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When onion cells are placed into a sodium chloride solution with a higher concentration than the cells, water leaves the vacuoles and the cells become plasmolysed.
You are provided with three samples of onion tissue, each soaked in a different concentration of sodium chloride solution, labelled S1, S2 and S3.
The three concentrations of sodium chloride solution are 0.25 mol dm^-3, 0.50 mol dm^-3 and 1.0 mol dm^-3.
The onion tissue in S1 is in 1.00 mol dm^-3 sodium chloride solution.
You are required to:
• observe and record the cells in S1 and the effect of adding water, W, to these cells
• observe and record the onion cells to identify the concentration of the sodium chloride solutions, S2 and S3.
You are advised to read steps 1 to 12 before proceeding.
Proceed as follows:
1. Label one dry and clean microscope slide S1 and put the slide on a paper towel.
2. Put a few drops of the sodium chloride solution from S1 onto the slide.
3. Remove a piece of the onion tissue from solution S1 and, using forceps or fingers, peel off the inner concave epidermis as shown in Fig. 1.1.
4. Cut one piece of the epidermis to fit under the coverslip. Replace the remaining epidermis into the solution in S1.
5. Place the epidermis into the sodium chloride solution on the slide as shown in Fig. 1.2. If the epidermis is folded, you may need to add more drops of sodium chloride solution so that it floats and uncurls. It is important to stop the epidermis from drying out.
6. Cover the epidermis on the slide with a coverslip and use a paper towel to remove any excess liquid that is outside the coverslip.
7. View the slide using the microscope. You may need to reduce the amount of light entering the microscope to observe the cells clearly.
(a) (i) Make a large drawing of two adjacent (touching) cells which show the effect of soaking the cells in the 1.00 mol dm^-3 sodium chloride solution, S1.
Label a cell membrane. [4]
(ii) Describe how you will obtain quantitative results to identify the concentration of the sodium chloride solutions, S2 and S3, compared with S1.
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(b) (i) Prepare the space below to record your quantitative results as you have described in (a)(ii).
8. Record the result for S1.
9. Take the slide off the microscope and remove the coverslip.
10. Remove as much of the 1.00 mol dm^-3 sodium chloride solution as you can and put a few drops of W onto the epidermis. Replace the coverslip and leave the slide until step 12.
11. Repeat steps 1 to 8 with S2 and S3 (note: you do not need to repeat (a)(i)).
(ii) Identify the concentrations of sodium chloride solution in S2 and S3. Complete the table.
[Table_1]
solution | concentration of sodium chloride solution / mol dm^-3
S1 | 1.00
S2 |
S3 |
[1]
(iii) Describe how you identified the concentrations of sodium chloride.
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12. Put the S1 slide onto the microscope and select two adjacent cells to show the difference in the cells after adding the water compared with those drawn in (a)(i).
(c) (i) Make a large labelled drawing of these two adjacent (touching) cells.
Label a cell wall. [3]
(ii) Use your knowledge and understanding of plasmolysis to explain how and why the cells changed when they were left in water.
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(d) Suggest how you would modify this investigation to find the water potential of the onion cells.
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.................................................................................................................................... [3]
588
522
503
You are required to investigate the effect of staining the tissues of sweet banana with iodine solution.
You are provided with a piece of sweet banana.
Proceed as follows:
1. Cut a transverse (cross) section of the banana, approximately 5 mm thick, including the outer skin, as shown in Fig. 2.1.
[Image_1: Fig. 2.1]
(a) (i) Draw a large plan diagram of the transverse section.
[4]
2. Put two or three drops of iodine solution on the surface of the transverse section of the banana and spread out the iodine solution.
(ii) Observe the effect of iodine solution on the different tissues.
Annotate the diagram in (a)(i) to describe the effect of staining with iodine solution on the different tissues. [1]
(iii) Describe the pattern of staining that you observe.
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(b) (i) Calculate the ratio of the mean of the total width of the section to the mean width of the outer skin.
You may lose marks if you do not show your working or if you do not use appropriate units.
[3]
(ii) State the value of the smallest division on the ruler used to measure the widths in (b)(i). ..............................
State the actual error in measuring the widths in (b)(i). .............................. [1]
Table 2.1 shows some of the components in 100g of plantain and sweet banana.
[Table_1: Table 2.1]
(c) Plot a chart of the data in Table 2.1.
[4]
The components in Table 2.1 make up a certain percentage of the total mass of the plantain and sweet banana. Water makes up the balance of the total mass.
(d) Calculate the difference in percentage water content between plantain and sweet banana.
You may lose marks if you do not show your working or if you do not use appropriate units.
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508
599
596
