To test the results of this onion investigation further I am going to conduct another experiment this time using sweet potato and normal potato. During this time I am going to place both the sweet Potato and normal potato into solutions of different sucrose concentrations and also one with distilled water. From here I hope to draw conclusions to the water potential of plant tissue and to see if any of my predictions are correct.
Predictions
- From what I have learned about water potential I would expect distilled water to have to highest water potential and therefore both the sweet potato and normal potato to take up the more molecules of water and have a larger end mass then what it began with in distilled water. The two samples of potato should also feel more turgid because of the extra water that has diffused into its cells.
- I think that when I use sucrose solution of 0.5m that the sweet potatoes mass should not change or only have a very small change. I believe this will happen because; as there should be an equal amount of sugar and water in the solution, and the potato. This should cause the particles to flow in and out of the potato evenly.
- As the normal potato contains starch this is not going to be able to move out of the cells as the molecules are too large. When the normal potato is in a solution with a high concentration of sucrose, the potato itself will have a higher water potential then the solution and will therefore end up lighter then its starting mass, because it has a higher water potential then the solution.
- Overall I believe that sweet potato will have a heavier end mass then that of normal potato because the normal potato will have a higher water potential and therefore lose mass when in solutions of a high sucrose concentration.
- I think that the normal potato will become isotonic with the solution, when the solution is around 0.25m in strength because once the solutions get any stronger then this then the water is going to flow from the potato into the solution.
Planning
When doing this experiment I will only be changing one variable and this going to be the concentration of the solution. I am going to be using 5 different strengths of solution; these will be distilled water, then 4 sucrose solutions of strengths; 0.25m, 0.5m, 0.75m and 1 molar (these solutions will have a negative water potential). To try and make the experiment fair I also kept some of the variables the same these included the size of the potato (as much as possible), the amount of the solution and the time that the potato was left in the different solutions.
There were not any major safety issues’ when doing this experiment, the main one was taking care when using the knife to cut the potato. To do this I placed a tile under the potato so that the surface was protected, and also took care while handling the knife so to protect myself and others.
Once the potato samples were cut, I weighed them and then placed them into test tube with 20cm3 of different strength solutions. The potato was then left in the solution for approximately 2 hours 30 minutes. On taking the potato out of the solution I blotted it and reweighed it, to find out the new mass.
Results
On the next page can see two tables of results, these are from the experiment that I done and also a copy of the results supplied by Imogen Freeman. The reason that I also used Imogen’s results was because I did not have time to conduct another experiment, so by using these results it is going to allow me to draw better conclusions, and also allow me to compare my results, and to look for consistencies or differences that might stand out.
Please see attached graphs for experiment one and two, where I have compared the percentage change in mass against the different solution strengths.
Sweet Potato Table of Results 1
Potato Table of Results 1
Sweet Potato Table of Results 2
Potato Table of Results 2
Analysis and Evaluation
As you can see from both graphs and as I predicted the sweet potato had an overall heavier end mass then that of the normal potato. This is due to the fact that the solution was hypotonic to the sweet potato cells and therefore had a greater water potential. This in turn has caused the cells mass to increase and its internal pressure to rise making the cells themselves more turgid.
During my predictions I also thought that the sweet potato would neither gain nor lose mass when in a 0.5 molar solution, from the graphs you can see that this is actually wrong. The point on the graph where the line cross zero is around 0.80 molar and 0.85 molar for experiment 1 and between 0.70 molar and 0.75 molar for experiment 2. To find the exact point that water potential is equal to that of solute potential plus pressure potential, it would be a good idea to try placing the sweet potato in solutions around these strengths but with smaller strength increases.
On experiment 1 the normal potato looks like it has an odd result when placed in a 0.25 molar solution, but because I have only got two normal potato samples in this strength solution, it is very difficult to tell if it really is an odd result. As I had predicted though the normal potato has lost mass throughout most of the experiment. The reason for this is that the sucrose solutions between 1molar and 0.5 molar are hypertonic to the potato cells. This causes the cells to become flaccid and feel floppy to touch, and also means that the potato cells have a higher water potential then the solution.
From looking at the results table for the normal potato it is hard to tell what point the water potential becomes equal to the solute potential plus the pressure potential, but once these results have been transferred to the graph, it is easier to see the rough point at which equilibrium is reached is approximately 0.2 molar for both experiment 1 and 2. As with the sweet potato it would be a good idea to try placing the potato in smaller increase of solution strength from around 0.2 moles to 0.25 moles.
In both experiment 1 and 2 the normal potato that was placed in the 1 molar sucrose solution has lost almost exactly the same amount of weight (just over 29% of its starting mass), leading me to believe the both samples have become fully plasmolysised, have minimum hydrostatic pressure and also feel flaccid. However because the sweet potato already contains sucrose, when it is in the 1 molar solution it only just begins to lose mass and is probably only at the point of incipient plasmolysis, to find out when the sweet potato samples become fully plasmolysised I would need to do another experiment with stronger strengths of sucrose solutions.
When both samples of potato are placed in distilled water they both gain in mass, however I don’t think that either of them has reached the point of full turgor. However if the samples had been left in the solution for longer they would have become fully turgid, and had maximum hydrostatic pressure. When either of the samples becomes fully turgid or total plasmolysised then they are not going to change any more. This is because when the cell is turgid, the vacuole has reached its maximum volume and no more water can enter the cell. When the cell is in a state of total plasmolysis then the vacuole has almost disappeared from the lack of water within the cell.
If I was to do this experiment again, there are several improves and changes that I would make. The first thing I would do is to use smaller increases in solution strengths, at the point where the line on the graph appears to cross 0, this would allow me to find the exact point that water potential is equal to solute potential plus pressure potential. I would also repeat the experiment more times to enable me to find an average result and also spot a spurious result. Something else I could do, would be to leave both potato samples in the distilled water for longer to see how long it takes for them to become fully turgid.
When doing this experiment you are never going to be able to all the samples of potato weighing and measuring exactly the same, and because of this the experiment is not going to be totally reliable. Nor are you going to be able to blot the potato sample’s the same why every time you remove them from the solutions. Again this is going to cause inconsistencies in the results.
References
Examination. (2004) Revise AS and A2 Biology. Uk: Hart McLeod
Indge, Rowland, Baker. (2000) An introduction to AS Biology. Italy: Hodder and Stroughton.
Roberts, Reiss and Monger. (2000) advanced Biology. Spain: Nelson.
Rowland,M. (1992) Biology. China: Thomas Nelson and Sons Ltd.