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. "Incipient" means, "about to be".
A diagram of the movement of particles in Osmosis:
= Water movement = Water molecule = Salt molecule Semi-permeable cell membrane
Osmotic equalibrium is the point at which the concentrations of the plant tissue and outside the plant are the same. This can be used to find out, for example the concentration of sugar in an apple.
The effects of extreme concentration in a plant cell of:
Salt Water An equalibrium
Pure water has the highest water potential, which is zero. If potato cores were placed into pure 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 cells by the process of osmosis. This will be visible as an increase in length and mass of the potato cores.
It is predicted that as the solute potential of the external solution is decreased (i.e. the solution becomes more concentrated) less and less water will move by osmosis into the cells and as a result the increases in length and mass of the potato cores will be smaller. This will continue until the isotonic point is reached, which is where the internal and external water potentials are equal, and will be visible as no change in mass or length of the potato cores. After this point the solute potential in the external solution will be less than that of inside the cell and therefore there will be a net movement of water molecules out of the cell, resulting visibly in a decrease in length and mass of the potato cores from their original size.
As osmosis is the diffusion of water molecules, and as diffusion is the random movement of particles from areas of high concentration to low concentration, it might be expected that any factors that speed up or slow down the movement of these particles will affect the rate of osmosis.
Plan:
Equipment: A knife (blade)
A potato
Cork borer
Measuring cylinders
Beakers
Safety specs
Ruler
Electronic weighing scales
To make this experiment a fair test I will:
- Conduct two experiments, one measuring length and one measuring weight of the potato cores. This will reduce the chance of getting any anomalous results.
- Keep the length/weight of the potato cylinders fixed.
- Have a control variable of the salt solution percentage.
Method: For my first experiment, with the weights, I will use the cork borer to get 5 potato cylinders. I will then use a knife/blade to cut the potato cylinders to about 3cm. Then I will weigh them all individually by the electronic scales, to two decimal places. After that I will mix the salt solutions up. To do this I will get the 20% salt solution (already mixed up) and put it into a measuring cylinder of 20cm3. I will then measure it up to 10cm3 and add water to the 20cm3 mark. This will make a 10% salt solution. I will keep doing this until I get a 5,10,15 and 20% solutions. These will be held in beakers and the solution at 20cm3. I will then put the potato cylinders into the different solutions and leave them for 2 hours. After 2 hours I will take the potato cylinders out of the solution, dry the surface with a paper towel and then weigh them with the electronic weighing scales. I will do this three times in total to get repeat readings to minimise anomalous results.
For my second experiment, with the lengths, I will use a cork borer to get 5 potato cylinders. I will then use a knife/blade to cut the potato cylinders to exactly 20mm using a mm ruler. I will then mix the solutions up. To do this I will get the 20% salt solution (already mixed up) and put it into a measuring cylinder of 20cm3. I will then measure it up to 10cm3 and add water to the 20cm3 mark. This will make a 10% salt solution. I will keep doing this until I get a 5,10,15 and 20% solutions. These will be held in beakers and the solution at 20cm3. I will then put the potato cylinders into the different solutions and leave them for 2 hours. After 2 hours I will take the potato cylinders out of the solution, dry the surface with a paper towel and use the mm ruler to measure the lengths. I will do this three times with two batches of 20mm potato cylinders and one batch of 30mm potato cylinders. To plot the graph I will need the differences of weights/lengths. So, I will add the three values up for the start weight/length and divide by three. I will do that for the end weight/length as well. I will then subtract the average start weight/length from the end weight/length.
Obtaining evidence:
Table for the weight of potato core:
Table for the average weights and weight difference:
Table for the lengths of potato cores:
Table for the average lengths and length difference:
Conclusion: My experiment showed that Osmosis does take place in plant tissue and that as a more concentrated solution is added the lower the weight becomes.
In pure water the greatest increase in mass and length occurred (0.17g and 3.67mm), due to the water potential inside the potato cells being far less than that of water. This caused a substantial influx of water molecules resulting in increases of length and mass.
The results confirm the prediction in that the solute potential of the solution decreased (i.e. the solution became more concentrated) and the changes in length and mass of the potato cores decreased. This was due to the difference in internal and external water potentials becoming smaller.
The increases in length and mass of the potato cores meant that the cells were in various levels of turgidity in the different concentrations of salt. These increases of size continued to increase until the isotonic point was reached, where both internal and external water potentials are the same. After the isotonic point was reached, the cells initially began to undergo slow plasmolysis. Plasmolysis is the shrinking of the cytoplasm away from the wall of a living cell due to outward osmotic flow of water. But this speeds up as the solute potential of the salt solution is further decreased and the solution becomes more concentrated. This can be seen as a decreasing length and mass from the start.
The isotonic point can be seen on the two graphs but varying slightly. It is the point at which the line passes through 0. The approximate isotonic point was at about 14% salt solution for changes in length and at 10% salt solution for changes in mass. Soon after the isotonic point the cells begin to show signs of plasmolysis occurring. The decreases of mass especially begin to decrease rapidly after the isotonic point.
The experiment does not state whether flooded plants that have become flaccid could recover if placed back into optimal conditions, i.e. whether cells of the plant had become fully plasmolysed. This is where 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 low external water potential. If the cells have not yet become fully plasmolysed then recovery could be possible and the effects of osmosis will not have been life threatening for the plant in the short term.
Evaluation:
I think that the experiment went very well, apart from some possible anomalous results. The differences in weight of the potato cores appeared to have some anomalous results. This could have been caused by the amount of solution left on the potato cores. When I weighed the potato cores I dried them with a paper towel. I could have left some solution on there or not got all of it off. Either way the weight of the potato core will have been different to the actual weight. This part of the experiment is difficult to come up with an accurate and fair method. Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result. I could also find the exact isotonic point (the point at which the internal and external. The graph that showed the difference in length of potato cores was very accurate and produced a best-fit line that did not go through the origin.
Other improvements I could make are:
- As the potato cores came from different parts or a different potato this could have varied the results. Ideally the potato cores should come from the same part of the potato so that they have the same texture.
- The room pressure and temperature would have varied in the 2 hours the potato cores were left. The temperature could have altered the rate of diffusion slightly.
I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, and then I would be able to see the cells in greater detail and draw some more observational results.
If I were to find out more about osmosis I would do a demonstration of how +osmosis works more clearly.
I would use Visking tubing to replace the plant cell wall and fill it with a sugar solution – Glucose. This is because the Visking tubing has tiny little holes in like a semi-permeable membrane. I will then place the tube in a beaker containing pure water.
Diagram:
Pure water
Visking tubing containing Glucose
Because there are more glucose molecules in the Visking tube, the water molecules pass through the semi-permeable membrane to ‘dilute’ the sugar concentration. There is a net flow of water molecules into the Visking tubing. The water molecules can travel both ways through the membrane. The Glucose cannot diffuse into the water because the molecules are too big to fit through the semi-permeable membrane. This causes the Visking tubing to fill up with water. Eventually the tubing will become so full up that it will burst.
Graphs for the two experiments
A graph to show the how the changing concentration of salt solution affects its mass in g
A graph to show the how the changing concentration of salt solution affects its length in mm
