I have been asked to investigate the water potential of a potato. During my investigation I will not actually be able to measure a value for, as it is a pressure value (kPa).
Osmosis is a special form of diffusion involving only water molecules. It is defined as “The passage of water from a region of high water potential to a region of low water potential through a partially permeable membrane”
Definition of Water Potential.
The potential for water to move across a selectively permeable membrane, where the osmotic potential of distilled water is 0 (kPa) and any water movement is measured with a negative value.
When a solute is mixed with a solvent the water potential will decrease, and will always be a negative value.
As the presence of a solute lowers the water potential, the more concentrated the solute is, the lower the water potential will be.
This is if pressure where applied to a solution it would resist the movement of water. There is an equation that can represent to relationship of water potential, solute potential and pressure potential:
= s + p
Osmosis and Plant Cells
Like animal cells, plant cells contain a number of solutes. When these cells are put into pure water, water moves into the cell because there is a lower water potential in the cell. But unlike animal cells, plant cells cannot control the amount of fluid surrounding the cell.
If a plant cell is placed in a solution with a lower water potential than the plant cell, water will move leave the cell, and the cell becomes plasmolysed (flaccid).
If the plant cell is place in a solution with the same water potential, there will be no net movement of water. The cell is then said to be incipient plasmolysis.
The table below shows what happens to the cell when water moves in and out of a cell.
Water potential of solution compared to cell Higher Equal Lower
Net movement of water Enters cell Neither enters or leave Leaves cell
Protoplast Swells No change Shrinks
Condition of cell Turgid Incipient plasmolysis Plasmolysed (flaccid)
A plant cell, unlike an animal will not burst when water moves into a cell, it becomes turgid. This is because the cellulose cell wall prevents it from bursting.
To find the water potential of a potato I will be placing fixed volumes of potato, with the same surface area, into different concentrations of sucrose solutions.
What I Did
I began by cutting cylinders of potato using a borer which had a diameter of 1cm. I then cut the cylinder of potato to a fixed length of 2cm. I then weighed the mass of the cut potato cylinder using a sensitive balance. I did this twice, so I had 2 cylinders of potato the same volume.
One of these potato cylinders was put into a beaker containing 0.2M sucrose solution and the other in a beaker containing 0.7M sucrose solution. Everyone put their potato cylinders in the same beakers.
They were left in the solution for approx. 48 hours, we then measured the mass of the potatoes again. We were unable to be sure which potato we were measuring the mass of, because they were un-marked. This gave us 2 sets of results for each concentration of sucrose solution. With these results I calculated the mean mass before and after the potato cylinders were put in the solution, the difference between the mean mass and I was able to determine if water entered or left the potato.
0.2M Sucrose Solution 0.7M Sucrose Solution
Before After 48hrs Before After 48hrs
Mass of Potato Cylinder 0.87 0.93 0.93 0.9
0.91 0.96 0.88 0.71
0.91 0.96 0.91 0.77
0.87 1.04 0.92 0.67
0.9 0.92 0.93 0.83
0.87 0.9 0.86 0.74
0.96 0.9 0.94 0.63
Mean Mass (g) 0.90 0.94 0.91 0.75
Change in Mass (g) 0.05 -0.16
From the results I can see the water moved into the potato after 48hrs in the 0.2M as the mass increased by 0.05g. I also saw that water moved out of the potato in the 0.7M solution after 48hrs as the potatoes mass deceased by 0.16g, it had Plasmolysed.
I have to keep the shape of each piece of potato the same to make the experiment as fair as possible and to ensure that the results I record are accurate and reliable. To do this I will be using a 1cm diameter borer so I am able to cut each piece of potato, and it will have the same diameter each time. I will then cut the length of potato will a sharp scalpel to a fixed length of 2cm, giving me a volume of 1.57cm3. I have decided to use these values because they were used in the prelim experiment and I felt that they worked well.
I am going to make a stock of 1M sucrose solution, that I will dilute using distilled water. From my preliminary result I can see that I do not need to use any sucrose solution below 0.2M or higher than 0.7M concentrations. This is because the prelim results showed that water moved into the potato when it was put in 0.2M solution and water moved out when placed in 0.7M solution. But I am planning to find out at what concentration here is no net movement of water which will be between these to values. The concentrations of solutions I plan to use are; 0.3M, 0.45M and 0.6M. I am also planning to place the potato in 30cm3 of sucrose solution.
In my preliminary experiment we left the potato tubes in the solution for 48 hours. I plan to do the same. This is because after a while there will be no net movement of water, as there is no water potential difference and the prelim did show a change in mass.
How I Am Going To Create the Stock Solution?
Amount of Stock required: 60cm3
Formula mass of sucrose:
C12H22O11 = 342g
1M dm-3 = 342g in 1000cm3
0.342g in 1cm3
60cm3 x 0.342g = 20.52g in 60cm3 = 1M solution
How I Am Going To Create the Different Concentrations:
Concentration (M) 0.3 0.4 0.5 0.6
Volume of 1M Stock Solution (cm3) 9 12 15 18
Volume of Distilled water (cm3) 21 18 15 12
I plan to put the potato cylinder in a boiling tube will 30cm3 of the different concentrations of solutions. Before putting the potato cylinders in the boiling tube I will weight the mass of the potato tubes, then after the potato tubes have been in the solution for 48 hours I plan to weight the mass again. But before doing this I must blot the potato with a paper towel to remove any water which may alter the mass. Using these values I will calculate the change in mass and will be able to determine in which direction the water moved.
To improve the consistency of my results I plan to put 5 potato cylinder into each boiling tube, so I will be able to calculate a mean mass. I am going to
do this to make my results more consistent, accurate and reliable. To make sure I will be able to distinguish between the different pieces of potatoes I am going to mark the end of each piece of potato with a different colour. This will allow me to me to make sure that I am measuring the mass of the same potato before and after being in the solution.
Using all of my recorded result I will be able to determine at what sucrose concentration would be required for no net water movement and therefore and equal water potential by plotting a graph. Then using that value I will using the NVB Roberts table to determine the water potential.
I predict that the 0.3M will give the smallest change in mass because in the preliminary experiment the 0.2M solution the potato only gained 0.05g, which suggests that water moved into the potato and it has become turgid. The potatoes which were in the 0.7M decreased in mass by 0.16g, so water moved out of the potato, causing plasmolysis of the plant cells.
I will require the following apparatus to complete my experiment:
Borer Allows me to cut cylinders of the same diameter
Sucrose Allows me to create the stock solution
Scalpel Allows me to cut an accurate length of potato
Cutting Tile Stops me from cutting into the work surface
Balance Scales Accurate weighing of the potatoes
Paper Towels Clean up any spillages and remove solution from surface of potato
Boiling Tubes Larger than test tubes so it will allow me put 5 potato cylinders in them
Boiling Tube Rack Allows me to keep the boiling tubes upright.
Stop Clock Accurately allows me to time the 48hrs
Distilled Water Dilute and make stock solution without adding additives that may be in tap water
Measuring Cylinders Finer gradations so more accurate volume can be measured
Permanent Marker To make the potato pieces and not be washed off
I must following safety precautions to minimise the risk of injury:
Clean up any spillages to prevent slipping
When not using the scalpel putting it in a safe place to prevent cutting me or anybody else.
When using the scalpel, use a cutting tile to prevent cutting into the work surface.
Wearing an apron/overalls and goggles.