To determine the water potential of a potato tuber cell using varying salt solution.

B (between 0.30 and 0.50 mole) = the amount of water potential outside the cell is equal to that of inside the cell therefore the cell will not lose or gain wait, this point is called incipient plasmolysis. When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. (the pressure potential is equal to zero so the Ψ = Ψs therefore there is no movement of water)

C (between 0.50 and 1.00 mole) = the cell has become flaccid or plasmolysed and has lost water. Therefore the cell has lost weight. I predict that between concentrations of 0.50 and 1.00 the potato cell will lose weight and become flaccid. Flaccid cells are when the cell has lost so much water that the inner membrane (the protoplast) pulls away.

With the information from the diagram my preliminary graph and scientific knowledge, I predict that the potato that goes into the 0.00 concentration, with the highest water potential will gain mass as more water will be entering the cell and I believe that the cell will then swell and become turgid because the water potential outside the cell is more then inside the cell therefore water will enter the cell as the theory of osmosis states water molecules from a region where it has higher water potential to a region where it has lower water potential and so I expect the potato cell that goes into the 1M solution to lose water and become flaccid as the water potential of the cell is higher then outside the cell so I predict that water will leave thus leaving the potato cell flaccid. I predict that between 0.00 and 0.35 the potato cell will gain mass and between 0.50 and 1.00 it will lose mass. I predict that between 0.35 and 0.50 concentrations incipient plasmolysis will occur giving a water potential of between    –970Ψ and –1450Ψ.

Preliminary

        From the preliminary these are the changes that have been made to insure an even more reliable and accurate experiment:

• In the preliminary the potato pieces were not cut in accurately sized pieces. For this experiment I will cut them accurately using a knife and ruler. I will also make sure they are all the same as the surface area is a variable that may distort the results.
• Another thing that will be changed is the sucrose solution. The solution is a good environment for bacteria to grow and therefore made the preliminary experiment inaccurate. The sucrose solution was also quite thick so when I weighed the potato pieces some sucrose solution that was still on the potato piece could have misrepresented the final mass of the potato. Therefore in this experiment I will use a salt solution that is of 2M.
• In the preliminary the experiment was not repeated so if something had gone wrong there was nothing for it to be compared to. This made it difficult to point of anomalous points. Therefore in this experiment I will repeat it so that it is a reliable experiment.
• Another point is using a syringe to measure out the salt solution instead of a measuring cylinder, which has a higher percentage error than a syringe.

Equipment

Safety

        To insure a safe experiment the following care will take place even though no harmless chemicals are being used:

• Normal laboratory procedure should be in order.
• First of all I will wear goggles, as I will be using a knife to cut the potato. Obviously extra care needs to take place while using the knife, as it is very sharp.
• Secondly a clean tidy environment is needed so that nothing is knocked over or spilt.
• Finally long hair will need to be tied back so that it does not interfere with the experiment especially when cutting the potato pieces.

Results Table

 

This is where I will record my results.

A table that shows the change in mass between potato cells placed at different salt solutions, in two runs and then finally showing the average change and average % change in mass between the potato cells.

Method

• First of all of the safety measures should take place. Clean tidy environment, hair tied back and goggles. Also all stools should be tucked in and any clothes and bags kept out of the way.
• Make sure a results table like the one above is ready before the experiment begins, also dilution table like this one should be prepared:

• Make sure the 12 test tubes are labelled with the concentration amount going from DW to 1M.
• Now set up the apparatus in the way shown in the diagram below:

• The dilutions are the first thing that you should do as if you cut the potatoes first they will be left outside and could get contaminated. Therefore you should cut the potatoes jus when it is necessary.
• Make sure all the boiling tubes are in the boiling tube racks.
• Take the bottle of distilled water and empty some out into one of the beakers. Then take the salt solution beaker and make sure you don’t forget which one is which. Now using the 20ml syringe take out 10ml of water and put it into the boiling labelled distilled water.
• For the rest of the boiling tubes, using the dilution table take out the required amount of salt solution and the required amount of distilled water until of the boiling tubes have been filled.
• Now get the potato and cork borer and cut the potato pieces.
• Using the ruler, knife and forceps cut the potato pieces in equal 2 cm long pieces, putting each one on a sheet of filter paper when finished.
• After that begin to weigh each potato piece, weighing the filter paper first and then the potato piece and the filter paper together. Then use this equation to calculate the mass of the potato piece on its own:

                 Mass of potato piece — Mass of filter paper = Mass of potato piece

                 and filter paper.

                 

                 Record these results on to the results table.

• Now place one potato piece into each of the boiling tubes closing it will the stopper. Note: remember which one you started with, as you will have to remove them in the same order.
• After this leave the potatoes in the boiling tubes for 45minutes.
• When the potatoes have been left for 45 minutes take them out in the order you put them in. to do this you will have to remove the excess solution into a beaker.
• Put each potato slice onto the filter paper it was on before and then weigh them like before.
• Record these results also on the results table.
• The experiment is now complete. Tidy the area and make sure everything is put away properly

Variables

 

Analysis

        From my graph and results I conclude that my prediction was relatively correct. There is a simple trend that can be seen from my graph, which is: the lower the concentration the salt solution the higher the water potential and the higher the amount of mass gained. The potatoes placed in a high water potential have gained mass and the potatoes placed in a low water potential have lost mass.

This trend can be seen throughout the graph:

At the point 0.00M the % change in mass is 14.71g, here the potato piece has gained mass. This is because the theory of osmosis states that water molecules will move from a region where there is higher water potential to a region where there is lower water potential. This means that, as I had predicted the water potential within the cell was less than outside the cell therefore water entered the cell. The water has entered the cell and caused the protoplast to press against the cell wall this is how the cell has swollen and turgid, this effectively increased the mass of the cell. At 0.00M the protoplast has been pushed against the cell wall:

And until 0.34M the graph shows a positive % change in mass. This means that until 0.34 the potato cells continued to gain water because the water potential inside the cell was lower then outside the cell an as the theory of osmosis states water travels from a region of high water potential to a region of low water potential.

At point 0.34 the pressure potential was zero. When the pressure potential equals zero the solute potential is equal to the water potential.

Ψ = Ψs + Ψp

Therefore if Ψp = 0 then Ψ = Ψs  

At the point 0.34 incipient plasmolysis occurred. Incipient plasmolysis is when there is no net movement of water into or out of the cell, the protoplast is no longer pressed against the cell wall and so the Ψp equals zero using this number and the formula I calculated my water potential using my reference graph.

        My reference graph shows the solute potential against the concentration. Using this graph I was able to read of the actual value of the solute potential and therefore the water potential. Using the reference graph I read of at 0.34M where my incipient plasmolysis had take place, I then concluded that the water potential of the cell was –950. I had originally predicted that it would be between –970 and –1450. I think my prediction is relatively close to the result.

At the 1.00M point the % change in mass is –24.24. Here the cell has become flaccid as I predicted. The inside cell has a higher water potential then the outside so the water has left the cell causing it to shrink. The graph shows that when the water potential inside the cell is higher than the outside i.e. after 0.34M, the cell begins to loose water instead of gaining it because the water molecules travel from a region of high water potential to a region of low water potential. Water has left the cell and as it leaves the protoplast has completely pulled away from the cell wall. This caused the cell to shrink and we say it has become plasmolysed.

 

Evaluation

Constant error table: (Errors which affect all the points)

Single Error table: (Errors that affect one or more single points. could be a cause for anomalous points.)

Accuracy

        I think my experiment was quite accurate although it seems as though most of equipment was quite accurate the % error does show some signs that it wasn’t.

The % error is calculate using this formula:

Divisions of equipment               100 = %error

Total capacity of equipment

The % error for the apparatus I used was:

Syringe 5ml with 1ml divisions:

1/5 x 100 = 20%

Syringe 20ml with 2ml divisions:

2/20 x 100 = 10%

Weighing scale with 0.001g divisions:

 0.01/1.7 = 1.7%

To calculate the overall % error for the whole experiment I have to add up all the % errors/ All together the % error for this experiment was = 31.7% which is a considerably large number. This means the experiment was not as accurate as I had predicted it would be. Using better equipment with a lower % error could have made the experiment more accurate.

Overall the experiment was accurate as the point of incipient plasmolysis

I had one anomalous point which was off the line of best fit this shows that one or more of the single errors must have distorted the result. The anomalous point that has been circled on my original graph shows this error bar graph however shows that although I have only circled one anomalous point originally with the error bar graph I can see that only 3 out of the 6 points were within a 5% margin of error.

Although I have only circled one anomalous point originally with the error bar graph I can see that only 3 out of the 6 points were within a 5% margin of error. The error bars are worked out by standard error. Standard error is the measure of error that could have occurred. The smaller the standard error the smaller the error bars. The smaller the error bars the more reliable the experiment. As you can see most of the error bars are quite small making my experiment accurate.  The error bars show how accurate the experiment was. These error bars show the accuracy within 5%. So if the point was affected by some errors where it could have been. Therefore the smaller the error bars the smaller the error.

Suitability of Method

        My method was quite suitable for the experiment it did allow me to gain a suitable and sufficient result. There were some limitations such as time and the range of equipment available such as buffer solution, but overall the method fulfilled its purpose to help me find out the water potential of the potato pieces.

Reliability

        I think my experiment was reliable to an extent as I did conclude a result that satisfied my prediction. I think my experiment was reliable because most of my equipment was consistent as I didn’t have any problems with any of the apparatus, also the fact that the experiment was repeated and similar results were achieved both times showed it was a relatively steady experiment. From the results I can see that only one of the point is an anomalous point this could have been caused from the fact that the experiment didn’t have a very good range of concentrations. I think I should have used a wider range of concentrations to see what would happen after 1Mole as the solution that was given was of 2Mole. Also as my constant error table shows, my I believe my most significant error was that of the 2Mole salt solution that was provided having said this my results are similar to others so the experiment must have been reliable.

To gain an even further understanding into water potential I could do experiments that change water potential like the affect of temperature on osmosis and the effect of different types of potatoes. I think this would give more accurate and reliable evidence to the conclusion of my experiment.

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