Finding the equilibrium between the sucrose solution and potato will identify the water potential of potato tissue.
I ran a preliminary experiment to test a suitable method that could be used to investigate water potential. For this I have chose to measure that amount of cells in which plasmolysis has taken place in a slice of potato using a microscope. I later discovered that using this was not effect because due to the colour of potato I was unable to identify cells that plasmolysis had occurred in. I decided to use a onion because it has similar properties and the purple pigment in onion would be a lot easier to identify under a microscope. The range of solutions we used were 0 Mol 0.5 Mol and 1 Mol and here are photos taken from these microscopes
To work a concentration of sucrose I had to work out what one mole of sucrose solution was. To do this I had to add the Fructose and Glucose compounds together:
C6 H12 O6 Glucose + C6 H12 06 Fructose = C12 H22 O11 Sucrose + H20 Water
Sucrose contains C12 H22 O11 H20
C12 H22 O11
(12z12) + ( 22x11) (11x16) = 342 so therefore is 342g of sucrose in 1000cm3 of water.
From sucrose I would need to work out the how many grams in 1000cm3. Of course this is to high to work with so by dividing both the Sucrose weight and cm3 of water by 5 I received and more workable amounts of 68.4 g of sucrose is 1 mole in 200cm3 of water.
The apparatus I used to for included:
- Sucrose
- Test Tubes
- Beaker
- Distilled water
- Potato’s
- Electronic balance
0 Mol 0.5 Mol 1Mol
Cells: 40 40 31
Plasmolysised: 0 5 19
% Polymolysised 0 % 12.5 % 61 %
Using the information I have gathered here it would appear that the equilibrium is somewhere between 0.5 Mole’s and 1 Mol. I do not believe it is entirely accurate to measure the plasmolysis this way so I have left the opened the next investigation from 0.25 – 0.75. I have also decided that I am going to measure mass change rather than count plasmolysised cells as I believe this will be more accurate.
I am now able to make a prediction using this information I have gathered. I believe that the equilibrium lies somewhere between 0.25 and 0.75 Moles.
Analysis and Conclusion
The point that passes the 0.0 Mass change and is 0.27Mol dm3- concentration. This therefore suggests that the potato tissue must contain a solution with the same water potential as a 0.27md. This is because at the 0.27 Mol solution no change in potato tissue mass appears to happen and this is because equilibrium between the potato tissue and the solution is reached. We can see that as you go above the 0.27 Mol mark the mass begins to significantly decrease. This is because there is a higher sucrose concentration outside the potato cell and therefore a lower water concentration. As I’ve said early “Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration”. Therefore the water from within the potato tissue its traveling outward into the solution making the cell flaccid and causing the decrease in potato mass. I can also see that below the 0.27 Mol concentration there is an increase in potato mass as water is traveling from within the solution into the potato cell due to osmosis making the potatoes cells to become turgid and causing and increase in potato mass
Here is the diagram I used before. Here it shows what happens when the sucrose solution concentration is above the 0.27Mol of the potato cell chip and vise versa if the solution is lower.
Here is the other diagram from before this is now relevant in that it show that at the point of equilibrium (0.25Mol) in this case. There is equal movement of water between the cell tissue and solution as there is nothing to be balanced out .
As you can see from the graph showing the kPa of the different sucrose concentrations 0.27 Mol dm-3 equals 720 kPa
This means that the potato chip has water potetinal 720kpa. The potato chip could potentials have a higher water potential but it contains starch molecules within the cells. This starch molecules are insoluble, if they were soluble they would mix with the water and the water potential would be increased and the cells could burst from the increased in turgor pressure within the potato cell
The starch molecules can be identified in this diagram I used before
