An Investigation into the water potential of a white potato
Biology Coursework
An Investigation into the water potential of a white potato
Ben Smith, 10S
Aim: To discover the water potential of a white potato, by investigating the extent to which osmosis occurs in sucrose solutions of differing concentrations.
Prediction: Osmosis is defined as "the spontaneous passage or diffusion of water or other solvents through a semi permeable membrane" (such as a cell membrane). The presence of a cell membrane is important because it allows only particles small enough to pass though it to escape, placing a limit on the extent to which osmosis occurs.
During my research into "stem tubers" (the part of the potato plant we eat), I found that the energy contained in them was converted into starch for storage. Starch is an insoluble material, with a fairly large size and therefore cannot pass through the membrane, and also cannot dissolve and then pass through the membrane. These characteristics of starch as a storage medium will limit how much osmosis occurs in the potatoes I am testing. However, I believe some osmosis will occur, as not all glucose will have become starch. I think there will be enough un-converted glucose for osmosis to have an effect. Using my knowledge of osmosis, and the chemical makeup of a potato, I think the water potential will lie below ?0.5, and possibly below ?0.4, depending on whether the potato is particularly moist or green.
Research:
Osmosis in plant cells:
Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" (swollen and hard) when they are put in dilute solutions. The pressure inside the cell rises and eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what causes the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrink and pull away from the cell wall. These cells are said to be "plasmolysed".
When plant cells are placed in a solution that has exactly the same osmotic strength as the cells, they are in a state called "incipient plasmolysis". ("Incipient" means "about to be".)
Potato Plants:
All plants obtain energy by photosynthesis, using sunlight to make glucose. This energy is used for activities such as growth, and also stored for future activities.
Potato plants store this energy in "stem tubers" - the actual potato we eat. In order to store stop loss of this valuable energy, it is converted from starch into glucose.
Plan:
We know that at the point of equilibrium (no loss or gain of water):
? potato cell is equal to ? sucrose solution
(? = water potential)
This knowledge is the basis for our investigation: We will find the water potential by discovering the concentration of the solution at which no osmosis occurs. Once we have found the concentration, the water potential of the sucrose solution can be found by:
? = -3150 X A
This formula gives us the water potential of the sucrose solution, and therefore (as explained above) ...
This is a preview of the whole essay
Plan:
We know that at the point of equilibrium (no loss or gain of water):
? potato cell is equal to ? sucrose solution
(? = water potential)
This knowledge is the basis for our investigation: We will find the water potential by discovering the concentration of the solution at which no osmosis occurs. Once we have found the concentration, the water potential of the sucrose solution can be found by:
? = -3150 X A
This formula gives us the water potential of the sucrose solution, and therefore (as explained above) the water potential of the potato.
However, it is highly unlikely that we will make up the exact solution needed, and so instead will find the results from a graph of other readings.
The actual experiment will involve placing pieces of potato (we will decide how big and how they will be cut later) in beakers of varying concentration sucrose solution. In some solutions the potato will gain water by osmosis - this means the water potential of the potato is lower than that of the sucrose. In other solutions the potato will loss water - indicating the water potential is higher than that of the sucrose. Once we have our results, we will calculate the percentage change in mass, and plot these figures on a graph. A line of best fit would pass through the value we are looking for.
Pilot experiments:
Before beginning the main experiment, we must decide on the best (most accurate) way of carrying out certain elements of it.
Experiment 1:
We have two options when cutting the pieces of potato. We can either:
* Cut small pieces using a scalpel, then measuring them (and trimming if necessary) to ensure they are the same size; or
* We can use a "cork borer", a tool that allows us to cut identical cores out of the potato, of a chosen diameter.
We are going to try each of these methods, to find which is more accurate or gives more concise results (none, or fewest anomalous results).
First dimensions (mm)
Mass (g)
End dimensions (mm)
Mass (g)
Difference (g)
% change
L
W
H
L
W
H
Cut with scalpel
30
30
05
.43
32
33
06
.77
+0.34
+23.7
30
30
05
.12
31
30
06
.31
+0.19
+16.9
40
40
05
2.45
43
44
07
2.81
+0.36
+14.7
40
40
05
2.64
41
41
06
2.94
+0.3
+11.4
Bored cores
30
03
N/a
0.34
32
04
-
0.35
0.01
3
30
03
N/a
0.36
33
04
-
0.37
0.01
3
40
04
N/a
0.98
44
05
-
0.02
2
40
04
N/a
0.95
43
06
-
0.99
0.02
4
I can see from these results that the pieces cut with a scalpel do NOT increase evenly in size or mass. This may be because the chunks are cut from different pieces of the potato: some pieces may come from the surface, which has a relatively high amount of water (and therefore will be effected by osmosis differently) compared with the middle. It is impractical to attempt to ensure the pieces come from similar areas, and this still does not guarantee accurate results. Much simpler is to use cork-borer-cut pieces, which I have found DO increase evenly in size and mass. However, because the changes in size are so small, and difficult to measure (dealing in millimetres), it will be simpler to just measure the mass. This will still produce fair results that will be easy to compare.
Pilot experiment 2
During the experiment I will be limited in time, and must perform the entire experiment, from preparation to final measuring within a double lesson.
Originally I had planned to measure all the concentrations between 0 and 1M, in 0.1M increments. However, In my hypothesis I have predicted that the water potential will lie under 0.5M. To save time during the actual experiment, I found what readings were redundant.
I did this by carrying out the experiment exactly as planned, but afterwards I saw that above a 0.6mola concentration, no significant change in mass was recorded. This indicates that above this concentration all osmosis change had finished, and these results were redundant. To save time, I will not carry out the experiment with these concentrations.
Constants and Variables:
In order for the experiment to be fair, I must ensure that each solution and potato pieces are tested under identical circumstances. These are the factors that I will keep the same:
* The temperature at which the experiment occurs: this will be room temperature for all of the solutions.
* The volume of the solution: there will always be enough to completely cover the potato pieces, and this amount will be kept the same across all the solutions.
* Type of potato: As stated before, I will always use the common "White" potato (or "Solanum tuberosum"). This is because different potatoes have a different makeup in the cells (Eg: differing % amounts of starch)
* Duration of experiment: All the potato pieces were left in the solutions for 1 hour.
* The balance used: The equipment may differ slightly in accuracy, so we will use the same balance for each measurement.
* One dependant variable is the mass of the potato pieces. They will all start at a recorded mass, but depending on the effects of osmosis may go up or down.
The only true variable is the concentration of the solution, and this will vary between 0 and 1 molas.
Safety considerations:
As I have decided against cutting pieces using a scalpel, there is much less danger. However, I must take care when using the cork-borer, as the ends are sharp, and the potato should be held down, to stop it moving. Also, to protect the lab benches a cutting mat or tile should be underneath.
Although not dangerous, sucrose is sticky and difficult to clear up. I must take care not to spill any, and will wear a lab coat to protect my uniform.
Carrying out the experiment:
Apparatus needed:
* A cork-borer
* Several "White potatoes", of similar size and age (i.e. not green)
* Several beakers, large enough to hold 100ml of sucrose-water solution.
* White tile to cut on
* Labels, to label beakers
* Distilled water
* Concentrated sucrose solution to make up solution
* Balance, accurate to 2 decimal places
* Measuring cylinder
* Syringe, for small adjustments to liquid amounts
* Concentrated sucrose solution(1M)
* Distilled water
Method:
o We will first prepare the potato pieces by cutting through using the cork borer, at 4.5mm diameter. All the pieces will then be trimmed to roughly the same length. To make comparison easier, we will adjust cut small pieces off the potato until they are all the same mass. As we are not measuring the length or width, the size does not matter.
o Next we will prepare the solutions we need. We decided to make up 7 solutions in all, with 0, 0.1, 0.2...0.6 mola concentrations. The solutions will be made up by putting the amount of sucrose (between 10ml and 60ml), measured in the measuring cylinder in a beaker, and topping up to 100ml with distilled water.
o The potato pieces will then be placed in the sucrose solution and left for 1 hour. There is enough solution to allow the potato pieces to be completely covered.
o Once the time has elapsed, we will remove all the pieces from the solutions and briefly dry them, removing the surface moisture before weighing.
o Each piece will be weighed and it's new mass noted.
The range of measurement we have chosen will provide enough data for a graph to be drawn, and comparisons to be made.
Each solution is tested with two pieces of potato, providing a replicate to minimise the effect of stray results.
Results:
We carried out the experiment as detailed above, and here are our results:
Concentration (molas)
Start Mass (g)
End mass (g)
Mean difference (g)
Mean percentage change (%)
0
2
2.53
0.485
24.5
2
2.44
0.1
2
2.38
0.36
8
2
2.34
0.2
2
2.14
0.11
5.5
2
2.08
0.3
2
2.09
0.08
4
2
2.07
0.4
2
.9
-0.16
-8
2
.78
0.5
2
.73
-0.3
-15
2
.67
0.6
2
.51
-0.5
-25
2
.49
As planned, to find the exact concentration we need to plot a graph.
We can clearly see that the point of equilibrium falls between 0.3 and 0.4, and I estimate it to be 0.31
Using the formula discussed previously, I can now calculate the water potential:
? = -3150 x 0.31
? = -976.5 Kpa
This is the water potential of the sucrose, and therefore also of the potato.
Conclusion
From the above results, it is evident we have been able to find the water potential of a white potato, by measuring the effects of osmosis upon it. The graph clearly shows how by plotting the results of different sucrose concentrations it is possible to find the "point of equilibrium" - at which no osmosis occurs - by drawing a line of best fit. The graph I have shows that at above the point of equilibrium, the potato loses mass through osmosis, and below the point of equilibrium, the potato gains mass through osmosis.
The result I have reached matches my prediction, being below ?0.4. This indicates that there was little glucose within the potato that could be affected by osmosis - probably because it had been converted into starch, a much better storage medium. Starch molecules are larger than glucose, so cannot escape out through the cell membrane. Additionally, they are insoluble, so cannot dissolve (as glucose does) and pass out through the cell membrane. The potato converts glucose, produced by photosynthesis, into starch for these very reasons - it won't lose its valuable, hard-earned energy reserves into the soil.
My final result supports my hypothesis, based upon the make-up of a potato.
Evaluation
By looking at the line of best fit I received, I can tell my experiment was fairly accurate, as all the results are close to the line. During the experiment I received two anomalous results, but fortunately I was able to repeat them and the repeated (and correct) results are included on the graph. I found that these "freak" results were a result of discrepancies between the measurements of the beaker (which were only approximate) and the measuring cylinder (which was far more accurate). Because the amounts of sucrose and water mixed was wrong, this gave a different concentration and could have heavily swayed my results if I hadn't corrected the error.
Looking at the graph, I believe I collected sufficient evidence to allow me to find a satisfactory result. The fact the figures obtained follow a line of best fit, and in every case both potato pieces tested the changes match means the evidence is quite reliable. To improve the experiment in future I could try testing multiple potato pieces in different beakers, to prevent any mistakes when making up the solution. However, for my experiment I don't think this would have helped, but it is something I could investigate.
I could also improve my experiment by attempting to be more consistent when drying excess solution before weighing. Although this only makes a slight difference, I may have accidentally dried one piece more thoroughly then the other, affecting my results. In future, I could find a different method, or attempt to be consistent. These are the only improvements I can think of, because my experiment was already quite reliable and accurate.
