Cambridge A Level Biology 9700 — 2019 May/June Paper 3 · Variant 4

1 Onion cells in sodium chloride solutions may show plasmolysis, where the cell surface membrane pulls away from the cell wall. Fig. 1.1 shows onion cells in different states of plasmolysis. • Cell A is showing no plasmolysis. • Cells B, C and D are all showing plasmolysis. no plasmolysis slight plasmolysis more plasmolysis complete plasmolysis A B C D Fig. 1.1 Different concentrations of sodium chloride solutions have different water potentials. When onion cells are put into a sodium chloride solution with a lower water potential than the cells, water leaves the vacuoles and the cells show plasmolysis. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 S1 0.50 mol dm–3 sodium chloride none 50 solution + onion tissue S2 sodium chloride solution none 50 + onion tissue S3 sodium chloride solution none 50 + onion tissue S4 sodium chloride solution none 50 + onion tissue If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. S1 is 0.50 mol dm–3 sodium chloride solution. S2, S3 and S4 are 1.00 mol dm–3, 0.25 mol dm–3 and 0.00 mol dm–3, but not necessarily in that order. You will need to: • observe a sample of onion cells from S1, S2, S3 and S4 • decide how many cells you will observe to record the states of plasmolysis • record the number of cells that show the different states of plasmolysis, A, B, C and D, as shown in Fig. 1.1 • use your results to identify the concentration of the sodium chloride solutions, S2, S3 and S4. Carry out step 1 to step 7 to prepare a slide of onion tissue. 1. Label one dry and clean microscope slide, S1. Put the slide on a paper towel. 2. Put a few drops of sodium chloride solution, S1, onto the microscope slide. 3. Remove a piece of the onion tissue from S1. Peel off the inner epidermis as shown in Fig. 1.2. The inner epidermis may start to separate or have become separated from the rest of the onion tissue, so the epidermis may be floating in the sodium chloride solution. Fig. 1.2 4. Cut one piece of the epidermis so that it will fit under a coverslip. Put the remaining epidermis into S1. 5. Put the piece of epidermis on the labelled microscope slide as shown in Fig. 1.3. If the piece of epidermis is folded, you may need to add more drops of S1 so that it floats and can be unfolded. piece of epidermis microscope slide sodium chloride solution paper towel Fig. 1.3 6. Put a coverslip over the piece of epidermis on the microscope slide. Use a paper towel to remove any excess solution that is outside the coverslip. 7. Observe the cells of the epidermis using the microscope. You may need to reduce the amount of light entering the microscope to observe the cells clearly. (a) You need to decide which objective lens you will use and decide the number of cells you will need to observe to give you confidence in your results. (i) State the objective lens you will use and the number of cells you will observe. Explain how this will give you confidence in your results. objective lens .................................................................................................................... number of cells .................................................................................................................. explanation ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... [2] 8. Using the objective lens and the number of cells you decided in (a)(i), count the number of cells for S1 that are in each state of plasmolysis, A, B, C and D (as shown in Fig. 1.1). Record your results in (a)(ii). 9. Repeat step 1 to step 8 for each of S2, S3 and S4. Record your results in (a)(ii). (ii) Record your results in an appropriate table. [5] (iii) Solution S1 is 0.50 mol dm–3. S2, S3 and S4 are 1.00 mol dm–3, 0.25 mol dm–3 and 0.00 mol dm–3, but not necessarily in that order. Using your results in (a)(ii), identify the concentration of sodium chloride solution in S2, S3 and S4. S2 ...................................................................................................................................... S3 ...................................................................................................................................... S4 ...................................................................................................................................... [1] Use a sharp pencil for drawings. You are expected to draw the correct shape and proportions of the different cells. (b) Observe the onion cells on the microscope slide you labelled S4. Select four adjacent, touching cells, with at least two cells showing plasmolysis. Make a large drawing of these four cells. Use one ruled label line and label to identify the cell surface membrane of one cell. [4] (c) A student investigated the effect of different concentrations of sucrose solutions on pieces of potato tissue. • Each piece of potato tissue was cut to exactly 50 mm in length and had a cross‑sectional area of exactly 10 mm × 10 mm. • Each piece of potato tissue was put into a different concentration of sucrose solution. • After 1 hour the length of each piece of potato was measured. The results of the student’s investigation are shown in Table 1.2. Table 1.2 sucrose change concentration in length / mol dm–3 / mm 0.0 +2.5 0.2 +1.5 0.4 0.0 0.6 –0.5 1.0 –1.5 (i) Describe the pattern of water movement shown in Table 1.2. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Using your knowledge of water potential, explain the result at 0.2 mol dm–3. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) The student decided to compare the effect of different concentrations of sucrose solution using a different type of potato. Complete Table 1.3 to suggest how three variables would need to be standardised. Table 1.3 variable how variable may be standardised [3] [Total: 19]

Mark scheme: 1(a)(i) 1. objective lens (×)40 + number of cells 5 or above ; 2. idea of cells being large enough to see plasmolysis clearly with ×40 lens or idea of having a large number of cells there are enough to count cells in different stages of plasmolysis ; 2 1(a)(ii) 1. heading for solution / sample / slide ; 2. (heading for dependent) state of plasmolysis ; 3. collects results for each solution ; 4. correct trend ; 5. records a number for each solution and each state of plasmolysis (at least 16 numbers in total) ; 5 1(a)(iii) correct results from candidates table ; 1 1(b) 1. lines are continuous, thin and sharp + minimum size ; 2. draws only 4 whole cells + each cell touching at least one of the other cells + at least two cells showing some degree of plasmolysis ; 3. cell wall drawn as two lines, 3 lines where two cells touch ; 4. label line and label to cell membrane ; 4 1(c)(i) 1. below 0.4 mol dm–3 water moved into the potato tissue or above 0.4 mol dm–3 water moved out of the potato tissue ; 2. no net movement of water at 0.4 mol dm–3 ; 2 1(c)(ii) 1. (the sucrose solution / 0.2 mol dm–3 solution) had a higher water potential than the potato cells / tissue ; 2. (so) water moved into the potato / cells / tissue by osmosis; 2 Question Answer Marks 1(c)(iii) one mark per row any three from: variable how variable may be standardised potato (same) age / from same plant / type / species / cut from the same potato ; surface area use ruler / calipers / cork borer ; length / width / diameter / cross section use measuring equipment, e.g. ruler, calipers ; volume (of sucrose solution) use a measuring cylinder / syringe / burette / graduated pipette to measure volume ; time timer ; temperature thermostatically controlled water-bath ; 3

More questions on Movement into and out of cells

2 (a) A student investigated the effect of different factors on transpiration. In the investigation, three identical plants were put in flasks containing 70 cm3 of water as shown in Fig. 2.1. plant layer of oil flask 70 cm3 water Fig. 2.1 The plants were each kept in different conditions: • 20 °C + still air + high humidity • 20 °C + windy + low humidity • 30 °C + still air + low humidity. All other variables were kept constant. After 4 hours the water remaining in each flask was put into beakers labelled P1, P2 and P3. (i) Suggest a suitable control for this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Describe how the student could have decreased the humidity. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] You have been given the beakers, labelled P1, P2 and P3, containing the water that was collected from each flask after the 4 hours. You will need to determine the volume of water lost by transpiration from each plant. (iii) Using the apparatus provided, measure the volume of water in P1, P2 and P3. Calculate the volume of water lost through transpiration by each plant. Record your results in an appropriate table. [2] (iv) State which plant was grown in high humidity. Give a reason for your answer. plant .................................................................................................................................. reason ............................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [1] (b) Plants lose water through stomata by transpiration. A scientist investigated the effect of carbon dioxide concentration in the air on the number of stomata present per mm2 of leaf area, in five plants. The carbon dioxide concentration was measured in parts per million (ppm). Table 2.1 carbon dioxide number of stomata / mm2 concentration / ppm plant 1 plant 2 plant 3 plant 4 plant 5 mean 288 294 307 286 301 292 296 306 271 285 266 278 270 274 328 254 241 265 263 242 253 357 242 237 297 235 246 383 234 227 231 234 219 229 (i) Complete Table 2.1 by calculating the mean number of stomata / mm2 at a carbon dioxide concentration of 357 ppm. Show all the steps in your working and use appropriate units. [2]

Mark scheme: 2(a)(i) no plant / fake plant or plant with no leaves or accept use only one / same plant (and put in each condition) / AW ; 1 2(a)(ii) use (calcium / sodium / potassium) hydroxide / chloride / oxide or silica gel / drying agent or use dehumidifier ; 1 2(a)(iii) 1. volume for P1, P2 and P3 + cm3 ; 2. records correct calculated values for each of the solutions for volume lost ; 2 2(a)(iv) correct answer from their results + least volume of water loss ; 1 2(b)(i) 1. correct answer 240 ; 2. shows 242 + 237 + 235 + 246 divided by 4 or shows 242 + 237 + 297 + 235 + 246 divided by 5 ; ecf 2 2(b)(ii) 1. x-axis labelled as carbon dioxide concentration / ppm + y-axis labelled as number of stomata (/)mm–2 ; 2. scale on x-axis 20 to 2 cm + labelled each 2 cm + scale on y-axis is 20 to 2 cm + labelled each 2 cm ; 3. correct plotting of points with a small cross or dot in circle ; 4. smooth, thin line, joined point to point, through 5 points / curve or line of best fit ; 4 2(b)(iii) as the concentration of carbon dioxide increases the number of stomata decreases ; 1 2(b)(iv) correct answer from candidates graph ; 1 Question Answer Marks 2(c) 1. minimum size + no shading + no cells ; 2. whole leaf drawn + at least 3 lines ; 3. correct leaf shape + at least 3 lines across width of vascular tissue in the centre ; 4. correct proportion of the diameter of the central region to the total diameter of the leaf section ; 5. label line and label to epidermis ; 5 2(d) 1. 17 (eyepiece graticule units) ; 2. shows multiplication of eyepiece graticule units by 39.7 ; 3. correct answer + correct units ; 3

More questions on Investigation of limiting