Osmosis In Plant Cells
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Osmosis is the diffusion of water molecules from an area of high concentration of sucrose solution to an area of lower concentration of sucrose solution, through a selectively permeable membrane. The water molecules move down a concentration gradient in osmosis. Two diagrams of osmosis are shown in figures 1 and 2. Figure 1 shows the diffusion of water molecules through a selectively permeable membrane from an area of highly concentrated sucrose solution to an area of lower concentrated sucrose solution. Figure 2 shows a concentration gradient, in which the water molecules are diffusing down the concentration gradient.
Depending on whether the higher concentration of sucrose molecules is on the outside of the selectively permeable membrane or on the inside of the selectively permeable membrane, the cell will either become turgid or plasmolysed. If the water molecules are diffusing into the cell through the selectively permeable membrane the cell will become turgid. If the water molecules are diffusing out of the cell through the selectively permeable membrane the cell will become plasmolysed.
1. A Turgid Cell
When osmosis takes place the water molecules will diffuse into the vacuole of the cell to an area of lower concentration. Due to the added water inside the cell it will swell and become turgid. When a cell is turgid the vacuole is large, as is the volume of cytoplasm. The cell membrane is pushed towards to cell wall and therefore the cell wall will bulge outwards. A diagram of such a cell can be seen in figure 3.
2. A Plasmolysed Cell
This happens when the water molecules diffuse out of the cell to an area of lower concentration of water molecules. As the cell is losing water it will shrink and become plasmolysed. When a cell is plasmolysed there is a small vacuole and a small volume of cytoplasm. A large area of the cell membrane loses contact with the cell wall and the cell wall becomes slightly bowed inwards. A diagram of such a cell can be seen in figure 4.
3. A Flaccid Cell
A flaccid cell is what the state of the cell is like in between the two previously discussed stages. The cell wall in a flaccid cell is neither bulging nor bowed inwards.
I predict that the higher the concentration of the sucrose solution the smaller the mass of the potato tissue will be at the end of the experiment. This is because the water molecules will diffuse out of the cell and it will then become plasmolysed, the higher the concentration of the solution the more water will diffuse out of the cell and the more plasmalysed it will become. A diagram of this can be seen in figure 4.
Independent Variable: There will be a range of five different bathing solutions – 2M, 1, 5M, 1M, 0.5M and 0M.
Control Variable: I will control the length of the potato cylinders used, I will measure the length of the potato cylinders accurately using callipers, the mass of each cylinder will be measured accurately using a balance, I will time the experiment accurately, and I will control the excess solution on the cylinders when they are removed from the boiling tubes by rolling them on blotting paper.
Dependent Variable: The mass of the tissue will be what is measured at the end of the experiment, and taken away from the measurements taken at the start of the experiment; I will then calculate change in mass for each duplicate in each concentration and then percentage change in mass for each duplicate. This will enable me to calculate the mean percentage change in mass for each concentration.
* 15 Boiling tubes
* Corer (10mm)
* Measuring cylinder
* Blotting paper
* One sided blade
* Cotton wool
* Salt solution
* Distilled Water
* The apparatus coloured orange will be used as basic equipment to set up the experiment.
* The apparatus coloured in blue will be used for extra accuracy, the balance is accurate to 0.01g and the callipers are accurate to 0.1mm. These pieces of equipment will ensure that measurements are taken as accurately as possible.
* The apparatus coloured in pink will be used to make the varied concentrations of salt solution.
It is important to take care of safety when doing any experiment. In this particular one, it is necessary to make careful use of the corer as it could be dangerous due to the sharp edge, the blade should be taken care of for the same reason and the glassware could break and therefore could cut someone. Long hair should be tied back, everything should be moved from the work area, so that no-one might trip, blazers should be hung up, stools should be pushed in, and ties should be tucked in.
1. Prepare five different concentrations of the sucrose solution using the table below, in order to make 40cm” of each solution. Prepare 3 of each.
Concentration of solution (M) Volume of 2m Sucrose solution Volume of water
2.0 30 0
1.5 22.5 7.5
1.0 15 15
0.5 7.5 22.5
0 0 30
2. Pour each solution into a boiling tube and mark its solution with a label.
3. Use the corer to create potato cylinders. (40.1mm long.)
4. Trim skin from each end using a one sided blade, ensuring the cylinders are all equal lengths by using the callipers.
5. Record the initial mass of the cylinders.
6. Place each potato cylinder into a boiling tube along with the solution submerged.
7. Leave for 48 hours.
8. Remove potato cylinders from boiling tubes. Do not squeeze!
9. Remove excess solution by blotting the cylinders with blue blotting paper.
10. Weigh each cylinder using the balance.
11. Calculate change in mass, percentage change in mass and mean percentage change in mass.
Strategy for ResultsI have chosen to use mean percentage change in mass to plot my graph as this will show my results more clearly. I will calculate this in the following way:
(Final mass – initial mass ∕ initial mass) x 100
Interpretation and Evaluation
As can be seen from my results, I repeated the experiment. This was due to anomalous results in the 1.5M and 2.0M concentrated solutions. From the graph it can be seen that the curve does not go through these two points, whereas it did with the other concentrations. The results in 1.5M and 2.0M solutions fit with the trend, but they are not cut by the curve because the difference between them is anomalous.
Trends and Patterns
The overall trend however can still be reached by looking at the progression of the results for each concentration, from the first three concentrations it can be seen that there should be approximately 10-30% difference between the results for each result, the graph shows this for these three results. This means that the more concentrated the bathing solution, the larger the change in mean % change in mass. The higher concentrations have larger changes in mass, but they are negative changes i.e.- the potato cylinders weighed less after the experiment had been carried out. Therefore the higher the concentration of the solution, the smaller the mass of the potato cylinder at the end of the experiment. From the graph it can be seen that as the concentration of sucrose solution increases the curve begins to level off, this is because the rate of water loss decreases, therefore the curve becomes less steep.
From the second experiment the same trends can be seen.
The trends which I have identified in this interpretation can be explained through osmosis, the higher concentrations caused the potato cells to become plasmolysed as in figure 4. Therefore the space between the cellulose cell wall and the cytoplasm became filled with sucrose solution. This is because there was a higher concentration of solution on the outside of the cell, so the water molecules diffused out of the cell through the selectively permeable membrane. The higher the concentration, the more plasmolysed the cell becomes and my results show this. The 0M bathing solutions in both experiments caused the potato cells to become slightly turgid, this means that there was a higher concentration of solution on the inside of the cell. So the water molecules diffused into the cell through the selectively permeable membrane. The rate of water loss decreases as the concentration increases because; as the cell becomes more plasmolysed it cannot lose as much water. Therefore it shrinks until no more water molecules can diffuse out of the cell.
From these explanations of osmosis and my results, I can accept my prediction, I predicted exactly what happened. The higher concentrations caused the cells to become plasmolysed as the water molecules diffused out of the cell. The lower concentrations caused the cells to become slightly turgid as the water molecules diffused into the cell.
Even though some of my results may not have shown this in the best possible way, it can be seen from my corrected results that this was true of my experiment.
From my results and my interpretation of them I can see that my method was not the most reliable.
Some ways in which I could have improved it might have been to have used more varied concentrations, for example 0M, 1M, 2M, 3M and 4M sucrose solutions would have shown mean percentage change in mass much more clearly, and this would have made the curve on my graph more accurate.
I also could have furthered the concentrations and therefore my results would have been a lot more accurate, for example- 0.1, 0.2, 0.3, 0.4, 0.5 etc.
If the bathing solutions had been left for a longer period of time it might have also benefited my results as the cell would have had more chance for osmosis to take place.
Some of the potato cylinders floated to the top of the sucrose solutions, depending on the concentration.
If the cylinders had been totally immersed at all times and none had floated to the top my results may have been more accurate.
The replicates for each concentration were reasonably close in initial mass and also on final mass, excluding a few. This might have been due to the equipment used to measure them and cut the cylinders. More accurate equipment could have been used.
The replicates which had large differences between masses might explain why some of my results were anomalous also. However, these anomalous results did fit with the trends of the experiment on a large scale as I have previously concluded.
I could also have improved reliability by using more replicates as this would have made my results more concrete.