In my experiment, I will vary the solutions and their molarity, as well as using two different kinds of plant tissue samples; potato and cucumber cores.
The dependant variables of this experiment are the changes in length and mass of the potato and cucumber cores, which should occur as a result of changing the dependant variables. However, calculating the change in length could lead to inaccurate results, as the measurements are more likely to be calculated imprecisely. Calculating mass, however, using a laboratory weighing scale, will give more accurate and detailed results, providing that each vegetable core is blot-dried before it is weighed the second time, in order to eliminate water which is present on the surface, which could give misleading results.
Hypothesis
Pure water has the highest water potential, which is zero. If potato and cucumber cores were to be placed in distilled water, the water potential inside the cells would be exceeded by the water potential of the external solution, resulting in a net flow of water molecules into the cell by Osmosis. This will show a considerable change in mass and length of the vegetable cores.
It is predicted that as the solute potential of the external solution is decreased (becomes more concentrated), less and less water will move into the cells by osmosis, and as a result the increase in length and mass of the cores should decrease. This will continue until the isotonic point is reached, and there will be no net movement of water molecules between the membrane, therefore almost no change in length and mass of the cores.
After this point the solute potential in the external solution will be less than the one inside the cell, causing the cores to decrease in size and mass, and as the water potential continues to decrease, the cells will become plasmolysed.
It is also predicted that the vegetable cores will lose water more easily in sodium chloride solution rather than glucose and sucrose solution, as sodium chloride is very corrosive, and, at a high concentration could damage the cell membranes, allowing more water to escape from the cell.
Apparatus
Potato (large)
Cucumber
Cork borer
Ceramic tile
Scalpel
Test tube rack
32 test tubes (used 3 times each)
Distilled water
Powdered Sodium chloride
Powdered Glucose
Powdered Sucrose
Forceps
3 measuring cylinders
Top pan balance
Method
Mix up correct molar quantities of sodium chloride, glucose and sucrose solutions in the molarity table as below using a measuring cylinder, and place into beakers.
Place the correct quantity of pure distilled water into a beaker, measured using a different measuring cylinder.
Place all test tubes in test tube rack.
Place 20cm of each solution into each of the test tubes, so that all 30 test tubes will be holding each a different concentration of any solution.
Place 20cm of distilled water in another two test tubes, these will be used as the control in order for the test to be fair.
Cut 16 potato cores and 16 cucumber cores and place them on a ceramic tile. Using a scalpel and ruler (calibrated in millimeters) cut the cores at 50mm, taking care that no peel is left and that all the cores are accurately cut at the nearest millimeter.
The cores will be individually weighed on a top pan balance to an accuracy of 0.01grams.
Each of the cores will then be placed into one of the 32 test tubes for 2 hours, timing kept by a stopwatch.
After 15 hours the cores will be removed from the solutions, blot-dried on top of a filter paper, and weighed straight away.
After the experiment has been completed, all apparatus has to be washed and properly placed away, and all organic material should be disposed of.
All results should be recorded on an appropriate table.
The experiment should be repeated 3 times, and a table of averages should be drawn up.
Reliability
There are a few things that have to be considered in order for the test to be fair. All variables, except for the independent variables, should always be kept constant. These variables include:
The temperature. If the temperatures were to be increased, the kinetic energy of the molecules would increase as well, causing the diffusion rate to increase as well.
The length, mass and diameter of the vegetable cores, in order to allow for uniformity.
The volumes of the solutions used, in order to allow for consistency.
The time the cores are left in the solutions for, as more or less exposure to the solutions will have a different osmotic effect to the cores.
The same apparatus should be used, in order to allow for consistency.
The same core samples to be used from the same potato and cucumber, in order to allow for consistency.
- The core samples will only be handled with forceps in order to avoid damaging the cell membrane.
- The solutions and test tubes should be covered in plastic film in order to prevent evaporation and the alteration of the different molarities.
Analyzing evidence and drawing conclusions
The water potential of pure distilled water is zero, as there are no solutes present. It was in pure water where the greatest increases in mass occurred (9.33% increase in average), due to the water potential inside the cucumber and potato cells being far less than that of the water. This caused a substantial influx of water molecules resulting in increases of mass.
The results confirm the hypothesis in that as the solute potential of the solution decreased (i.e. the solution became more concentrated) the changes in mass of the potato and cucumber cores decreased. This was due to the difference in internal and external water potentials becoming smaller.
The increases in mass of the potato and cucumber cores meant that the cells were in various levels of turgidity in the different molar concentrations. These increases in size and mass continued to decrease until the isotonic point was reached, where both the internal and external water potentials are the same. After the isotonic point has been reached, the cells initially begin to undergo slow plasmolysis but this speeds up as the solute potential of the sodium chloride solution is further decreased and the solution becomes more concentrated. This can be seen as a decreasing in mass from the original.
Between 0.0 molar concentration and 0.10 molar concentration, the changes in mass of the two vegetables appear to be very different. The potato cores’ mass seem to drop only by about 2% in Glucose and Sodium Chloride solution, compared to the distilled water, and about 4% in Sucrose solution. The cucumbers however, at 0.10 molar concentration in all three solutions, seem to have reached an isotonic point, Sucrose and Glucose being just under this point by having decreased by 1% from their original mass, and Sodium chloride just being above the isotonic point by 1%. The potatoes, according to the graph, seem to reach their isotonic point at about 0.5 molar concentration in all 3 solutions.
By looking at the graph, a very distinct pattern is shown: the cucumber cores are being more plasmolysed by the solutions than the potato cores. These results suggests that there is a higher water potential in cucumber cells than in potato cells. This could be due because cucumber cell have a larger vacuole than potato cells.
The maximum decrease in mass of potato cores occurs at 0.9molar concentration in all solutions. The scenario does not state whether the flooded plants that have become flaccid in the 0.9 molar concentrations could recover if placed back into optimal conditions, i.e. whether the cells of the plant had become fully plasmolysed. This is where the connections between cells by cytoplasmic strands called plasmodesmata are broken and the cell is non-recoverable. If the cells have become fully plasmolysed then the plant cells are unable to cope with the low external water potential. If the cells have not become fully plasmolysed then recovery could be possible and the effects of osmosis have not been life-threatening for the plant in the short term.
Evaluating evidence and procedures
- It would have been beneficial to have repeated the experiment more times in order to insure a greater accuracy in the results, making sure that the results were not gained through chance or by an external factor.
- Ideally, all samples should have come from the same part of the potato or cucumber, as this would have decreased chances of inaccuracy of results. However, this was impossible to do as 48 plant tissue samples were needed for each vegetable.
- A variety of other similar plant roots could have been placed through the same procedure in order for comparison of different tissues.
- A larger cucumber should have been used, in order to cut the chips out of the same tissue, and trying to exclude the seeds and placenta, as this could have lead to inaccuracy in the experiment.
- Some results could have been inaccurate due to not allowing excess solution on the external surface of the cores from draining away before placing it onto the top pan balance. This superficial water would be measured as extra mass by the sensitive weighing scales, even if the cores have been blot-dried on filter paper beforehand.
- Once the cells have been plasmolysed, it is possible that some of the solution has entered the cell between the cell wall and the cell membrane, given that the cell wall is fully permeable. This could lead to inaccuracy of results, as part of the mass in the plasmolysed plant cores would be caused by this intake of water, which has not been caused by osmosis.
- The experiment was also lmited by the accuracy of the top pan balance, which showed mass in grams to one decimal place.