Investigating Osmosis: The Molarity Of A Potato Cell.

Daniel Rios-Perez

Investigating Osmosis: The Molarity Of A Potato Cell

Introduction

Knowing that osmosis (diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells, they will either become turgid if water flows into them, or plasmolysed if water flows out of them, therefore changing their volume.

I want to test the hypothesis that:

If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will decrease in mass, and if the concentration is less than that level it will increase in mass.

Prediction

I believe from my past knowledge of a similar experiment involving raw eggs that the potato cylinders placed in the distilled water will be hypertonic, therefore the water will diffuse by osmosis into the potato from an area of high concentration down the concentration gradient. This will result in a gain of mass for the potato.

However in the 1.0M solution I think that the opposite will occur as there will be a higher concentration of water within the potato than the solution, this will make the potato hypotonic and it will lose its water content as the water diffuses out. A difference between the egg experiment and the potato experiment is that the potato cells will not reach a dynamic equilibrium (become isotonic). This is because plants have a cellulose cell wall which cannot expand beyond a certain point as it is turgid, therefore the contents will never be able to be equal to the outside or it will not be able to give out all its water as it has a rigid structure. It is the pressure created by the cell wall which stops the cell reaching a dynamic equilibrium.

I think that the molarity of the cell will be about 0.3M concentration as the point at which the potato cylinders will not change in mass will be between 0.2M and 0.4M.

Method

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I think that the molarity of the cell will be about 0.3M concentration as the point at which the potato cylinders will not change in mass will be between 0.2M and 0.4M.

Method

The experiment was set up to be as fair as possible. A cork cutter was used to make 6 potato cylinders. These were not the same size, so the cylinders were then all accurately cut to 4cm with a razor blade. Once the weight of each potato cylinder was measured and recorded, the potato chips were placed into their 10ml test tube solutions of 0.0M (distilled water), 0.2M, 0.4M, 0.6M, 0.8M and 1.0M of sucrose. Then the tubes, each containing one potato cylinder were left for 15minutes. When the time was over the potato was removed with a pair tweezers and placed on a paper towel to try and dry off any excess water. The potatoes were then weighed with some electronic weighing scales and their new weights were recorded. The change in mass was worked out into a percentage and recorded in the table shown below. The experiment was then repeated a second time.

Results

First attempt:

Second attempt:

Conclusion

The first observation I made was after the potato cylinders were placed in their solutions I noticed that the potato cylinders in the 0.0M and 0.2M solutions were floating and the ones in the 0.4M, 0.6M, 0.8M and 1.0M solutions were at the bottom of the test tube, this led me to drawing my first conclusion.

The graph (see attached) shows that the potato in the 0.0M solution and in the 0.2M is hypertonic, as I said in my prediction, this means that there is a higher water potential in the distilled water and 0.2M solution than in the potato, which has a high concentration of solutes. Therefore this is why the water diffuses by osmosis down the concentration gradient from high concentration to low concentration, resulting in the potato gaining mass.

The opposite occurs in the solutions where the molarity is higher 0.4M, 0.6M, 0.8M and 1.0M, the potato in these is hypotonic because there is a higher concentration of water inside the potato than in the solutions. Therefore the water diffuses out of the potatoes by osmosis, down the concentration gradient and into the solutions outside, this resulted in the loss of mass.

Another important fact is that the graph makes a shape that will result in the potato not being able to take in any more water or lose any more. Like I said in my prediction the cell wall causes the pressure that prevents this from happening, therefore the cell will never become isotonic like an egg.

The point on the graph where the line crosses the x axis is where the potato neither loses nor gains mass. This happens at 0.32M, therefore the molarity of the potato cells in molar strength is 0.32M. This result is very close to my prediction of 0.3M and it shows that the molarity of the cell is between 0.2M and 0.4M which is why in these concentrations the potato cylinders gained and lost mass.

Evaluation

I think that the experiment went well. I obtained odd results in my first attempt at the experiment. I think that this was because the potato cylinders and sucrose solutions were not measured out accurately, therefore it was not a fair test. But on the other hand, my second attempt at the experiment went very well and my results produced a good graph that agreed with my predictions. However, there were a few areas where there could be improvement. Firstly, when I dried off the excess water on the potato cylinders after the experiment and before I weighed them, I used a paper towel. This might have either taken some water out of the potato or it might have left some excess water on the potato. It is difficult to come up with an accurate and fair method to do this part of the experiment as other ways would also lead to some slight mistakes.

Also the potato itself was not definitely from the same potato and was not exactly the same size, although I did try to cut them to 4cm each. This could have affected the amount of water gained or lost.

Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result.

Also the relatively low number of solutions tested (6) suggests that there is a range of possible values for the molarity of the potato cell, therefore that we cannot accurately predict values for the change in mass at different concentrations. Finally, the results obtained in this experiment support my hypothesis, and I was also able to discover the approximate molarity of a potato cell.

Teacher Reviews

Here's what a teacher thought of this essay

Stevie Fleming

29th of May 2013

*** This candidate has collected some valid data, analyzed it fairly well and come to a valid conclusion. The understanding of osmosis seems poor in some sections of the report. To improve: Planning: The background theory on osmosis was confused in several places and was not explained clearly in all paragraphs. The planned experiment could be used to collect valid data but there was insufficient consideration of the key variables and the written method lacked key details. No risk assessment was carried out and this is awarded marks under newer schemes. Carrying out: Some data was collected but there very different sets of data between the two experiments. The candidate needed to take more care to ensure that table headings contain full descriptions and units for each column. Analysis and Evaluation: The graph was not included. The data seems to have been interpreted correctly and a sensible value obtained for the molarity of the solution inside the potato cells. The conclusion does refer back to the prediction and contains an adequate amount of background theory. The evaluation could be improved by including more detailed suggestions for improving the method or extending the range of the investigation.

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