Variable I will change:-
I will change the concentration of sucrose solution in my experiment, because I believe the results of that experiment will tell me the water potential of the potato cells.
Fair Test
I will carry out all my experiments at the same temperature, 20°C, because that is room temperature, and I will not have to heat up anything. I will cut my chips to the same dimensions every time, using a ruler, and a sharp knife (for accuracy) – to measure the cross-sectional area I will use a calculator. I will also put all my chips in at the same time, and take them out at the same time, therefore giving them an equal amount of time for the reaction to occur. I will always use a total volume of 100cm³ of sucrose solution (for the sake of argument). I will also dry each chip before weighing after being in a solution, as the chip will be heavier due to the liquid if it is dripping. I will ensure that the potato chips will have no skin on them at all by peeling the potatoes before the experiment begins, as the skin cells of the potato may have different properties.
Preliminary Work
Before I carry out my experiment I must do some preliminary work to give myself some practise in using the equipment, so that I can do the main experiment quickly enough so that I do everything I need to in the time given. I also need to determine what the best size of chip is to use (the best being the size which gives the biggest difference in results). I also need to establish that a) the chip shrinks in 1.0M sucrose solution
b) the chip grows in distilled water.
I cut two each of four different sizes (long and thick, short and thick, long and thin, short and thin) , measuring each chip carefully, with a ruler, and weighing them on a digital top-pan balance. I then attached thread to the chips and put one set in 1M sucrose solution and the other in distilled water. The chips were left for three hours.
It is clear from these results that the long and thick chips give the greatest difference in results, possibly because they have the greatest surface area, so they can take in more water. The results also prove that the chip shrinks in 1.0M sucrose solution and grows in distilled water.
Method
I have now chosen values for the variable I am investigating. I will repeat the experiment using sucrose concentrations of 0.0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M in my investigations.
Equipment:
- 1.0M Sucrose Solution
- Distilled water
- Potatoes
- Knives
- Thread
- Six (250 ml) beakers
- Ruler (accurate to 1mm)
- Digital top-pan balance (accurate to 0.01g)
-
Measuring cylinders (50ml)
- Clock
- Firstly I will cut eighteen chips out of a potato using a sharp knife and a ruler, all the same length (40mm), making sure that there are no skins left on, which would change the chip’s weight.
- Tie lengths of cotton thread onto each chip, in three different colours – 6 chips with green thread, 6 chips with pink and 6 chips with red.
- Carefully measure out 100 cm3 sucrose solution using a 50ml measuring cylinder, and put into a 250ml beaker. Do the same for the other 5 solutions which include different measurements of distilled water.
- Weigh each chip using the digital top-pan balance and note down which solution it will go into e.g. Red Chip 4.30g – into 80cm3 sucrose solution.
- Put each chip into its correct solution at the same time. Leave for two hours.
- After two hours return to experiment. Remove three chips from their solution using thread, dry them off with a paper towel, and weigh them one by one. Repeat for the remaining chips (in the correct order).
Reliability of results
To make results as reliable as possible, I intend to use three chips in each concentration of solution, and take the average (mean) measurements of changes in length and mass of the three.
How my results will be useful
In order to find the water potential of potato cells, I will need to process my results after obtaining them. I will plot a graph of change in mass or length against concentration of solution, which should reveal the concentration where the mass or length doesn’t change. I will then use a table to convert this concentration into a value of the water potential of the solution.
Prediction
I predict that the chips in distilled water will have increased in mass and length, while the chips in 1.0M sucrose solution will decrease in mass and length. I believe this because according to the diffusion theory of Osmosis, if a plant cell (potato cell) is surrounded by water, the water will pass into the vacuole, which will expand and press outwards on the cell wall, therefore increasing the size of the potato chip.
If the solution outside the cell is more concentrated then the cell sap, the opposite should happen. Water diffuses out of the cell vacuole, which then shrinks, pulling the cytoplasm away from the cell wall, therefore decreasing the size of the potato chip.
I also predict that there will be a concentration, between 0.0M and 1.0M, where the chip will stay the same mass and length. Just for equality, I predict that it will be around 0.5M.
Results
Raw data
To process these results, I will use these formulae:-
Molarity of sucrose solution = Volume 1M sucrose ÷ total volume
Average change in length = Average final length – Average initial length
Average change in mass = Average final mass – Average initial mass
Processed data
Graphs
Analysis
My results (and graph) show that the chips which were placed in the more dilute sucrose solution increased in length and mass. This backs up the water potential theory, as the water potential inside of the cell was less than the water potential outside, so water moved into the potato cells(by osmosis), which made the cells turgid, which increased the size of the chip. In the more concentrated sucrose solutions, the chips placed inside decreased in mass and length. This again backs up the water potential theory, as the water moved out (by osmosis) of the cells, which made the cells become plasmolysed, decreasing the size of the chip. This makes my prediction correct, although I was not specific in what would happen to the chips which were placed in solutions 0.2 – 0.8.
The line of best fit on my mass change graph allows me to calculate the concentration sucrose where there would be no change would be where the mass change is 0 – a concentration of 0.38M. If I were to calculate this from the length change graph, it would be 0.69M, but obviously, this is more unreliable, as I used a more accurate form of measurement for the mass, with a much smaller margin of error. Therefore, I shall rely on the from the mass change graph (0.38M). I can then use this table to convert my sucrose concentration into a figure of water potential:
Because my concentration figure is not quite on this scale, I need find out the exact solute potential (we know it is between -970kPa and -1120kPa). I am now going to do a calculated estimate of what it is:-
1120 – 970 = 150
150 / 5 = 30
30 x 3 = 90
970 + 90 = 1060
I conclude that the water potential of the sucrose solution that caused no change in the mass of the chips was – 1060kPa. Therefore, because there was no change, the water potential inside of the chips must also have been 1060kPa.
Evaluation
I believe my results have given a fairly accurate figure for the water potential of potato cells, although I do not know exactly how accurate they are as I do not know the figure given under better conditions, performed by professionals. But I believe that my results must be quite reliable, as I took several precautions to ensure that my experiment was fair. I increased the accuracy of my measurements by using a 50cm3 (+/-1.0cm3) measuring cylinder for measuring the volumes of solution. This means that there could only be a maximum error of 5.0% (1.0/20.0). For measuring the masses of the chips, I used a digital top-balance (+/-0.01g) instead of conventional weighing scales. This means that there could only be a maximum error (0.01 ÷ mass of chip x 100)%. I suppose the only measuring tool I used that could be disputed in terms of accuracy was the ruler I used to measure the chip’s length, but I don’t think that matters because to get the result of the water potential I used my results for mass, rather than my results for length.
Also, I think that the fact that I put three chips in each beaker instead of one must have improved my reliability. The three results were also averaged, which minimises the problem of anomalous results. I think that there are no anomalous results for the change in mass, all of them seem to fit a certain pattern, but I think in the change in length results the results for 0.4M and 0.8M seem to be anomalous. This was probably due to human error more than anything else, I may have not measured those chips accurately enough. They seem to be the opposite to what they should be. If you were to put the result for 0.8M at -1mm and the result for 0.4M at 1mm then it would be fine on the graph. They could have been swapped the wrong way round by accident.
Despite this, there are things I could have done (and not done) which might have lowered the accuracy of my results. For example, I may have unknowingly not had the right line of sight when using my measuring cylinder, or when measuring my chips with a ruler. Or, I may have not dried some of the chips well enough before measuring their mass. Many of these results can be changed quite easily just by simple human error.
My experiment could have been improved by using a more sensitive balance to measure the mass, and I could have used a burette or pipette for volumes of liquids. I also could have used a sharper knife to cut the lengths of the chips. All of these things would have made my experiment more accurate. I also could have done a few more averages and repeats, to decrease the margin of error in my experiment, making my results more reliable.
For further work, I would like to see if in my experiment I could actually get the result in my experiment where chip does not change at all, by putting it in the correct Molarity of sucrose solution. I would do this by putting it in different concentrations, ranging from 0.1M to 0.4M, and see what happened. I would also like to do the original experiment again, but using the improvements I listed above, and see how much of a difference there was in the results with the improvements.