Osmosis in Potatoes.

GCSE Science Coursework: Osmosis in Potatoes

Planning

Introduction:

Osmosis is the special name used to describe the diffusion of water across a membrane, from a dilute solution to a more concentrated solution. In biology, this usually means the diffusion of water in and out of cells. Osmosis is just one special kind of diffusion because it is only water molecules we are considering.

Molecules will diffuse from where there is a lot of them to a region where they are fewer in number; that is, front a region of highly concentrated molecules to a region of lower concentration (below).

In a plant cell, when water molecules enter it, the vacuole will fill and push against the cell wall. The cell will become very firm, or turgid.

If a plant is in an environment where there is not much water, water will leave the plant cells and move into the cells with less water. The vacuole will shrink and the plant will wilt. When even more water leaves the cell, the cell membrane tears away from the cell wall. This is called plasmolysis.

To prove this, I will be performing an experiment in which I will place potato cylinders into various sugar/water solutions for 24 and observing the effect this has on the potatoes.

Method:

Use the boring rod to cut out 15 cylinder shaped pieces of potato of equal volume, cut them into 5cm strips.
Dry the chips with a paper towel, and record their weight.
Put three pieces of potato each into 5 tubes.
Label each tube with 0.0m, 0.25m, 0.5m, 0.75m and 1.0m.
Pour the correct solution into the corresponding tube, just covering the potato.
Leave the tubes for 24 hours. After 24 hours is up, dry the chips with a paper towel, then weigh each potato chip and record the weight.
Also record the texture of the chips.
Work out the change in weight (M2-M1 / M1 x 100)
Record these results.

Fair Test/ Safety

This is a preview of the whole essay

Fair Test/ Safety

To keep this test reliable and accurate, I have to consider many factors. Doing all the tests at one temperature will control the temperature. For the purpose of my experiment I am going to do all the experiments at room temperature.
To keep the water potential of the potato initially will be kept the same by using the same type of potato, which have been treated in the same way, e.g. have all been cut without being washed and peeled.
The size of the potato is very important. This is because if the size of the potato varies,
so will the amount of mass is looses or gains. I will cut all the pieces of potatoes, to the
same size. We will be given potato chips with the same width and dimensions, but the
length of them will differ, (due to potatoes being of uneven shapes). The potato chip will be 5cm in length. The type of potatoes we use are going to be kept the same, because different potatoes may absorb at different rates. For this experiment I am going to use the same brand.
The time the potato chip is left in the solution must be kept the same in each
experiment. This is because more or less solution may be absorbed depending on time. I
am going to keep each potato chip in each solution for 24 hours, in a boiling tube.
The surface area of the potato chips will be kept the same by having all the chips the
same size. This must be kept the same because the amount of surface area exposed to
the solution may effect the rate of osmosis. The thickness of the chip will already be the
same, so I will cut the length so that each chip is 5cm long.
To make the mass readings more fair, I will take each chip of the solution, roll all the surfaces gently on a paper towel, to remove all excess solution, and I must not squeeze the chip.
I am also going to use the same balance to weigh my potato chips. This is because the measurements may slightly vary between scales.

To ensure safety , the boiling tubes will be kept in beakers so that they can not be knocked over.

Prediction

I predict that the higher the concentration of sugar solution the more water will move out of the potato chip. I also predict that the more sucrose the solution contains, the softer the chip will be, due to loss of water from a cell. The less sucrose the solutions contains, the firmer the chip will be. We know that osmosis the flow of one solvent, (water) of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. As the potato is a plant cell, it contains a vacuole and a cytoplasm. The cell membrane is also partially permeable i.e. it lets some substances in but not all substances. This means that water paeticalscan diffuse into cells by osmosis if the cells are surrounded by a weak solution. (Osmosis is the movement of water molecules from a high area of water potential to an area of low water potential through a selectively permeable membrane.) If the cells are surrounded by a stronger solution, e.g. sugar water, the cells may lose water by osmosis.

Obtaining Evidence

Analysing Evidence

See graph attached.

My prediction was accurate, the potato chips in the solutions with less sucrose gained in mass, and became firm. The potato chips with more sucrose lost mass as became soggy and flabby.

My graph shows the molarity of the solution plotted against the percentage of change in mass. There was one anomalous result, which was plotted, but not joined up to the curve. The graph shows that when the sucrose in the solution reaches higher than 0.25 percent, the potato chips start losing weight. The most dramatic difference was between 0.25 and 0.5 percent, when the graph dipped suddenly.

Overall, the graph corresponds to my prediction that the higher the amount of sugar in the solution, the higher the loss in mass. It also proves that this happens rapidly. Although I didn’t predict that.

Evaluating Evidence

My results were, on the whole, accurate. Apart from one anomalous result at 1.0 molarity, which was disregarded. I believe this happened because I dried the chips too vigorously when I took them out of the solution, which would have a great bearing on the results.

I believe the method we used to investigate Osmosis was the best possible and I can’t really think of a better one right now.

We tried to carry out the experiment as fairly as possible. However, we ran out of potato a few times and had to keep using different potatoes, which may have affected our results slightly.

I think that if ten solutions were used instead of five (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 ) were used, we would have got a much more accurate overlook, especially when it came to the transition between 0.25 and 0.5 molarity.

I believe that even with the anomalous result, the measurements were good enough to support my prediction. Maybe to get a view of how Osmosis works in different situations, a different plant could be used.