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What is the isotonic point of a plant cell?

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Introduction

What is the isotonic point of a plant cell? The aim of this investigation is to determine the isotonic point of a plant cell. First thing to do therefore is to establish what an isotonic point is. This is the point at which there is a balance between the concentration of water on either side of a semi-permeable membrane (see diagrams on reverse for more details). This goal could be approached in any of a number of ways, but I have chosen to perform it as explained below. I would just like to point out first that I will do two experiments on two different potatoes so that I can establish whether the isotonic point varied from one plant's cells to another's. I would ideally like to repeated each of these again so that I have a more accurate result, but due to limited time, this will not be an option. Instead, I shall just have to use some common sense, if the results are really close together than I shall assume that the difference is the result of an inaccuracy of the equipment or human error. If the results are fairly far apart however (as I expect them to be (see predictions)) then I shall assume that the isotonic point varies from one potato type to another. Equipment. A full list of the equipment I'll be using is given below. N.B. This experiment, as with many, would be best performed with equipment that would give as accurate a measurement as possible. Unfortunately, also like other experiments, there is a limit on the accuracy of the equipment due to several reasons ranging from cost to current technological levels; the equipment listed below is therefore as accurate as I could get in our labs with the time and resources allotted to me. Experiment 1 * Potato (white) * 6 cups * Sucrose solutions in concentrations of * 0.2 molars * 0.4 molars * 0.6 molars * 0.8 molars * 1.0 molars * Distilled water * Knife * Cutting mat * Paper towels (approximately 4) ...read more.

Middle

As it turned out, the scales agreed with each other almost all of the time, but it was worth doing this because it gave a more accurate view of the ones that were measured differently and thus made my work more reliable. Now is as good a time as any to mention what I think will happen when I perform these experiments. I know from a carrot experiment that I did in class a few weeks ago that a plant cell in sugarless water gains weight, one in slightly sugared water gains less weight in the same amount of time and one in heavily sugared water (near enough at the saturation point) loses weight. This is because weight change through osmosis (the movement of water molecules from a region of high concentration to a lower one via a semi-permeable membrane) only ever occurs if there is a difference between the concentration of sugar outside the cell and inside it; even then only going one way (from a higher water concentration to a lower one). This is because the particles in the fluids (both inside the cell and out) are moving about. The water molecules, unlike the sugar ones, are small enough to pass through the cell membrane and thus gain access to the other side. The side with less glucose molecules would therefore have a larger amount of water molecules moving out because there are fewer holes in the membrane being blocked by them. After this process has gone on for a while however, it gets to a point where the larger amount of water molecules on one side compensates for the fact that there are less holes to get through on that side (due to the glucose molecules) and so the rates of transfer balance. We call this a solution in dynamic equilibrium (see diagram). Based on this, I think that a graph representing the changes in weight from my experiment should look something like the one below. ...read more.

Conclusion

It is therefore better to compare these results with those of other people who had to perform the same experiments under the same restrictions. Compared with these people, my method is good and I seem to have removed more variables than some of those I spoke to (e.g. using two scales to minimise chances of inaccuracy). On top of this, it would appear that my results were very accurate, especially when compared to some of the other people. The other thing that stands out in my experiment is the accuracy of the predictions that I made; I predicted not only that the 1 molar solution chips would lose weight, but also the general S shape of the graph, something that quiet a few people didn't predict. As a result, there were no readings that I would consider to be anomalous in my work. Two points that Ii would like to make here are: 1. Bearing in mind that the question calls for finding the isotonic point of a plant cell, it is impossible to answer because no two types of plant cells necessarily have the same isotonic point. In an ideal world, we would answer the spirit of this question by performing it with every plant type on earth, but obviously this is impractical and 2. Should I have more time and resources in which to discover the isotonic point of any plant cell, I would first perform the experiment as above for each plant cell and then would narrow the range of sucrose solutions accordingly. (so in the case of the white potato, I would repeat the experiment with sucrose solutions ranging from 0.N1 molars and 0.N9 molars).this would allow me to more accurately pinpoint the isotonic point of the cell/cells in question and, if time were still available, I could narrow it still further by using a still thinner range of sucrose solutions (e.g. 0.201molars - 0.209molars). ...read more.

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