Fair Test, Control Variables
To make this a fair test there are some things that need to be controlled. The volume of sugar solution and dilute water in the main experiment must be controlled, as this will mean there will be the same amount of glucose and water in the solution. The chip sizes must be the same size in surface area and more importantly in weight, because the weight is what will be measured.
This is the method for this experiment:
- Using the core borer, take a cylindrical piece of potato. Do this 18 times.
- Then cut the potatoes into smaller pieces making sure they are approximately the same size.
- Measure the pieces to make sure that they are all exactly same size.
- Measure the mass of each piece of potato making sure they are the same mass. They were all made the same weight as best as possible to make the test fair.
- Each piece of potato was then placed in a test tube with the solutions.
- 6 measuring cylinders were taken to measure 15cm3 of solutions of 0.2M, 0.4M, 0.6M, 0.8M and 1.0M.
- 3 potatoes were put into 3 measuring cylinders with the same molar-mass. The potatoes were then left for a small period of time.
- Measure the potatoes by removing excess water from the surface of the potato.
- Note the new mass.
- Work out the average change in mass for each of the 6 molar solutions.
Preliminary Test
The preliminary test for this experiment will help to identify the concentration of sugar solution in cell sap.
The results of the preliminary results are shown in the table below.
From the preliminary experiment I have decided that for the main experiment to use 15 ml of sugar solution for each concentration. The potato was left in the solution for a total period of 60 minutes, and it was decided that this was a sufficient enough time to get the good results. The aim of the experiment is to find the point where the potatoes neither gain nor lose weight. For the concentration of sugar solution I have decided to use a range of different molar solutions – Dilute Water, 0.2M, 0.25M, 0.3M, 0.35M and 0.4M.
I think that from this experiment a potato piece that is left in dilute water will gain mass because the water molecules enter the cells by osmosis. Then as the concentration of solution increases there are less water molecules in the solution so the potato cylinders will shrink. This is because the net movement of water molecules in the potato will decrease and water molecules will leave the potato. This will therefore cause a mass decrease. This mass loss will be proportional to the increase in concentration of the solution. At the same time, there will only be a gain in mass when the sugar solution is 0M, i.e. dilute water.
Results of the Investigation
The following table shows results of the main experiment.
The graph above shows a general trend with relation to the concentration and percentage change in mass of the potato chip. As the concentration of sugar solution increased the percentage mass change decreased. So when the solution is dilute water, the percentage change in mass is a gain in mass because the water has been take in by the potato cells, from the dilute water surrounding it. At 1M, the percentage change in mass is -40%, because water molecules are rare in the sugar solution and therefore they move from the potato chip to the surrounding (sugar solution of 1M) and therefore there is a reduction in the surface area and mass of the chip in the 1M solution. Also, there is a loss of mass in the 0.4M, 0.6M and the 0.8M concentrations.
The aim of the experiment is to find the isotonic point of potato cell sap, where there is neither a gain nor loss of mass. By looking at the graph, I can see that the isotonic point is the point where the line of best fit crosses the X-axis. This is at 0.46M. At 0.46 Molar concentration, there is no net movement because the percentage weight change would be 0.0 if this experiment was carried out. Above and below this point there was either a gain or a loss in mass of the potato chip.
Potato cells contain a vacuole which contains cell sap. This cell sap is a solution with solutes dissolved in it. When tissues in the potato are surrounded by sugar solution like the one used in this experiment, water molecules will move between the outside sugar solution and the solution inside the potato cell vacuole (cell sap) because of osmosis. The potato cells have a semi-permeable membrane, in the cell membrane and the vacuole membrane. Therefore when the solution has got a less concentration than the solution in the vacuole, then the water molecules will move into the potato tissue by osmosis. This will make the potato have an increased mass. The rate of net movement depends on the concentration in the solution in the vacuole and the sugar solution used.
Evaluation
In the main experiment, there were no anomalous results. The preliminary experiment contained one where the percentage change in mass increased even though the concentration increased.
One problem with this experiment was that not enough results were taken to the stage where the graph was drawn, so the isotonic point was not as reliable as it could have been. However, the experiment was done to the fairest possible. Every precaution was taken to make sure that all the control factors were kept the same. The results are very reliable because there were 3 potato chips for each concentration so that the average could have been taken. This allowed the risk of anomalous results to be reduced. So the evidence in the form of the results and graph can support a very firm conclusion.
To improve the investigation, I would have taken the isotonic point that I concluded to be, and experimented with this concentration. So I would have taken the 4.6 mole concentration and seen how accurate this experiment was in finding this as the isotonic point.
If I were to do this project again I would change one thing to extend the experiment. This would be to use another plant to compare the results with. For example, we could have used an onion or carrot as they are both under-growing vegetables, so they have a lot in common with each other. By doing the same experiment again with a similar vegetable we could have seen whether it is a similar case for other similar plants.
