Investigation into the water potential of celeriac.

Investigation into the water potential of celeriac

PLAN

AIM:

This is to investigate the water potential of celeriac when placed in different concentrations of sucrose solution. To do this I will be studying the process of osmosis to explain my prediction. Celeriac, being a root vegetable means it has a cell like any plant with a permeable cell wall and a selectively permeable cell membrane. The cell membrane only allows for water molecules to pass through which is why osmosis, the movement of water from a high water potential to a low water potential can be used. There is NO movement of sucrose molecules.

HYPOTHESIS:

I predict that as the sucrose concentration decreases in the solution the celeriac would increase in mass. The celeriac placed in pure water (weak solution) would cause the greatest change in mass as the water potential is the highest it can be (0). This is because the inside of the celeriac cells would have a concentration of substances (starch, sugars, other substances,) while the pure water has none. As a result, water molecules would diffuse in to the cells to increase the water potential of the celeriac. As the sucrose concentration increases in the solution, the celeriac’s ‘increase in mass’ should start to decrease to. As a result the solutions solute potential will increase too. Once the solute potential increases this lowers the water potential of the solution (solute potential is always negative). Assuming that the water potential inside each sample of celeriac is the more or less the same, this would account for the ‘increase in mass’ to decrease as the molarity of the solution increases. There will come a point where the celeriac will have no change in mass or will have a ‘decrease in mass’. This is due to the celeriac cells own water potential being greater than the surrounding solution’s water potential. The sucrose solution had increased so much that the solute concentration in the solution would be higher than that in the cells. This solution is known to be a strong solution.

In addition to my previous gained knowledge I have noted previous experiments carried out to back up my statement. One is using the erythrocytes in a lab that can change in mass depending on the water potential of the blood plasma. When red cells are placed in a dilute solution the cell takes up the water molecules until it can take no more and bursts. Then they were placed in a solution where the concentration was equal to their own and the red blood cell retained its shape and mass. Finally it was placed in a concentrated solution where it shrank as water was passing out from it into the solution.

This is a preview of the whole essay

The second experiment is the use of red onion cells under a light microscope. When red onion cells were placed in pure water or any weak solution just like any other cell the water was taken in through the cell membrane and into the vacuole. As the vacuole expands, the volume of the cell increases and pushes against the cell wall. When the cell has reached its maximum inflation it is turgid. This is used in nature for plants to keep stable. As expected when the red onion cells were placed in a low water potential solution than the cell, water from the cell passed into the solution. As seen under the microscope the vacuole shrank (plasmolysed). The higher the concentration of the solution the smaller the size of the vacuole became. As an onion is a root vegetable as is the celeriac I expect a similar result.

MATERIALS: EQUIPMENT:

Celeriac (cut into 14 cylinders of equal size) Electric Weighing machine (g)

1M Sucrose 250 cm³ 14 McCartney bottles

Distilled Water 250 cm³ Scalpel

Tile

Ruler

2 X 30ml syringe

Test tube holder

Cork borer

Beaker

Cling film

METHOD:

Collect bottles and label 2 sets of 7 bottles each with, 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 M
Place bottles in test tube rack
Form all solutions using the table below. NB Place distilled water in a beaker so that water can be drawn up with syringe.

NB A separate syringe should be used for the distilled water and sucrose.

Once placed in bottles, removed 25ml of a solution and place in corresponding bottles. Be sure to wash out syringe every time you have finished with a solution.
Using the cork borer take out 14 samples of the celeriac.
Cut each tube using the scalpel on a tile into 2 cm each. Note any differences from 2cm for any of the cylinders.
With blotting paper, blot each cylinder to remove as much water as possible.
Weigh each cylinder making sure I know which one is going into which solution. Make sure to place a filter paper on weighing machine.
Place the celeriac cylinders into all solutions beginning with 0.6M and then descending from that and screw the lid.
Use cling film to firmly seal each bottle.
Place in a safe corner and leave overnight approx 20 hours
Remove cylinders from bottles beginning from 0.6M and then in descending order.
Blot each cylinder carefully and weigh each one placing a filter paper on machine.
Calculate change in mass for all solutions and find an average. As a result calculate the percentage change in mass.
Plot a table of results and a graph of percentage mass.

SAFETY TEST (RISK ASSESSMENT)

If any sucrose was to spill I should wipe this immediately as it can be very sticky.
I must use the scalpel carefully by using it on a tile so it is on a stable surface and does not damage the table underneath.
When carrying the scalpel around I must point the sharp edge downwards.
Bottles and other glass wear should be placed away from edges so these do not drop and break causing further accidents.
The celeriac being quite a large vegetable can be hard to handle so when using the cork borer I should hold this securely.

FAIR TEST – VARIABLES

Volume of sucrose solution should be constant in both tests. This allows for both tests to be valid.
The length & diameter of celeriac cylinders must be kept constant. To ensure this the same cork borer should be used. The lengths should not range more than 0.1 of a cm from 2. The increased length means there is more surface area. One of the factors that diffusion depends on is the surface area and if this is increased this can increase the rate of diffusion.
All test tubes should be left in the solution for the same time. This needs to be controlled as near as possible by placing the all celeriac as soon as the first one has been placed. They should be removed in the same order.
It is highly likely to be quite a range of mass of the cylinders. This is why it is essential that the ‘mass of change’ is to be recorded to get a valuable result. However I must take care to record the correct weight after with the corresponding weight before.
The samples of celeriac should be from around the same place. Different parts of the vegetable vary in water potential. It is best to extract samples from places of nearly equal water potential.
Temperature can control the rate in which the water molecules can diffuse. The test tubes must be placed in an area where they are all equally subjected to the same temperature. Test tubes exposed to higher temperatures will increase the rate of diffusion, as the water molecules will have more energy to move.
Another precaution I took was not to expose the solution by placing cling film on top. Apart from disallowing any spillage, this prevents any dirt or grit coming inside the bottle. This would inevitably make the solution more ‘concentrated’ and affect the osmosis.

RESULTS

Results Table of Weight Before and After for Each Celeriac

N.B 1&2 =0.60, 3&4=0.50, 5&6=0.40, 7&8=0.30, 9&10=0.20, 11&12=0.10 & 13&14=0.00

Results Table showing the Average Change in Mass per gram in %

I decided to convert the average change in mass per gram to percentage because I decided the results would be easier to read. By looking at percentage I would be able to see the relation between the change in mass to the Weight Before for each sucrose solution.

CONCLUSION

After carrying out two sets of the experiments I have decided that my results are quite reasonable.

As I had predicted, a general trend can be seen, that as the sucrose concentration decreases the average change in mass (%) increases. In my plan I stated that when the sucrose concentration in the solution is its highest the change in mass will be a decrease (-4.56%) and as the concentration is its lowest the change in mass will be an increase (11.55%). Both results tie in with my prediction. When celeriac is placed in a strong solution this means there are less water molecules in the solution than in the celeriac itself. This forms a diffusion gradient from the cell pass the semi permeable membrane towards the solution. Water that leaves the cells reduces the mass explaining about the negativity. On the contrary, when celeriac is placed in weak solution or pure water the reverse will take place. Less water concentration will be in the cell and a diffusion gradient will be from the solution to the cell showing the positive results.

The results also show that at 0.6M the change in mass was –4.56% and when the solution was at 0.5M the change in mass was –3.9%. This also backs up my prediction that at the negative change in mass at a higher concentration will be a GREATER LOSS in mass than the one below. However this case becomes the opposite once there is a GAIN in mass. As can be seen when the solution was at 0.3M the change in mass was 1.95% but at 0.2 M the change in mass was 4.8%. This shows that a positive change in mass at a lower concentration there will be a GREATER GAIN in mass. There is a point in the 0.30M where there is no change in mass. This shows that the water potential for the solution and celeriac cells are the same.

EVALUATION

It is evident by looking at the graph that my results are not perfect as some differ a lot from the trend. This would be dependent on a couple of anomalies. In the ‘Change in mass per gram’ column it can be seen that with both sets of results there is quite a difference. This is particularly evident in the 0.1M solutions, where the first result shows a gain in mass 0.106 and the second result shows a loss of mass -0.009.

The reasons for this could be:

The celeriac had not been clotted as thoroughly as they should have been particularly after being left overnight. This would mean the actual mass of the celeriac would be less than what was taken down as a result.
Not all celeriac had been extracted from the same place.
Although after taking precautions it could have been possible that the lengths of the cylinders were not as similar as it should have been.
When making up the solutions there were times I had to use more than 30ml of sucrose or water. Therefore when drew up 30ml and then an extra 10ml it could have been possible some residue of water or sucrose could have been left. The solutions were then not made as accurately as intended.

To improve this experiment I will certainly need to take more readings for each concentration. For example the average percentage chamnge in mass was greater for 0.4 M than it was for 0.5 M, which does not tie in.

I should decide to blot each cylinder for exact equal times as I suspect this greatly disrupted the results.

I have obtained outside data to form a graph of the water potential of various sucrose solutions.

When I look at my graph I see that at 0.33M there was no change in mass as the point crosses the x-axis. On the line of best fit this shows as 0.35M. The potential must be somewhere between 0.3M and 0.4M. I can therefore suggest that as there is a range, the celeriac’s water potential must be between –850kPa to –1075kPa. My experiment was not accurate enough to show an exact number which is why a range has been provided. By increasing the number of results would increase the accuracy of the water potential.