Required Practical- Osmosis and the Effect of Solution Concentration on Plant Tissue
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of osmosis and plant cell structure to interpret experimental results and draw conclusions.
What is osmosis?
The movement of water from a dilute to a concentrated solution across a partially permeable membrane
Why do plant cells not burst in pure water?
Their cellulose cell wall provides strength and structural support which prevents bursting
What happens to a plant cell in a concentrated solution?
Water leaves the cell by osmosis. The cell loses mass and may become plasmolysed
Topic Explainer Videos
Check out these @JoeDoesBiology videos that explain the required practIcal- Osmosis and the effect of solution concentration on plant tissue, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Required Practical - Osmosis
Aim:
- To investigate the effect of different concentrations of sugar solution on the mass of plant tissue due to osmosis and to identify the concentration of sugar inside the plant cells.
Method Summary:
- Cut equal-sized cylinders of potato using a cork borer and a ruler.
2. Measure the initial mass of each cylinder using a balance.
3. Place each piece in a test tube containing a different concentration of sugar solution.
4. Leave for at least 30 minutes (or longer for best results).
5. Remove, blot gently to remove surface solution, and re-measure the final mass.
6. Calculate the percentage change in mass:
|
Sugar Solution Concentration (mol/dm³) |
Initial Mass (g) |
Final Mass (g) |
Change in Mass (g) |
% Change in Mass |
|
0.0 (pure water) |
1.50 |
1.65 |
+0.15 |
+10.0% |
|
0.2 |
1.50 |
1.58 |
+0.08 |
+5.3% |
|
0.4 |
1.50 |
1.50 |
0.00 |
0.0% |
|
0.6 |
1.50 |
1.42 |
-0.08 |
-5.3% |
|
0.8 |
1.50 |
1.35 |
-0.15 |
-10.0% |
|
1.0 |
1.50 |
1.30 |
-0.20 |
-13.3% |
Independent variable (the one you change) = Concentration of sugar solution.
Dependent variable (the one you measure) = Change in mass / % change in mass.
Expected Results:
- In dilute solutions (e.g. pure water):
Water enters the potato by osmosis → mass increases. - In concentrated solutions:
Water leaves the potato by osmosis → mass decreases.
Control Variables & Improving Validity
To ensure that the experiment is fair and the results are valid, all variables except the concentration of the solution must be controlled. It is really important that you include controlled variables when planning any investigation.
Key Control Variables in this practical:
|
Variable |
How to Control |
|
Size and shape of potato |
Use the same cork borer and cut pieces to the same length with a ruler, to control the surface area:volume. |
|
Volume of solution |
Use the same volume of solution (e.g. 10 cm³) in each test tube. |
|
Time in solution |
Leave all pieces in solution for the same amount of time (e.g. 30 minutes). |
|
Temperature |
Conduct the experiment at room temperature, or use a water bath set at the same temperature throughout. |
|
Surface drying |
Remove the excess solution from the surface by blotting the cylinders with a paper towel. Not doing this would introduce error into the final mass measurement. |
|
Same potato source |
Use the same type or potato to reduce variation in plant tissue. |
Improving Validity:
- Repeat the experiment at least three times at each different concentration and calculate a mean to reduce the effect of any random errors.
- Any obvious anomalies should be discarded and not included in the mean calculation.
- Use percentage change to be able to compare the potatoes even if they had different starting masses.
Graphing Your Results
- Plot percentage change in mass (y-axis) against concentration of sugar solution (x-axis).
- The point where the curve crosses 0% change represents the concentration inside the potato cells.
- We know this because there was no gain or loss of mass, therefore no net movement of water by osmosis. This indicates the concentration of the sugar solution must have been equal to the concentration inside. the potato cells.
- In the graph below the concentration inside the potato is 0.27 mol dm-3
Key Term
- Osmosis – diffusion of water across a partially permeable membrane, from a dilute to a more concentrated solution.
No answer provided.
Exam Tips
- Dry the potato cylinders carefully before reweighing to remove excess solution stuck on the outside, this avoids inaccurate mass readings.
- Plot % change – not just final mass – so results are comparable between potato cylinders.
- Always say repeat and calculate a mean.
In your exam, they may use potatoes, carrots or even eggs with their shells removed! You can still apply your knowledge of this practical and of osmosis to these different contexts!
No answer provided.
Practice Questions
A student places potato cylinders in sugar solutions of different concentrations. In the 0.8 mol dm-3 solution the potato lost mass? Explain why. (3 marks)
Model Answer:
- Water leaves cells by osmosis,
- Across the partially permeable cell membranes,
- As the concentration inside the cells is more dilute than the concentration of the solution outside.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!