Variables:
I am going to vary the concentration of the solutions but to ensure a fair test, I will keep the number of chips in each solution the same and I will keep the amount of solution I use in each test tube constant. Similarly, I will try to keep the length of the chips roughly the same and the diameter of the chips by using the same cork borer. For more accurate results, I will place three chips in each solution and calculate their average.
Method:
- Start off by cutting out eighteen chips from the potato using a cork borer. I will slice off the ends of the chips because they will have skin on them which could affect the results. I will also cut them to roughly the same length.
- Weigh each chip individually and record its mass. Measure out 10cm³ of each solution and pour into each test tube. Label each test tube with the concentration of solution in it so that I don’t forget.
- Place three chips in each test tube and wait for an hour.
- Take chips out after an hour and blot them to remove excess solution so that it doesn’t affect my results.
- Weigh and measure each chip and record the results.
- Calculate the averages for each solution and calculate the percentage increase/decrease in length and mass.
Scientific Theory / Prediction
I predict that when the potato is placed in a concentrated sucrose solution, it will decrease in size and mass because when osmosis occurs, the water particles will move out of the cell and into the solution through the cell membrane (from a high concentration to a low concentration). This is a diagram to illustrate my prediction which is based on osmosis:-
My prediction is based on the preliminary experiment which I have carried out already. In that experiment, I used four concentrations of sucrose solutions (0.0, 0.1, 0.5, and 1.0) and three chips in each one. These are my preliminary results:-
After drawing a graph, the results show that the ratio of water moving in and out of the cell membrane is equal at 0.22M. The preliminary experiment went well and the results were as I expected them to be. This has helped me with my plan because now I have an idea of what range of concentrations I want to use and when I carry out my experiment, I will know how to set everything up. This will help me a lot because it will save time and it should be easier for me.
Obtaining
(See results table on the next page)
Analysis
I have found out that when the potato chips are placed in a strong sucrose solution, they lose water through osmosis, therefore the mass and length decreases. When the chips are placed in pure water, they gain water through osmosis, so their length and mass increases. For instance, when the chips were placed in a 1.0 molar solution, the mass of the potato chips before was 1.37g and after was 0.96. It is clear that they lost weight. When they were placed in pure water, their average mass before was 1.33g and after was 1.45, therefore they have gained weight.
The graph I have drawn shows a pattern. The pattern is that as the sucrose concentration increases, the percentage change in mass and length decreases. As the sugar concentration of the solution decreases, the percentage change in mass and length increases. This is because when the chips were placed in a concentrated sucrose solution, the concentration of water is lower than that of the cell sap. Therefore the water moves out from the cell sap, through the cell membrane and into the solution. The cell then becomes plasmolysed. This means that the cytoplasm shrinks away from the wall of the potato cell due to the water loss caused by osmosis. When the potato chips increase in mass and length, the cells become turgid. This means that it was swollen or full because of the increase in water molecules.
This is a diagram to show turgid and plasmolysed cells:
Conclusion
In conclusion, when the solution which the chips are placed in is high in sugar, osmosis will occur. This is the movement of water molecules from a region of high concentration to a region of low concentration through a partially permeable membrane. In this case, the partially permeable membrane is the cell membrane of the potato cell. Therefore the water molecules in the cell sap of the potato will move out through the cell membrane and into the solution. This is why the chips decreased in mass and weight.
My prediction was that when a potato is placed in a solution of water it will gain weight and length because of osmosis. When it is placed in a sugar solution, it will lose weight and length because of osmosis. My conclusion supports my prediction and my prediction was correct according to my results.
From my graph, I have found out the isotonic point. This is when the concentration of the solution in the potato cells balances the concentration of the solution outside the potato. So in conclusion this is the concentration of the cell sap. The isotonic point for the mass is 0.15 M and for the length it is 0.11 M. the fact that they are very close proves that my results are reliable.
Evaluation
The procedure I used was quite successful because everything went according to plan. I think that my results are of good quality because the line of best fit fits nearly all of the results. I carried out my experiment with 3 chips in each concentration so that I could have more accurate results by finding out the averages. They are all quite close together and that is proof that they are fairly reliable
My results match my prediction apart from one result which is anomalous. This result is circled on my graph which is 1.0 M of mass change. This result is anomalous because it doesn’t fit in with the line of best fit. There are also two anomalous results in the length graph. This is for 2.0 and 1.0 molar. I think that this is probably due to human error. This proves that the results for mass are more reliable than length, probably because the mass was measured by electronic scales, so they are more reliable.
In my opinion, the procedure I used was quite suitable although if I wanted very accurate results I could have produced more repeats or increased the range of concentrations of solutions. I wouldn’t say that my results are very reliable because of the three anomalous results, but overall they do fit the pattern, therefore they are reliable to some extent.
From drawing the graph, I can see that the mass results are more reliable than those of the length. This is because I had to measure the length using Vernier Calipers which are quite difficult to read off. I think the anomalies are therefore down to human error. My results are still sufficient for me to form a conclusion. It was easier for me to obtain the results of the mass, which is why I would concentrate mainly on mass if I had to repeat the experiment. I could carry out further work to extend this investigation. For instance, I could place potato chips in a solution which has the concentration of the isotonic point. If the mass and length do not increase or decrease then it would prove that my results are extremely accurate.
