Throughout this experiment I shall be investigating how different solution concentrations can affect osmosis. For this I have chosen to use sucrose solution, and cucumbers.
Diffusion is the movement of a substance through a fluid (i.e. liquid or gas). In a solution, diffusion happens when there is a higher concentration of a substance in one part of the solution than in another. It spreads until the concentration is the same throughout the solution.
Osmosis is the diffusion of water from an area of low concentration (i.e. high concentration of water) to an area of high concentration through a selectively permeable membrane.
In both plant and animal cells water can pass through but in the latter there is no cell wall to keep the cell from bursting. So if the water concentration is higher outside the cell, then water will pass until the cell bursts. (Unless a stable point is reached before). However in plant cells, the cell wall, which is entirely permeable and very strong, can keep the cell from bursting.
Turgor and plasmolysis. These are the two different “states” in which a cell can be left if it’s concentration within is not the same as outside. (Note, animal cells can go flaccid but not turgid, they burst instead). Flaccid means “floppy” and turgid means “hard” or “stiff”.
* Petri dishes (x10)
* Cucumber slicer
* Sucrose solution (1M)
* Filter paper
* Measuring cylinder
The Petri dishes will be lined up on the table in two rows of five. The second row is the one in which the repeats shall be done. Then the 1M sucrose solution shall be diluted into 0.75, 0.5 and 0.25 M. 20cm3 of the solutions shall be placed in the appropriate dish (as shown in the diagram).
Then the cucumber shall be cut up using the slicer into ten equal pieces. These shall then have their mass recorded, one by one, and keeping track of which slice is which placed in the Petri dishes and left there for 20 minutes. This will ensure that the test is fair.
After the twenty minutes the pieces shall be quickly removed, and dabbed on the filter paper to remove the water on the surface, then re-massed and weight recorded.
Time ………………………. Controlled Variable
Concentration …………. Independent Variable
Mass of slice …………… Controlled Variable
Volume of solution…… Controlled Variable
Surface area of the slice and volume ratio ……..(see evaluation)
There are no major safety issues regarding this experiment except that the teacher will be making the cucumber slices to avoid accidents.
Hypotheses and predictions
From the evidence shown in my introduction it was seen that:
* Water will move across the semi permeable membrane leaving it’s solute behind to find a balance in concentration, and
* Bigger molecules such as Glucose or Fructose cannot make it through the barrier because they are too big.
I am therefore going to deduce the following:
* When the slice of cucumber is placed in D.W. it will gain mass because the concentration of water will be weaker inside it’s cells than outside.
* When the solution is very weak (i.e. not much Sucrose) then the cucumber might still gain mass depending on how high the concentration is inside the cucumber.
* As the concentration increases, the loss of water from the cells will increase. This will be shown by a change in mass.
The relationship between concentration and %change in mass will be of the following:
As you can see from the above graph, there shall be a point where the concentrations of the solutions inside and outside the cucumber slice will be equal. At this point the solutions will be in equilibrium. This point on the graph is marked as “X”. From the graph I will also be able to work out the water potential of the cucumber cell sap.
Another quick idea about the way that the experiment will be carried out; the time between which the slice has been made, weighing and putting it in the petri dish should be minimal be if the slice is loosing mass during this time, the results will be distorted
I have already carried out a similar experiment before using potatoes. The results from this experiment have helped me find a suitable range of concentrations to be used in my final experiment.
The experiment was carried out as described in the Plan
The ten petri dishes were laid out on the table and the 1M Sucrose solution was diluted into 0.75 M, 0.5M and 0.25M. Distilled Water was used as the solution in the last two petri dishes and the solutions were put into the petri dishes as shown in the diagram of the planning section. Then the cucumber was sliced into ten slices made as even as possible, massed and put into the appropriate petri dishes. After twenty minutes they were extracted, dabbed on filter paper, re-massed and this was recorded. (NOTE. It was important that the different pieces of cucumber are kept track of so that the change in mass can be measured)
From this data a % change in mass was be calculated for each slice. Averages were also calculated.
There were no particular safety issues in the carrying out of the experiment except that the cucumber slices were made by the teacher to avoid accidents.
My results were as follows:
% Change in mass
% Change in mass
% Change in mass
See also the graph plotted by hand on the next page.
Reminder of hypothesis:
* When the slice of cucumber is placed in D.W. it will gain mass because the concentration of water will be weaker inside it’s cells than outside. (This was true)
* When the solution is very weak (i.e. not much Sucrose) then the cucumber might still gain mass depending on how high the concentration is inside the cucumber. (This was also true)
* As the concentration increases, the loss of water from the cells will increase. This will be shown by a change in mass. (This was also true)
It has been found that there is a relationship between the concentration of a solution and the change in mass of a plant cell. It has been found that a weak solution (i.e. only water) will give the cell water because the concentration of “compounds” in solution is greater inside the cell than outside it. Because only water particles can cross the membrane it then swells up to equalise the concentrations as much as possible. In this particular experiment it was found that using distilled water, after twenty minutes, the slices of cucumber had absorbed nearly 10% of their original mass in water (about 0.30 grams in this test). The opposite occurs when the concentration of impurities is in solution outside the cell. The cucumber loses water and becomes flaccid. In this experiment, using a sucrose solution of 1M it was found after twenty minutes that the cucumber slice lost nearly 5% of it’s original mass.
The graph in Section “O” shows the relationship between concentration and % change in mass. This is not one of direct proportionality. The graph shows how the cell first gains water, then loses it as the solution concentration increases. My prediction given in section P and this graph are quite similar although my prediction doesn’t have any units because they are impossible to estimate. Therefore my results are consistent with my predictions. (see top of page).
Because it is not possible to know the concentration of solution in the cells at any given time it is difficult to estimate when the cell will be losing water to it’s surroundings or whether it will be gaining it. In this test, there was no change in mass when the concentration of the Sucrose solution was 0.25M. This suggests that the concentration inside the cell is around 0.25M too, but it isn’t quite that simple because there is not only sucrose inside the cell. If there are other chemicals these would also effect the way in which osmosis happens. So technically it would be possible to work out exactly how much gain (or loss) in mass there should be but this is pointless because there would be experimental error anyway, and this type of information is not useful.
The results support the original prediction which I made, except I wasn’t expecting the line to fall so fast from above the line to the point where it stabilises.
As in my prediction, there was a point where the two solutions, i.e. the one inside and outside the sap cell are isotonic and in equilibrium. This was proven by there being no change in mass at this point. This is shown on the diagram at the point (0.25;0). My original predictions were suitable but could be slightly adjusted for my final conclusion. I think that the evidence is reliable and that the predictions were quite accurate.
*Evaluation of evidence*
The experiment was not as accurate as it could have been because of the types of equipment used. For example the measuring out of solutions was not done very accurately and with primitive equipment.
The procedure used was suitable to acquire the information necessary. It is not really possible to make any improvements to things like the amount of time between slicing the cucumber, massing it and putting it in the petri dish. Any ameliorations would result in people rushing across the lab, making it unsafe.
There is also one thing which would be hard to make better which is the ratio between surface area of slice and volume. If the Volume is smaller (making the sample lighter) and the surface area is larger, then the percentage change in mass would be bigger because the surface area is what effects the speed at which water is absorbed. This would produce a distortion throughout the results. This is because the cells at the surface of the slice will be in contacts with the water but those deeper inside the slice will not be able to absorb as much water as much water as those on the outside. This can only be improved on by making all the slices identical… good luck!
There is also a problem when massing the end mass. This is because the cucumber has just come out of the petri dish, and there is water “stuck” to the surface. The problem is that there is no way of making sure how much water you are removing and whether or not it is the right amount. I cannot think of an improvement for this weakness in the results. Any other methods of drying the outside of the slice would cause it’s own problems. The best solution (which is what I did) is to give the cucumber slice a quick shake and dab it on filter paper.
The experiment could be improved by trying a wider range of solutions (not just sucrose) and at different concentrations. The tests should be repeated as many times as possible and a longer time should be used to let the slice absorb or eject water. This should be done because it would eliminate any extreme end results and give better averages.
I think that my results were accurate enough to reach a firm conclusion which is probably correct as it would agree quite strongly with my background information.
Further work could include researching what chemicals are in solution inside the cell and whether using different solutions would effect osmosis. Also the cucumber slices could be tested at different stages of development to test for a specific time period during which the starch in the sap cells is developing. Also another test could be to do exactly this same experiment after having left the slices of cucumber for different periods of time in sunlight and or shade to see how the concentrations inside the inner sap vacuole would change as the cells were dying. The tests would be almost identical to this one with perhaps a few changes in their preparation.