Osmosis experiment Osmosis is the passage of water molecules, which have moved from a high concentration to where there is a low concentration through a semi - permeable membrane.
Aim:
We have been asked to investigate the effect of osmosis on potato chips, and therefore the effect of osmosis in plant cells. My main aim is to determine whether osmosis occurs in a potato and how it affects the potato in different molar solutions of sucrose and distilled water.
I shall be experimenting with a baking potato, and will be using a range of different concentrations of sucrose, and distilled water. By measuring the change in mass and length of the potato cylinders I should also be able to find the isotonic point. An isotonic point is a point where the mass does not increase or decrease from its original mass.
I must also experiment with at least 5 types of concentrations, to have sufficient results ,and to see if my prediction is correct I am going to use 6.
The diffusion theory of Osmosis: (from 'GCSE BIOLOGY' by D.G. Mackean.Page 36.)
Partially permeable sugar molecules pass
Membrane through pores more slowly
Fewer water molecules more water molecules
go in this direction going in this direction
Hydrated sugar molecule.
Variables:
We could:
* Look at the type of temperature.
* Compare a potato with a parsnip for example.
* See if the potato changes with different solutions
* I have actually chosen to investigate different molar concentrations of sucrose - by measuring the mass and length of the cylinders.
Fair test:
To create a fair test certain aspects of the experiment have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sugar solution. This will give me a varied set of results from which I hope to make a decent conclusion. If any of the non-variables are not kept constant it would mean that the test was not fair. For example if one of the potato chips was 1cm longer than the others the surface area would be larger, and therefore there would be more space for osmosis to occur.
To ensure my experiment is fair I will make sure I:
* do not change the potato
* fill the flat bottomed tubes with the same amount of each solution
* do not change the chemical balance in between weighing the chips
* don't change the constant variable
* cut each chip exactly 30mm long to ensure more accurate results
* leave the tubes in the same place for the same time.
* Do all tests at the same temperature
Pilot experiment:
From the pilot experiment, the potato chips I will use for the full experiment will have a diameter of 8mm and a length of 30mm each. I chose these measurements because from the pilot experiment I conclude that the potato chips which had an 8mm diameter, and a 30mm length gave a more accurate result than the remaining chips which were being tested.
Method:
. We plunged the cork borer into the potato 18 times so that we had long potato chips with a diameter of 8mm and a length of more than 30mm.
Careful not to push the borer towards the palm of your hand because it could cause a serious injury. Make sure that you don't change the potato at any stage because they may differ in density and one may be drier than the other etc
2. Once we had 18 potato chips we cut them so that each one measured 30 mm in length.
Keep your fingers well away from the scalpel, as it is exceptionally sharp and could very easily cause an accident.
3. We measured the mass of each potato chip on the chemical balance and then recorded it in a result table.
Before weighing each chip make sure that the chemical balance has gone back to weighing 0 grams. E.g. check that the balance doesn't say 1.79g on them for example. Do not change the balance at any stage as the balances may vary in measurements slightly.
4. We then placed each potato chip in a separate flat-bottomed tube.
We added distilled water to 3 of the tubes,
0.1 sugar solution to 3 more,
0.2 sugar solution to the next 3 tubes,
0.3 sugar solution to the next 3 tubes,
0.4 sugar solution to the next 3 tubes,
0.5 sugar solution to the next 3 tubes.
Fill each of the tubes with 140 mm of your chosen solution.
E.g 140 mm of distilled water. Don't change it because it may affect the final results.
5. We finally placed the lids on the flat-bottomed tubes so that none of the solution evaporated.
6. We then placed the named test tubes in a tray by the window.
7. After the 24 hours we drained out the solutions in the sink and placed all the chips, in order of concentration, on layed out paper towels.
8. We dried each chip with the ...
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Fill each of the tubes with 140 mm of your chosen solution.
E.g 140 mm of distilled water. Don't change it because it may affect the final results.
5. We finally placed the lids on the flat-bottomed tubes so that none of the solution evaporated.
6. We then placed the named test tubes in a tray by the window.
7. After the 24 hours we drained out the solutions in the sink and placed all the chips, in order of concentration, on layed out paper towels.
8. We dried each chip with the paper towels and then re weighed them on the same chemical balance. The weights were recorded once more in a results table.
Precautions:
As was stated I the "fair test" section of my coursework, I had to keep all the non - variables the same in order for my experiment to be fair. Whilst using the scapel extreme precision had to be used to cut the potato as it could easily cause a serious wound. The measurements for the solutions had to be perfect so not to change the out come of the experiment. I had to ensure that everytime I handled the potato chips my hands were clean and dry. This was to stop any kind of contamination, and made sure no water could pass through from the potato from my hands.
Apparatus:
* Square tile - to cut the potato on
* 8mm diameter cork borer - to cut out potato cylinders
* Scalpel - to cut the potato
* 18 flat bottomed tubes - to hold the solutions and potato chips
* Potato - part of the experiment
* Pusher outer - to help push the potato cylinders out of the cork borer
* Forceps - used for picking up the potato cylinders
* Chemical balance - used for weighing the potato cylinders etc
Here is a diagram to show the apparatus I will be using:
Prediction:
I predict that somewhere at some point I can find the solution, which has the same concentrations as the potato solution (isotonic). The isotonic point is the point where the mass and length has not changed from its original mass at all.
Distilled solution
I predict that the more dilute the solution (the solution with more water than sugar in it) the more the potato's mass will increase. I base this prediction on the knowledge I have about osmosis. Osmosis is the term given for the process of water passing through a semi - permeable membrane from a less concentrated solution to a more concentrated solution.
The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. I predict that the cells should increase in length volume and mass because of the extra water.
Here is a diagram to show a turgid plant cell. (No.2.6 from a worksheet about plant cells)
Cell wall Cell surface membrane
Most concentrated solution
Osmosis takes place first the cell shrinks slightly, and
becomes flaccid; then the cell m membrane pulls away from the
Water diffuses into the cell wall, and the cell is plasmolysed.
Cytoplasm and vacuole
through the partially the cell becomes turgid and swells.
permeable
It shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. I predict that the cells will become turgid and swell because this is what happens when water enters the cells by osmosis. Osmosis is the passage of water molecules, which have moved from a high concentration to a low concentration, through a semi - permeable membrane.
I think that if the potato chip has a higher sugar concentrate than the sugar solution surrounding it, osmosis will take place into the potato, causing the potato in the solution to swell. This is because the plant cells - which make up the potato, have become turgid, so that the cytoplasm is pushing outwards onto the cell wall.
Sugar solution
I predict that the potato chips in the sugar solution will decrease in mass.
A solution that contains less solute particles than another, and is hence less concentrated, is referred to as being hypotonic. The chips in 0.1M sugar solution will be quite hypotonic and should decrease more in mass, diameter and length than the chips in distilled water. I do not think that they will decrease as much as the chips in 0.2M sugar solution etc because I predict that:
The more hypotonic the solution : the more the cell will increase in mass
The more hypertonic the solution : the more the cell will decrease in mass
Plant cells plasmolyse in concentrated solutions. Therefore the chips in the stronger concentration (the chips which are more hypertonic - most concentrated) should show a greater sign of being flaccid (the cell is flaccid if it has been plasmolysed) than if they were in a weaker solution. Flaccid is the complete opposite of turgid. When plant cells are placed in a solution that has exactly the same osmotic strength as the cells they are in a state of flaccidity and turgidity. We call this incipient plasmolysis. "Incipient" means about to be.
Here is a diagram to show a flaccid plant cell. (No.2.7 from a worksheet about plant cells)
Cell surface membrane Concentrated solution
Less concentrated solution
Water diffuses out of the first the cell shrinks slightly, and
Cytoplasm and vacuole becomes flaccid; then the cell t through a partially membrane pulls away from the Ppermeable cell membrane cell wall, and the cell is plasmolysed.
Osmosis takes place
Appearance:
Over 24 hours I think the colour of the potato chip will get lighter and fade in the distilled water and possibly 0.1M solution. E.g. if the potato chip was at first a golden yellow colour I think that after 24 hours it would change to a pale white colour. This is because the cell in the potato has become turgid, and water has entered the cell by osmosis. I also predict the chips in 0.2M - 0.5M solution should be dark in colour, and bendy and brittle in texture. This is because the cell has become flaccid. The cell will become flaccid when plasmolysis has occurred. The cell will become flaccid when plasmolysis has occurred. Plasmolysis is when the cell membrane peels away from the cell wall. A flaccid cell is the complete opposite of a turgid cell.
Results:
This is a table to show the change in mass in the potato chips.
Solution used
Original Mass (g)
Final mass (g)
Change in mass (g)
Average change (g)
% change in mass
Average % change
.73
.9
0.17
9.83
Distilled water
.66
.82
0.06
0.1
3.16
5.3
.6
.69
0.09
5.63
.67
.76
0.09
5.39
0.1M sucrose
.68
.74
0.06
0.06
3.57
8.19
.77
.82
0.05
2.82
.65
.7
0.05
3.03
0.2M sucrose
.69
.75
0.06
0.04
3.55
7.13
.83
.83
0.03
.64
.8
.71
-0.09
-5
0.3M sucrose
.82
.67
-0.32
-0.19
-16.6
-25
.66
.49
-0.17
-10.2
.76
.49
-0.27
-15.3
0.4M sucrose
.74
.51
-0.23
-0.29
-13.2
-41.5
.79
.44
-0.39
-21.8
.86
.5
-0.36
-19.4
0.5M sucrose
.92
.52
-0.4
-0.37
-20.8
-46.8
.85
.46
-0.36
-19.5
% change in mass = change 100 anomalous result.
original
THERE ARE TWO GRAPHS INCLUDED IN THIS INVESTIGATION.
I OBSERVED THE RESULTS FROM THESE GRAPHS AND HAVE RECORDED MY FINDINGS AND OBSERVATIONS BELOW.
The graph to show average change in mass in the chip.
This graph shows a set of results over the course of a 24-hour experiment.
In this graph there were no results which were very anomalous thus showing that my results have been fairly accurate. My anomalous results were 0.06g for 0.1M solution, and 0.04g for 0.2M solution. I thought that these results were very similar and they didn't change as dramatically as I would have predicted. This may have been because the potato chips were dabbed too much or too little. These results were not extremely anomalous which meant my results weren't too unreliable and were surprisingly accurate.
This graph drawn is a line that slopes downwards and does not go through the origin. Because the line is not very straight and does not pass through the origin, it means that the percentage gain and loss in mass are not directly proportional. However, there is a pattern on my graph, and this is, as the concentration of the solution increases, the average change in mass decreases. From the line of best fit that has been added in, it can be seen that all of my points were fairly close to creating a reasonably smooth line. This shows that my results are fairly reliable.
The graph to show average % change in mass in the chip.
Like the graph to show average change in mass there were no very anomalous results. This shows that my results were not completely in accurate and unreliable. The anomalous results were in solution 0.1M. It was 8.19%, and 0.2M solution. It was 7.13%. These results were very similar and I would have predicted that they would have varied to a greater extent then they actually did. Because an anomalous result is a result which does not follow the pattern of the other results, I found that my results must have been fairly in accurate although fairly reliable too. I have found a pattern in my graph, and this is, as the concentration of solution increases, the average percentage change in mass decreases.
From my results we can see that the more water in the solution, in comparison to the sugar, the more weight is put on by the potato. This is caused by osmosis. Osmosis is the movement of water from a low concentration to a high concentration through a semi - permeable membrane.
Conclusion:
When the concentration reaches above 0.22M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed. From the graph an estimate to the concentration of the potato cell can be made as 0.22M, as this is the point where the potato is not increasing or decreasing in mass, this is known as the isotonic point. This is where no osmosis is taking place, both the potato and the solution have an identical concentration.
The time I used to test the potato chips was plenty of time to allow sufficient osmosis to occur as. However if I was to repeat this investigation I might decrease the time of the experiment because I don't think it would have taken 24 hours as I believe osmosis would occur within 2 hours. Therefore I would test the potato chips for 2 hours as I think that the cells would be fully plasmolysed fairly quickly (the rate of plasmolysis varies depending on the type of solution).
From my results we can see that the more water in the solution, in comparison to the sugar, the more weight is put on by the potato. This is caused by osmosis. Osmosis is the movement of water from a high concentration to a low concentration through a semi-permeable membrane. When there is more sugar, in comparison to the water, weight is lost. This is because the water that is already present in the potato is passed through to the solution.
The predictions I made about the appearance of the potato chips were very accurate. I predicted that at the end of the experiment "the colour of the potato chip will get lighter and fade in the distilled water and possibly 0.1M solution. E.g. if the potato chip was at first a golden yellow colour I think that after 24 hours it would change to a pale white colour. This is because the cell in the potato would become turgid, and water water will have entered the cell by osmosis. I also predict the chips in 0.2M - 0.5M solution would be dark in colour, and bendy and brittle in texture. This is because the cell has become flaccid. The cell will become flaccid when plasmolysis has occurred. Plasmolysis is when the cell membrane peels away from the cell wall. A flaccid cell is the complete opposite of a turgid cell." This predictions were almost completely accurate.
I took three samples of each solution. I think this was a satisfactory amount of samples for the amount of concentrations that I was using. Although, if I was to do the experiment again I would probably take more samples. This would hopefully ensure more reliable results.
The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.1M, 0.5M, 1.0M, and so on. This way would have allowed me to also find the isotonic point far more accurately as the one that I estimated is very approximate.
The evidence obtained from this investigation supports the prediction made. It shows that the potato cells increase in mass, when they are in solutions with a high water concentration, and decrease in mass in solutions with a low water concentration. At concentrations above 0.5 M, I would predict that no further water would be lost, as the cell would be almost fully plasmolysed.
I do not know if this prediction is correct, however, because my solutions only went up to 0.5M solution and I cannot know what stage plasmolysis would be complete.
From the graph an estimate to the concentration of the potato cell can be made as 0.22M. As this is the point where the potato is not increasing or decreasing in mass. However, it is important to realise that this is only an estimate as the potato cells will not be uniform in their concentration.
Evaluation:
Overall I think this experiment was successful.The accuracy of the investigation was adequate. I obtained a large quantity of accurate results from which I was able to create two informative and accurate graphs
I was also extrememly satisfied with my prediction as it was very successful and accurate. The most accurate predictions I made, however, were about the appearance of the potato cylinders.
My prediction was almost completely successful. Because my predictions came true I was not surprised with the results of my experiment as these results results were what I had anticipated before hand.
The cutting of the potatoes was a very dificult part of the experiment as although I was recording my results by mass, it could have well affected the surface area and so the overall rate of osmosis. Osmosis is the passage of water molecules from where there is a high concentration to where there is a low concentration through a partially permeable membrane. If I were to repeat the experiment I would have perhaps found a machine to cut the potato as it would ensure that each potato cylinder would be the same dimension and mass. I could have also found a more accurate way to measure out solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale. E.g. 0.000g not 0.00g.
I was also very surprised as I only had two small anomalous results which means my results were probably fairly reliable. To observe these results look at my graphs or results table. An anomalous result is a result which does not follow the pattern of the other results and so is abnormal or irregular. If I was to carry out this investigation again I could use different varieties of potato or different plant tissues e.g. carrot instead of potato. This way I would be able to compare the results and see what differences and similarities were between the two types of plant tissue.
The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.10M, 0.50M, 1.0M, and so on. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate. Three repetitions of each concentration are sufficient but unfortunately there were anomalous results (circled on graph). This part of the investigation may need to be repeated, as it did not seem accurate.
I think I took just enough results but if I were to do this investigation again I would certainly take more so that I would have a greater chance of obtaining more reliable ones. There were no out of the ordinary results, but some were not as close to the lines as others. This may have been caused by human. When the potato chips were removed from the flat bottomed tubes and dried i may well have dried some more thoroughly than others and so some would have more excess water, which would add mass. If the experiment was repeated I could find another way to dry the potatoes which would ensure that all were dried in the same way for the same amount of time.
Although I could not draw a firm conclusion I could see a trend and I was very satisfied with the complete comparison of my results with my initial prediction.
