Osmosis in potato chips.

Muhammad Khasimullah

GCSE Biology Coursework

Osmosis in potato chips

Osmosis is by definition, the diffusion of water from a weaker, or more dilute solution, to a stronger solution (i.e. down the water concentration gradient) through a partially permeable membrane, and it is this principle which I want to study. My teacher has done the planning to me for investigating the effect of osmosis on potato chips. In order to observe any results, and draw a conclusion from them, I need to thoroughly research osmosis and it’s effect on plant cells. Osmosis can occur in plant cells, and allows processes such as transpiration to occur. It also can change the structure of a cell, to a point where the cell can no longer function. Osmosis is the only way that water can diffuse in and out of a cell.

When the solutions on either side of the partially permeable membrane are equal (in strength), or isotonic, the net movement of water is zero. As there is no difference between the solutions, no water needs to be moved in order to dilute, and osmosis does not occur.

Prediction:

In this experiment, I would expect the different concentrations to show fairly noticeable effects on the lengths of the potatoes, with the water diffusing into the potatoes at a high rate in the pure water solution, and therefore making their size bigger than at the start. As the solutions become stronger, I expect the rate of osmosis to decrease (and as a result, make the potatoes themselves comparatively smaller at the end). Also the potatoes will be quite turgid at the start of the experiment, there will be already some water inside, and because of this I expect, in the stronger solutions, for there to be decrease in length of the potato, due to a reversal in the effect of osmosis. I know that water can pass in both directions (from my biology class notes) and that direction depends on the relative concentrations inside and outside the potato. Using this all information, I can also predict my graph, which should show a negative correlation between % change in length (y-axis) and concentration of the solution (x-axis)

Method:

Firstly, I will take the potato from which my samples will be taken, peel it (so that the skin does not interfere with the experiment), and use a kitchen knife to take 25 cylindrical potato samples. These are then to be cut to 50mm in length. I will then take five clean petri dishes and will label them individually according to the concentration of salt solution e.g. 0.1M, 0.2M etc.

Finally, 5 potato samples are added to each solution, ensuring that the potatoes are all completely underneath the solution. After 30 minutes, the potatoes will be removed from their solutions, and will be measured with a ruler, to find any change in length. These results will be then taken into a table, and the potatoes and solutions will be disposed of.

Fair test:

My independent variable will be the concentration of the liquid that I will place my potatoes into. This is because it is the effect of this on osmosis that I am studying.

Other variables are temperature, volume of the solutions, the time for which the potatoes are left in the solutions, and finally, the number of potato pieces in each petri dish.

Temperature will be kept constant at the average room temperature 21º C.
Volume of the solutions will be measured (50ml) and kept constant across all five solutions.
The potatoes will be left in their solutions for 30 minutes, staggered so as to allow time for each solution to be completed and then emptied at the end of the experiment.
There will be five potato chips in each solution, meaning that there will be five results for each solution. These results will be averaged for each solution, so the effect of any anomalous result on my results would ...

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Temperature will be kept constant at the average room temperature 21º C.
Volume of the solutions will be measured (50ml) and kept constant across all five solutions.
The potatoes will be left in their solutions for 30 minutes, staggered so as to allow time for each solution to be completed and then emptied at the end of the experiment.
There will be five potato chips in each solution, meaning that there will be five results for each solution. These results will be averaged for each solution, so the effect of any anomalous result on my results would be reduced.
The potatoes will all have to be underneath the surface, so as to ensure that the full surface area of the potato is in contact with the solution.

Safety:

Obey the laboratory rules.
Take care while cutting the potato chips with knife, as it is sharp and can cause serious wound.
After finishing the experiment put the apparatus away and clean the working area.

Obtaining evidence:

Results:

Raw data: Initial length of each potato chip = 50mm

These are my results. As you can see, the table relates to one particular potato sample, each one with a starting length of 50mm. Each solution has five samples.

Calculated data:

Each solution has five samples, and the results are then averaged, and this average is used to calculate the change, both numerically and as a percentage. I have also drawn a graph from my data, which shows the pattern of the results more clearly than these tables, and allow me to see the effect of concentration on change in length.

There is one anomalous result, in the 0.2M solution, where the length has decreased significantly more than the other four samples in the solutions.

How the table was made?

E.g. for 0.0M Solution

Final total length = 54 + 55 + 56 + 55 + 55 = 275 mm

Average final total length = 275 ÷ 5 = 55 mm

Change in length = Average final length - Average initial length

= 55 - 50 = +5

% Change in length = (Change in length ÷ Average initial length) * 100

= (5 ÷ 50) * 100 = +10%

Analysing evidence and drawing conclusion:

My results show me that there is an obvious trend in my results; osmosis has worked at an increasingly faster rate going into the potatoes, before reversing, and steadily decreasing in rate diffusing out of the potatoes. This proves my prediction which was that if the solution surrounding the potato chip(s) is weak (more water molecules) then water will diffuse into the potato (length of potato chip will increase), and if the solution surrounding the potato is strong (less water molecules) then water will diffuse out of the potato (length of potato chip will decrease).

I can see that in the first solution, the potatoes have increased in length. This is because they have gained water. The only way they could have gained water is by osmosis, as my background information states. Osmosis has occurred, and water has diffused from the solution into the potato, in an effort to dilute the solution in the potato. This is because the solution is of a weaker concentration (more water molecules) than the solution inside the potato.

This has caused the potatoes to become turgid, with the inner turgor pressure pushing the cell membranes up against the cell walls. This makes them stiffer and much like the stems of plants, which, use their turgid state to hold the plant up. This is different to the other four solutions. In these solutions the potatoes have become limp, decreasing in length, again due to water loss. Again, the only way that water can be gained or lost from a plant cell is by osmosis. However, instead of there being osmosis into the potatoes, the water is diffusing out of the potatoes. This is because the solution inside the potatoes is now of a weaker solution (more water molecules) than the solution in the petri dish, and as a result, the water in the potatoes’ solutions has diffused through the partially permeable membranes of the potato cells, and into the outer solutions. As a result, the inner turgor pressure has reduced, a little in the second and third solution, but a progressively more in the fourth and fifth solution. I think this reduction in pressure has made the cells’ membranes pull away from the cell’s walls, as there is no turgor pressure, and the cell becomes plasmolysed, whereby the plant cannot expand and take in any more water. This makes the potato limp, and weak, much like a plant’s stem which, without water, begins to wilt because there is too little turgor pressure to hold it upright.

The curve on my graph also shows up an interesting pattern and also proves my prediction, which was that the graph would have a negative correlation between percentage change in length (y-axis) and concentration of the solution (x-axis). I can see that my graph starts from top left and finishes at bottom right (negative correlation). The gradient of the graph is decreasing i.e. the graph is flattening off. The reason for that is after a certain concentration the potato cannot lose any more water i.e. maximum water loss. After that if I increase the concentration of the solution surrounding the potato, it will not lose any more water, therefore flattening off the graph.

In the graph, there seems to be one anomalous result. This is on 0.2M solution. There is almost perfect correlation, providing an almost smooth curve. The curve also provides us with an estimate for the osmotic potential, at almost exactly 0.04 M. This is shown by the fact that the curve crosses the original length (x-axis) close to where the 0.04 M bar is. This tells me that isotonic status would be reached at 0.04 M concentration, this being the approximate concentration inside the potatoes.

My experiment shows me that the concentration of a liquid has a very noticeable effect on osmosis, as the bigger the difference between the solutions on either side of the partially permeable membrane, the higher the rate of osmosis down the concentration gradient. I know this because my graph shows a strong negative correlation between concentration and the rate of osmosis (% change in length).

Overall, considering these facts I am satisfied with my investigation and the results obtained, they were proved to be reliable and accurate. I think my results are sufficient to prove my prediction and support a good conclusion.

Evaluation

This experiment went very well, and provided us with results that proved that the concentrations of liquids have an effect on osmosis. My results also proved my prediction that the net movement of water molecules is always from weak solution to strong solution (i.e. down the concentration gradient). I can see this effect more clearly on graph paper, which shows negative correlation between % change in length (y-axis) and concentration of the solution (x-axis) and also proves my prediction. The interesting thing about the graph is that it is starting to level of. I think this is because the loss of water from potato is reaching to a critical value after which it cannot lose any more water (maximum water loss).

However, the experiment was not perfect, and could have been improved. This is most obvious where I have an anomalous result, which does not tie in with the four other results for that solution (0.2M). This anomalous result could have been the result of human error. However, there are many factors that could have been improved upon. Firstly my measurements could have been more accurate; I might have used a micrometer instead of a ruler to measure the length. I could have also measured the width of the potato, adding an extra dimension to my results and therefore more accuracy in my findings. Using the extra information I could have than calculate the volume of the potatoes using the formula lπr2, the formula for cylinders. This would give a completely accurate result, which would justify any change in size, be it in any direction.

Therefore, if one potato increased in width but not length and another increased in length but not width, both changes would be accounted for. The potatoes could also have been cut with a scalpel instead of a knife, making the original measurements even more accurate.

I could also have changed the structure of the experiment to ensure more accuracy. I could have tested more solutions, which would have given me a more accurate picture of how the concentrations affect osmosis. I could also have used more samples to test each solution, which would iron out any anomalous results even more efficiently, and produce a more accurate result for each solution. I could have worked with a larger group so did not all have to share a limited amount of potatoes. I could have pooled results from other groups, to get a wider range of results (Either independently or as a whole class swap results.) I could have used a burette to accurately measure the amount of sugar solution out. Coming back to the cylindrical extractions, I went through the core of the potato, but then to obtain more I used different parts of the potato, the core would be the most saturated water, therefore the concentration of the cut pieces would vary. I could have used cloned potatoes to make them as similar as possible.

The experiment could have taken place over a longer period of time, to accentuate my results, as osmosis would continue until the solutions were all of equal concentration, and the effects of the original solutions would be highly visible. The fact that it did however must be taken into account; so more caution must be used for future experiments.

With the sources available, I think this experiment was reliable and accurate. If we assume that with theses sources if someone does not get any anomalous result, his/her experiment is 100% accurate/reliable then my experiment is 80% accurate.

Total results = 5 Anomalous result = 1

Reliability = (5-1) ÷ 5 *100 = 80%

If I were to repeat the experiment

If I were to repeat the experiment I would not do the length and instead I would do the mass because this would give more accurate readings (more decimal points).

If the inquiry were to be repeated I don’t think I had recommended a potato because it is too fiddly (an irregular shape) difficult to cut and there is too much room for error, but if it was possible with sufficient time and more careful planning and accuracy, sugar solution could be used instead of salt to see if this produced the same results as salt and verifying the hypothesis that osmosis will occur. If osmosis really does occur when water gradient is less than other, the trend should be evident with any substance soluble in solution. We could even change the type of potato to see if all potatoes behave the same during osmosis, providing we conduct the experiment correctly.

Even better, a vegetable that is easy to cut such as a cucumber, something already cylindrical and repeat the investigation (mass not length) on that. At least the slice would lay flat say in a petri dish and maybe completely submerged (encouraging osmosis) producing a more accurate result. I also know that cucumbers have high water content so any change should be more apparent.

However you propose to discover osmosis in potatoes there is always room for human error and under classroom conditions and limitations in time, (which we need more of) to complete our experiment to the standard of something substantial proving or disproving my hypothesis is very difficult.

Secondary sources used:

Advanced Level Biology

GCSE Biology