When plant cells are placed in a solution which is exactly the same concentration as the cytoplasm, they are between turgidity and flaccidity. This is called incipient plasmolysis. “Incipient” means “starting to happen”.
Prediction
I predict that the more concentrated the solution is, the more the mass of the potato cylinders will decrease. I think this because osmosis occurs when the concentration of the solution surrounding the cell is different to the concentration of the cytoplasm. If the solution in the beaker is more concentrated, water from inside the cell will diffuse out of the potato in order to dilute the solution and balance the concentrations. When the water flows out, the potato cylinders will decrease in mass.
Variables
To ensure this experiment is fair I must take into account several aspects. Only one variable is to be changed, the rest must remain constant throughout the experiment. If this is not observed, the results would not be reliable. For instance if the potato cylinders were not equal is size, the surface areas would be different and some would have more space for osmosis to occur than others. Doing all the tests together should control the surroundings, duration and temperature.
For this experiment I will change the following:
- The concentration of the salt solutions.
For this experiment I will keep the following the same:
- Type of potato
- Surface area of potato cylinders
- Volume of solution
- Duration of experiment
- Temperature
- Solution
- Weighing scales
- Surroundings e.g. if it is in direct sunlight etc.
I will be investigating the effect of different concentrations of salt solutions on potato cylinders. There will be 5 beakers in which the different strengths of solution will be placed. I will use 0.0, 0.2, 0.4, 0.6 and 0.8 molar salt solutions.
Equipment Required
- 1.0 molar (M) salt solution
- Distilled water
- Potatoes
- Cork borers
- 15cm syringes
- 5 Identical beakers
- Electronic balance
- Square tile
- Scalpel
- Watch
- Any other normal laboratory apparatus.
Measurements
In order to keep my results as accurate as possible, appropriate measuring equipment must be used to ensure the test is as fair as possible. Each beaker will have 3 cylinders of potato in it and an average will be taken to increase accuracy and the cylinders will each be weighed on a sensitive balance before and after being placed in the solution. The balance must be zeroed after each cylinder. After first weighing the cylinders, I will place a coloured pin in each cylinder in order to tell them apart. I will remove this again before weighing the second time. This will give us 3 results for each different strength. There will be 5 beakers each containing a different strength of solution ranging from 0.0 molar to 0.8 molar of salt solution. The cylinders will be left in the solution for 3 hours.
Method
- I will first collect all the equipment needed for the experiment.
- I will take an average sized potato and using a cork borer I will cut 15 cylinders, equal in diameter. I will then cut them using a scalpel to 3cm in length on a square tile. This must be done very accurately as a change of surface area may affect the amount of osmosis which occurs.
- I then prepared the salt solutions by adding varying amounts of distilled water to varying amounts of 1.0 molar salt solution. I prepared 15cm of each strength.
To make 0M, I will use 0cm 1.0M and 15cm H O.
To make 0.2M, I will use 3cm 1.0M and 12cm H O.
To make 0.4M, I will use 6cm 1.0M and 9cm H O.
To make 0.6M, I will use 9cm 1.0M and 6cm H O.
To make 0.8M, I will use 12cm 1.0M and 3cm H O.
I will measure these using a 15cm syringe each time and will measure it directly into the beaker.
- I must then label each beaker so as to avoid confusion.
- I will then measure each potato cylinder and then place a coloured pin in it, recording the information carefully in the table shown below.
- I then will place the cylinders in the beakers and begin timing.
- While waiting I will tidy up and prepare labeled filter paper to set the cylinder on when I remove them.
- After 3 hours, I drained out the solutions and placed the cylinders on the filter paper to remove any excess solution, being careful to put the correct cylinders on the matching filter paper to avoid confusion. If the excess solution is not removed, it may affect the results depending on the amount of excess solution.
- I will remove the coloured pins and re-weigh each cylinder and record the results.
- I will the repeat the experiment if possible to increase accuracy.
Safety
All usual laboratory safety rules must be taken into account, including care whist using a scalpel or cork borer.
Diagrams
Sample Results Tables
Sample Graph
Results
I worked out the averages by adding the 3 results together and dividing by 3. I found the change in mass by subtracting the Average Initial Mass from the Average Final Mass. I calculated the % Change in Mass by dividing the Change in Mass by the Average Initial Mass and multiplying by 100.
Analysis
The graph above shows the strength of the solution against the % change in mass. As the solution gets stronger, the mass of the potato cylinders decrease. Of the 5 experiments, the most osmosis occurred at 0.8 molar salt solution and the least at 0.2 molar salt solution. The points at 0.6 and 0.8 are very similar, which shows that only a certain amount of water can diffuse out of the cylinder. After that, it reaches a point where no more water can be lost.
At 0.0 molar salt solution, there was a higher % change than 0.2 but this is a % gain rather than a % loss. This shows that osmosis occurs in pure water as well as in salt solution, but water will have diffused into the cylinder instead of out of it.
I have been able to see this by drawing a line of best fit on my graph. The curve slopes downwards but does not go through the origin and is not straight. This tells me that the % change in mass and the strength of solution are not directly proportional. The only patterns are that the stronger the solution, the more decreased in mass the cylinder is and that the curve is steeper at the top than the bottom showing that the change is more extreme at the beginning of the experiment. This is because at the bottom of the curve, the cylinder is becoming as flaccid as it possibly can.
The points on my graph are fairly even, however, the point at 0.2 molar salt solution is slightly off. This may be because of many reasons. Perhaps there was a difference in surface area in that experiment or maybe the solution was inaccurate.
From this graph I estimate that the concentration of a potato cell is approximately 0.14M, as this is where the line crosses the X-axis and the mass is constant. This is called the isotonic point, where no osmosis is occurring and the potato and surrounding solution are of equal concentrations.
Conclusion
In conclusion, when a potato cell is placed in a solution that is more concentrated than the cell itself, the mass of the cell will decrease. The more concentrated the solution is, the more the mass of the cell will decrease. Osmosis occurs in order to balance the concentrations of the cell and its surroundings, so if the cell is placed in a concentrated solution, water will diffuse out of the cell to dilute the solution, causing a loss in weight. If the cell is placed in pure water, the water will diffuse into the cell to balance the concentrations, causing a gain in mass. Osmosis moves water from areas of high concentration to areas of low concentration.
My original prediction matches this statement and my results and graph also show this information.
When the concentration reaches above 0.8M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed.
Evaluation
In my opinion this experiment was very successful as it served its purpose in showing and proving the effects of osmosis. I obtained some accurate results from which I was able to draw an informative graph. The range of concentrations I chose gave me a good series of results and the time I allowed for the experiment was long enough to allow sufficient osmosis to occur.
If I were to repeat this investigation, I would perhaps leave the experiment longer to allow more osmosis to occur in the hope of more significant results and possibly find out the saturation point of the potato cylinders. I could also increase the number of different concentrations which would give me a vast variety of results. This would allow me to find the isotonic point much more accurately than what I estimated, which was very approximate. I would also cut the potato cylinders more accurately because although I was recording the mass, the surface area still plays an important part in keeping the test fair. The potato cylinders should be of equal weigh, size and shape. As well as the potato, the solutions could be measured more accurately by perhaps using a burette to determine the molar concentrations. This would ensure an accurate amount of liquid in each beaker. I could also weigh the cylinders on a more accurate scale, for instance 0.0000g rather than 0.00g.
I could perhaps find a more accurate way of removing the excess liquid from the potatoes after the experiment which is more accurate. Also, I did not have time to repeat the experiment a second time. This would be a good idea if I were to do the experiment again.
Only one of my results seemed to be slightly inaccurate. This could have been because of a number of reasons. Maybe I didn’t cut the cylinders equally or the solution was inaccurate. Perhaps I did not dry it as thoroughly as the other, which would have added to the mass.
With all this said I think my experiment went very well and I am pleased with the outcome.
Jennifer Humphrey