Investigation into the Rate of Osmosis in Potatoes and to find out the Sucrose Concentration of the Potato.
Investigation into the Rate of Osmosis in Potatoes and to find out the Sucrose Concentration of the Potato.
Introduction
In order to carry out this investigation, I first have to define the terms of the objectives so that an understanding is present and a clear process to acquire the answer can be devised.
What is Osmosis?
Osmosis is the process by which water molecules pass through a partially permeable membrane from an area of high concentration of water to an area of lower concentration of water (e.g. from distilled water to concentrated sucrose solution). The reason for this is because of the natural tendency of water to spread from regions of high concentration to regions of low. This spreading process is called diffusion.
Several examples of osmosis can be seen in nature that can give a better idea of what is happening during osmosis. E.g. if water is withheld from a flowering plant then the plant will wilt. If bacterial cells are placed in concentrated salt-water solution, they collapse and die. Human blood cells placed in fresh water expand and burst.
The reason for these events is because of the partially permeable membrane present in osmosis. Water molecules are very small (one oxygen atom and two hydrogen atoms) while sugar molecules are many times larger. The membrane has very small holes or pores in it that allow water molecules to pass through them because they are small enough, but do not allow sugar molecules to pass through because they are too big. Therefore when the water molecules are diffusing through the membrane, the side of the membrane that is gaining water will expand and increase in mass and volume, as the other side decreases in mass and volume by losing the water molecules. Thus osmosis is the movement of water molecules only across a membrane. The water molecules will continue to diffuse across the membrane until equilibrium between concentrations is established.
Below is a diagram showing osmosis and the movement of the water molecules as described above:
Osmosis takes place in both plant cells and animal cells. Animal cells do burst in pure water. The cytoplasm inside the cell is quite a concentrated solution of proteins and other large substances. Therefore osmosis will take place between the cell and the surrounding fluids via the partially permeable membrane (the cell membrane). The cell will soon burst because of the swelling from the increase in water.
Plant cells do not burst in pure water, though, because they have a cell wall that is fully permeable so osmosis does not take place across it. It is bypassed and osmosis takes place between the cell membrane in the same way as an animal cell, but with the resistance of the strong cellulose of the cell wall to keep the cell from bursting. When a plant cell is pushing out against the cell wall (like an inflated football) because of the expansion by osmosis, then it is said to be turgid. It is the turgidity of a plant's cells that helps it to stay upright if it has no wood. Plant cells are usually turgid.
Below is a diagram of a turgid plant cell in pure water:
Another term is flaccid, which is the state of the plant cell when it has lost water so that it stops pushing out against the cell wall. The cytoplasm and the vacuole shrink, then the plant loses its firmness and begins to wilt. However, when the plant cell reaches the extremity of water loss through osmosis then it is said to be plasmolysed. This happens when the plant cell is in a very concentrated solution and the water has diffused out of the cell causing the cytoplasm and the vacuole to shrink even further. The cell membrane tears away from the cell wall, and on this the membrane becomes damaged and the cell dies. Plant cells do not normally become plasmolysed naturally because they are never usually in such strong solutions but they can be plasmolysed with intervention. Below is a diagram of a plasmolysed plant cell in a concentrated solution:
Hypothesis
The aim of the investigation is to explore osmosis in potatoes and find its sucrose concentration. With regard to the osmosis, it will be taking place across cell membranes in the potato. The cells in the potato will be similar to the plant cells explained previously in terms of how osmosis occurs. Therefore I predict that water will diffuse into the potato if it is put into pure water. Also I expect water to diffuse out of the potato if it is put into a very concentrated sucrose solution. This is because there must be a considerable amount of water in the potato cells already seeing as a potato is a root vegetable and absorbs water into the ground.
The sucrose concentration of the potato will be found when the potato is put into a concentration of solution in which no osmosis or diffusion of water takes place. From my own general knowledge I know that the water content of potatoes is approximately 70%. This is a notorious quantity of water found in nature e.g. the composition of humans is 70% water, the composition of the Earth is 70% water etc. So from this information I can predict that osmosis will not take place in a solution that is approx. 70% water and the sucrose concentration in ...
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The sucrose concentration of the potato will be found when the potato is put into a concentration of solution in which no osmosis or diffusion of water takes place. From my own general knowledge I know that the water content of potatoes is approximately 70%. This is a notorious quantity of water found in nature e.g. the composition of humans is 70% water, the composition of the Earth is 70% water etc. So from this information I can predict that osmosis will not take place in a solution that is approx. 70% water and the sucrose concentration in the potato will be approx. 30%.
In connection with the pattern of osmosis at different solution concentrations, I think that if the potato is put into pure water then xcm3 of water will diffuse into the potato, and if the potato is put into a concentration half way between that of itself and pure water then 0.5xcm3 of water will diffuse into the potato. If the potato is put into a concentrated sucrose solution then ycm3 of water will diffuse out of the potato, and if the potato is put into a concentration half way between that of itself and the concentrated sucrose solution then 0.5ycm3 of water will diffuse out of the potato.
Preliminary Experiment
Aim:
To find the optimum conditions and values of variables for the main experiment.
Apparatus:
electronic weight balance; filter paper; hole-borer; knife; measuring tubes; pipettes; potato; ruler; sucrose solutions: 0?2M, 0?4M, 0?6M, 0?8M, 1?0M; 6 test tubes; tile; water.
Diagram:
Method:
I began by taking the potato and by using the hole-borer I cut out cylinders from the potato on the tile. Altogether I cut out six cylinders, all of which were at least 5cm in length. Then I used a ruler and a knife to measure them and cut them all down to 5cm in length and kept these six cylinders aside. Next I used a pipette to take sucrose solutions from each of the 5 containers (0?2 - 1?0 M) and filled five test tubes up with one concentration of solution in each. In a sixth test tube I used the measuring tube to fill up the test tube with water, which is in effect 0?0M sucrose solution. Coming back to the potato cylinders, I recorded the weight of each cylinder using the electronic balance and then put one cylinder in each test tube with the solutions in. When weighing the cylinders I put filter paper onto the balance to keep it clean.
After leaving the test tubes for 24 hours for osmosis to take place I came back to the potatoes, drained the solution out and took the potatoes out of the tubes. Then I reweighed each cylinder and wrote down the weight next to the first recording.
Results:
Below is a table showing the results I obtained from the preliminary experiment:
Volume of
Concentration of
Start Potato
Finish Potato
Change in
Percentage Change
Sucrose Solution (ml)
Sucrose Solution (M)
Mass (g)
Mass (g)
Mass (g)
in Mass (%)
5
0.0
2.84
3.18
0.34
1.97
2
0
0.2
2.81
2.88
0.07
2.49
3
0
0.4
2.78
2.68
-0.10
-3.60
4
0
0.6
2.80
2.21
-0.59
-21.07
5
0
0.8
2.78
.76
-1.02
-36.69
6
0
.0
2.84
.86
-0.97
-34.51
These results can also be plotted on a scatter graph as shown on the next page.
The results delivered a fairly sufficient distribution with the exception of the reading for 1.0M sucrose solution. However, that point is not as important as where the curve crosses the X-axis at just below 0.3M sucrose solution. This tells me that at 0.3M there is no change in mass of the potato and therefore no osmosis is taking place. The aim of the investigation is to find out the sucrose concentration of the potato and my preliminary results show that this value will be in the vicinity of 0.3M. In order to find a more accurate and detailed value, when carrying out my main experiment I should concentrate the studies around this point. So instead of using solutions ranging from 0.0M to 1.0M concentration, I will use solutions in the narrowed down range of 0.2M to 0.4M.
In terms of the actual experiment itself, I will need to create more accurate concentrations of the sucrose solution myself, rather than use concentrations that have 0.1M difference between each. This gap is too big so I will start with 1.0M sucrose and dilute it with water to the exact concentrations I need. I have also realised that the surface area of the potato cylinders is going to affect osmosis. If the 24 hour period in which the potatoes are kept is not long enough for full osmosis to take place then in order to have comparative results, I have to make sure that the rate of osmosis is constant across all the samples. The greater the surface area, the faster the rate of osmosis. So when cutting the cylinders I have to keep the surface area the same. This can be done by cutting cylinders with diameters of the same width out of the potato and keeping them all the same length.
In each test tube I will only use 10ml of solution because in the preliminary, when I used 15ml for the first tube, it was too much and the liquid level came near to the top of the tube. 10ml is adequate for the cylinders to be completely submerged. In order to dampen the effect that anomalous results have on the experiment I will carry out the experiment for each concentration twice and use the average as the final result.
Main Experiment
Aim:
To find the sucrose concentration of the potato.
Apparatus:
3 beakers; electronic weight balance; filter paper; hole-borer; knife; labels; pipettes; 2 potatoes; ruler; stirrer; 1.0M sucrose solution; 12 test tubes; tile; water.
Diagram:
Method:
Use the hole-borer to cut out 12 cylinders from the two potatoes making sure that all the cylinders are at least 5cm long. Then using the ruler and knife, crop all the cylinders down to exactly 5cm in length on the tile. Keep the 12 cylinders aside. Now you have to make up six different concentrations of sucrose solution using the 1.0M solution provided and diluting it with water. To make the concentrations I used the following proportions:
.0M Sucrose Solution (ml)
Water (ml)
Resulting
Concentration
0
40
0.20
2
2
38
0.24
3
4
36
0.28
4
6
34
0.32
5
8
32
0.36
6
20
30
0.40
I made the solutions by using pipettes to measure the sucrose solution in one beaker and the water in a second beaker, then mixing both in a third beaker with a stirrer. With another pipette I put 10ml of each concentration into two test tubes so that there were two tubes with 0.2M, two tubes with 0.24M etc. I measured the mass of the potato cylinders and recorded the values, before putting the cylinders into the solutions, one cylinder in each test tube. I put labels on each tube to show which tube contained which concentration so that there was no error in mixing up the results.
After leaving the potatoes for 24 hours (probably sufficient time for osmosis to take place) I drained away the solutions from the tubes, took out the potato cylinders and reweighed them, recording the new values next to the previous ones.
Results:
Below is a table of the final results obtained from the main experiment:
Volume of
Concentration of
Start Potato
Finish Potato
Change in
Percentage Change
Sucrose Solution (ml)
Sucrose Solution (M)
Mass (g)
Mass (g)
Mass (g)
in Mass (%)
Test 1
0
0.20
3.84
4.11
0.27
7.03
2
0
0.24
3.81
4.05
0.24
6.30
3
0
0.28
3.88
4.03
0.15
3.87
4
0
0.32
3.76
3.90
0.14
3.72
5
0
0.36
3.83
3.88
0.05
.31
6
0
0.40
3.81
3.71
-0.10
-2.62
Test 2
0
0.20
3.92
4.16
0.24
6.12
2
0
0.24
3.85
4.08
0.23
5.97
3
0
0.28
3.82
4.01
0.19
4.97
4
0
0.32
3.80
3.92
0.12
3.16
5
0
0.36
3.84
3.81
-0.03
-0.78
6
0
0.40
3.82
3.74
-0.08
-2.09
Since two tests for each concentration were taken, I have to find the average change in mass. The table below shows the final change in mass:
Concentration of
Percentage Change
Percentage Change
Sucrose Solution (M)
in Mass (%)
in Mass (%)
0.20
(7.03 + 6.12) / 2
6.58
2
0.24
(6.30 + 5.97) / 2
6.14
3
0.28
(3.87 + 4.97) / 2
4.42
4
0.32
(3.72 + 3.16) / 2
3.44
5
0.36
(1.31 - 0.78) / 2
0.26
6
0.40
(-2.62 - 2.09) / 2
-2.36
Analysis
Below is a graph plotting my final results:
What did I find?
From the graph plotted above, the main aspect that I found was of course the point at which no osmosis took place. As explained before, this is the point where the curve crosses the X-axis. This is because at this point there is neither an increase nor decrease in the mass of the potato so there would have been no diffusion of water molecules across the potato cells into or out of the sucrose solution.
The curve crosses the X-axis in between the readings for sucrose solutions of concentration 0.36M and 0.4M. When a straight line is drawn between these two points the X value when the line crosses the X-axis is 0.364. This would indicate that the sucrose concentration of the potato is 0.364M. However, the black line of best fit drawn on the graph could alternatively be used as an average of the final results. The X value when the line of best fit crosses the X-axis is 0.368. This is 0.004 more than the value between the two points plotted on the graph. For reasons including in order to identify anomalous results and to distinguish a trend in the diffusion of water molecules, I will use the line of best fit to analyse the results plotted and so I will take 0.368M as being the sucrose concentration of the potato.
Sucrose Concentration of Potato = 0.368M
What is happening?
As you can see from the graph above, osmosis has taken place across the potato cells. This can be seen by the way in which the mass of the potato decreases as the sucrose concentration increases. The reason for osmosis taking place with the potato is simple. When the sucrose concentration is less than 0.368M there is a concentration gradient for water molecules present. This is because then there is a greater ratio of water molecules in the solution than in the potato so water molecules travel down the gradient from the solution into the potato cells. The diagram below represents this gradient:
Some more key terms that would be useful in explaining the situation at different points would be hypotonic, hypertonic and isotonic. The sucrose solution would by hypotonic if it was less than 0.368M (concentration is less than that of the potato) and would be hypertonic if the solution was more than 0.368M (concentration is more than that of the potato. So for the potato to remain without osmosis taking place, it would have to be in an isotonic solution that would have the same sucrose concentration of 0.368M.
From my knowledge of the process of osmosis, I assume that the change in mass of the potato would be inversely proportional to the change in concentration of the sucrose solution. This is based on the fact that if, for example, the concentration of solution increases, then there will be a smaller percentage of water molecules in the solution and therefore more water molecules would be diffusing out of the potato in order to balance the system; consequently the mass of the potato would go down as the concentration went up. The case would be the same vice versa. This is the reason why in perfect conditions I think the graph would exhibit a straight line like the line of best fit on my graph.
The gradient of this straight line could then be used to find the change in mass of the potato per change in solution concentration during osmosis. I have applied this theory to my own results as you can see below:
Gradient = Y1 - Y2
X1 - X2
= 6.00 - 3.00
0.23 - 0.30
= 3 ? -0.07 = -42.86
If the gradient is -42.86 then this shows that for every 0.1M increase in sucrose solution concentration, there is a 4.29% decrease in the mass of the potato. Also if you take 1.0M - 0.368M = 0.632M then (0.632/0.1) x 4.29 = 27.11% shows that the maximum percentage decrease in mass would be 27.11% in 1.0M sucrose solution. Similarly (0.368/0.1) x 4.29 = 15.79% maximum increase in mass of potato in water.
Review of hypothesis.
When reviewing my hypothesis, I found that on the whole my predictions were correct. Firstly I stated that water would diffuse into the potato if it were put in pure water. From my preliminary experiment where water was used, this can be seen happening because the mass of the potato does increase in water. Also I said that if it were put into a very concentrated solution then water would diffuse out of the potato. This prediction was correct, too, because in the preliminary where 1.0M sucrose solution was used (very concentrated) the mass of the potato decreases due to water diffusing out of it.
Then I went into more detail to say that the actual sucrose concentration in the potato would be in the region of 30%. This estimation proved to be quite accurate because the actual concentration was 0.368M or 36.8%, only 6.8% out.
The last part of my hypothesis focused on patterns in the diffusion of water in which I said that if the concentration of the solution is doubled then twice as much water will diffuse out of the potato and likewise if the concentration is halved. Since my results gave a strong correlation, they could be based around the line of best fit, and since the line of best fit is straight then this pattern does occur.
Evaluation
I would consider the experiment and investigation as a whole to have gone underway the way I expected it to without any major errors or disturbances. However, even though it may have seemed to go to plan, there is still the underlying question of how accurate and more importantly how reliable the tests and the results are.
Several factors could have affected and introduced errors into the accuracy of the testing. The surface area is the first. Being able to cut all the potato cylinders so that they had the exactly the same surface area would be very hard and so there may have been some deviation. Surface area affects the rate of osmosis so if 24 hours was not enough to allow full osmosis to take place then the surface area should be taken into account as an erroneous factor. The recording of the masses of the potatoes would also be severely flawed. During the time lag between weighing and using the potato there could have been a loss in mass due to evaporation of moisture or water being absorbed out of the potato by contacting surfaces. Since the differences in start and finish mass are quite small, the loss of water from the potatoes before and after osmosis would have a visible effect.
In order to make the results more reliable, a repeat of each test at each concentration was taken. Finding the average of these would diffuse any errors from anomalous results. While on the subject of anomalous results, a few of the points when plotted on the graph strayed from the line of best fit. Attacking the points raised above more carefully could combat anomalous results.
Had I the chance to repeat the experiment I would not have changed the method itself to find the sucrose concentration of the potato since this seemed like a viable method. However, in extension to the task, I could record the mass of the potatoes at equally measured intervals throughout the duration of the osmosis process. This would give a better idea of the rate of osmosis. I could also expand by applying the same method to other plant cells e.g. the use of different vegetables etc.
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