If you place a plant cell, or even an animal cell in a medium, different things could happen to it depending on its situation. If the water concentration of the cells cytoplasm is lower than the concentration of the medium surrounding the cell, which is a hypotonic solution, then osmosis will result in the cell gaining water. As this carries on, it could result in swelling up the cell or even bursting it. This is because the water in the cell comes in faster by osmosis, than it leaves the cell by diffusion. If the water concentration inside the cell is the same as that of the surrounding medium, which is now an isotonic solution, a dynamic equilibrium will occur between the number of molecules of water entering and leaving the cell. This basically means the rate of osmosis is even on both sides of the partially permeable membrane. By doing this, the size of the cell will remain the same. If the concentration of water inside the cell is higher than that of the medium, which is now hypertonic, then the number of water molecules diffusing out will be more than that entering, and therefore the cell will shrink and shrivel due to osmosis.
Plant cells have a strong cell wall surrounding them in comparison to animal cells. When they consume water by osmosis they start to swell. The cell cannot burst as it is protected by a cell wall which regulates the concentration inside the cell. If the concentration is high, then the cell wall will release substances. If it is too low, then the cell wall will allow substances to enter. Plant cells become turgid when they are put in dilute solutions. The pressure inside the cell increases, and eventually the internal pressure of the cell are so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant arouse into the sunlight.
When plant cells are placed in concentrated sugar solutions, such as sucrose, which I am studying, then they lose water by osmosis and they become flaccid, which is the total opposite of turgid. After placing the plant cell in sugar solution, the cells become plasmolysed. This is the process of plasmolysis. This basically means that the contents of the cell have shrunk, and moved away from the cell wall. The vacuole shrinks due to loss of water, and the cytoplasm peels away from the cell wall.
But these cells only become turgid and flaccid in those conditions. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells, they are in a position between turgidity and flaccidity. This is called incipient plasmolysis. "Incipient" basically means that they are about to be something.
Prediction
When putting a potato chip in a medium, either one of two things could happen. The potato chip could either loose mass or gain mass. It will loose mass if the concentration of water is higher in the potato chip, than it is in the medium. So therefore, I think that if the concentration of sucrose is less that means that there is a higher concentration of water in the liquid, meaning the net movement of water molecules will be from the solution, to inside the potato chip, which will lead to a gain in mass of the potato chip. If the sucrose solution has a higher concentration of sucrose, than this means that the concentration of water will be less, meaning that the water molecules from inside the potato chip will diffuse by osmosis out of the chip, therefore the net movement will be from the chip to the solution, resulting in a loss of mass.
Method
Underneath my method you will find a list of apparatus which I used to conduct this experiment/investigation.
I will start by collecting all my apparatus which can be found on the following list. I am going to use three test tubes for each concentration of sucrose so that I can get an average, and therefore it will be a fair test. I will fill my test tubes to the 20ml point with the given amount of sucrose concentration, which is either, 0%, 10%, 20%, 30% or 40%. The reason for these different amounts is so that we have a fair test, and so that we can see the affect of increasing concentration. I will then label them to prevent confusion. The volume of solution will stay the same at all times, at 20ml, so that the concentration gradient is the same. I will then cut out cylinders from my potato chips using a cork borer. At this point the potatoes will be on the white tile. I will cut them into equal lengths of 50mm using a ruler and a scalpel. The reason why they have to be equal lengths is to keep a fair test. I will weigh my potato chip on the top pan balance before I do anything and record it. I will then place the potato chip in the medium and leave them for 24 hours. I will weigh the potato chip once again using the top pan balance, the following day, and record it. Before I do that though, I will blot of the excess water from the chips to prevent an extra mass. After obtaining all my results, I will use the following formulae to work out the percentage change in mass:
Initial Mass
Final Mass
In my experiment, I will keep these the same:
- The surface area of the potato chips, therefore the same lengths. This can be done using a ruler.
- 20ml of solution in the beaker for each experiment.
- The same person timing, and conducting the experiment.
- The same beaker, (washed after each experiment.)
These are the variables:
- The concentration of sucrose in the solution:
0%, 10%, 20%, 30%, and 40%
Safety Precautions:
- Wear a lab coat at al times during the experiment to protect clothing and skin.
- Be careful with sharp instruments such as the cork borer. Hold them by the handle, and keep away from eyes, and other people.
- Bags and coats under the tables to prevent people falling over, and tripping.
- Be careful when pouring the hot water from the kettle.
List of apparatus:
- 15 x test tubes (With Lids)
- Test tube rack
-
Potato
- Sucrose solution
- Water
- Stop watch
- Cork Borer
-
White Tile
- Scalpel
- Ruler
- Electronic Top Pan Balance
Results
These are the results I gained during the experiment. I have shown them in a results table and a scatter graph.
(Scatter Graph on next page)
Evaluation:
My results have come out as expected in the prediction. I stated that as the concentration of sucrose increased, the rate of osmosis from the potato to the sucrose solution would increase. As you can see this is true. The first sucrose solution of 0% is mostly known as water. So therefore, this is the only situation where the net movement of water molecules is from the sucrose solution to the potato. Despite this concentration, the other concentrations worked out fine. In the 10% sucrose solution test, the mass of the potato decreased by 3%. As the concentration got higher, the mass of the potato dropped more rapidly. This is simply because of particles. If the concentration is low (10%) then there is a fair amount of water particles and sucrose particles. At this point, the water particles inside the potato are at a higher concentration than there is in the solution, therefore osmosis takes place from the potato to the solution. If the sucrose concentration is even higher (40%) then the concentration of water particles is even higher in the potato, meaning the rate of osmosis is faster.
My test could have been improved by using different apparatus. For example, we could have used a lot more hi-tech equipment, such as data logging equipment or scientific computers to conduct the experiment. I could have also repeated my test a dozen times to be sure and also to create a fair test. The reason that these ideas were not approached was because it is time consuming to repeat the experiment so many times, and it is too expensive to buy such hi-tech equipment.
I think that I could have improved in one or two areas; such as wiping of the excess water from the potatoes using a towel. The towels could have either wiped off and absorbed a bit of water or just pushed the water into the potato. If this did happen, then it would change the mass of the potato. 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.
Basically, my test was a success, as the results and graph shows. I did not get any anomalies, and I did not make any mistakes.