Question 126 marks
Question
You are going to investigate the effect of the concentration of sugar solutions on osmosis in potato cells. Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results in the space provided in 1(a)(ii). You should use the safety equipment provided while you are carrying out the practical work. Step 1 Label four test-tubes A, B, C and D and put them into the test-tube rack. Step 2 Use the volumes of 1 mol per dm3 sugar solution and distilled water shown in Table 1.1 to make solutions containing different concentrations of sugar in each test-tube. Use the syringes provided to make your solutions. (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 Put the potato cylinders on a white tile and cut each cylinder to exactly 40 mm in length. Step 4 Add one potato cylinder to each of test-tubes A, B, C and D. Step 5 Leave the potato cylinders in the test-tubes for 20 minutes. While you are waiting continue with the other questions. Step 6 After 20 minutes empty the contents of test-tube A into the container labelled waste. Step 7 Use the forceps to remove the potato cylinder from the waste container and place it on the white tile. Step 8 Measure the length of the potato cylinder from test-tube A and record this measurement, in millimetres, in your table in 1(a)(ii). Put the potato cylinder from test-tube A onto a paper towel. Step 9 Repeat steps 6, 7 and 8 for the potato cylinders in test-tubes B, C and D. (ii) Prepare a table to record your results in the space provided. [4] (iii) Explain why it was important that the potato cylinders were all cut to the same length in step 3. …… [1] (iv) Identify the variable that you changed in this investigation (independent variable). …… [1] (v) Suggest two improvements that you could make to the method you have used in this investigation. 1 …… 2 …… [2] (vi) Describe one safety precaution you took while preparing the potato cylinders in step 3. …… [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. 1.1 is a photomicrograph of some plant cells that contain starch grains. starch grain cell X A B magnification ×800 Fig. 1.1 (ii) Draw a large diagram of cell X in Fig. 1.1. 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. 1.1. length of line AB …… mm Calculate the actual diameter of the starch grain using your measurement for line AB, the information in Fig. 1.1 and the formula: length of line AB on Fig. 1.1 magnification = actual diameter of the starch grain Give your answer to two significant figures. [3] [Total: 26]
Mark scheme
1(a)(i) 0.4 ; 1 1(a)(ii) table drawn with a minimum of two, columns / row, and a header line ; 4 headers with units ; MP2 concentration (of sugar) / mol per dm3 or test tube / AW and length (of potato after 20 minutes) / mm four lengths recorded ; expected trend: potato cylinder in A is shorter than D ; check supervisor’s report 1(a)(iii) so that the results were comparable / so sugar concentration is the only 1 variable that is changed / to ensure the surface area was the same / so surface area doesn’t affect rate of osmosis / AW ; 1(a)(iv) concentration of the sugar solution ; 1 1(a)(v) any two from: 2 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 same temp ; dry cylinders before measuring ; ensure timing is the same for each test-tube ; AVP ; 1(a)(vi) cut away from body / use wooden board or tile / cut on flat surface / keep 1 blade away from body / dry before cutting / hold potato with forceps ; 1(b) independent variable: 6 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 ; 1(c)(i) iodine solution ; 2 blue-black ; 1(c)(ii) lines: clear, single, unbroken with no shading ; 5 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 ; 1(c)(iii) (length of line AB) 10 ±1 (mm) ; 3 MP1 for correct measurement 0.013 (0.011–0.014) mm ;; MP2 for correct calculation MP3 for correct rounding to two significant figures ecf MP2 & MP3 from previous step