- Mass of the potato. This is the outcome variable and the mass change in the potato is being measured as a percentage.
Theory: The process that takes place in this investigation is osmosis. Osmosis is the word used to describe the movement of water molecules from a high water potential to a low water potential through a selectively permeable membrane. In biology, this usually means the diffusion of water in or out of cells. Osmosis is a special kind of diffusion as it is only water molecules and their movement that we consider. Therefore in this investigation we are examining the movement of the water molecules in and out of potato tissue cells. If the concentration of sugar is greater inside the potato cell than outside the cell membrane, water molecules from the surrounding solution diffuse into the cell to dilute the more concentrated sugar solution. As a result, the potato tissue cell becomes turgid. However, if the concentration of the sugar in the cell sap is more dilute than the concentration of the solution outside the cell membrane, the water molecules from inside the cell diffuse into the solution to dilute it. This is known as plasmolysis.
The cell membrane is called partially permeable because it allows water molecules to pass through more easily than sugar molecules. Allowing only the smaller molecules to pass through is typical of most partially permeable membranes in biology.
Prediction: My prediction is based on a previous experiment we have performed involving osmosis in a visking tube and the theory explained above. The visking tube contains small gaps that allow water molecules to pass through more easily than larger sugar molecules. The cell membrane in the potato tissue cells shares similar properties to that of the visking tube.
A glucose solution was placed in the visking tube, which was weighed at 13.1g. This was then placed in a large beaker of water and left for a certain amount of time. After removing the visking tube from the beaker it was rinsed to eliminate excess water or sugar molecules on the tubing and dried to ensure the water it was rinsed with would not add extra weight. The final mass was found to be 15.3g and subtracting the initial weight from this showed the change in mass was 2.2g. As a percentage this was calculated to be 16.8%. The results showed that water from the beaker flowed through the partially permeable membrane of the tube and in addition to the mass change, an increase in the amount of water molecules inside the visking tube was also observed. Therefore my prediction is supported be the previous experiment.
I predict that potato tissue cylinders in test tubes labelled ‘A’ containing distilled water will gain weight, generally feeling firm to the touch and potato tissue cylinders in ‘B’ containing 0.2 molar sugar solution will lose weight, feeling slightly flabby when compared to ‘A’. This prediction regards the results of the first test in this investigation, as mentioned in the method.
The reason for the weight gain and firmer texture of potato tissue cylinder ‘A’ is that if the cells were not fully turgid at the beginning of the experiment then they would take in water by osmosis (water moves from a high water potential outside the cell to a lower water potential inside the cell through its selectively permeable membrane). This would then increase the turgor of the potato tissue cylinder and also cause a rise in weight.
In potato tissue cylinder ‘B’ the sugar solution is stronger than the cell sap of the potato tissue cells. Thus these cells would lose water by osmosis. This results in the cylinder feeling flabby due to the lack of water causing a low turgor pressure (the cells become flaccid). The potato tissue cylinder would also become lighter at the end of the experiment because of the water loss through osmosis.
Therefore from this prediction it is possible to assume that the more concentrated the sugar solution is, the more mass the potato tissue cylinder loses. For example, if the mass lost by the potato tissue cylinder is 2% for a concentration of 0.4 molar sugar solution, you can predict that a greater loss of mass, say 7%, would occur in a more concentrated sugar solution of 1.0 molar. Although it is impossible to link a mathematical theory with the mass loss of the potato tissue cylinder and the sugar solution concentration, it is however feasible to predict it by the formation of a graph, which would produce a curve as seen below:
As the graph shows, the loss of mass is gradual at first for the weaker sugar solutions. This is because their concentrations are almost equal to or not much greater than the concentration of the potato cell sap itself; meaning that any weight loss would be small. During the central stage of the graph the increase is fairly steep as the concentration of the sugar solution far exceeds the concentration of the potato cell sap, meaning a large mass loss. Once the sugar solution concentrations reach a maximum towards the end of the graph, little mass is lost, as the potato cells have become completely flaccid with barely any cell sap available to lose.
Results: The results of the investigation are shown in the table below. The difference in mass was calculated by subtracting the final mass from the initial mass and the percentage change in mass was obtained using the calculation (Difference in mass/Initial mass of potato) x 100 to give a positive or negative answer. The number after the letter in the column labelled ‘potato’ donates which potato (1, 2 or 3) the tissue cylinder came from:
Each set of results can be collectively presented on a graph, as shown on the graph paper. The results are plotted in a scatter arrangement and a best-fit line is added to display any trends or patterns. I found this to be an inefficient way of presenting the results as many of the points failed to correspond with the best-fit line. I have therefore plotted a second graph using the average value of each set of results and drawn a best-fit curve. This has proven to be much more accurate and could be used for predicting results more successfully. I have also added a best-fit line to compare.
Analysis and Conclusion: The first obvious relationship I can find from my results is between the concentration of the sugar solution and the potato cell sap. The mass of the potato increases when the tissue is in a solution less concentrated than the cell sap. The more concentrated the sugar solution becomes the less mass is gained. Once the concentration of the solution is greater than the concentration of the cell sap, the mass of the potato tissue decreases and gives a negative result.
If the potato cell sap is stronger than the solution, for example water, then the solution will diffuse into the cells to help dilute the more concentrated cell sap. It will continue to do so until either the concentrations of both solutions are equal, or no more of the water can enter the cell as it has reached full turgor (the cell is full and cannot absorb any more water).
However, if the sugar solution is more concentrated than the cell sap, the water present in it will diffuse into the solution. This causes the cell to lose its turgor and dilute the sugar solution in addition to making the sugar in the cell sap more concentrated. This process will continue until the concentration of the cell sap is equal to the concentration of the sugar solution. The movement of water from a high water potential to a low water potential across a selectively permeable membrane – in this case, the potato cell membrane – described here is called osmosis and is the process that occurs in this investigation, as explained in the theory.
When arranged in a graph, the results show how the loss of mass is considerably large in the low sugar solution concentrations, while after a concentration of between 0.6 molar and molar the loss in mass becomes much more gradual, producing a shallow curve. This is due to the potato cells having reached a flaccid point where they have little more water to diffuse.
Because the potato tissue cells lose water as it diffuses into the sugar solution, they eventually lose their turgor pressure and become flaccid. Once this occurs they have little more water to lose thus explaining why the loss of mass curve becomes far more gradual. From the graph it also appears that the cells become flaccid in a concentration of approximately 0.6 molar.
Evaluation: To begin this evaluation I must first consider whether my experiment supported my original prediction. I am generally pleased with the accuracy of my prediction based on a previous experiment, the theory and my graph. The basic forecasts of mass being gained by the potato tissue in solutions less concentrated than the cell sap and mass being lost from the potato tissue in solutions more concentrated than the cell sap were correct. The concentration of the cell sap seemed to be between 0 and 0.2 molar. The quantity of mass gained or lost I predicted was slightly incorrect – approximately double as much mass was gained or lost in each experiment than I had foreseen. I am also pleased with the graph, as the curve was especially accurate.
I believe that this experiment could have been improved if I had taken more care in processing the measurements and observations. For example, I should have been more accurate when weighing the mass of each potato tissue cylinder before and after the experiment. The electronic scales fluctuated on numerous occasions and because of time restrictions, I had to remove the potato tissue cylinders before an exact weight was established. When drying the potato tissue cylinders, I could have possibly soaked up more of the liquid in one cylinder than another, resulting in a slight weight difference. Also, I may have measured the distilled water and sugar solution to 0.25ml either way of the required volume. Due to these inaccuracies my investigation was less efficient than it should have been.
In this experiment I do not feel that there were any major anomalous results that I had not predicted, although some were rather surprising. These could have been due to the tissue cylinders being taken from a less turgid potato, as opposed to the others. To overcome this, all the cores should have been soaked in a beaker containing distilled water prior to the investigation and then gently dried in filter paper immediately before each individual experiment. The investigation could have been further improved if I had managed my time more efficiently to begin with, preventing hurried measuring and weighing of the potato tissue cylinders. If I would have left each cylinder in the solution for a longer period of time, for example 24 hours, more accurate and definite results could have been achieved. A wider range of solutions, including 0.1 molar, 0.3 molar, 0.5 molar, 0.7 molar and 0.9 molar would also help to produce a more precise graph.
I am satisfied that stronger solutions will take water from weaker solutions until their concentrations are equal. The evidence that I have obtained is sufficient to support my conclusion, although I did refer to other information sources to confirm and reinforce each theory.
In order to extend the investigation I could test other plant tissues. It could prove interesting to see their viability in cases of drought. For example, I could propose the question ‘do plants living in high drought areas have more concentrated sugar solutions than plants living in wetland areas, such as cacti and watercress?’ I could also involve marine plants that live in a salty environment and examine what would happen to them in terms of osmosis.
