Plasmolysis may be seen in onion cells that have been put into a sodium chloride solution. Plasmolysis occurs when water moves out of the cells and the cell surface membrane pulls away from the cell wall. You will observe the effect of different concentrations of sodium chloride solution on onion cells in samples of onion tissue. You will use your observations to determine the concentration of sodium chloride in three solutions, U1, U2 and U3. You are provided with onion tissue in approximately 50 cm³ of each of the three different concentrations of sodium chloride solution. If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You will need to: • prepare microscope slides of the three samples of onion tissue in U1, U2 and U3 • observe 15 cells on each microscope slide and record how many of these cells show any sign of plasmolysis. Carry out step 1 to step 11. step 1 Label one clean and dry microscope slide U1. Put the slide on a paper towel. step 2 Put a few drops of the solution in the beaker labelled U1 onto the microscope slide. step 3 Remove one piece of onion tissue from the beaker labelled U1. Cut the piece of onion tissue so that it is between approximately 0.5cm × 0.5cm and 1cm × 1cm. Put the remaining onion tissue back into the beaker labelled U1. step 4 Peel off the inner epidermis from the piece of onion tissue. step 5 Put the inner epidermis on the microscope slide, as shown in Fig. 1.1. If the piece of epidermis is folded, you may need to add more drops of solution. The inner epidermis will float and can then be unfolded. [Figure 1.1] step 6 Put a coverslip over the piece of inner epidermis on the microscope slide. Use a paper towel to remove any excess solution that is outside the coverslip. step 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. step 8 Using the ×10 objective lens, observe 15 cells. You may need to move the slide to change the field of view. step 9 Count how many of these 15 cells are undergoing plasmolysis. Record your results in (a)(i). step 10 Repeat step 1 to step 9 using the onion tissue in U2. step 11 Repeat step 1 to step 9 using the onion tissue in U3. You will need to use slide U1 again for (a)(iii). (b) A student investigated the effect of different concentrations of sucrose solution on the mass of potato pieces. • Five different concentrations of sucrose solution were used. • The masses of five potato pieces were measured before and after soaking in these different sucrose solutions. • The percentage change in mass was then calculated for each potato piece. • The procedure was repeated three times. • The mean percentage change in mass as a result of soaking was then calculated for each concentration of sucrose. Table 1.1 shows the results of this investigation. Table 1.1 sucrose concentration /mol dm⁻³ | trial | initial mass /g | final mass /g | change in mass /g | percentage change in mass | mean percentage change in mass --- | --- | --- | --- | --- | --- | --- 0.0 | 1 | 2.4 | 2.5 | 0.1 | +4.2 | | 2 | 2.5 | 2.8 | 0.3 | +12.0 | +8.2 | 3 | 2.4 | 2.6 | 0.2 | +8.3 | 0.2 | 1 | 2.4 | 2.5 | 0.1 | +4.2 | | 2 | 2.4 | 2.5 | 0.1 | +4.2 | | 3 | 2.3 | 2.4 | 0.1 | 0.4 | 1 | 2.5 | 2.5 | 0.0 | +0.0 | +1.3 | 2 | 2.5 | 2.6 | 0.1 | +4.0 | | 3 | 2.5 | 2.5 | 0.0 | +0.0 | 0.6 | 1 | 2.6 | 2.5 | -0.1 | -3.8 | -6.7 | 2 | 2.5 | 2.3 | -0.2 | -8.0 | | 3 | 2.4 | 2.2 | -0.2 | -8.3 | 0.8 | 1 | 2.4 | 2.2 | -0.2 | -8.3 | -9.9 | 2 | 2.4 | 2.2 | -0.2 | -8.3 | | 3 | 2.3 | 2.0 | -0.3 | -13.0 |