When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become flaccid; this is the exact opposite of turgid. If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells, they are in a state between turgidity and flaccidity. We call this incipient plasmolysis.
Variables: Independent – The concentration of the water
The molarity of the solution
The salt concentration
Dependent – The change in mass
Controlled – The amount of potato used
The amount of solution used
Prediction: I think that the lower the concentration of the sugar solution in the test tube the larger the mass of the potato will be. This is because the water molecules pass from a high concentration, i.e. in the water itself, to a low concentration, i.e. in the potato chip. Therefore, the chips in higher water concentrations will have a larger mass than in higher sugar concentrations.
Method: Apparatus: Sugar solution, distilled water, potato, cork borer, mass balance, boiling tubes, knife, labels and a measuring cylinder.
Plan: A range of sugar solutions will be prepared with concentrations from 0.0M to 0.8M, in intervals of 0.2M, by adding varying amounts of distilled water. Sections of potato will be cut using a cork borer and knife to equal lengths of 5cm each, this will keep the surface area constant. 10 ml of each concentration of sugar will be placed in five test tubes and labelled. To each test tube the 5cm piece of potato will be added and its mass recorded. These will be left for 24 hours. Then the potato pieces will be removed, the surface solution removed using paper towels and weighed. We repeated the experiment twice at each interval.
Safety: In order to keep the experiment as safe as possible we took care when cutting the potato and also worn goggles to prevent the sugar solution entering our eyes and causing irritation.
Reliability of results: As it is difficult to get the cut potato pieces to the same mass it was decided that a percentage change in mass would be used to compare the data in the results, as this would be far more accurate. We also made sure we done the experiment at the same time and at the same temperature and we repeated the experiment twice at each interval.
Strategy for dealing with results: This is how I will record my results.
Table showing the percentage change in mass
Table showing the average percentage change in mass
Graph to show Average % change in mass against Concentration
Average % change
(g)
Concentration (M)
Results: Here are my results.
Table showing the percentage change in mass
Table showing the average percentage change in mass
Conclusion: The evidence obtained from this investigation supports the prediction I made. It shows that the potato cells increase mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. At concentrations above 0.6 M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed.
Evaluation: The graph obtained in the results has a strong resemblance to the one in the prediction. This suggests that the experiment was fairly accurate. The accuracy of the investigation was adequate, however the concentrations were measured using a measuring cylinder and could be made more accurate using titration, also when the potato was dried to remove surface liquid it was not necessarily done the same on each potato, a more accurate and uniform way of drying would improve the accuracy further. Although 2 repetitions of each concentration are sufficient, there was no anomalous result. Further work could be carried out to include concentrations that increased in 0.1 M rather than 0.2 M. This would increase the accuracy and improve the results and graph. Other investigations could include using different varieties of potato or different plant tissues e.g. carrot.
