To ensure the test remains fair I will do all my experiments at room temperature, use the same type of potato, which will be kept at the same size and be put into the pertri dish for the same amount of time. I will also use the same volume of the solution in the test tube so it is kept fair, and also use three chips for each solution, as it will provide me with a varied range results.
Precautions:
*- Measure all the potato chips with a ruler, before placing them in petri dishes, making sure that they are totally submerged in the solution.
Scientific Theory:
*- Osmosis is defined as the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. In a high concentration of water the amount of solute (e.g. salt) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. When two such solutions are divided by a partially permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium). The partially permeable membrane in this case will be the potato cell, as if I were to use an animal cell; it would burst.
Knowing that osmosis will occur across a partially 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 flaccid if water flows out of them, and thus change their volume, we want to test the hypothesis that:
If the concentration of a solution into which a chip of potato is placed is greater than a certain level the chip will contract, and if the concentration is less than that level it will expand. This can be seen in living cells. The cell membrane in cells is partially permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the partially permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in volume and mass because of the extra water. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as flaccid. The potato cells will have decreased in volume and mass.
The greater the concentrations of water in the solution the greater the amount of water that enters the cell by osmosis. The smaller the concentrations of water in the external solution the greater the amount of water that leaves the cell.
Many variables effect osmosis. These variables include:
Temperature
Volume of a solution
Concentration
Weight of potato
Surface area
I have chosen to change the concentration because I hope this will give me a good varied set of results to work with. The concentration will be measured in ml.
Prediction:
*- I predict that at first the mass of the potato tissue will increase, however, as the concentration of salt in the solution increases past a certain point the mass of the potato chips will decrease. Water particles pass from a high concentration to a low concentration through the uptake of water through a semi-permeable membrane. I predict that the lower the concentration of the salt solution in the test tube the larger the length of the potato will be. So in a high salt concentration the potato will have a smaller length. You see When you put an animal or plant cell into a liquid containing water one of three things will happen:
*- If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up and become turgid.
*- If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
*- If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.
Secondary Sources Used:
*- Internet.
*- Ms Amboule’s Biology Lessons.
*- Heinemann Modular Science for AQA.
*- CGP, The Revision Guide.
Safety Precautions:
*- Be careful with a knife and scalpel.
Table of Results:
*-
Use of I.C.T:
*- There was no use in terms of I.C.T.
Analysis:
Patterns and Trends:
*- All of the solutions gained in length, therefore the concentrations was lower outside the potato chip than inside the potato. In order to get an equilibrium (where the line crosses the x axis, the concentrations are the same), I would need to use a more concentrated solution.
Conclusion:
*- From this a firm conclusion can be drawn, if the amount of water in the petri dish is greater than the amount of salt, then the potato chip will increase in size and the more water, the more growth. The opposite is true for salt; the more salt, the more the potato tissue contracts. Therefore the petri dishes containing more water than salt produced an increase in the length of the chip while in the petri dishes in which the amount of salt was greater than the amount of water the potato tissue contracted.
This is in accordance with my prediction and therefore proves that the potato chip will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration).
In my prediction I stated that the change in length would increase then decrease as the amount of salt increases, This is in agreement with my conclusion.
The main anomaly was the potato chip in the 2.5% solution which actually decreased even though there was an equal amount water and salt in the solution. I have attributed this to the fact that there was already water in potato before it was placed in the solution.
Evaluation:
*- I was pleased with the results that I obtained from my experiments; they proved to be accurate as my repeat readings were very close to each other, there were also, as far as I can tell, only one anomalous results, (An anomalous result being a result that does not agree with the general trend of the other readings) however I did not re-measure my points. There is however, always room for improvement. Firstly, I did not take into account the effect of fluctuations in the room temperature on my experiment, though I could not have controlled the temperature inside the laboratory, I could have covered the test tubes with cling-film to prevent any solution escaping through evaporation. Although I did do all my experiments at room temperature, these factors could have been taken into account more thoroughly. Another problem was the fact that I was forced to rely on imprecise tools to cut the potato chips such as rulers, scalpels and my own judgement. Another I could have measured the potato chips more accurately could have been to use a calliper. I was able to partially remedy the problem of different sized chips by calculating the percentage change in length as opposed to the actual change. Yet another complication is that there is no guarantee that the potato was the same density throughout, in fact it is highly unlikely. A final obstacle was that some of the chips remained in the solution while measured the others, an partial answer to the problem would be to empty all of the chips out of the petri dishes first, then measure them all at once.
In terms of expanding my investigation, I could introduce other variables such as temperature (Would osmosis slow down or even fail to occur in the low temperature of a refrigerator?) or the weight of the chip (Would the weight of the potato chip affect the speed of osmosis or prevent it from occurring at all and would a potato chip cut in two react differently when compared with a regular chip?)or even comparing the results yielded by a) one or two chips in separate test tubes and b) one or two chips in the same solution, the possibilities are endless. There was always a chance that the potato that I used was abnormal so a good idea would be to test a series of potatoes. It would be interesting to investigate whether or not other varieties of potato yield different results and trends. And why stop at potatoes? Why not try other similar vegetables such as sweet potatoes, carrots, turnips and parsnips.
By
Tyrone Sinclair
