Skill Area P: Planning and Experimental Procedures
Aim
The aim of this experiment is to demonstrate the process of osmosis in potato tissue.
To do this I will cut strips of potato, immerse them in various concentrations of sucrose solution and examine the change in length of the strips, if any.
Preliminary Study
To set up a range and interval for my experiment I carried out some preliminary work to give me an indication of what the change in length would be like. I used three concentrations, 0, 0.5 and 1.0 molar (1M) and immersed three 50mm strips in each solution for 15 minutes.
The results are shown in the following table: -
Molarity
Length after immersion
Average length after immersion
Change in length
Percentage change
st
strip
2nd
strip
3rd
strip
0
53mm
50mm
47mm
52mm
2mm
4%
0.5
51mm
49mm
49mm
50mm
0mm
0%
.0
52mm
51mm
48mm
48mm
-2mm
-4%
In this preliminary study, the equilibrium concentration, that is where there is no change in length, is approximately 0.5M. A range of concentrations either side of this value should therefore give me a satisfactory set of results.
Apparatus/Materials
6 test tubes
Test tube rack
Scalpel
Cutting board
Vernier
20cm3 Graduated pipette
Stopwatch
Various sucrose solutions
Potato
Distilled water
Planned method
I will first obtain 5 sucrose solutions - 0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M. (These are all available from stock and made up by science technicians). I will use a graduated pipette to accurately measure 20cm3 of each solution and place in a test tube. The tubes will be labelled and placed in a test tube rack to ensure they do not get mixed up.
Then, using a vernier, I will measure and cut 15 strips of potato 50.00mm long, 3 for each concentration. I will use 3 strips per concentration, as this will give me more representative results.
The three strips will be placed in each solution and left for 20 minutes. After this time, the test tubes will be drained and the strips will be laid out on paper towels and re-measured immediately. These measurements will be recorded in a table, as this is the best way to record results clearly, accurately and easily.
Safety measures
For safety, the test tube rack will be kept well away from the edge of the table. To prevent injuries when the scalpel is being used, the cutting will be done well away from large numbers of people and on a cutting board.
Identification of variables and ensuring a fair test
To make this experiment fair and accurate, I will need to identify the variables to find out which ones should be kept constant. The variables are: -
* Volume of sucrose solution
* Length of potato strips at the beginning of the experiment
* The concentration of the sucrose solutions
* The length of the potato strips at the end of the experiment
* The temperature of the solution
* The light intensity
* The type of potato
* The time the strips are immersed in the solution
I am only altering the concentration of the sucrose solutions so this is my manipulated independent variable. The outcome of my experiment is the length of the potato strips at the end of the experiment. This is my dependant variable. All other variables are my constant independent variables and should not be changed in order to keep the test fair.
To make sure the same volume of sucrose solution is used throughout my experiment a 20cm3 graduated pipette will be used. To make sure the potato strips are of equal length; I will be using a vernier, as this is able to measure to a greater accuracy of 1/100th of a millimetre. The experiment will be conducted at room temperature. To keep the light intensity the same, the room lights will be kept on throughout the experiment. I will be using the same potato throughout and I will not wash the potato, which could alter the water potential. ...
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To make sure the same volume of sucrose solution is used throughout my experiment a 20cm3 graduated pipette will be used. To make sure the potato strips are of equal length; I will be using a vernier, as this is able to measure to a greater accuracy of 1/100th of a millimetre. The experiment will be conducted at room temperature. To keep the light intensity the same, the room lights will be kept on throughout the experiment. I will be using the same potato throughout and I will not wash the potato, which could alter the water potential. Finally, I will keep the potato strips immersed in the solutions for the same amount of time.
Hypothesis + Predictions
Water molecules can move through plant tissues by a process called osmosis. In "GCSE Biology", osmosis is defined as,
"The passage of water across a partially permeable membrane from a dilute to a concentrated solution."
I have explained this process simply in the following diagram.
A plant cell has a cell wall, a cell membrane and a vacuole containing cell sap. The cell wall is fully permeable to water and dissolved substances. In a plant cell it is the cell membrane which acts as a partially permeable membrane i.e. one that is permeable to molecules of water but not to molecules of solute.
Osmotic relationships of plant cells are generally described in terms of water potential. The water potential of a system is the "tendency of water to leave a solution" ("Understanding Biology").
Water potential is measured in Pascals (Pa). Pure water has the highest water potential of 0KPa. Solute molecules prevent water from leaving a solution and make it have lower water potential i.e. a negative value. The more concentrated the solution, the more negative the water potential. Water will diffuse from a region of higher to a region of lower water potential.
I predict that the potato strips in the pure water will increase in length because I believe the cell sap in the potato is more concentrated than the external solution giving it a more negative water potential. Since it has a more negative water potential, water will enter the cell by end-osmosis. As the potato cells take in water and swell, they will become more turgid. A turgid cell is one where the vacuole is pushing out on the cell wall and exerting a turgor pressure. This pressure is as a result of taking in water.
As cells on the surface of the potato take in water, they will gain in water potential thus water from these cells will pass by osmosis to neighbouring cells down an osmotic gradient.
As all the cells swell, this will make the overall size of the strips increase.
"Lowson's Botany" states that the dynamic equilibrium of a typical plant cell is equivalent to a sucrose solution of 0.4M. Therefore I predict that in the solution of concentration 0.4M, there will be little or no net movement of water between the cell and the sucrose solution and there will be no change in length of the potato strips at this concentration. This stage is termed incipient plasmolysis and the cells in the potato tissue would look like this:
I also predict the strips in the 0.2M solution will increase in length as the cells in the potato strip will still have a lower water potential than the external sucrose solution. As the difference between the water potential is less however, I think less water will be taken in than in the 0M solution and the increase in length will be smaller.
In solutions stronger than 0.4M, I predict the potato strips will decrease in length. This is because above 0.4M the external solution will be of a higher concentration than the cell sap. The sucrose solution will have a more negative water potential than the cell. This means that water will leave the cell by ex-osmosis and give a plasmolysed state to the cell. A plasmolysed cell is one where the protoplast is completely pulled away from the cell wall (see diagram below).
As the concentrations increase, the difference between the two water potentials of the cell and the solution will increase so I predict more water will leave the cell.
Skill Area O: Obtaining Evidence
I followed my original plan and did not make any changes to the proposed method. I used all the apparatus listed in the planning section. All safety procedures were carried out and no-one was injured during the experiment.
All results obtained are shown in the table below.
Results
Molarity
Length at end of experiment (mm)
Average length after immersion (mm)
Trial 1
Trial 2
Trial 3
0
53.46
53.24
52.40
53.03
0.2
51.52
52.63
52.24
52.13
0.4
49.96
50.71
49.68
50.12
0.6
48.64
48.13
48.84
48.54
0.8
48.45
48.39
48.15
48.33
.0
47.80
48.09
48.23
48.04
Skill Area A: Analysing Evidence and Drawing Conclusions
In the table below, I have calculated the actual change in length and the percentage change in length of the potato strips to enable me to plot the graph on the next page.
The percentage change was calculated by dividing the average change in length by the original length (50.00mm) and multiplying by 100.
Molarity
Average change in length (mm)
Percentage change
0
3.03
6.09
0.2
2.13
4.26
0.4
0.12
0.23
0.6
-1.46
-2.93
0.8
-1.67
-3.34
.0
-1.96
-3.92
The experiment showed that the concentrations of sucrose solutions did affect the osmotic activity in the potato strips.
My results show: -
. That osmosis actually took place in this experiment since the potato strips changed in length.
2. Pure water produced the greatest increase in length. Increasing the concentration of sucrose solution decreased the overall increase in length of the potato strips.
3. There was an equilibrium concentration of sucrose, which produced no overall change in length. My graph showed this to be at 0.41M.
4. In concentrations greater than 0.41M, the potato strips decreased in length.
5. In concentrations less than 0.41M, potato strips increased in length.
These results confirm all my original predictions. The process of osmosis causes water to pass through the partially permeable cell membrane of the potato cell from a region of low concentration to one of high concentration. When the sucrose concentration is higher than the solute concentration of the potato cell, water passes from the cell causing the strip to contract. When the sucrose concentration is lower than the solute concentration of the potato cell, water passes into the cell causing the strip to enlarge.
In terms of water potential, in the hypotonic sucrose solution, the water potential is higher than in the cell so water enters the cell (because water will flow from a high water potential to a lower). This makes the cells swell and the potato strip to increase in size overall. In the hypertonic sucrose solution, the water potential is lower than in the cell so water leaves the solution. This makes the cell shrink and the size of the potato strip to decrease overall.
There is a point where there is no overall change in length of the potato strips. This equilibrium point is where the water potential inside the cell is equal to water potential of the solution outside the cell. The two solutions inside and outside the cell are said to be isotonic. On my graph this point is 0.41M and using the graph on the next page, I have been able to calculate an approximate water potential value of the cell. I have found this to be approximately -1100KPa.
At first there is a substantial change with the potato strip increasing by 6.09% (indicating there must be a big difference between the water potential in the cell and in the solution). The first two points are spread out indicating there was a large change in length. Towards the end, the last few points are very close together and the curve is levelling out. This indicates that there is a limit to the amount of water the cell can lose. This means that the potato strip would soon stop decreasing in length. From my graph I estimate this to be when the decrease is 4% which means when there is an overall decrease in length of 2mm.
Skill Area E: Evaluating Evidence
The investigation went well as results supported the original predictions. The process of osmosis was found to occur and the percentage change in length of the potato strip was found to decrease with increased molarity of the sucrose solution.
The results appear to be quite accurate as most fall neatly on the curved line of best fit. There is one anomalous result which was obtained from the strips in the 0.8M solution. I believe this might be due to inaccuracies during the experiment: -
* The test tube containing the 0.8M test tube cracked and some liquid may have been lost when transferring this to another test tube. This might have slightly reduced the amount of sucrose solution in the test tube
* Human error
* Incorrect measuring using the vernier
* I may have damaged the potato tissue whilst preparing it.
Since all measurements taken from the strips in the 0.8M solution were very similar (48.45mm, 48.39mm, 48.15mm) I believe the anomalous result was caused by a factor that affected them all i.e. the cracking of the test tube.
In spite of the anomalous result, I am satisfied that the results confidently prove the predictions. I believe the experiment was as accurate as could be expected using the equipment available.
Also, there were several difficulties I came across in the experiment method:
* The vernier was difficult to use, as it was the first time I had used this equipment.
* Difficulties also arose when trying to make sure all potato strips were immersed for the same amount of time as it was impossible to place all 15 strips in the solutions at the same time. To try and overcome this and make it as fair as possible, the strips which I put in first, I took out first etc.
* It was difficult to keep the surface area constant as time restrictions meant width and depth were hurriedly measured.
If I were to redo this investigation I would suggest the following changes:
* I would make sure a number of people were placing the strips in the solutions reducing the risk of one set of strips having more time for osmosis to occur.
* I would use a cork borer as this would give me a uniform shape and keep the surface area constant.
* To improve the reliability of my results, I could increase the number of concentrations by going up in intervals 0.1M not 0.2M (i.e. 0M, 0.1M, 0.2M, 0.3M...). This would give me more points to plot to improve my line of best fit.
* If I didn't have time restrictions, I would leave the potato strips in the solutions for 24 hours, as this would give more time for osmosis to occur and stabilise.
* Finally I would put strips in separate test tubes, as this would prevent the risk of strips touching and reducing the surface area for osmosis.
To provide additional evidence for conclusions, I could repeat this investigation measuring the mass of the potato strips because this would mean that all over change was taken into account (change in all 3 dimensions).
I could also investigate the effect of various concentrations of sucrose solution on different types of potatoes (e.g. sweet potato, new potato). Also I could compare the effect of various concentrations of sucrose solution on different types of vegetables (e.g. swede, parsnip, and carrot) and also see if the dynamic equilibrium varies with different tissues.
References
GCSE Biology D.G.Mackean
Understanding Biology G.Toole and S.Toole
Lowson's Botany Simon, Dormer and Hartshorne
Internet http://www.biology.demon.co.uk/
Biology/mod1/osmosis/osmosis.htm
Class-work notes
