The following is a diagram showing how the apparatus should be set up:
In terms of safety, I must make sure that I am very careful with the scalpel as it may be very sharp, and I must also make sure that I do not drink any of the sucrose solutions.
Prediction: I predict that for low concentrations of sucrose solution below a certain point (the isotonic point), the mass and length of the potato cylinders will increase, and I think the lower the concentration, the more the mass and length will increase.
“Osmosis makes plant cells swell up if they’re surrounded by weak solution and they become turgid.” (Parsons)
This is because when there are low concentrations of sucrose solution, there is a higher amount of water outside the potato than in it, so osmosis takes place from the solution to the potato, so water molecules enter the potato, therefore increasing its mass and length.
And I think for high concentrations of sucrose solution above the isotonic point, the mass and length of the potato cylinders will decrease, and I think the higher the concentration, the more the mass and length will decrease.
“Osmosis makes plant cells become looser, if they’re surrounded by strong solution and they become flaccid.” (Parsons)
This is because when there are high concentrations of sucrose solution, there is a lower amount of water outside the potato than in it, so osmosis takes place from the potato to the solution, so water molecules enter the potato, therefore increasing its mass and length.
I also predict that the isotonic point is approximately 0.15mol.
I am basing this prediction on a preliminary experiment that I have already performed.
The following graph represents my prediction:
This experiment should give a set of results showing the mass and length change of the potato cylinders, after osmosis, with the different sucrose solutions.
OBTAINING EVIDENCE
Modifications:
In my plan I stated that I would place the potato cylinders in the following concentrations of sucrose: 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0. However, I was informed by my partner that the isotonic point is not around 0.15 mol as I had thought, but is more around 2.5 mol, therefore I decided to use a 0.3 mol concentration instead of the 0.1 mol concentration.
I also said that I would use 25cm³ of sucrose solution to put the potato cylinders in, but I used 100cm³ instead because 25cm³ of solution did not fully cover the potato cylinders in solution, given the beakers we were using.
I also had to devise a way of being able to match up the starting mass of each individual potato cylinder, to it’ finishing mass, given that we were putting 3 potato cylinders in 1 beaker. I attached a pin to the 3 potato cylinder, keeping them in order and noting this down. I never mentioned a way of doing this in my plan.
I also said that I would leave the potato cylinders in the beakers for a 20 minute time period, but I realised that this is not long enough to obtain decisive results so I left the cylinders in the beakers for 24 hours.
I also said that I would repeat the whole experiment 3 times, but I did not have nearly enough time to do this, but I still used 3 potato cylinders for each concentration of solution as I said I would in my plan, and I think this ensures accuracy and reliability enough.
Here is a table showing the results I obtained:
The following table shows the mass and length changes of the cylinders:
When I refer to ‘conc.’, I mean the concentration of sucrose solution.
Also, for each concentration, I have 3 sets of results, as these are for the 3 different potato cylinders I used for each sucrose concentration.
The following table is that which I will use to draw my conclusions from. I have averaged the percentage mass and length change for the 3 cylinders I used for each concentration.
ANALYSIS
I have decided that the best way of presenting my results is by constructing a line graph, with a line of best fit as well as by joining the points. I have done one of these graphs for both average percentage mass change, as well as average percentage length change. The line graphs showing my results are on a separate sheet of paper.
My results show that with sucrose concentrations of 0, and 0.2, the mass of the potato cylinders increases, and with all the other concentrations higher than these, the mass decreases. My graph showing percentage mass change shows the isotonic point and the potato cylinder concentration, to be 0.22. This means that referring to my graph, all concentrations of below 0.22, the potato mass increases, and for all concentrations greater than this value, the potato mass decreases. This means that generally, the closer the sucrose solution concentration is to 0.22, the less the mass change is going to be.
My results also show that with sucrose concentrations of 0, and 0.2, the length of the potato cylinders increases, and with all the other concentrations higher than these, the length decreases. My graph showing percentage length change shows the isotonic point and the potato cylinder concentration, to be 0.23. This means that referring to my graph, all concentrations of below 0.23, the potato length increases, and for all concentrations greater than this value, the potato length decreases. This means that generally, the closer the sucrose solution concentration is to 0.22, the less the mass change is going to be.
However, there is one problem with my results. Both my graphs show that sucrose solution concentration is not actually proportional to the mass or length change. There is a fairly strong negative correlation shown by both graphs, but it is by no means even close to perfect as the line of best fit does not pass through all of the points. One reason for this is human error and inaccuracies which I will later discuss in my evaluation. Another reason may be plasmolysis.
“Plasmolysis is when water moves out of the cell, to such an extent, that the cell wall is pulled away from the cell wall. The cell may be damaged as a result.” (Mr.Greenaway)
I think this explains why both graphs show that for sucrose solution concentrations of about 0.6 and upwards, the potato cylinders do not lose mass or length at such a rapid rate, that is why the points form a curve at the end. I think this is due to plasmolysis, the cell becoming damaged as it loses such a large amount of water to the high sucrose concentrations.
Both sets of my results, for mass and length change, show very similar patterns.
Conclusion: I conclude that for low concentrations of sucrose solution below a certain point (the isotonic point), the mass and length of the potato cylinders increases, and I think the lower the concentration, the more the mass and length increases.
“Osmosis makes plant cells swell up if they’re surrounded by weak solution and they become turgid.” (Parsons)
This is because when there are low concentrations of sucrose solution, there is a higher amount of water outside the potato than in it, so osmosis takes place from the solution to the potato, so water molecules enter the potato through the partially permeable membrane, therefore increasing its mass and length.
And I think for high concentrations of sucrose solution above the isotonic point, the mass and length of the potato cylinders decreases, and I think the higher the concentration, the more the mass and length decreases.
“Osmosis makes plant cells become looser, if they’re surrounded by strong solution and they become flaccid.” (Parsons)
This is because when there are high concentrations of sucrose solution, there is a lower amount of water outside the potato than in it, so osmosis takes place from the potato to the solution, so water molecules leave the potato through the partially permeable membrane, therefore decreasing its mass and length.
I have also found that the isotonic point, the point where osmosis does not take place as there is no net movement of water because the sucrose concentration is the same on the outside as it is in the inside of the potato, to be 0.22 or 0.23 molar concentration.
I think that my results may also show that plasmolysis takes place in potato tissue when it is surrounded by concentrations of sucrose higher than about 0.6, but this s not certain as my experiment was not designed to find this out.
I think some aspects of my prediction were correct, and some were wrong. I predicted correctly the pattern, of the higher the sucrose solution concentration, the less the mass and length increase or decrease is. However, I predicted the isotonic point incorrectly. I thought that it would be around 0.15, but I have actually found it to be 0.22 or 0.23. Also my prediction did not take plasmolysis into account so I expected the solution concentration and the mass and length chain to be a little more proportional.
EVALUATION
Overall I think that the experiment went quite well. I am pleased with my results and I think that they are fairly accurate, especially as they proved the general pattern that I predicted. However, my experiment was not absolutely perfect and I think there are many faults and ways in which I could have improved it.
Experiment Method Criticism:
To start with, I do not think the experiment was accurate enough. The Vernier callipers for measuring length, and the top pan balance for measuring mass, were very accurate. However, the measuring cylinder was not accurate enough. We were making up concentrations of sucrose solutions, and the measuring cylinder was not precise enough to get the exact volumes and concentrations correct. I think the measuring cylinder created too much room for human error to take place. I think this contributes to the reasons why the line of best fit on my graphs does not go through all the points. I would improve on the experiment by using a pipette to measure out the volumes of sucrose and water.
I think that the experiment is actually quite reliable. If you look at my results tables, all the replicates for each concentration give pretty much the same results.
I do not think the experiment was very suitable. This is because plasmolysis takes place after a certain concentration, so it is difficult to establish a trend when this happens. Also, the percentage mass and length change should be approximately the same at least, but looking at my table, most of the time the percentage values are not even similar. I think this may be because he experiment is not suitable.
I think it would be very interesting to do the experiment again with many improvements and modifications. I think it would be interesting to use more sucrose concentrations, especially around 2.22, the value I found to be the isotonic point. I think I would be able to find a more accurate value. It would also be better to use more replicates. I would also use more accurate equipment, mainly a pipette rather than a measuring cylinder. I think with the new experiment, up till the point of plasmolysis, I may be able to get a line of best fit that runs very close by all the points.
I think it would also be interesting to investigate more about plasmolysis, and the point where the cell tissue plasmolyses. I could use more concentrations around 0.6, the point I approximate to be where the potato tissue starts to plasmolyse.
It would also be interesting to use a different plant tissue, possibly a similar root crop. I think the pattern would be the same, but the actual values and the isotonic point, would be different.