After 40 minutes, forceps were used to remove the 2 cylinders of potato from the specimen tubes. These cylinders were then blotted with filter paper to remove excess solution. New filter paper was then placed on the balance and the final mass was then recorded to 2 decimal places. The change in mass and the average % change (to 1 decimal place) in mass of potato tissue were then calculated.
Results
A table to show the change in mass of potato cylinders placed in solutions of different sucrose concentrations.
Conclusion
The graph shows that the Ψ of the potato is about 0.15M because this is where the line crosses the x-axis and therefore this should be the point where there is no change because in theory this is where the Ψ of the potato is the same as the Ψ of the sucrose solution. However in practice, this is likely to be inaccurate because there are only two points on the graph (at 0.0M and 1.0M) so any mistakes could seriously distort the graph.
Evaluation of Preliminary Experiment
I think that this experiment could be quite inaccurate because there are only two points on the graph and if these are very inaccurate, the results could be skewed and if this is the case, the supposed Ψ of the potato would be totally wrong. I think there was also a great possibility of error because the experiment was only repeated twice and with only two different concentrations the Ψ will not be entirely accurate. Also the experiment was only left for 40 minutes so osmosis may not have finished when the ‘final mass’ was taken.
Background Theory and Prediction
Osmosis is a specialised for of diffusion. It is the movement of H2O molecules from an area of high water potential (Ψ) to an area of low Ψ through a partially permeable membrane. Osmosis is a passive process. This means that it does not need to have ATP energy. Movement of H2O molecules is as a result of the molecules having kinetic energy. They will continue until they have reached equilibrium.
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Equilibrium
At equilibrium, both solutions either side of the partially permeable membrane have identical Ψ. As the H2O molecules still have kinetic energy, they can still move but if one H2) molecule moves from the right hand side of the partially-permeable membrane, another molecule moves across from the left hand side to maintain the balance. N.B. In osmosis, only the H2O molecules are small enough to pass through the partially permeable membrane. Sucrose is too big to pass through.
Osmosis in plant cells
Water moves into the cell by osmosis because the Ψ inside the potato cell is less than the Ψ of the 0.0M solution. . The volume of liquid inside the cell increases and thus the pressure exerted by the cell membrane exerted on the cell wall is increased as well.
When this happens, the cell is TURGID.
If a plant cell is placed in a concentrated sucrose solution, it will lose water by osmosis. This is because the Ψ of the sucrose is lower than the Ψ of the cell cytoplasm, so water moves out of the cell.
This will cause the volume of the cytoplasm to decrease. The membrane is pulled away from the cell wall as its contents shrink. The cell is then said to plasmolysed.
At the point between turgidity and plasmolysis, a cell is said to be flaccid. The plant cell’s wall is not involved in osmosis because it is freely permeable. A cell becomes flaccid when it is placed in a solution which has the same Ψ as the cytoplasm/vacuole. This is because although the H2O molecules are continually moving due to kinetic energy, there is no net change in the volume of either the cell or in the solution. This is equilibrium.
Prediction
My prediction is that the water potential of the potato will be equivalent to a 0.2M (allowing for experimental error). I predict this because according to my graph for the preliminary osmosis experiment, the line on my graph crosses the x-axis between 0.1M and 0.2M (at 0.15M). However this prediction will not be totally accurate because there were only two points on this graph so the results may be distorted and the line may cross the x-axis at a different point but this is the only information available to me.
Apparatus
Goggles, Borer, 10.0 cm³ measuring cylinder, glass rod, scalpel, 45x filter paper, a ruler, 30.0 cm³ 0.1M sucrose solution, 30.0 cm³ 0.2M sucrose solution, 30.0 cm³ 0.3M sucrose solution, 30.0 cm³ 0.4M sucrose solution, 30.0 cm³ 0.5M sucrose solution, 1x potato, balance, washable OHP pen, 5 forceps, 15x specimen tubes with lids, clock/timer, white tile to cut on when using scalpel to cut cylinders of potato.
Diagram
Method
Using a borer, cut sections of potato from a potato and then extract them from the borer using a glass rod. Then remove the skin of the potato off the ends (using a scalpel on the white tile) to ensure a fair test is carried out. Now divide these cylinders into fifteen 2.0cm long cylinders of potato (again on the white tile). Cut them again and again until there are 75 x 0.4 cm cylinders of potato (do this to create a greater surface area for osmosis to occur). Place a clean piece of clean filter paper on the balance and tare the balance. Then mass each pair of cylinders twice (to 2 decimal places) to make sure that the mass recorded is as accurate as possible. It is imperative to keep each group of cylinders together because if they get mixed up, they will have a different mass and/or density and this will affect the results because the percentage change could become significantly greater or less. Take the different concentrations of solution, and measure out 10.0 cm³ of each solution (01M, 0.2M, 0.3M 0.4M, 0.5M) using a measuring cylinder. Then add a group of cylinders to each specimen tube and label the specimen tube with the cylinder’s initial mass and the solution’s concentration with a washable OHP pen. Then leave the tubes overnight for allow time for the process of osmosis to be completed.
For each specimen tube, use a different pair of forceps to remove the group of cylinders of potato from each tube. Then remove all excess solution from the cylinder by blotting the potato dry with a clean piece of filter paper. Place another clean piece of filter paper on the balance and tare it. Then mass each group of cylinders twice to make sure of the final mass and record to two decimal places. Finally calculate the change in mass for each beaker, the % change in for each beaker and also the average % change in mass for each concentration of sucrose (0.1M, 0.2M, 0.3M, 0.4M and 0.5M solutions)
Fair Test and Safety
To ensure a fair test:
- Do not mix up the cylinders of potato.
- Put lids on the specimen tubes to prevent contamination.
- Cut the skin off the end of the potato cylinders as they have a ‘waterproof’ layer which prevents osmosis from occurring properly.
- Make sure there is the same volume of solution in each specimen tube.
- Ensure that the potato cylinders are kept in the solutions for approximately the same amount of time.
- Ensure that the potato cylinders are of the same size and have the same surface area.
To perform the experiment safely:
- Wear goggles and take care when handling the sucrose as it is an irritant.
- Take care when using the scalpel as it is sharp.
Results
