SAJC H2 Prelim P4 2023 Mark scheme for sharing

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 1 1 You will investigate the water potential of potato cells. When pieces of potato are put into a sucrose solution, water will move by osmosis into and out of the potato cells. The overall direction of water movement depends on the difference between the water potential of the potato cells and the water potential of the sucrose solution. • If the overall movement of water is out of the potato cells, the sucrose solution will become less concentrated. • If the overall movement of water is into the potato cells, the sucrose solution will become more concentrated. Fig. 1.1 shows how the change in concentration of the sucrose solution after 15 minutes can be assessed. A blue dye is added to the sucrose solution around the potato pieces at the end of the 15 minutes. The blue dye does not affect the concentration of the sucrose solution. After mixing, a drop of the blue sucrose solution is added to the original sucrose solution in a separate test-tube. The movement of the blue drop is then observed. Fig. 1.1 • If the sucrose solution has become less concentrated, then the density of the sucrose solution will have decreased. • If the sucrose solution has become more concentrated, then the density of the sucrose solution will have increased.

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 2 (a) (i) When two solutions of different density are added to one another without mixing, the denser solution will sink to the bottom and the less dense solution will rise to the top. Complete Fig. 1.2 by drawing an arrow on each test-tube, as shown in the key, to predict how you expect the drop of blue solution to move. Fig. 1.2 1 All three directions correct ; in order from left to right , ,  [1]   

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 3 You are provided with: • 5 potato cylinders, in a Petri dish labelled P • 150cm3 1.00 mol dm-3 sucrose solution S, in a container labelled S • 250cm3 distilled water W, in a container labelled W • 20cm3 blue dye M, in a container labelled M. If M comes into contact with your skin, wash it off immediately under cold water. It is recommended that you wear suitable eye protection. You will carry out a serial dilution of the 1.00 mol dm -3 sucrose solution, S, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of sucrose solution in addition to the 1.00 mol dm-3 sucrose solution, S. After the serial dilution is completed, you will need to have 50.0 cm3 of each concentration available to use. (ii) Complete Table 1.1 to show how you will make the concentrations of the sucrose solutions needed. ……………………………………………………………………………..……………………..[3] Table 1.1 sucrose solution concentration of sucrose solution / mol dm-3 1.00 0.50 0.25 0.125 0.0625 concentration of sucrose solution to be diluted / mol dm-3 1.00 0.50 0.25 0.125 volume of the sucrose solution to be diluted / cm3 50.0 50.0 50.0 50.0 volume of distilled water to make the dilution /cm3 50.0 50.0 50.0 50.0 1 Correct concentrations 0.50, 0.25, 0.125 and 0.0625 mol dm-3 ; REJECT if not by serial dilution, as specified by question stem 2 Correct volumes of sucrose solution to be diluted (accuracy to 1 d.p if using measuring cylinder) to form total volume of 100.0cm3 for each concentration ; 3 Correct volumes of distilled water W (accuracy to 1 d.p if using measuring cylinder) to form total volume of 100.0cm3 for each concentration ;

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 4 Read steps 1-22. Proceed as follows. 1 Prepare the concentrations of sucrose solution, as decided in (a)(ii), in the beakers provided. 2 Label five boiling tubes with the concentrations of sucrose solution prepared in step 1, including 1.00 mol dm–3. 3 Put the five potato cylinders onto a white tile. (iii) The potato cylinders all have the same diameter. State the method that you will use to standardise the surface area of all five potato cylinders. ………………………………………………………….…………………………………………[1] 1 Cut all potato cylinders to the same length (e.g. 6.00cm) ; 4 Carry out the method stated in (a)(iii) for each of the five potato cylinders. 5 Cut one of the potato cylinders into eight pieces of approximately the same length. 6 Put the eight pieces of potato into one of boiling tubes labelled in step 2. Record the length of each potato piece. length of potato piece = ………………………………….cm Markers’ guide : Maximum length = 1.00cm (maximum length of potato cylinders provided to candidates is 8.00cm) 7 Repeat step 5 and step 6 for the four other potato cylinders so that there are eight pieces of potato in each of the boiling tubes labelled in step 2. You will put sucrose solution into each boiling tube to just cover the eight pieces of potato. You will need to standardise the volume of sucrose solution put into each of the boiling tubes. (iv) State the volume of sucrose solution you will use to just cover the eight pieces of potato in each boiling tube. volume = …………………………………… [1] 1 appropriate volume (range : 2.0cm3 – 15.0cm3) with units (REJECT ml) ; Markers’ guide :

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 5 0.20cm / piece – 2.0 (use of syringe) to 5.0cm3 0.50cm / piece – 5.0cm3 to 8.0cm3 0.75cm / piece – 8.0cm3 to 10.0cm3 1.00cm / piece –10.0cm3 to 15.0cm3 8 Put each concentration of sucrose solution prepared in step 1, including 1.00 mol dm –3, into the appropriately labelled boiling tube. For each boiling tube, use the volume of sucrose solution stated in (a)(iv) to cover the potato pieces. 9 Leave the pieces of potato in the sucrose solutions for 15 minutes. While you are waiting, continue with step 10 to step 13. 10 Label five test-tubes with the concentrations of sucrose solution that you have used in step 8. 11 Put a mark 5.0 cm from the bottom of each of the test-tubes, as shown in Fig. 1.4. Fig. 1.4 12 Put 15.0cm3 of 1.00 mol dm–3 sucrose solution into the appropriately labelled test-tube. 13 Repeat step 12 with each of the other concentrations of sucrose solution. 14 After leaving the pieces of potato for 15 minutes in step 9, put 1.0 cm3 of the blue dye, M, into each of the boiling tubes containing eight pieces of potato in sucrose solution. 15 Swirl the contents of the boiling tubes to mix M with the sucrose solution. The blue dye may not mix in completely. This will not affect the results. 16 Use a pipette to remove a sample of the blue solution from around the pieces of potato in the boiling tube to which 1.00 mol dm–3 sucrose solution had been added. Throughout step 17 to step 20, the pipette must be held still so that its position does not change. Drops can then be released and observed without disturbing the sucrose solution. 17 Put the end of the pipette into the small test -tube containing 1.00 mol dm –3 sucrose solution. The end of the pipette should be level with the mark on the small test -tube, as shown in Fig. 1.5.

2022 9744 Preliminary Examination H2 Biology Mark Scheme (SAJC) 6 Fig. 1.5 18 Keeping the end of the pipette as still as possible, release a drop of the blue solution from the pipette. 19 Observe the direction and time taken for the drop of blue solution to reach the top or bottom of the test tube. 20 Repeat step 18 and step 19 two more times. 21 Record your observations in (a)(v). 22 Repeat step 16 to step 21 for the other concentrations of sucrose solution. In step 17 and step 18, make sure that drops of the blue solution from each boiling tube are released into the small test-tubes labelled with the same concentration of sucrose. (