wall ; detail two: minimum of five starch grains around perimeter of cell ; label: one starch grain labelled ; 5 1(c)(iii) (length of line AB) 10 ±1 (mm) ; 0.013 (0.011–0.014) mm ;; 3 MP1 for correct measurement MP2 for correct calculation MP3 for correct rounding to two significant figures ecf MP2 & MP3 from previous step 4 A either orientation

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2 (a) A student monitored their pulse rate after exercise. The student’s pulse rate before exercise was 62 beats per minute. The results are shown in Table 2.1. Table 2.1 time after exercise pulse rate / minutes / beats per minute 0 156 1 108 2 78 3 66 4 62 5 62 (i) Plot a line graph on the grid of the data in Table 2.1. Include a line of best fit. [4] (ii) Describe the relationship shown in your graph, between pulse rate and time after exercise. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Calculate the percentage change in pulse rate from 0 minutes to 5 minutes using the data in Table 2.1. Give your answer to two decimal places. Space for working. ............................................................ % [3] (iv) State the variable that was measured (dependent variable) in this investigation. ..................................................................................................................................... [1] (b) The student monitored their pulse rate after exercise on three separate days and calculated their average pulse rate from the data they collected. The results are shown in Table 2.2. Table 2.2 pulse rate average time after / beats per minute pulse rate exercise / beats per / minutes day 1 day 2 day 3 minute 0 156 154 158 156 1 108 107 106 107 2 78 80 76 78 3 66 67 65 66 4 62 120 64 63 5 62 60 61 61 Explain why the student correctly calculated the average pulse rate at 4 minutes after exercise as 63 beats per minute rather than 82 beats per minute. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 2(a)(i) axes: labelled with units as time (after exercise) / minutes and pulse rate / beats per minute or bpm ; scale: evenly scaled and plots occupying half of the grid in one direction ; plots: all correct ± half a small square ; line: smooth curve (through at least 4 plots) or plot points joined with ruled lines ; 2(a)(ii) any two from: pulse rate decreases as time increases / AW ; biggest decrease in pulse rate is in first, one / two, minute(s) / initially ; plateaus or levels off / returns to pre-exercise pulse rate / returns to 62 (bpm) (at 4 mins) ; 2 Question Answer Marks Guidance 2(a)(iii) 60.26 (%) ;;; 3 MP1 selection of correct values from table 156 and 62 MP2 correct calculation e.g. (94 ÷ 156) × 100 = 60.2564 MP3 correct rounding to two decimal places ecf MP2 and MP3 from previous step 2(a)(iv) pulse rate ; 1 2(b) 120 bpm is an anomalous result / AW ; (idea that anomalous / AW data), should not be included in an average ; 2 2(c) similarity, max one from ; both have, a wall / layers both have, central space / lumen / AW both contain blood (cells) differences, max two from ;; artery wall is thicker / vein wall is thinner inner surface of artery more irregular / AW ora lumen AW, smaller in artery / ora vein (overall) is bigger / ora artery (overall or lumen) circular / AW, and vein irregular (in shape) / AW 3

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