Prediction: I predict that the process will reach an Isotonic point where the sugar concentration will be the same in the potato as in the sucrose solution. However I do not know what this point is.
Analysis: We found from our results table and our graph that water is indeed transferred by osmosis in potatoes. I was correct in predicting that an isotonic point would be reached, we calculated that this point is at approximately 0.37M of sucrose solution. We discovered this by drawing a graph with ‘Average Percentage Change’ on the y axis, and ‘Concentration of Sucrose’ on the x axis. We did this by plotting the points after calculating the average percentage change in mass for each of the two samples (by dividing the difference between before and after by the original mass and then multiplying it by 100) and then matching it with the concentration of the solution used for each of the samples. We plotted the points on the graph and then we drew a line of best fit through the points. We used our line of best fit to calculate the isotonic point, by noting where the line cut the x axis. We measured along the x axis and discovered the point was at approximately 0.37M. If a solution is on the isotonic point then the solution is known as an isotonic solution to the potato. This means the concentration is exactly the same and there is no net movement of water. This is why the y axis shows the average percentage change to be 0%, because there is no net movement. This means that anything above this concentration of sucrose is a ‘hypertonic’ solution to the potato, and anything below this sucrose concentration level is a ‘hypotonic’ solution to the potato. A Hypertonic solution is one where there is less water and more sucrose, this means that water is transferred to the solution from the potato cells by the potato until the two concentrations are the same. This causes the cells to lose water and become flaccid. If the solution outside the potato is hypotonic then there is less sucrose concentration in the solution outside then there is in the potato cells. This causes water to be transferred to the cells of the potato by osmosis to level the two concentrations out. Osmosis is therefore the process of transferring water into and out of the cells until the solution outside is isotonic (the isotonic point is reached, meaning the solutions both inside and outside are equivalent in concentration). Water will continue to be transferred (through a partially permeable membrane) until this is achieved. A plant converts sucrose molecules to starch molecules for storage in the roots because starch is an insoluble molecule so it does not affect osmosis. Osmosis is found mainly in root hair cells in plants as a means to take water into the plant from the ground.
A TABLE TO SHOW OUR RESULTS
EVALUATION
Our graph seems to be in order with our predictions and a line of best fit can be drawn easily through the points. Except, our water result seems to be slightly lower than the rest and sits far below it’s place on the line of best fit. Our water result seemed more fitting for a solution f roughly 0.22M, not sugar free distilled water. Also the potato samples for the water felt less turgid than they should’ve been, almost like the 0.2M samples. The only explanation I have for this is that we used contaminated water. I would say that apart from our water result our experiment was fairly reliable as all our other points sit on a line of best fit and no others are anomalous. I feel that two samples for each concentration is enough. Our two results for each concentration are fairly close together and none of them deviate largely from the mean average:
E.G Our results for 0.6M:
Sample 1 % Change: -17.00
Sample 2 % Change: -15.70
Average % Change : -16.35
Average % Change - Sample 1 % Change = -0.65
Average % Change – Sample 2 % Change = 0.65
Therefore each sample only deviates from the mean average by 0.65, this shows that both samples reacted in similar ways. Results were similar between samples for all concentrations with no massive deviation from the mean. This is reflected in our graph and our line of best fit. This is why I feel 2 samples for each concentration is quite enough for a fair test.
There are ways we could yet further improve the accuracy and fairness of this test. We used a scalpel to cut the pieces of potato. I cut them myself which means there is a margin however slight for human error. We could eliminate this possibility by using a cork borer to make each piece of potato the same. They would all have the same weight and surface area, exactly the same.
There are some variables that we could change to investigate the conditions that affect osmosis. Our experiment was conducted at room temperature. We could conduct the same experiment at different temperatures to see if the process is affected. Our experiment was left for 24 hours. We could try leaving it for a shorter time to see how long the process takes. We could also try a different volume of liquid or a different plant to investigate different species’ rates of osmosis. We could use this experiment to find isotonic points for different cells. An alternative way of looking at osmosis is to peel away the epidermal layer plant such as onion, leek or rhubarb and put them in varying concentrations of sucrose solution. Then examine the samples under a microscope and observe osmosis happening. When the cell loses water the membrane caves in while the cell wall remains rigid. The cell innards shrivel but the wall keeps the shape so the cell appears normal from the outside whereas an animal cell (because they have no cell wall) just shrivels. This is known as plasmolysis.
