Osmosis Between Potato Tubes and Sugar Solutions.

Phil Harford 11O

Investigation – Osmosis Between Potato Tubes and Sugar Solutions

Aim: We are aiming to investigate the effect of different sugar concentrations in distilled water on osmosis in small potato tubes.

Trial Run: We did a trial run, and found that it would be much more efficient to put three potato tubes in each petri dish rather than just one, so we are using fewer petri dishes and less solution. We will then take the average weights of the three potato tubes before and after each experiment. It also means we will know that each potato tube in a certain petri dish is under identical conditions. We also found that when we made large amounts of solution much of it was wasted, as we only required very small amounts. Although we are dealing with small amounts of sugar that are difficult to weigh out, we will only make enough solution needed for the experiment to avoid wastage. By using electronic scales instead of manual scales we can accurately weigh the small amounts of sugar required.

Background

Knowledge: I know that when osmosis takes place, water moves through a semi-permeable membrane from a low concentration of a solution to a high concentration of a solution, and although the water molecules move both ways, there is a net movement from a region with lots of water molecules to a region with fewer water molecules. If sugar molecules are involved, water will move to a more sugar-rich region, so diluting the high concentration solution to try and make the concentration on both sides equal. Sugar molecules are too large to pass through a semi-permeable membrane. The structure of a potato is such that it is effectively a semi-permeable membrane.

Prediction: From the scientific knowledge I already have, I can make a prediction on this experiment. Osmosis will be very important in describing what I think will take place. The description of osmosis above shows that when there are two different sugar solutions on either side of a semi-permeable membrane, water tries to move across the membrane to even out the concentrations, and this is what I predict will happen in our potato tubes. The fibrous structure of the potato forms a semi-permeable membrane, and I know that potatoes contain sugar (in the form of starch, a large group of sugar molecules) and a certain amount of water. So I predict that when the concentration of sugar in the potato tubes is greater than the concentration in our sugar solution in the petri dish, water will diffuse into the tubes and so the mass will increase. When there is more sugar in our solution in the petri dish than in the tubes, I predict that water will diffuse out of the tubes, and the mass will decrease.

Key Factors: We could change:

Length of potato tube
Diameter of potato tube
Concentration of sugar
Amount of solution
Type of potato

We are only changing the concentration, and keeping the other four key factors the same. It is very important to keep the other four the same, because they could easily affect the results of our experiment. For example, a different potato may have a different amount of sugar in it, and so the amount of diffusion would be different from one potato to another, so making our results inaccurate.

We have decided to vary the ...

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Key Factors: We could change:

Length of potato tube
Diameter of potato tube
Concentration of sugar
Amount of solution
Type of potato

We have decided to vary the concentration of the solution in our experiment because doing this should hopefully give us some interesting results that we will be able to use to draw a firm conclusion on the effect of osmosis in potato tubes. We know that the concentrations can be measured very accurately, using electronic scales, so this should give us accurate and fair results.

Method: We are firstly going to cut out 33 small potato tubes, 2mm in diameter, and about 20mm in length, group them into three’s, weigh each one separately, and record the weights. We will put each set of three potato tubes in order in a petri dish and assign them a concentration. This is so we know which potato tube is which so our results will correspond. We will then make 10ml of each of the eleven concentrations of sugar solution we are using, from 0% to 10% sugar inclusive at 1% intervals. We will then put the solutions in the corresponding petri dishes at staggered intervals of 1 minute, and then leave each for 30 minutes with the lids on.

We will then take the tubes out of the dishes, dab them dry to get rid of any excess solution that could affect our results, weigh the potato tubes again and record the results in the second half of the table. We will work out the change in mass, then work out the percentage change and record the results.

Equipment:

Safety: To make sure the investigation is safe we will be careful when using sharp knives to cut the potato and wear goggles to stop harmful substances getting in our eyes.

Results: These are the results from the experiment: The numbers are the weights of the potato tubes in grams. We put three tubes in each petri dish and then took the averages. The last two columns are the change in mass in grams and the percentage change:

Using the last column, I plotted the results in a graph of percentage change against the concentrations and then used the computer to work out the line of best fit. I was then able to see where the line crossed the x-axis to give a percentage of sugar in the solution where there would be no change in mass, i.e. there would be no diffusion of water from or into the potato tubes.

I worked out the Change by subtracting the Average Before from the Average After:

e.g. 0.273 – 0.26 = 0.013

I worked out the percentage change by dividing the Change by the original (Average Before) and then multiplying by 100:

e.g. 0.013/0.26*100 = 5.128%

Conclusion: From my results, it is difficult to draw a firm conclusion, as there is a very large spread in the range of my results. The graph shows points plotted from the percentage change in mass at the different concentrations of sugar solutions, worked out from our results. There was no very obvious pattern or trend in my results. However, I have still found there is a gradual downward trend and there is a line of best fit on the graph that shows this, indicating that the mass change in the potato tubes was decreasing as the concentration of sugar solution was increasing. If I extrapolate this line forward a few units, the line crosses the x-axis, and we can work out this exact point by using the equation of the graph as derived by the computer:

y = -0.4872x + 6.3025

The next step is to work out what x is equal to, when y = 0.

-0.4872x + 6.3025 = 0

-0.4872x = -6.3025

x = -6.3025/-0.4872

x = 12.93617

This is the solution to our investigation that would indicate that there is about 13% sugar in a potato.

My conclusion has generally supported the prediction that I made earlier. I predicted that when the sugar concentration in our solution was less than in the potato, water would diffuse into the potato and the mass would increase, and when the concentration in my solution was less than in the potato tubes, water would diffuse out and the mass would decrease.

Evaluation: The results of my investigation were not as good as were expected. There are a number of individual points on the graph that didn’t support the theory, but the line of best fit does give the expected result. On the positive side, I think my group worked well together, and we all made sure we carried out the same task each time to make it a fair test. However, I don’t think our results are as reliable as they could have been, and there were some anomalous results. This could have been for a number of reasons.

The first reason why an error could have occurred was in the final weighing. We had decided before we started that we would dab dry the potato tubes, to get rid of excess water. We were very careful while doing this not to squeeze any water out of the tubes to avoid making our results inaccurate. The absorbing paper towels we used weren’t particularly good though, and it was very easy to either just dab it very gently and so not remove much water or press a bit too hard and squeeze out some of the water inside the tubes. Either scenario would make our results inaccurate.

Another problem we had was not being able to take the samples out of the solution and weigh them after exactly thirty minutes. This was due to working with lots of other groups also wanting to use the scales, so it was difficult to weigh the potato tubes at the correct ending times. Also we only wrote down the starting times in hours and minutes, so if some tubes were put in at the beginning of the minute, and not weighed until the end of the corresponding minute half an hour later, some tubes could have been in the solution between one and two minutes longer, so making our results inaccurate.

Some of the tubes had pieces of skin at the end and others didn’t, meaning tubes without skin at the end would have been able to take in or give out water more easily as there wasn’t an additional layer of skin to get through.

We only had time during our lessons to leave the potato tubes in the solutions for 30 minutes, so we had time cut the potato tubes, make the solutions and weigh the tubes before and after. However, when the sugar concentrations in the tubes and the solution were about equal, osmosis may not have had time to fully complete only been taking place slowly, as we know that water molecules move both ways during osmosis. This could also have made our results inaccurate.

I don’t think our experiment was carried out as well as it had the potential to be and I would make several changes if I were to do it again. Firstly, I would leave the potato tubes in the solution for an hour to give more time for osmosis to take place. Secondly, I would make sure we were very careful with the tubes when we were doing the final weighing. I would use a better absorbing paper towel, like kitchen roll, that would soak up excess water without affecting the water inside the tubes. We could also make bigger tubes which were more sturdy, from which it would be more difficult to squeeze out any water. Thirdly, I would make sure we staggered the weighing of the tubes more effectively, with greater time gaps to avoid any delays in taking the final weighing. To help this problem I would also work with the rest of the people in the classroom so we didn’t all need to weigh our tubes at the same time. I would also record the time to the nearest second at which we started so the weighing times would be more accurate. I would also use a syringe to measure the amount of solution we put in each dish so it was exactly the same each time. Next, I would make sure there were no little bits of skin left on the potato tubes that could affect the results.

To further improve my results, I would leave more time to complete the experiment and leave all the potato tubes in the dishes for an hour to give more time for the osmosis to fully finish. I would also extend the range of concentrations of solutions beyond 13%, to 20% and hopefully we should find that the later results do show a mass loss, and it should hopefully create a more conclusive line of best fit.

To extend the experiment, I would have to measure the mass change with a greater range solutions, such as 0.5%, 1.5% etc. This would give more results and hopefully would create a better line of best fit, so I would be able to draw a firmer conclusion.

I could also extend the experiment by using the technique with different types of potato to see if they had different concentrations of sugar in them. A final way to extend the experiment would be to change the temperature of the sugar concentrations and to see if this had an effect on the rate of osmosis, as at higher temperatures the water molecules will have more thermal energy and will move faster.