To Investigate the Rate of Osmosis In Potatoes and Find Out the Sucrose Concentration of the Potato.
INTRODUCTION
Aim:
To investigate the rate of osmosis in potatoes and find out the sucrose concentration of the potato.
Osmosis is the movement of water molecules from a solution of high water potential to a solution of low water potential across a partially permeable membrane. Water potential is the ability at a solution to lose water. A concentrated solution has a low water potential and a dilute solution has a high water potential. The diagram below illustrates the concept of osmosis.
The diagram above shows a concentrated sucrose solution and a dilute sugar solution separated by a partially permeable membrane. This membrane allows small molecules, such as water, to pass through it but it does not allow larger molecules like sucrose through it. The water moves from the right to the left so that the sucrose concentration on the left is decreased. The concentrated solution becomes more dilute due to the extra water molecules entering it.
In plant cells the cell changes when the concentration around it changes. A plant cell can become turgid or flaccid. When the concentration of water is higher outside the cell than inside the cell, water will move by osmosis down its concentration gradient, through the cell membrane and into the cell. As water goes in the vacuole and cytoplasm will swell. As the cell has cell wall, which is fully permeable, it will not burst. The cell's volume and mass will increase. The cell will expand to its maximum size and it is then said to be in a turgid state.
When the concentration of water outside the cell is lower than inside the cell, the opposite happens. Water moves out of the cell by osmosis and into the concentrated sugar solution. The cell decreases in volume and is said to be flaccid. If an excessive amount of water is lost from a cell, the cell membrane will pull away from the cell wall. The cell is then said to be plasmolysed.
Turgid Cell
> Higher water concentration outside cell
> Water diffuses into cell
> Cell expands and its volume increases
> Cytoplasm presses against cell wall
> Cell becomes turgid
Flaccid cell
> Lower water concentration outside cell
> Water diffuses out of the cell
> Cell decreases in volume
> Cell becomes flaccid
> Cell membrane pulls away from the cell wall
PRELIMINARY EXPERIMENT
A preliminary experiment was carried out to found out the range of different sucrose concentration solutions that were to be used for the main experiment. A smaller and more accurate range needed to be found in order to find the concentration of the cell sap in the potato, which was going to be done in the main experiment. There were several variables that needed to kept constant during the main experiment. The exact measurements of these variables needed to be decided upon on the basis of the preliminary results.
Aim: To find out the range in which the concentration of the cell sap lay in by investigating the process of osmosis of pieces of potato in different sucrose solutions.
Apparatus:
- Potato x 1
- Pair of borers
- Knife
- White tile
- Test tubes x 6
- Test tube rack
- Sucrose solutions (0.2, 0.4, 0.6, 0.8, 1.0 molar)
- Syringe (10ml) x 1
- Forceps
- Filter paper
- Electronic weighing scale
Method:
. 6 tubes were cut from the potato using the pair of borers.
2. Each potato cylinder was cut into 4cm lengths using the knife on the white tile.
3. Each of the potato cylinders were weighed and their masses were noted
4. The test tubes were each half filled with different sucrose solutions using the syringe. The concentrations of these were 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 molar.
5. Each test tube was marked with the concentration of the sucrose solution inside it.
6. Each potato cylinder was placed into each test tube using the forceps.
7. The test tubes were left for 24 hours.
8. After 24 hours, each potato cylinder was removed from the test tube using the ...
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3. Each of the potato cylinders were weighed and their masses were noted
4. The test tubes were each half filled with different sucrose solutions using the syringe. The concentrations of these were 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 molar.
5. Each test tube was marked with the concentration of the sucrose solution inside it.
6. Each potato cylinder was placed into each test tube using the forceps.
7. The test tubes were left for 24 hours.
8. After 24 hours, each potato cylinder was removed from the test tube using the forceps.
9. The surface liquid from the potato cylinders was dried off using filter paper
0. In turn, each potato cylinder was weighed again and there masses noted in the results.
Results:
The table below shows the mass before and mass after of the potato cylinders for the preliminary experiment. The change and percentage changes were calculated.
Graph:
Conclusion:
From the preliminary results you can see that as the sucrose concentration increases, the % mass change decreases. I have drawn a line of best fit to indicate this correlation. At the concentrations of 0.0 and 0.2 M the potato gained in mass and at the concentrations of 0.4, 0.6, 0.8 and 1.0 M the potato lost mass. Water either moved in to or out of the cells by osmosis due to the concentration of the external sucrose solution. From the preliminary experiment I needed to find out what conditions would be best for the main experiment. The point at which the line crossed the x-axis should give the concentration of the cell sap. From the line and the line of best fit this value is between 0.28 and 0,34 M. This range helped me to plan the main investigation. The potato cylinders used gave good results with mass changes upon which I was able to spot a trend. Therefore these conditions didn't need to be changed.
Planning for the main investigation
The following variables needed to be kept constant in the main experiment in order to keep it a fair test. The values for these variables were based upon the preliminary results. The variables were:
- Length of potato cylinder
- Volume of sucrose concentration
- Range of concentrations to be used
Length: 4 cm
In the preliminary I used 4cm lengths. This gave results from which I could see clear pattern and therefore this variable did not need to be changed. For the main experiment I planned to use 4cm potato cylinders.
Volume of solution: 10ml
The volume of sucrose solution to be used had to completely cover the 4cm potato cylinder inside the test tube. Therefore 10ml was chosen as this volume met these requirements.
Range: 0.25 to 0.35 M at 0.25 M intervals
The preliminary results indicated that the concentration of the cell sap was between 0.28 and 0.34 M. I chose this range because it was slightly wider than the range I had concluded previously. It was also still narrow enough to be able to find out an accurate value for the concentration of the potato cell sap. This range at these intervals would give 5 experiments and at least 5 points need to be plotted on a graph in order to be able to spot and account for a strong trend.
Repetitions: 3 tests per concentration
Three tests per concentration would be carried out. By doing this I would be able to calculate averages and use these average values to plot a graph. Averages would give more accurate results. I would also be able to spot and isolate anomalous results.
Prediction
Based on my background knowledge and the preliminary results I am able to make the following predictions:
> In 0.25 M solution the potato will gain in mass and in 0.35 M solution the potato will have a negative mass change.
> The concentration of the potato's cell sap will be approximately 0.3 M.
The first prediction is based around the preliminary results. According to the graph from the preliminary this statement is true. But, this can also be explained by using the background knowledge. When the solution is 0.25 M it is lower than the concentration of the potato so water diffuses into the potato cells by osmosis, and the cells become turgid due to the excess water.
When the solution is 0.35 it is higher than the potato so water diffuses out of the potato by osmosis and the concentration of the solution is reduced. As the cells lose water they become flaccid.
There is no water movement when the concentration is the same of the potato. From the preliminary I predict that the concentration of the cell sap is 0.3 M. However this may not be exactly correct but only approximate as a wide range of concentrations was used in the preliminary. The diagram below demonstrates this:
0.25 M 0.3 M 0.35
MAIN EXPERIMENT
Aim: To find out the concentration of the cell sap in a potato by investigating the movement of water within a potato.
Apparatus:
- Potato x 1
- Pair of borers
- Knife
- White tile
- Test tubes x 15
- Test tube rack x 3
- 1 molar sucrose solution
- Water
- Syringe (10ml) x 2
- Forceps
- Filter paper
- Electronic weighing scales
- Sticky labels x 15
Method:
. Using the knife the potato was cut into three pieces.
2. 15 cylinders were cut from the potato using the pair of borers. (3 cylinders per piece of potato)
3. The 15 potato cylinders were all cut to a length of 4cm using the knife. This meant that all 15 had the same surface area.
4. All 15 potato cylinders were weighed and their masses were noted.
5. The concentrations of 0.25, 0.275, 0.3, 0.325 and 0.35 molar were written on to the sticky labels. Each concentration had three labels and these were stuck on to the 15 test tubes.
6. Diluting 1 molar sucrose solution made the different sucrose solutions. The solutions used were 0.25, 0.275, 0.3, 0.325 and 0.35 molar and the table below shows what proportions of water and sucrose solution were used. They were measured using a 10ml syringe.
Concentration (molar)
Amount of sucrose (ml)
Amount of water (ml)
0.250
2.50
7.50
0.275
2.75
7.25
0.300
3.00
7.00
0.325
3.25
6.75
0.350
3.50
6.50
7. All test tubes had 10ml of sucrose solution in them
8. The designated potato cylinders were placed into the designated test tubes using the forceps.
9. 3 tests were done for each concentration so that the results would be as accurate as possible and averages of the 3 would be used in the analysis.
0. The test tubes were left for 24 hours
1. After 24 hours, the potato cylinders were removed from the test tubes using the forceps.
2. The surface liquid from the potato was dried off using the filter paper.
3. In turn, each potato cylinder was weighed again and their masses were noted in the results' table.
Results:
The results' table below shows the masses of the potato cylinders before and after the 24 period when they were left in their sucrose solutions. Changes in mass and percentage changes were calculated. Averages of the 3 tests were used to calculate the percentage change.
RESULTS
Concentration (molar)
Mass before (g)
Mass after (g)
Mass change (g)
% mass change
Average %
0.250
3.27
3.72
0.45
3.76
2.96
3.39
3.83
0.44
2.98
3.38
3.79
0.41
2.13
0.275
3.40
3.81
0.41
2.06
9.97
3.32
3.67
0.35
0.54
3.29
3.53
0.24
7.29
0.300
3.39
3.50
0.11
3.24
4.70
3.39
3.55
0.16
4.72
3.59
3.81
0.22
6.13
0.325
3.42
3.49
0.07
2.05
3.50
3.37
3.54
0.17
5.04
3.52
3.64
0.12
3.41
0.350
3.29
3.18
-0.11
-3.34
--3.18
3.44
3.31
-0.13
-3.78
3.32
3.24
-0.08
-2.41
Value highlighted represents an anomalous result
GRAPH
ANALYSIS
As the concentration increases, the percentage mass change decreases. This means that when the concentration of the sucrose is higher outside the potato, the potato looses more water. From the graph I can see the points do show a clear correlation. The line going through the points crosses the x-axis at 0.39 M. However the line of best fit indicates that it crosses the x-axis at 0.36 M. This means that the concentration of the cell sap in the potato lies between these two points.
Positive mass change
When the mass change is positive the potato gains mass by taking in water. It takes water in from the solution which has a lower concentration than the cell sap of the potato. The solution outside the potato is more concentrated than the potato itself. The water molecules move by diffusion into the potato so that the cell sap has a higher water potential.
CELL SAP SUCROSE SOLUTION
As the potato cells take in more water, they begin to swell and increase in mass. The cell expands in size as well and the cytoplasm presses against the cell wall. The cell is said to be turgid as my prediction stated.
Negative mass change
When there is negative mass change the potato looses water. Water molecules diffuse through the cell wall (the partially permeable membrane) into the sucrose solution because the sucrose is more concentrated then the cell sap. The water moves down the concentration gradient, as the water potential is higher outside the potato cells than inside. The potato cells become more dilute and have a lower water potential. As the cells loose water they decrease in volume and size. They become flaccid. If the concentration of the solution was higher, the cells may have lost so much water that they would have become plasmolised, where the cell membrane pulls away from the cell wall.
CELL SAP SUCROSE SOLUTION
No % mass change
There would have been no mass change if the sucrose concentration was 0.336 M according to the line of best fit. This means that the concentration of the cell sap was 0.336 M. When the concentrations are the same the water molecules would not diffuse elsewhere because that would imbalance the solutions. The cells would not become turgid or flaccid, but they would stay they same.
CELL SAP SUCROSE SOLUTION
EVALUATION
My investigation illustrated how a potato was affected by being emersed into different sucrose solutions. I found that the potato's mass did change according to my prediction. At 0.25 M it did gain mass and at 0.35 M it did lose mass. When the concentration was higher than the potato, the potato lost mass as water diffused out of it by osmosis. The opposite happened when the concentration was lower than the potato, water diffused into the potato by osmosis. However I predicted that the concentration of the cell sap would be roughly 0.3 M but I found that it was higher - 0.336 M. My predicted value was only an estimate and in the main experiment the controls were kept more tightly than in the preliminary. This may have affected the two different values I obtained for the concentration of the cell sap. The method used did have a number of inaccuracies and problems that are discussed below.
Problems / Inaccuracies
There were a number of inaccuracies in measurement of the various constants. The sucrose solutions were not all at the exact concentration stated. Using the syringes was accurate but a syringe with a smaller scale could have been used instead. Also the surface area of the potato cylinders were not all exactly the same. They had different masses and were only roughly the same size. In the experiment the surface liquid of the potato cylinders was removed by using filter paper but this was not done accurately. The potato cylinders were dried to different extents and therefore their mass change was affected. Nevertheless the method still gave reliable results, despite the inaccuracies, with which I was able to draw conclusions from. If I were to carry out this experiment again, there are a number of features that I would improve in order to improve the accuracy of the results.
Anomalous results
I didn't get any major anomalous results that swayed the trend between the mass change in the different solutions. As I had done three tests for each concentration the results were more accurate as averages of the three tests were used to plot the graph. The third test for when the concentration was 0.275 M was anomalous as it was much lower than the other values (tests 1 and 2). Also, the average % mass change for when the concentration was 0.325 M was quite high and it was the furthest point from the line of best fit. These anomalous results were most probably caused by human error i.e. inaccurate measurement.
Improvements
. Instead of using potato cylinders I would use smaller discs. The cylinders would be cut into equally sized smaller discs, which would increase the potato's surface area. This would mean that the mass change of the potato would be more substantial and the results would how a clearer trend.
2. The variables would be kept constant in order to keep it a fairer test. This means that the concentrations, amount of solution, size of discs, time would all be measured more accurately. The temperature and pressure would also be monitored.
3. The potatoes would be dried using the filter paper evenly so that the results would not be largely affected.
Extending the investigation
The investigation could be extending in a number of ways b8ut still based around osmosis. In my investigation I investigated how the concentration of sucrose solution affected the movement of water into and out of the potato. Instead of the concentration of a sucrose solution, other variables such as temperature, pressure and humidity could be varied to see how they affected the rate of osmosis. Different vegetables could be used to see how they compare to how the potato was affected. Various root vegetables could be used such as carrots, onions, potatoes.
