The water source for making up the solutions will be kept the same to minimise the chances of differences in the number of impurities between the solutions. Greater impurities would lower the water potential.
The concentrations of the solutions used will have to be carefully administered. Inconsistencies between the concentrations used will affect the results of the experiment. The different concentrations will be made up separately for each repeat run of the experiment and will be made using syringes and measuring cylinders accurate to 1ml.
Minor precautions will include, making sure that the apparatus used is clean and not contaminated.
Apparatus
The equipment I will use is as follows:
Ruler
Cork borer
Boiling tubes
10ml syringes
25ml measuring cylinders
Knife (for cutting potato cylinders)
Clock
Balance (accurate to .01g)
Initial setup and method
I will use the cork borer to cut cylinders from the potato. These cylinders will be trimmed all to the same length. Every sample will be weighed individually.
I will then put these potato samples in separate boiling tubes containing different concentrations of sucrose solution. The boiling tubes will all be labelled and left for osmosis to happen.
The potato samples will then be taken out of the boiling tubes, excess water will be removed and the samples will be weighed again.
The percentile change in the mass of the potato will be calculated and the results will be displayed on a graph. A line of best fit will indicate where the overall change in mass of the sample is zero.
The experiment will be repeated.
Dealing with Problems – Preliminary Experiments
Some initial experiments were performed to tackle any unforeseen problems.
I experimented with potato samples of different surface areas, the time the samples were left for osmosis to happen and with the consistency of the controls mentioned above for different variables.
My first set of tests was to identify a suitable size for the cylinder of potato sample and to agree on a reasonable length of time to leave the potato samples.
I found it difficult to accurately measure out the water and sucrose solution to create the different concentrations needed. To overcome this, I found that it was easier to create a large amount of the concentrations needed than to create only what was needed (e.g. 15ml). My apparatus was accurate to 1ml and measuring of 0.5 ml would have caused inconsistencies.
After successfully making up the different sucrose solutions needed for the first trial (1M, 0.75M, 0.5M, 0.25M & pure water), I set up the experiment using different size cylinders of potato samples. I used cork borers with radiuses of 2mm, 5mm & 12mm.
The samples were left for 30 minutes and 24 hours.
The sample with radius 12mm was cut open to see if the cells in the middle of the sample had been affected. The sample used here was the one left in 1.0M sucrose solution. The middle of the sample was slightly turgid; the outer part of the sample was flaccid. I will not be able to use 12mm radius samples in the experiment, the sample is too thick and osmosis would not happen in most of the cells.
The same was done with the samples of 5mm and 2mm radiuses. The 2mm sample had been split sometime during the experiment, but; the 5mm sample was fine. The inside of the sample had the same ‘toughness’ as the outside. I will use samples of potato with 5mm radiuses. A length of 2.5cm should be sufficient; it is enough to be covered by all 15ml of solution.
The samples will be kept overnight. The samples from the first set of results showed little sign of change when left for 30 minutes; 24 hours is a better option. It gives the water molecules enough time to diffuse through the whole of the sample.
My second set of tests was to help me narrow down the range of solution I would use in the final experiment. The first potato samples’ water potential was the same as (approx.) 0.3M sucrose solution. If the second potato samples’ ideal solution is close to this, I can use concentrations closer to this figure.
The results from the second set showed equilibrium of osmosis at 0.45 (approx.). Because the ideal concentrations are quite far apart, I will have to experiment using a wide range of solutions.
Because of this I can narrow down my prediction and say that the sucrose solution with the same water potential as the potato sample is between 0.2 M and 0.6 M. This would be a water potential between -540 Ψ and -1800 Ψ.
Altered Method and Setup
The apparatus will be setup as before.
The potato will be cut using cork borers and a knife to a cylinder with radius of 5mm and length 2.5cm.
The amount of sucrose solution used will be 15ml, and the concentrations used will be from 0.2M to 0.6M at 0.05M intervals.
The samples will be left for 24 hours all together.
The experiment will be run twice, but the samples will be cut from the same potato and both runs of the experiment will start at the same time and end at the same time. The concentrations will be made up twice.
There will be at least 40 samples of potato cut and weighed. These will be sorted into groups according to their mass. The samples picked will be the ones with masses closest together; this will keep the start mass sufficiently consistent.
The boiling tubes will be labelled with the mass and concentration. The samples will be taken out after 24 hours and then weighed again.
The percentile change in mass will be calculated for each sample and the results will be checked over for anomalies. The results will then be displayed on a graph and the ideal solution concentration will be noted and read of against a chart detailing the water potential of different concentrations of sucrose solution.
Results
Table showing percentile change in mass of potato samples left in sucrose solutions for 24 hours.
Other useful observations made whilst recording results
I noticed that the potato samples were which were in the pure lower concentrations of sucrose solution were slightly turgid and ‘stiffer’ than the rest of the samples. These samples had gained water, and the density had increased.
I also noted that the samples from the higher concentrations of sucrose solution were weak and flaccid, they had a rubbery feel. These samples had lost water, the density had decreased.
Conclusion
The graph displaying the percentile change in mass of potato samples in different solutions shows that there is no overall change in mass at 0.365 molar sucrose solution.
When this value is read of against the chart of osmotic pressure of different sucrose solutions; the water potential of this solution is given as -1020 Ψ.
My prediction stated that the water potential of the potato sample would be between -540 Ψ and -1800 Ψ. The prediction was correct.
We can say that the water potential of the potato cells is the same as that of the solution where there was zero percentile change in mass; because a zero change in mass would indicate no overall movement of molecules between the potato sample and the solution surrounding it. Water moves naturally from a high concentration to a low concentration, and because there was no change in mass, we know that the concentration of water in both the potato sample and the solution are equal.
Evaluation – Reliability of evidence, accuracy and strength of conclusion
There were 2 anomalous results. These are identified in the table. These results are likely to be caused by human error. A simple mix up when calculating the concentration of the solution may have been made. It is also possible that the solutions were contaminated, but this seems unlikely because the rest of the results show the expected trend and ‘agree’ with each other and the repeat experiments.
Repeat readings were taken for all samples, all repeat and original readings (except anomalous) were incredibly close together.
I believe the degree of accuracy of my conclusion to be ±200 Ψ. There are several reasons for the stated degree of accuracy.
The syringes used to measure out the sucrose concentrations would have introduced some inaccuracies. It is these inaccuracies and also taking account of the fact the water used to make up the solutions may well have had some impurities, which leads me to state the above level of accuracy.
Something else worth considering would be the temperature at which the samples were kept at. The samples were left for 24 hours and may have decayed slightly in that time.
There were some other minor factors. The balance used had a 0.01g degree of accuracy. This would have affected the calculated percentile mass change. This change would however be so small is can be disregarded.
Some parts of the potato cut would have had a different makeup to other parts of the potato. There could be impurities or air pockets within samples. This would affect the mass of the sample; this in turn would result in a random error.
The cork borer would have cut into the potato at different angles. Since the potato cells are arranged in a particular way, cutting through at different angles would have altered the number of cells exposed and would have killed a different number of cells. This would have produced a slight random error.
The potato samples were trimmed to lengths of 2.5cm, the cutting was done by hand. This would have also created a small random error.
Excess water was removed from the potato samples before weighing them a second time. This was done by rolling them down a paper towel. This would have resulted in a small random error.
If I was to repeat the experiment, I would use distilled water instead of regular tap water and would make sure that the temperature of the solutions was constant.
