Variables
My independent variable will be the concentration of the sucrose in which the potato is incubated. From a preliminary experiment, I know that the concentration of a potato cell lies between 0.0M sucrose and 1.0M sucrose. I will use ten values in between 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, 0.9M and 1.0M. The range was decided by the information I gathered during a preliminary experiment. I will make these concentrations by diluting down sucrose from 1M to the desired concentration. I will keep time of incubation constant, as if I leave the experiment for too long, the potato and solution will reach equilibrium any way, or if I don’t give long enough the readings will all be similar, as they won’t have time to take effect. Also I must keep the type of potato, e.g. King Edward, Maris Piper, etc the same, as different types of potato will have different concentrations. It is a necessity to keep the temperature constant throughout, as changing the temperature can have two effects; 1) if the temperature increase it changes the permeability of the membrane, so osmosis is made easier and therefore quicker, and 2), when the temperature is increased, the kinetic energy of the particles is increased, so they have more energy and travel quicker, which, again, speeds up osmosis. The volume of sucrose must be kept the same, as if there is more sucrose there are molecules to travel in and out of the potato cell. The surface area and potato size will need to be kept the same, because a larger surface area gives a larger partially permeable membrane, so more osmosis can happen at a time.
Presentation of results
I will present the raw data in the form of a table, giving a separate column for averages. The average data will be placed onto a graph where the X intercept can be used to find the isotonic level of the potato.
Prediction
From a preliminary experiment, I know that the concentration inside a potato cell lies between 0M sucrose and 1M sucrose. I predict that the isotonic level will be approximately 0.5M sucrose, because the majority of substance inside a potato cell is water, and 0.3M has a lot of water compared to a small amount of sucrose. I predict that any potatoes left to incubate in sucrose of concentration less than 0.5M, which is a high water potential (Ψ), will have a higher net movement of water molecules into the potato, thus making the cell swell and become TURGID. If the Ψ is considerably high, the cell would burst if it was an animal cell, but as a plant cell has a strong cell wall, bursting is prevented, though pressure potential does increase. E.g.
Potatoes that are left to incubate in sucrose of concentration more than 0.5M (lowΨ) will have a net movement of water molecules out of the potato cell, which has the effect of making the cell membrane shrivel away from the rigid cell wall through dehydration, therefore the percentage weight change will decrease. A cell in this state is PLASMOLYSED. E.g.
The potatoes left in sucrose concentration 0.5M will remain normal, and have a minimal weight percentage difference as the net movement will keep the ratio of water molecules inside and outside the potato the same through out the experiment.
Therefore I predict the graph will be linear an proportional because the net movement will be proportional, thus making the graph look like this;
The point at which the Data Series crosses the x axis is 0.5M; therefore 0.5M concentration is the isotonic point.
Sources
To gain additional information and background knowledge for this Plan, I have used;
Jones and Jones Biology, pages 8-11
Taylor and Jones Foundation Biology, pages 63-64
Obtaining Evidence and Observations
Results
The results that I obtained from my experiment are as follows:
From looking at the table I can see only one result that looks like it may be an anomaly, but, the result is anomalous in both the original test and the repeat test, which means that either the result is correct, or there was an error in setting up the experiment, most probably the concentration of the sucrose was not measured with enough care.
Analysis
Results
From the results gained from the experiment I can plot the following scatter graph with a straight line of best fit.
Trends and Patterns
There is a strong downward trend to the line, and it is inversely proportional, e.g., as the concentration increases, less mass is gained, and more mass is lost. I think that the reason it is not in a completely straight line, with a regular difference between points is because each potato cell is individual, and will work at a different rate to others, making some more efficient than others.
Analysis of Information
The point where the line crosses the X axis (concentration of solution) is approximately 0.2M. I will take this to be the isotonic point of the potato and the sucrose. Therefore, I believe that the concentration of inside a potato cell is 0.2M. Any potato left in a solution with concentration less than the isotonic point will gain more water, as more is let into the cell to even out the concentration inside the cell and outside it. If the concentration is considerably low, e.g. a high ψ the cell will take in too much water and become turgid, only the cell wall stops it from bursting. A potato cell left in solution with a higher concentration than the isotonic point, a low ψ will lose water, and therefore mass, because it needs to let out water in order to get a higher concentration inside, so it matches the concentration outside it.
Conclusion
I can confidently conclude that the concentration inside a potato cell is approximately 0.2M. I cannot be completely certain, as all potatoes are different, so the concentration really depends on the potato. However, 0.2M is a very sensible answer that can be backed up by scientific information, and is very close to other results from other experiments. I feel that my result makes sense, as I know that a vast majority of solution inside a cell is made up of water, as it is essential to prevent dehydration. Only a small amount of salts and sugar are present to keep the potato cell nourished.
Did the Results Fit My Prediction?
I predicted that the isotonic point would be 0.5M, which is fairly close to the actual result. My reasons behind my predicted result have not been undermined by the experiment; they have been supported, but not completely confirmed. I also predicted that the cells left in solutions where the concentration is higher than the isotonic point would become plasmolised, and the cell left in solutions of low concentrations would become turgid, both of which were confirmed, because the cells left in a highly concentrated solution (higher than isotonic level) lost weight, and cells left in a weak concentration (lower than isotonic level) gained weight.
Evaluation
How Effective Was My Procedure?
My procedure worked very well, as it was done with ease, but also allowed me to obtain a good set of results to a high level of accuracy. There were however, some areas that could be improved to be more efficient in gaining results, e.g. the draining of water from the potato, and achieving the desired concentration of sucrose by dilution. However, the method used was very good considering the equipment that we had the option of using.
How Reliable are my Results?
All but one my results followed a trend, which was inversely proportional. The result that didn’t seem to fit in was an anomaly, but as the corresponding result in the second experiment was similar, and also an anomaly, it lead me to believe that the problem was neither recording the weight, or the particular piece of paper, but was probably the sucrose that it was left to incubate in. The sucrose was probably not measured accurately enough, and as both tests were done in the same test tube, both pieces of potato would produce similar results. Apart from that anomaly, my results appear to be very reliable, and recorded to the highest possible level of accuracy, as the liquids were measured in the most accurate measuring vessels we had, and the most accurate scale was used to weigh the potato, as well as using the most accurate timer. As the results are quite reliable they can be used to back up and support a firm conclusion on my experiment.
How can I Improve my Experiment?
There are a number of things I can do to improve the experiment or procedure. I can do more repeat tests, and start anew each time, rather than using the same solution for two pieces of potato. I can have a wider range, with smaller intervals between each reading, so that I get a bigger view of the way a cell works in different concentrations, and the level of accuracy is much higher, so I can get a more precise conclusion. When the potatoes are being dried, I could try and be more delicate, so as not to squeeze any water out from the cell. In any measuring devices, the more accurate it is, the higher level of accuracy I will achieve overall
How can I extend My Work?
To gain a better knowledge of the subject, I can perform similar subjects, which examine other aspects. I could try the same experiment with a different type of potato, or completely different vegetable, e.g. carrot, parsnip, etc. I could use vegetables in different solutions, e.g. a solution of salt and water, rather than sucrose and water. I can change many different factors, and find common links trends or patterns.
