Of course there is a limit to the growth of the potato, at a certain point the vacuole has grown to such a size that it is pressed against the cell wall, which is not letting it grow any further it becomes totally saturated and turgid. There is also a limit to the size the cells can shrink also.
A similar preliminary experiment proves this prediction; here are the results from that experiment:
This table shows the growth of the potato in distilled water and it shrinking in the sugar solutions.
Based on these preliminary results it is obvious that precise and reliable equipment must be used as measurements as small and 0.01 grams can mean a lot. The use of chosen concentrations of sugar in water can be justified by saying that with higher concentrations, such as in the preliminary work, there will be a greater difference in mass afterwards and a pattern may be sketchy and unclear. However, with lower concentrations a detailed pattern can be drawn up of the relation between mass and concentration.
Variables
The things that can be possibly changed in the experiment are the size of the potato chip, concentration of water, amount of solution and the length of time the experiment will be left for. The variables I decided to keep the same are the size of the potato chip and the amount of solution, which will be 25cm³. I will be changing the concentration of the water.
Equipment
Boiling tubes
Boiling tube holders
Potato
Scalpel
Borer
Electronic scales
Measuring Cylinder
Distilled water, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M sugar solutions
Method
The equipment was set up as shown:
First attempt to cut 18 exact chips of potato using the borer and scalpel and weigh and record the mass of each chip. Fill 18 boiling tubes with 25cm³ of distilled water, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M sugar solutions. Put one of the chips in each of the boiling tubes and leave them in the boiling tube racks for approximately 48 hours. After two days record their masses again.
Safety
To make the experiment safe, do not ingest any of the potatoes and be practise basic safety skills when using the scalpel.
Reliability
To make sure the results are precise and reliable repeat the tests as many times as possible, in this case, three times for each concentration and record the masses to two decimal places. Then averages all three of the results, this will give a more even picture of what’s happening, try and ignore any anomalous results, as they tend to interfere with the averages. This proves that osmosis exists: the passing of water through a semi-permeable layer.
Summary
In summary I predict that the potatoes exposed to higher concentrations will loose the most water thus decreasing most in mass and those exposed to the lower concentrations will not shrink and may even grow in mass. Here is a diagram to show how the potato’s plant cells would react in a high concentration.
Analysis of Evidence
From my table and my graph I can see that there is a negative correlation in the way that the average difference in mass of the potato chip gets smaller and smaller. The line of best fit on my graph travels negatively down the graph proving there is a correlation between sucrose solution water and change in mass of potato. This means that the higher the molar of sugar concentration in water, the smaller the difference in mass. Even to the extent where the potato chip actually gets smaller in mass than before it was put in the solution. This means that my prediction was correct in the way that I said the potato would grown in low concentrations and shrink in high ones. This is due to Osmosis, which affects a potato because its plant cells have vacuole, which stores water, and as a vegetable (plant) needs water anyway which is why the cell wall is semi-permeable.
The shrinking and growing would’ve been caused by the water stored in the cell’s vacuole diffusing by osmosis through the semi-permeable membrane in an attempt to even out the concentration of water and sucrose solution.
By looking at the graph you can easily see the concentration of the sugar already in the potato by looking across from the origin to where the line of best fit crosses 0 on the X-axis. This value on my graph is 0.4 Molar sugar.
As with all experiments I found some anomalous results, these can be quite easily seen on the graph. One obvious problem with my table is where I have entered “N/A” – this is because when we were measuring the new masses after two days, a member of our group managed to drop a potato down the sink by accident, this should not affect our results as I can and did average the two remaining potato chip’s weights. The change in chip size between 0.4M and 0.6M is so marginal that it invokes thought to why this happened. My opinion on this is that whilst we were filling the boiling tubes we used the wrong solution, meaning it is human error rather than something wrong with the potato. This could either be caused by the mislabelling of the beaker containing the 0.6M or someone not paying attention whilst setting up the experiment and using the wrong solution. Either way the results for 0.6M are not correct to my prediction and do not follow the pattern that the other solutions tend to follow. Another anomaly is the result average for 1.0M, it has more mass than the preceding 0.8M sugar solution, where my theory dictates that it should be smaller. This could be caused by another human error, although there is a scientific explanation: the potato has become so saturated and the vacuole has pushed so hard against the cell wall that it has become turgid and caused plasmolysis. Plasmolysis; dictionary definition:
Shrinkage or contraction of the protoplasm away from the wall of a living plant or bacterial cell, caused by loss of water through osmosis.
The American Heritage® Dictionary of the English Language, Fourth Edition
Meaning that the cells could have begun to loose water causing them to shrink and even die in some cases.
Evaluation
The experiment was set-up and carried out efficiently and safely with exactness in mind. The results are correct with the prediction apart from the anomalous results: the mass change was only marginal between 0.4M and 0.6M sugar solution and as I explained earlier; this could be due to human error through the mislabelling of the solutions in the science lab. The anomalous result for 1.0M: where the mass is larger than the 0.8M where it should be smaller could be caused by plasmolysis (the cell has become turgid and began to die and loose water).
Overall I think the approach to the experiment was as accurate as possible with the equipment we had available to us. The measuring cylinders we used could have been replaced by burettes or measuring syringes for optimal results, although the difference it would have made would have been marginal. The electronic scales we used were extremely accurate, recording mass in grams to 2 decimal places.
In summary this experiment isn’t very accurate compared to it being done in better-controlled conditions, such as a university laboratory.
On a whole I think the experiment was suitable as it properly showed how osmosis worked across a concentration gradient (ignoring the anomalous results). In general it would be useful for other science groups to do the same experiment to find evidence of osmosis and I encourage teaches use this experiment as an example.
There are several improvements that could be made to the experiment, for example: if I were to do this experiment again I would use more exact equipment, such as burettes for measuring out the solution. I would also design a set time to leave the experiment instead of roughly estimating two days like we had to because of social limitations. To create a more detailed graph I would use more concentrations of water, such as 0.3, 0.5, 0.7, 0.9 as well as the original concentrations. Also I would do higher concentrations as to find the exact point of total saturation of the potato.
In summary we managed to prove the existence of osmosis and how it changes under different concentrations. The main idea of this experiment was to prove that water will attempt to even out concentrations of itself to create an equilibrium and I think we did this.
Bibliography
Microsoft Word used for presentation.
I mainly used the Internet to do this coursework, primarily theses sites:
“Biology for Life”