Required Practical 3 Osmosis
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of osmosis. You can test your knowledge on this below.
What is osmosis?
Osmosis is the diffusion of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
What happens to an animal cell when placed in a solution with a higher water potential?
Water enters the cell via osmosis, causing it to swell and potentially burst (lysis) due to the lack of a cell wall.
What happens to plant cells when placed in a hypertonic solution?
Water leaves the cell by osmosis, causing the cytoplasm to shrink away from the cell wall (plasmolysis).
Topic Explainer Video
Investigating Osmosis in Plant Cells
Aim:
To investigate the effect of different concentrations of a solution on the mass of plant tissues (e.g., potato cylinders).
Method:
- Use a cork borer to cut cylinders of potato tissue with the same diameter.
- Use a ruler to cut all cylinders to the same length. This ensures all potato cylinders are the same size and have the same surface area. Remove any skin.
- Measure and record the initial mass of each potato cylinder.
- Place each potato cylinder into different sucrose solutions of known concentration (e.g., 0.0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1.0M).
- Leave the cylinders in the solution for a set time (e.g., 30 minutes).
- Remove, blot dry, and remeasure the mass.
- Calculate the percentage change in mass. Using the equation: (difference / original) x 100.
- Plot results on a graph with concentration of sucrose on the x-axis and percentage change in mass on the y-axis.
Results & Interpretation
- Increase in Mass: The solution has a higher water potential than the potato (hypotonic), so water moves in by osmosis.
- No change in Mass: The solution is isotonic to the potato, meaning the water potential is equal inside and outside the cells, so there is no net movement of water. On the graph above, the solution which is isotonic to the potato tissue has a concentration of 0.28 mol dm⁻³.
- Decrease in Mass: The solution has a lower water potential than the potato (hypertonic), so water moves out by osmosis.
How can you use your graph to find the water potential of the potato tissue?
- Find the point where the percentage change in mass is 0. This will be where the line of best fit crosses the x-axis. On the graph above, this is at a sucrose concentration of 0.28 mol dm⁻³.
- At this concentration, the solution is isotonic to the potato tissue, therefore there was no net gain or loss of water by osmosis and no change in mass.
- The water potential that equates to this concentration of sucrose can then be looked up in a table.
- This will be the water potential of the potato tissue.
Key Points
Why are the potato cylinders blotted dry after removing them from the sucrose solution?
- To remove excess sucrose solution on the surface of the potato cylinders that could artificially increase mass measurements.
- Ensures that only the water absorbed by osmosis is measured.
Why is percentage change in mass calculated instead of just mass change?
- Different potato cylinders may have different starting masses.
- Percentage change standardises the results and allows for comparison.
Controlled variables (to ensure a valid experiment):
- Same volume of sucrose solution for each potato cylinder.
- Same temperature (osmosis is affected by temperature).
- Same duration for all potato cylinders in the solution.
- Use of the same type/age of potato (as different potatoes may have different initial water potentials).
- Same surface area of potato cylinders (cut to equal size with a cork borer).
- All potato cylinders have all skin removed- as skin will act as a barrier to the movement of water by osmosis.
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Key Terms
- Water potential (Ψ): The tendency of water to move from one place to another.
- Hypotonic solution: Higher water potential than the cell; water enters the cell.
- Hypertonic solution: Lower water potential than the cell; water leaves the cell.
- Isotonic solution: Water potential inside and outside the cell is equal.
- Plasmolysis: Shrinking of the cytoplasm due to water loss in a hypertonic solution.
- Turgid: When a plant cell is swollen due to water uptake.
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Exam Tips
When explaining osmosis, always refer to water potential rather than concentration. Examiners expect precise terminology at A-level.
Make sure you can explain how to use the graph plotted in this investigation to find the water potential of potato tissue.
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A student investigated osmosis by placing potato cylinders into different concentrations of sucrose solution. After 30 minutes, they recorded the percentage change in mass.
(a) Explain why the potato cylinders in the 0.2M sucrose solution increased in mass. (2 marks)
(b) Explain why the potato cylinders in the 0.8M sucrose solution decreased in mass more than the potato cylinders in the 0.6M sucrose solution. (3 marks)
(a) The 0.2M sucrose solution has a higher water potential than the potato cells.
Water moves into the cells by osmosis, causing an increase in mass.
(b) The 0.8M sucrose solution has a lower water potential than the potato cells.
Water moves out of the cells by osmosis, causing a decrease in mass.
There is a steeper water potential gradient at 0.8M than at 0.6M so the rate of osmosis is faster and more water moves out by osmosis.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!