Biology 9700/32 — May/June 2020

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Use of the Light Microscope · Analysis, Conclusions and Evaluation · Presentation of Data and Observations

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsUse of the Light MicroscopeAnalysis, Conclusions and EvaluationFree sample

When plant cells are placed into sodium chloride solution, osmosis occurs and water will enter or leave the vacuoles. Some cells may become plasmolysed. A cell is described as being plasmolysed when the cell surface membrane detaches from the cell wall.

You will investigate the effect of different concentrations of sodium chloride solution on onion tissue.

You will need to:

prepare different concentrations of sodium chloride solution using proportional dilution of $4.0\%$ sodium chloride solution
observe and record the effect of adding these concentrations of sodium chloride solution to the onion tissue.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
X	2 pieces of onion tissue in distilled water	none	—
S	$4.0\%$ sodium chloride solution	none	50
W	distilled water	none	50

It is recommended that you wear suitable eye protection.

You will need to prepare different concentrations of sodium chloride solution, S, using proportional dilution. You will need to prepare $10\ \text{cm}^3$ of each concentration.

(a)

(i)

Table 1.2 shows how to make up two of the concentrations of S you will use.
Decide which three other concentrations of S you will use.
Complete Table 1.2 for the other concentrations you will use.

Table 1.2

percentage concentration of sodium chloride solution	volume of $\mathbf{S} / \text{cm}^3$	volume of $\mathbf{W} / \text{cm}^3$
4.0	10.0	0.0



0.0	0.0	10.0

(ii)

Carry out step 1 to step 14.

Prepare the concentrations of sodium chloride solution, as shown in Table 1.2, in the beakers provided.
Put one clean and dry microscope slide on a paper towel.
Put a few drops of W onto the microscope slide.
Remove one piece of onion tissue from beaker X. Peel off the inner epidermis as shown in Fig. 1.1.

Fig. 1.1

Cut one piece of the inner epidermis so that it will fit under a coverslip. Put any remaining epidermis into the beaker labelled For waste.
Put the epidermis into W on the microscope slide, as shown in Fig. 1.2. If the epidermis is folded, you may need to add more drops of W so that it floats and uncurls. It is important to stop the epidermis from drying out.

Fig. 1.2

Put a coverslip over the piece of epidermis on the microscope slide. Use a paper towel to remove any excess W that is outside the coverslip.
Observe the epidermis using the low power lens of the microscope. You may need to reduce the amount of light entering the microscope to observe the cells clearly.

You will need to observe and record the effect of adding W and the different concentrations of sodium chloride solution on the onion tissue. You will do this by counting the number of plasmolysed cells within a sample of cells.

Decide the total number of cells in your sample.

State the total number of cells in your sample ______

(iii)

Count the number of plasmolysed cells observed in your sample for W. Record your results in (a)(iii).
Take the microscope slide off the microscope and remove the coverslip.
Use a paper towel to remove W from around the epidermis.
Put a few drops of the lowest concentration of sodium chloride solution you prepared in step 1 onto the epidermis.
Repeat step 7 to step 11 using the lowest concentration of sodium chloride solution instead of W.
Repeat step 12 to step 13 using the remaining concentrations of sodium chloride solution. The $4.0\%$ sodium chloride solution is used last.

Record your results in an appropriate table.

(iv)

Using the high power lens, select three adjacent, touching cells that show the effect of adding $4.0\%$ sodium chloride solution.

Make a large drawing of these three adjacent, touching cells. Use a sharp pencil for drawing.

Use one ruled label line and label to identify a cell surface membrane of one cell.

(v)

Use your knowledge of water potential to explain the appearance of the inner epidermal cells in $4.0\%$ sodium chloride solution.

(vi)

Suggest how you could modify the procedure to have more confidence in your results.

(b)

A student investigated the effect of different concentrations of sucrose solution on pieces of potato tissue. The student used the results to determine the mean percentage change in length of the pieces of potato tissue.

Pieces of potato tissue were cut to exactly the same length and cross-sectional area.
Each piece of potato tissue was put into a different concentration of sucrose solution for 1 hour.
After 1 hour the length of each piece of potato tissue was measured and the percentage change in length was calculated.
Five replicates were done for each concentration.

The student then calculated the mean percentage change in length of potato tissue for each concentration of sucrose solution.

The processed data are shown in Table 1.3.

Table 1.3

concentration of sucrose solution / $\text{mol dm}^{-3}$	percentage change in length					mean percentage change in length
0.0	+2.60	+1.90	+2.80	+2.60	+2.80	+2.70
0.2	+0.25	+0.35	+0.35	+0.30	+0.25	+0.30
0.4	-1.40	-1.40	-1.35	-1.40	-1.45	-1.40
0.6	-2.20	-2.25	-2.20	-2.15	-2.20	-2.20
0.8	-2.60	-2.50	-1.60	-2.60	-2.70
1.0	-2.90	-3.00	-2.85	-3.15	-3.10	-3.00

(i)

Complete Table 1.3 by calculating the mean percentage change in length of the potato tissue in $0.8\ \text{mol dm}^{-3}$ sucrose solution.

(ii)

Plot a graph of the mean data in Table 1.3 on the grid in Fig. 1.3.

Use a sharp pencil for drawing graphs.

Fig. 1.3

(iii)

Fig. 1.4 is a calibration curve of sucrose concentration against water potential.

Fig. 1.4

Use the graphs in Fig. 1.3 and Fig. 1.4 to determine the water potential of the cells in potato tissue.

Show on the graphs how you determined your answer.

water potential of cells in potato = ______

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