Prediction
I think that as the concentration of the sucrose solution around the potato cylinder decreases below 0.3M the potato will gain mass as the water in the sucrose solution will enter the potato cylinder. I think this because I know roughly that a potato has a solute concentration of more than 0.3M. As the concentration goes above 0.6M I predict the potato cylinder will loose mass because the potato is most likely to have a smaller concentration than 0.6M. I know the concentration of a potato roughly but all the potatoes are different so I have left a space in my concentration of a potato cell prediction as the real result will vary between these values. My prediction is that the potato's solute concentration will be somewhere in between 0.3M and 0.6M. This leads me to think of the resulting graph to have a best fit line which has negative correlation and falls as the concentration decreases. I know this because osmosis works in a way that water molecules move from a low solute concentration to a high solute concentration.
Preliminary experiment
For this I was advised to use 5cm of peeled potato (a diameter of 1.2cm), distilled water and 1M sucrose solution. I had to make two potato cylinders and leave one in water and one in a 1M solution (each 30ml) for 15 minutes. I got the percentage change by measuring the potato's length before and after the experiment. I did this to give me a rough idea of what to expect from this experiment and to change any variables to keep constant for my main experiment, like the time or the initial length. I carried this out and got this result (on a graph):
This shows a very simple conclusion that the potato's solute concentration is 0.6M because when the line passes 0% of an average change then there is no net movement past the semi-permeable membrane so the solutions are isotonic. It is only a prediction and the actual concentration might be more or less than this, because this is only a best fit line against only two high and low points so it is not accurate enough. I also did a control test (look at method) with this and there was no average change.
I changed some variables for my real experiment once I looked into more accuracy and ease. The first thing I changed was to use the weight of the potato for the percentage change. It is much easier because an electric balance is much more accurate than measuring the length as this can lead to errors in judgement. I also changed the diameter of the potato cylinder, the length of the potato cylinder, and the time the potato cylinder had to be in the solution for. These were mostly because it made the results much more accurate and reliable (time and weight), but I chose some to be changed while I was doing the experiment (diameter and length of the potato cylinder).
Method
Apparatus list:
Potatoes
Cutting tile
Boiling tubes
Test tube rack
Measuring cylinder
Cork borer
Electric balance (to the nearest 2 decimal points)
Scalpel
1.0 M sucrose solution
Distilled water
Stopwatch
I started off the experiment by getting all the equipment and using the cork borer to cut a cylindrical area of the potato. The size of the borer was a 12 gauge and I used the scalpel to make the length of each piece 4cm each time. I had to also take off all of the remaining skin on the potato piece because that is a stronger and different permeable membrane that I wanted to test. I wanted the all the surface area of the potato to be the same type of membrane and changing it will change the results. I did the cutting on a cutting tile to refrain the scalpel from cutting the table. Then I used the balance to weigh the potato cylinder in grams (to the nearest 2 decimal places). The amount I measured for each test tube was 30ml of solution. To make the various concentrations I had to dilute it with a curtain amount of distilled water (e.g. 15ml of solution and 15ml of distilled water to make a 0.5M solution). The range of concentrations I tested were 0M (water), 0.1, 0.2, 0.5, 0.6, 0.7, 0.8 and 1M. Once I put the potato cylinder in with the sucrose solution in the boiling tube, I started timing and put the boiling tube in the test tube rack. The time limit for the potato to be in the solution was 30 minutes because this is enough time for some significant change to happen and to get a good graph.
When the 30 minutes was up I took the potato cylinder out of the boiling tube and weighed it again, making sure I dabbed the potato piece to get rid of excess solution so I don't weigh the solution with the potato. From the results (initial mass (at the beginning) and final mass (at the end)) I could work out the percentage change. This is because I could not make every potato piece the exact same weight and percentages are much easier to work with because when it is on a graph it is easier to see the trends and it is so widely used. I used this formula to work it out and round it up to the nearest 2 decimal places if required:
% Change = Mass after adding solution - Mass before adding the solution x 100
Mass before adding the solution
OR
Percentage (%) Change = Initial Mass - Final Mass x 100
Initial Mass
A control is necessary for this experiment to find out if there are any other variables in the air changing the weight of the potato by any other way. If there is no change when the potato is in the air for 30 minutes I then know the variables in the room are constant enough for a fair test.
I had to make sure I took the right safety precautions as well. I was working with a scalpel so I had to be careful when I am cutting the potato cylinders and when I am walking around the room with it. The balance was electric so I had to take care when handling the solution and distilled water around it.
Fair test
To make a reliable and accurate test I had to conduct a fair test. The first thing that should always be done after an initial test is a retest to find out an average of the two so I could straighten out any anomalous results. I also made notes on what I used in the experiment, like what cork borer gauge I used and the length of each piece so when I repeat the test again and I can keep these variables constant. This is important as I want to only change the concentration of the sucrose solution and if I don't keep the other variables constant I will have very unreliable results with unexpected outcomes. The only thing I changed was the concentration of the sucrose solution and nothing else that I did not mean to change, e.g. different potatoes. Variables that I kept constant were the potato, temperature, and gauge of cork borer. The temperature is important because it can damage the cell membrane if it is too high or it can reduce the activity of the water if it is too cold, so doing all the experiments in the same room and as close as possible is important. Changing the cork borer can also change the surface area and thickness which are both important variables to keep constant.
Different potatoes could have different concentrations of water so if I change the potato it could affect the result but sometimes there is not enough in one potato to use so I had to change potatoes but only when I really needed to. No experiment is ever perfect and sometimes I can put different amounts than the intended of the liquid in the solution. This can happen but it does not make a significant change if I do some repeat experiments and try to keep the other variables constant. The way I avoided this was to carefully measure out the solutions with a measuring cylinder to the drop, so I got near exact measurements.
Table of Results
As you can see I have done 16 experiments including repeat ones and put them on this graph as a whole. I have worked with percentage change most of the time and found out the average. For the control, the mass did not change and so I know that the variables in the room for those 30 minutes of the experiment were constant. The graphs I have done are an average percentage against concentration and my other one is all the results from all my experiments (including repeats, but not average %) against concentration and added error bars to analyze the accuracy of my repeat results to my first results.
Concentration (M)Initial mass (grams)Final mass (g)Change in mass (g)Percentage Change (%) correct to 2 d.pAverage percentage change (%)
0.0 (water)9.49.60.22.132.075
9.910.10.22.02
0.110.010.10.11.281.195
99.10.11.11
0.28.78.80.11.151.1
9.59.60.11.05
0.510.19.8-0.3-2.97-4.01
9.99.4-0.5-5.05
0.69.39.0-0.3-3.23-3.915
8.78.3-0.4-4.60
0.798.9-0.1-1.11-2.12
9.69.3-0.3-3.13
0.89.18.8-0.3-3.30-4.28
9.59.0-0.5-5.26
1.010.39.7-0.6-5.83-5.855
10.29.6-0.6-5.88
ControlInitial mass (g)Final mass (g)Change in mass (g)
2.72.70
Conclusion
On my average graph the main trend correlation is negative. The percentage change falls as the concentration rises and this makes the two approximately proportional when the best fit is added in. The best fit line passes through the concentration (M) line at approximately 0.25M. This is the concentration of a potato cell that I was trying to achieve (the isotonic point), where the concentration of the sucrose solution is the same as the potato's solute concentration. It means that the potato cell has a solute concentration between 0.20M and 0.30M approximately. I can say that the sucrose concentrations below 0.25M is the hypotonic solution and sucrose concentrations above 0.25M is the hypertonic solution (approximate values) when it is put in with a potato.
The error bar graph shows how different each repeat experiment was and this tells me how accurate I was in my experiment on each concentration. Each point has a line with a marker at the end (to identify when it starts and ends), which is to represent the error margin, and the further away the these lines are from each other then the less accurate the experiment. These are not very far away on the smaller concentrations like 0M up to 0.2M (very accurate results), but higher up on the concentration scale the results seems to be varied a bit more, but the lines are still touching on some concentrations (some variation on the results). It is definitely because of one or more variables changing. This could be because of different conditions, different potatoes or anyone of those. There were some anomalous on the graph in this same area as well. They still fit in the trend but do vary quite a lot around the mid point of each concentration.
My prediction was nearly right in thinking that the range of concentration was between 0.3M and 0.6M. The actual concentration was 0.25M so I was near the range of results. I was right about the trend of the best fit line being in a negative correlation.
Evaluation
My procedure in the experiment showed up as good in the error bars, but there are some variables that I should have controlled better. I should have chose a smaller gauge for the potato cylinder to keep the results fair because I had to change the potato more often than if I was to use a smaller gauge (not enough potato to do all the initial and repeat experiments). I could have tested more concentrations like the ones in between 0.2M and 0.5M (0.3M and 0.4M) or maybe even more (0.25M) to get a better best fit line. Cutting the potatoes was the hardest part of the procedure as I had to get the length exactly right and make sure I took off all the skin as well. If it was available I would have used a machine to cut it accurately as to get it the same weight and length to get a very fair experiment for all the concentrations I tested.
When I was ready to weigh the potato cylinders I could have accidentally dried some potatoes out more when they came out of the sucrose solution than others, therefore making a difference in weight. If I had the equipment and the time I could dry out the potatoes in a very fair and even way to ensure that they are all treated the same. Although the procedures could use some improving the experiment was a success.
Extension Work
For extension work I could change another variable like temperature of the sucrose solution to see how it affects water passing though a partially permeable membrane by osmosis. I could test much more concentrations (0.05M, etc.) and take more repeat experiments. It could be left for longer in the solution to get more net movement (if there is any) and get more curtain results over a large percentage change scale.
