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When onion cells are placed into a solution, water will move depending on the water potential inside the cells and the water potential of the surrounding solution.
The water potential of a solution depends on both the concentration and the type of molecule dissolved.
You are required to:
• identify S1, S2 and S3 by carrying out tests for carbohydrates, using the reagents provided
• investigate the effect of water and each of the solutions S1, S2 and S3 on onion cells.
You are provided with:
| labelled | contents | hazard |
|---|---|---|
| S1, S2 and S3 | see below | none |
| W | water with one piece of onion tissue | none |
Each of S1, S2 and S3 contains a piece of onion tissue and one of the following:
• a mixture of sucrose solution and glucose solution
• sodium chloride solution
• starch solution
By using only two of the biological molecule tests for carbohydrates, you should be able to identify the solutions, S1, S2 and S3.
(a) (i) Identify S1, S2 and S3 by carrying out tests using the apparatus and reagents provided. Decide which biological molecule to identify in the first test.
Complete the following:
First test: Test for .................................................
Describe how you will use the reagent(s) to carry out this test.
...................................................[1]
Carry out the first test and record your observations in (a)(iii).
(ii) Decide which biological molecule to identify in the second test.
Complete the following:
Second test: Test for ..............................................
Describe how you will use the reagent(s) to carry out this test.
...................................................[1]
Carry out the second test and record your observations in (a)(iii).
(iii) Prepare the space below and record your observations for each test.
(iv) Complete the table to identify S1, S2 and S3.
| molecule(s) | solution |
|---|---|
| mixture of sucrose and glucose | |
| sodium chloride | |
| starch |
(v) State the hazard with the greatest level of risk when using the apparatus and reagents to test S1, S2 and S3 for biological molecules. State the level of risk of the procedure: low or medium or high.
hazard .........................................................
level of risk ..................................................[1]
You are required to investigate the effect of water, W, and each of the solutions, S1, S2 and S3, on the cells of the epidermis of onion.
(b) (i) Complete the sentence below by choosing from the following words:
enters leaves plasmolysed turgid
You may use each word once or more than once or not at all.
When onion cells are placed into a solution with a more negative water potential than the cells, water ....................... the vacuoles and the cells become ................... .[1]
(ii) Prepare the space below and record your results.
(iii) Identify one significant error in measuring the dependent variable.
..................................................[1]
You are required to use a sharp pencil for drawings.
(iv) Make a drawing of one cell from the epidermis in W and one cell from the epidermis in S1. The cells you choose must show the effect of W and S1.
Use one ruled label line and label to identify the cytoplasm in each cell.
cell from epidermis in W
cell from epidermis in S1[4]
(v) Explain, using your knowledge of the movement of water, the effect of S1 and S3 on the cells of the epidermis.
S1
.....................................................
S3
.....................................................[3]
521
533
579
(a) A student used a model to study the loss of water from stomata in leaves.
At the start of the investigation, the model was set up as shown in Fig. 2.1.
The student investigated the effect that different numbers of holes in the lids had on the evaporation of water from the tube.
Each tube was set up with a lid with either 5 or 10 or 15 or 20 holes.
The tubes were left for 9 days and then the volume of the water remaining was marked with a second line on the tube. The volume of water at the start, $V_0$, and the volume of water after 9 days, $V_9$, were measured.
The volume of water lost by evaporation was calculated.
You are provided with four tubes labelled 1, 2, 3, and 4. Each tube is marked with lines to show the volume of water at the start and after 9 days.
You are required to:
- measure the volume of water at the start of the investigation in each tube, $V_0$
- measure the volume of water remaining in each tube after the 9 days, $V_9$
- calculate the volume of water lost by evaporation in each tube (processed results).
(i) Describe how you will measure the volumes $V_0$ and $V_9$ (raw results) using the apparatus provided.
.................................................................................................................
.................................................................................................................
................................................................................................................. [1]
Carry out your method as described in (a)(i) for each of the four tubes and record your raw results and processed results in (a)(ii) on page 9.
(ii) Prepare the space below and record your raw and processed results. [4]
(iii) Using your processed results, complete the table below to show which lid was used to cover which tube.
[Table_1]
Fig. 2.2 shows four lids labelled $P$, $Q$, $R$, and $T$.
[4]
(iv) A control experiment is set up to make sure that the trend in the results is caused by the variable under investigation and not by some other variable.
Describe a control experiment for this investigation.
.................................................................................................................
................................................................................................................. [1]
(v) The student wanted to increase the rate of evaporation from the tubes, without removing the lid or altering the number or size of the holes.
Describe how the student could have changed the procedure to increase the rate of evaporation.
.................................................................................................................
.................................................................................................................
.................................................................................................................
................................................................................................................. [2]
A student investigated the relationship between the total circumference of the holes and rate of evaporation.
The student repeated the investigation with different lids with 0 or 5 or 10 or 20 holes. All the holes had the same diameter.
The student processed the results by calculating the total circumference of the holes and the rate of evaporation of water per day.
The processed results are shown in Table 2.1.
[Table_2]
(b) Plot a graph of the data shown in Table 2.1.
(c) Fig. 2.3 is a stained transverse section through a plant stem. You are not expected to be familiar with this specimen.
Draw a large plan diagram of the quarter of the stem as shown in Fig. 2.4.
[4]
(d) A student calibrated the eyepiece graticule in a light microscope using a stage micrometer scale so that the actual length of the tissues could be found.
The calibration was: one eyepiece graticule division equal to 0.024 mm.
(i) The use of the unit mm is not the most appropriate unit for use with the light microscope.
State which unit is most appropriate for use with the light microscope and show how 0.024 mm is converted to this unit.
You may lose marks if you do not show your working.
unit .............................. [2]
(ii) Fig. 2.3 shows a photomicrograph taken using the same microscope with the same lenses as the student.
Use the calibration of the eyepiece graticule division from (d)(i) and Fig. 2.3 to calculate the actual total width of the pith, shown by line Y.
You may lose marks if you do not show all the steps in your calculation and do not use appropriate units.
actual total width .................................................. [2]
600
500
514
