Download Expt - Cytoplasmic Concentration of Onion Cells

Topic 2 – Cells
International Baccalaureate - Biology
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Design
Aspect
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Data Collection
and Processing
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Conclusion
Evaluation
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Manipulative
Skills
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Partial/1
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CYTOPLASMIC CONCENTRATION OF ONION CELLS
INTRODUCTION
When a plant cell is surrounded by a solution of lower concentration is plasmolyses, that is , its cytoplasm losses water and
contracts, gradually separating the plasma membrane from the cell wall until it rounds off in the centre. When plant cells are
placed in a range of solutions of different concentrations, the solution just strong enough to make the plasma membrane
separate from the cell in places (incipient plasmolysis) can be regarded as having the same concentration as the cell
cytoplasm.
This provides us with a way of estimating the solute potential of a plant tissue, which is the aim of this practical.
In practice, the cells in a piece of plant tissue plasmolyse at different rates. We can regard incipient plasmolysis as taking
place when half of the cells in the tissue exhibit visible plasmolysis.
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Topic 2 – Cells
International Baccalaureate - Biology
METHOD
1. Into each of the six specimen tubes, place about 10 cm3 of a different sucrose solution; 0.3, 0.4, 0.5, 0.6, 0.7 and 1.0
mol.dm-3; and label each tube appropriately.
2. Cut the onion and obtain 12 specimens of onion epidermis, approximately 5 mm by 5mm. Ensure that each piece is only
1 layer of cells thick. Then place 2 pieces of epidermis into each of the specimen tubes, stopper the tubes and gently
shake the contents to submerge the tissue. Note the time.
3. After 20 minutes, remove one piece of epidermis from each tube and mount it on a labelled microscope slide, beneath a
coverslip, in a drop of the solution in which it has been immersed. Observe under low power. Look for plasmolysed cells
see the figure above. If plasmolysed cells are not clearly visible in the more concentrated sucrose solutions add a few
drops of dilute iodine solution to stain the cytoplasm, or view the second epidermal sample.
4. Count all the cells visible within the low power field of view. Now count all those which are plasmolysed. Include those
which show a visible separation of the cells contents from the cell wall, however slight. If any reading is unexpected
compared to the other samples, repeat with the second piece of epidermis.
5. Record the results in a table. Plot a graph of percentage plasmolysis against concentration of sucrose solution and join
the points by straight lines.
6. Read off your graph the molarity of sucrose which corresponds to 50 per cent plasmolysis.
regarded as having the same solute potential as the cytoplasm of the tissue.
This solution may be
RESULTS ( DATA COLLECTION AND PROCESSING)
1. Recording Raw Data: Record your raw data and quantify the uncertainties.
2. Processing Raw Data: Work out any calculations you think are necessary.
3. Presenting Processed Data: Decide how it would be best to present your data.
CONCLUSION
1. Concluding: Analyse your results and explain the shape of your graph.
EVALUATION
2. The design and method of the investigation must be commented upon as well as the quality of the data. You must not
only list the weaknesses but must also appreciate how significant the weaknesses are. Comments about the precision
and accuracy of the measurements are relevant here. When evaluating the procedure used, you should specifically look
at the processes, use of equipment and management of time.
3. Improving the Investigation: Suggestions for improvements should be based on the weaknesses and limitations identified
in aspect 2. Modifications to the experimental techniques and the data range can be addressed here. The modifications
proposed should be realistic and clearly specified. It is not sufficient to state generally that more precise equipment
should be used.
MANIPULATIVE SKILLS
1. Following Instructions: Work independently to assemble your equipment, carry out the experiment orderly and follow
instructions accurately.
2. Carrying Out Techniques: Complete the experiment competently.
3. Working Safely: Work safely at all times.
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