A Flaccid Cell
The water was stored in cell sap in the large vacuole, when placed in the strong salt solution, it osmosed out of it, causing it to shrink. This reduced the volume of the cell’s contents, so the cell membrane was no longer pushed against the cell wall.
Diffusion Out of Cells
If a cell is exposed to a hypotonic solution, one that contains a low concentration of solute relative to the cell's cytoplasm, water will diffuse into the cell, causing the cell to swell and become turgid.
A Turgid Cell
Diffusion into Potato Cells
There is also a third type of solution, these are isotonic solutions, they contain the same concentration of solute as the cell's cytoplasm When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate creating a dynamic equilibrium.
Variables
In the experiment, the variable I am changing is the concentration of salt solution around the piece of potato; this will tell me what the concentration of water in potato cells is. The concentration in which no mass is gained or lost will be the same concentration as the potato cells. It is also easy to measure under lab conditions, as what we are measuring is change in mass.
To make the experiment a fair test, it is necessary to keep all other variables the same for each test tube.
1. The temperature of the solution – in a hotter solution, the molecules will move faster, causing more molecules to diffuse across.
2. Surface area of potato piece – with a greater surface area, more potato will be exposed to the solution, causing osmosis to happen faster.
3. Amount of solution – although solution is in excess here, there should be 40cm³ of solution in each test tube.
4. Time left for – if one solution is left for a longer period of time, more osmosis may occur.
5. Age of potato - as potatoes get older, they are likely to start to dry up, meaning they may take on more water. This is why it is necessary to maintain the same aged potato for each experiment.
6. Type of potato – some breeds may osmose faster or slower than others as they may have different concentrations.
Apparatus
- 1 Potato
- Potato Borer
- Scalpel
- 5 Test Tubes
- Distilled Water
- 1M Salt Solution
- Measuring Cylinders
- Top-Pan Balance
- Ruler
- Stopwatch
- Calculator
Prediction
I predict that the stronger solutions will be hypertonic, the water will diffuse out of the cell, as the solution in the cell will be stronger than that outside the cell. This will cause the cell to become flaccid (see above), and will, consequently, lose mass.
I predict that the weaker solutions will be hypotonic, the water will diffuse into the cell, as the solution in the cell will be weaker than that outside the cell. This will cause the cell to become turgid (see above), and will, consequently, gain mass. The concentration where the mass of the potato doesn’t change has the same concentration as the potato cells, as equal amounts of osmosis are happening in and out of the cells.
I am using salt solutions with concentrations of 0M, 0,125M, 0.25M, 0.5M and 1M. 0M is distilled water, so it would be impossible to use a weaker solution. I am not using a solution stronger than 1M because preliminary work revealed that exposure to a 1M salt solution caused potato cells to become very flaccid. Taking 5 measurements will be enough to pinpoint the concentration of potato cells. I intend to do 3 repeats of the experiment, this will create a larger sample and lead to a fairly accurate average result being taken.
Through a preliminary investigation, I have decided not to measure change in length in the potato chips as in the preliminary investigation length change was too small to be measured by school rulers, meaning that results would be inaccurate.
Evaluation
My experiment worked well, all three of my sets of data were quite similar. Our graph shows a curve of best fit, meaning that they supported the theory that there is a connection between the concentration a solution and mass change in potato cell. However, as the graph was a curve my results suggest that surrounding concentration and mass change are not directly proportional.
I did have a slightly anomalous result, the average mass change for 0.125M was +4.77, when our graph suggested it should have been the point of dynamic equilibrium, this may have been because of the mass of excess water which had not been osmosed into the cell.
Faults in the Method
The method used was simple, it was resticted by boundaries of time and resources. In retrospect, there were several faults which, if corrected, would provide more accurate results.
Firstly, equipment used was not always sufficiently accurate or precise to give good results.
- I used a school ruler to measure length, the smallest denomination on these is a millimetre. For more precise measurements, I could have used callipers.
- I used measuring cylinders to measure the volumes of solution used, this could mean that the concentrations of solutions were not accurate. The actual concentration of one concentration of solution may not be consistent over the three repeats. To combat this I could have made batches of solution, giving greater accuracy and consistent solutions.
- I used a top-pan balance accurate to one decimal place to weigh the potato pieces, this is not accurate enough to give good results as changes in mass were small.
- It was difficult to cut the lengths of potato exactly straight, this caused a difference in the surface area of the pieces. We could have cut them more accurately by using a tri-square.
- There was excess moisture left on the potato pieces after they had been left in the solutions, it was very difficult to remove the same amount of water from each potato piece. We could have dipped the pieces in water quickly before we weighed them for the first time.
There are further ways we could have improved our experiment:
We could have left the potato chips in the solutions for a greater amount of time allowing for more osmosis, and thus, greater differences in mass, to occur.
We could have measured more concentrations between 0M and 0.5M, this would have given us greater precision in finding the point of dynamic equilibrium.
Making potato chips with a greater surface area would allow more osmosis to occur and would give more obvious results.
Further results
Although we can draw a conclusion from our experiments there is a lot more investigation that could be done into the osmosis theory.
- Investigation into the link between surface area and size change due to osmosis. An experiment could be set up where pieces of potato, all with different surface areas, are put into a 1M salt solution and the change in mass is recorded.
- Investigation into different types of potato. Does osmosis affect different strains of potato differently? Does the age make a diiference?
- Investigate more closely the area of concentration around where the graph crosses the x axis. This would give a more accurate picture of what the solute concentration equivalent is to potato sap.
