Variables and Fair Test:
- The concentration of the solution.
This will be changed accurately and with the same measuring cylinder. If it were not kept the same it would affect the rate of osmosis, as the cells would be moving quicker, colliding quicker, and osmosis would be quicker.
- The surface area of the cylinder of potato.
The surface area of the potato pieces must be kept the same because if one chip were one centimetre longer than another it would have more area for osmosis to occur on. It will be controlled by me measuring the potatoes accurately with a ruler, and lining it up against it has previously cut potato piece, to double check it is the same size.
- The duration of the experiment.
The longer that the experiment lasts the more osmosis is going to occur, until it is fully plasmolysed or turgid. To counter this all experiments will be taken out of the solution at the same time.
- The temperature of the solution and potato.
The temperature will affect the rate of reaction, and keeping all experiments at room temperature will keep all potato pieces the same.
- The accuracy of the weighing scales.
I am also going to use the same balance to weigh my potato pieces. This is because the measurements may slightly vary between scales.
I am going to keep the solution volume the same at 100ml.
I am going to keep the water potential of the potato the same by using the same type of potato, which have been treated in the same way, e.g. have all been cut without being washed and peeled.
I am going to measure the pieces in grams.
I am going to use the same apple corer, so that all the cylinders are the same width.
Key Factor:
The factor that I am going to vary is the concentration of the solution, this is because it will give me a varied set of results, from which I can make a decent conclusion.
Equipment:
Scalpel
Weigh Scales
Five plastic cups
Cylinder
Sugar Solution
Water
Potato
Apple Corer
Tile
Paper Towel
Analysis:
This graph shown separately gives the line of best fit for the percentage change in mass of the potato chips over the course of the experiment. My computer-drawn graph shows a very slight curve on that is leading to complete plasmolysis, whereby the potato cannot expand and take in any more water, but this is not evident enough to prove anything, and is more or less a straight line. There is a pattern on my graph, and this is, as the concentration of the solution increases, the percentage change in mass decreases. The graph shows that the percentage gain and loss is inversely proportional to the concentration. The gradient does change in my graph. The potato chip is becoming as flaccid as it possibly can, and so the change in mass of each cylinder concentration is becoming closer and closer together. From the line of best fit that has been added in, it can be seen some of my points were very close to creating a perfectly smooth curve. This shows that my results are somewhat reliable. My graph fits in with my prediction and with the predicted graph. It shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. From the graph an estimate to the concentration of the potato cell can be made as 9%, as this is the point where the potato is not increasing or decreasing in mass, this is known as the isotonic point. This is where no osmosis is taking place; both the potato and the solution have an identical sugar concentration. My results also match with my initial predictions. As you can see as the sugar, concentration increases the change in mass decreases. The graph shows a clear indication that there was an overall change in mass during the experiment.
At 20% sugar volume the results/weights for after the experiment is much lower than the ones for before the experiment unlike any of the others. This is because the water potential of the sugar solution is higher than that of the potato chip.
Evaluation:
The experiment was very successful in my opinion. However, a small problem I came across was that not all my potato pieces weighed the same. The total difference was 1.2g between the heaviest and the lightest piece. This is quite a big difference considering the results would be precise. Nevertheless, I did use an average of the three. I obtained a large quantity of very accurate results from which I was able to create informative graphs. I think I took easily enough results for the number of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 2.5%, 5%, 7.5% and so on. This way it would have allowed me to obtain results that are extremely more accurate and the graphs would be easier to see anomalous results on. It would also have allowed me to find out the isotonic point far more accurately as the one that I estimated is approximate. I have also drawn a maximum and minimum isotonic point – (Minimum: 7%, Maximum: 10%) – which shows the range of concentration that I should, if I was too repeat the experiment, use. This would give me a more precise isotonic point, rather then an approximate one. The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment, I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato, I could have found a more accurate way to measure out the solutions and to determine the sugar concentrations. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.0g but to 0.000g. I could also have improved my results by looking at the experiment again, but in more detail around the isotonic point. There were not any out of the ordinary results, but some were not as close to the line as others were. This may have been caused by human error. Also, the bigger the potato cylinders, the more accurate the results are, as there is more surface and it will easier to notice the change in mass. When the potato chips were removed from the test tubes and dried, I may well have dried some potatoes more thoroughly than others may and therefore some would have extra mass, from the excess water. If the experiment was repeated, I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time.
However, with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.
