The rate at which osmosis takes place is affected by the concentration of water in the two solutions on each side of the cell membrane.
If the selectively permeable membrane separates two solutions, water moves through it in both directions at the same time. Although more water leaves the high water concentration and passes into the low water concentration solution than enters it. The water seems to move across the membrane in one direction, but is moving in both directions with more water moving one way than the other. When the concentration of water on both sides of the membrane is equal, the movement of water will be the same in both directions and therefore have reached the equilibrium.
When plant cells absorb water by osmosis they start to become swollen as the vacuoles are swollen with water. This causes pressure to be built up and this is known as turgor pressure. The cell wall of the cells in a plant is made up of strong inelastic cellulose. This stops too much swelling up of the cell. Turgor pressure supports the plant cells. We know this because when a plant is watered well the cells become turgid and this helps to make leaves and stems stiff. But when a plant loses water the cells become flaccid and there is less turgor pressure and so the plant starts to wilt.
As an animal cell does not have a cell wall when too much water is taken in by osmosis the cells can burst.
But the amount of water entering and leaving the cells must be controlled and this done through the job of the Guard cells otherwise called stomata.
Stomata are specially shaped cells lying side by side in the epidermis of a leaf. These cells form a stomatal pore between them. Guard cells are able to adjust the size of the stoma. The cell walls of these cells are not of the same thickness all the way round. Those nearest the stoma are thickest and so cannot stretch very much. The rest of the cell wall is much thinner and therefore is capable of a lot of stretching.
When there is light, the guard cells photosynthesise as they have chloroplasts and so produce sugar through the process. A high concentration of sugar is developed in the guard cell. The surrounding epidermal cells cannot produce sugar, as they do not photosynthesise. Due to the difference in concentrations osmosis takes place. So water is passed from the epidermal cells to the guard cells. As a result the cells become turgid. As I mentioned earlier the cell wall is not of the same thickness all the way round and the turgor pressure makes the cells become kidney shaped causing the stoma to become wider.
So when guard cells lose water they become flaccid and less kidney shaped and so the stoma is smaller.
Prediction:
I predict that when the potato chip will be placed in high water concentration then it will gain mass. This is because inside the potato chip there is less water concentration and more solute particles such as sugars and salts and therefore the potato has lower water potential than its surroundings. The water molecules pass through the selectively permeable membrane from an area of high water concentration to an area of low water concentration by osmosis. The vacuoles will collect more water and will swell up and therefore the potato cells will become turgid and become stiff and heavier.
I also predict that when the potato chip will be placed in high solute concentration the potato chip will lose mass. Again there is a concentration gradient. There is less solute concentration in the potato cell and more water concentration therefore osmosis occurs causing the potato chip to lose water and lose mass. So the cells in the potato chip will become flaccid.
Below is the graph that I think will look like after the experiment.
I think this because as the concentration of water increases the amount of water taken up by the potato cells increases. So the cells become turgid and gain mass. Whereas if the potato chip is placed in high solute concentration the potato chip cells will lose water and become flaccid and therefore lose mass.
Plan of Experiment:
Diagram:
Apparatus:
- Testube
- Testube rack
- Potatoes
- Tissues
- Stopwatch
- Scales
- Water
- Salt Solution
- Potato corer
- Measuring cylinder (50ml)
Method:
- Get a potato and using a potato corer take out six chips of potato.
- Weigh each potato and try to make each potato chip weigh the same as the lightest potato chip or to the nearest 0.01g.
- After that measure out the concentration for each testube in the diagram and then set up the apparatus as shown in the diagram.
- Leave potatoes in testubes for 20 minutes.
- After twenty minutes have past on the stopwatch take out each potato and roll gently on tissue to wipe off any excess water or salt solution
- Record the results and repeat experiment three times.
- Take measurements before and after the experiment and work out mass difference in percentage as well.
- Plot graph and analyse.
Safety:
As I am working with liquids I shall not work near any sockets as that may be very dangerous. I will not walk around with equipment when I have wet hands as that could cause me to break equipment.
Variables and Fair test:
There are several variables to this experiment which are listed below:
- Light
- Concentration of water
- Surface area of potato chip
- Size of chip
- Temperature of water
I have chosen to vary the concentration of water and salt solution. In order to carry out a fair test I shall keep the other factors under control and the same. I shall place all the chips in the same area of light and keep the surface area of the potato chip the same to the best of my ability and shall cut the same size potato chips. The water and salt solution will be kept at the same temperature for a fair test.
Results:
Table 1:
Table 2:
Table 3:
Table 4: Table of Averages
Now using my results I plotted two graphs. One graph has ‘Concentration of water’ running along the x-axis and the other has ‘Concentration of Salt running along the x-axis. Please turn over to see the graphs.
Conclusion:
After doing my experiment I proved that as the concentration of water increased the amount of mass gained by the potato chip increased as well. And as the concentration of the salt solution increased the amount of mass lost by the potato chip increased.
Firstly if you look at my table of averages you will see that the potato chips in the first three testubes gained mass. This was because in the first three testubes there was a higher concentration of water than in testubes 4,5 and 6. This proves that with higher concentration of water the higher the mass that the potato chip gains. The potato chips in testubes 4,5 and 6 lost mass as there was a higher concentration of solute solution than in the first three testubes. This, proving my point, means that the higher the concentration of solute solution the less mass gained by the potato chips.
Secondly, Graph 1 shows the mass difference when the concentration of water varied. The line of best fit is a straight line and is sloping upwards. There is a linear relationship with the concentration of water and mass difference. So as the water concentration increases the gain in mass increases. It has a positive gradient of 0.44.The positive gradient shows that there is an increase in mass as there is a steady increase of concentration of water. This is because the potato chips have lower water potential than the surrounding liquid in the testubes 1,2 and 3. As the potato chips have a selectively permeable membrane it allows water molecules to pass through. The water molecules move from an area of high water concentration to an area of low water concentration across a semi-permeable membrane by osmosis. Therefore as the vacuoles in the potato cells draw in more water they become swollen with water and are turgid cells and the contents of the cells started pushing against the cell wall. This builds up turgor pressure and plant cells rely on this pressure for support. As there is an increase in turgidity the potato cells become stiffer and there is an increase in mass.
Graph 2 agrees with this statement as well. Graph 2 shows the mass difference when the concentration of the salt solution varies. The line of best fit is straight and is a slope going downwards. There is also a linear relationship with the concentration of salt solution and mass difference. It has a negative gradient of-0.9. This shows that the potato chips lost mass as the concentration of the salt solution increased. This is because the potato cells have higher water potential than the surrounding liquid. Due to osmosis the water molecules move from an area of high water concentration, the potato cells, to an area of low water concentration across a selectively permeable membrane. Therefore the cells in the potato lost water and became flaccid cells. There was a decrease in turgor pressure and the cells were not very stiff. We found this out because the potato chips in testubes 5, 6 and 7 were not very firm and were very floppy, as the cells could not support themselves.
I predicted rightly. Looking back at my prediction my theory was correct and was proven through my table of results and graphs. I predicted that as the concentration of water increased so did the mass gained by the potato chips. And vice versa. My results proved me right. My predicted graph was that similar to Graph 1 except the gradient wasn’t as big as in Graph 1. But I was on the right lines.
Evaluation:
I think that my experiment was excellent.
I followed my method and did exactly what I was meant to. I tried to take measurements as accurately as possible.
If you refer to my graphs you will see that in Graph 1 only one point is on the line of best fit. But this does not mean to say that my results were not very accurate because they were. The rest of the points are close to the line of best fit showing that my experiment was fairly accurate. But I feel that the measurements taken for testubes 3 and 3 I was not as accurate as I was for the other testubes. This may be due to an error on the scales or how much force I put when rolling the potato chips dry. In future I shall try to be more accurate at doing both things.
In Graph 2 again only one point goes through the line of best fit and the other five points are fairly close. This shows that my experiment was quite accurate giving me fairly accurate results.
I think the reason that my results were as accurate as they were because I repeated my experiment three times which meant that I was able to do better each time I repeated the experiment. In future I will repeat my experiment more than three times as this will give me more accurate results.
I think the biggest disadvantage for this experiment was that I did not let the experiment run for very long. I think with more time I would have been able to support my theory better and my results may have been more accurate. I think with the results that I have I was able to make quite good conclusions but for maximum accuracy and better conclusions I would have to let the experiment run for longer.
Overall the investigation was quite satisfying.