Variables
Concentration of solution, size of potato chips, volume of solution, time left for osmosis to occur.
I am going to keep the size of the potato chips, the volume of solution I keep the potato chips in and the time left for osmosis to occur all constant. I shall be varying the concentration of the solution that the potato ship is immersed in. I shall use the following concentrations: distilled water, 0.2 moles, 0.4 moles, 0.6 moles, 0.8 moles and a 1 molar solution. I will need to make sure all the other variables remain constant so that it is a fair test.
Prediction
I predict that when the potato chip is immersed in distilled water than it will absorb more water and therefore grow in length. Therefore I can also predict that the exact opposite will occur when the potato chip is kept in a salty solution; water will leave the cell and cause the cell to become flaccid. This will cause the potato chip to decrease in length.
I am now going to research in depth the reasons behind what will happen, in order to support my prediction.
Firstly I will need to look at what is happening when the potato chip is in the solution. This is called osmosis. Osmosis is the diffusion of water molecules, and occurs when a partially permeable membrane separates two solutions. Here is a definition I found in a textbook: “Osmosis is the passage of water molecules from a weaker solution into a stronger solution through a partially permeable membrane.”
All cells, belonging to both animals and plants, are surrounded by a cell membrane. This cell membrane is partially permeable, so it is full of tiny holes that allow small molecules to pass through, when large molecules cannot, as they are too big.
So I know that as the water molecules diffuse from a high water concentration to a lower concentration. There is a higher concentration of water molecules in distilled
water than there is in the various salty solutions, so there is going to be more water molecules moving from the solution into the potato cells. This is going to mean that the cells become turgid as they swell with water, causing them to increase in length.
As I have already said, when cells are placed in water, the water enters the cell because the cell sap contains a strong solution of water. As the water enters the cell it causes it to swell up. The water acts against the cell wall, pushing against it until the cell can hold no more water. The cell is then fully turgid. When a cell is turgid it is firm, and rigid. This is why in my method I have said that I should also observe how rigid the potato chip is, as it gives me an indication as to what has happened in the cell. The opposite to turgid is flaccid. When potato cells are placed into a stronger salt solution, water passes out of the cells by osmosis. The permanent vacuole begins to decrease in size, causing the cells to turn limp. The cells are no longer firm, this means they are flaccid. You can see examples of turgid and flaccid cells below.
Obtaining
I have followed all the safety rules I set at the very beginning of this investigation, and ensured a fair test at all times. I feel that I collected a suitable amount of results for this experiment, and they are shown in the table below.
Solution in molars measurements (mm) Average (mm)
Distilled water 55.9 58.8 55.9 56.9
0.2 53.8 53.7 52.2 53.2
0.4 52.2 53.2 53.3 52.9
0.6 48.4 48.4 50.0 48.9
0.8 45.0 49.2 47.4 47.2
1.0 46.3 45.2 48.6 46.7
I am very pleased with these results; I have one slightly anomalous result, which I will be able to comment on in my evaluation. I believe my results to be as accurate and as reliable as I could have made them, all the ranges are appropriate and suitable. I have taken an average of my results, and will use the average results when plotting my graph.
To give me a better idea of the size changes I shall also do a table showing the percentage change for each result (average).
Percentage increase = Increase in length X 100
Original length
Solution Percentage Increase (%)
Distilled water 11.78
0.2 molar 4.5
0.4 molar 3.9
0.6 molar -3.9
0.8 molar -7.3
1 molar -8.3
During this experiment I have learnt about osmosis, and cells being turgid/flaccid. I have learnt that water molecules move from a weaker solution into a stronger solution through the partially permeable membrane. I have tested these facts out for myself and confirmed that they are indeed correct.
Look at the graphs attached. The graph showing the average measurements of potato strips kept in various solutions is a relatively straight line, with a line of best fit drawn across it. This graphs shows that as the concentration of the salty solution increased, the average length decreased. Therefore we can draw the conclusion that potato chips decrease in size when placed in salty water, but increase in size when placed in distilled water. The gradient is steady, and it looks as if it would continue to be steady, this tells us that the subjects of the axis must somehow be proportional.
The second graph looks very different. It is a graph of the percentage increases, looking at this graph, there seems to be an anomalous result, the results would form a considerably smoother curve without the result for the 0.4 molar solution. This is most likely to have been caused by leaving the potato chips longer than twenty-four hours. However, a line of best fit still fits the results with ease so it hasn’t affected my investigation significantly. The graph has the same conclusion as the last graph, but is presented differently.
Scientific Conclusion
This is basically what happened, and is the reasoning behind both my prediction and my conclusion: The water enters the cell because the cell sap contains a strong solution of water. As the water enters the cell it causes it to swell up. The water acts against the cell wall, pushing against it until the cell can hold no more water. The cell is then fully turgid. When a cell is turgid it is firm, and rigid. This is why in my method I have said that I should also observe how rigid the potato chip is, as it gives me an indication as to what has happened in the cell. The opposite to turgid is flaccid. When potato cells are placed into a stronger salt solution, water passes out of the cells by osmosis. The permanent vacuole begins to decrease in size, causing the cells to turn limp. The cells are no longer firm, this means they are flaccid.
Also I can support my conclusion with all the research I used to support my prediction. Also, I mentioned that I would observe the firmness of the potato chips as well as their lengths. I found that the potato chips kept in the distilled water were as predicted firmer than they had begun, although I could not feel any difference in firmness when observing the potato chips kept in the 0.2 molar solution, it was only with the potato chips kept in distilled water. Again I saw a difference with the potato ships kept in the 1 molar solution, but none in the chips kept in the 0.8 molar solution. The chips were considerably flexible, and bent with no effort.
My results support my original prediction, as does my conclusion.
The method of results was good, and everything went according to plan. I consider my results to be above average in accuracy as I used accurate measuring equipment such as a borer to get equal potato chips, used millimetres and always recorded results to one decimal place. The only area that wasn’t as accurate as it should have been is the timing, and is how I got the slightly anomalous result. I would need to have a special timer so that the test could be as fair as humanly possible.
The evidence here is plenty to come to a definite conclusion. Osmosis occurs at a greater rate when there is a lack of salt molecules. Osmosis causes cells to become turgid or flaccid depending in the circumstances. I feel that there is enough evidence to support my conclusion, and feel happy with the final product of the investigation.
