There should also be a dynamic equilibrium where, at a certain concentration of sucrose solution, osmosis does not occur at all. This is because there is an equal concentration of water inside the potato as well as outside it.
Method
- Peel a potato so there is absolutely no skin left
- Cut out a cube of potato 1cm x 3cm x 3cm using a knife
- Measure the mass of the piece of potato using a mass balance
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Measure out the volume of the sucrose solution using a 25cm3 measuring cylinder
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Measure out the volume of water using a 25cm3 measuring cylinder
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Pour both of the liquids into a beaker and stir them thoroughly. Together the solutions should both equal a volume of 20cm3. The amount of water and 1 molar sucrose solution will be altered to alter the concentration of the sucrose solution. The concentration of the solution will range from 20cm3 of the 1 molar sucrose solution, in other words all sucrose solution, to 0cm3 of sucrose solution, i.e. all water. The concentrations that will be used are: 0mol, 0.25mol, 0.5mol, 0.75mol and 1 mol of sucrose solution.
- Place the potato in the petri dish and carefully pour the sucrose solution over it
- The solution and potato are left uncovered for 20 hours on a tray
- The potato is taken out of the solution and weighed using a mass balance
- This experiment will be repeated using different strengths of sucrose solution.
- The whole investigation will be repeated at a later date but under the same condition in order to compare results to find an average.
Fair test
To make sure that the experiment is a fair test the following actions are taken:
- The length of time the anyone potato piece is in the solution is 20 hours.
- The volume of solution in the petri dish is always the same
- The measurement of the potato is always the same
- The potato is always skinless
Obtaining Evidence
After steadily changing the concentration of the sucrose solution I found out the following results before repeating the experiment another time.
A Table To Show the Change in Mass of Potatoes in Different Concentrations of Sucrose Solutions for the first experiment
A Table To Show the Change in Mass of Potatoes in Different Concentrations of Sucrose Solutions for the second experiment
For the next table I calculated the averages of the change in mass of potatoes in different concentration of sucrose solution of the first two experiments. However, when working out the averages I did not include the anomalies. An anomaly is something that you do not expect; an irregularity.
A Table To Show the Change in Mass of Potatoes in Different Concentrations of Sucrose Solutions for the average of the two experiments
A Table to Show the estimate of the Isotonic Point of pieces of Potato in a Sucrose Solution
Analysis
I found out that as the concentration of sucrose in the solution increases then the mass of potato decreases. The results suggest my prediction may be correct. This is shown on the graphs on the following pages.
The mass of the potatoes increases in the 0.25molar sucrose solution. This means that at this concentration there is more water outside the potatoes than inside them. Due to this water enters the potatoes cells in an attempt to equal the amount of water inside and outside of the potato, the potato cells become turgid (i.e. full of water), the cell has gained mass and does not burst because it has a strong cell wall; lysis does not occur at any concentration by looking with the naked eye. This has happened because water molecules diffuse down a concentration gradient, from high to low concentration of water.
However, the potatoes in solutions above the concentration of 0.25molar sucrose solution have had a decrease in mass as water leaves the potatoes cells as there is less water outside of the potatoes, they become flaccid i.e. when the vacuole shrinks because it gives out water and the cell is no longer firm, it is limp. Incipient plasmolysis (as the vacuole gives out to much water the cytoplasm starts to peel away from the cell wall) does not occur at any concentration by looking with the naked eye.
In conclusion, as the concentration of sucrose in the solution increases then the mass change of a potato decreases from the mass change when placed in 0molar sucrose solution. This should be valid for all of the concentrations of the sucrose solution up to 1molar sucrose solution. At a concentration between 0.25mol and 0.3mol dynamic equilibrium occurs; i.e. at this point the potato neither increases or decreases in mass as the concentration of the water in the potato cells is the same as that in the sucrose solution so there is no movement of water molecules between the two, i.e. there is no concentration gradient as the potato has reached its isotonic point. Unfortunately I could not manage to establish the precise concentration of the sucrose solution dynamic equilibrium. Therefore between these two points the water concentration inside the potato is equal to the water concentration outside the potato so no water moves across the potatoes semi-permeable membrane. However, from my graph it can be interpreted as being 0.26molar sucrose solution.
Evaluation
I believe that the experiment went well and that the results I obtained were as accurate as possible. The concentration of the solution was experimented every 0.25 moles, including 0 and 1. Two more concentrations were also experimented on these were 0.24M and 0.3M solution, these were taken as it was estimated that either one of these or a value between the two would make the potato stay the same mass and size therefore no osmosis occurring as the concentration of water in and outside the potato would be the same. For some reason two measurements, 0.25M and 1M solution, had a higher or lower mass than was expected in the trend. These are called anomalies.
I could have obtained better results if I had performed more experiments, to do this I would have needed more time; more tests would have made the average more accurate. With more time I could have experimented with more concentrations. With these extra concentrations a trend in the results would be easier to see. If a microscope was used to view the potato cells after each experiment we may have noticed if the cells were either plasmolysed or had under gone lysis.
As you can see the experiments were repeated twice, each time the results are similar. This should show that they are reliable enough to make a firm conclusion.
There are a number of alterations that would allow us to investigate further into osmosis.
- Test how different semi-permeable membranes could effect osmosis. This could be done by using other organisms instead of a potato.
- Investigate the concentration at which the potatoes will undergo plasmolysis.
- Investigate how changing surface area of the potato pieces and different volumes of solution affect the rate of osmosis.
- Investigate what affect temperature has on osmosis
- Investigate what affect sunlight has on osmosis
- Investigate the size of the pieces of potato that osmosis occurred most rapidly
- Investigate how long the poato piece had to be in a certain solution before it plasmolysed
- Investigate how long the poato piece had to be in a certain solution before it under went lysis
