The same thing occurs when cells are placed in a concentrated
sugar solution. Osmosis occurs in the opposite direction, and the
cells begin to lose water, and become ‘flaccid’. This is the exact
opposite of ‘turgid’, the contents of the cell shrinks, and pulls
away from the cell wall. These cells are said to be called
‘plasmolysed’. When plant cells are placed in a solution which
has exactly the same concentration as the cells, they are in a state
called ‘incipient plasmolysis’, or isotonic.
Variables
To ensure a fair test, certain aspects of the experiment will have to be kept the same whilst one key variable is changed (the concentration of sucrose). I will vary the concentration of the sugar solution. This will give me a vary varied set of results from which I hope to make a decent conclusion. If any of the non-variables below were not kept constant, then the test would not be fair. For instance if one of the potato chips was massively larger, area of the chip would be larger and there would therefore be more space for osmosis to occur. Doing all the tests at one temperature will control the temperature.
1. For the purpose of my experiment I am going to do all the experiments at room temperature.
2. To keep the water potential of the pieces the same, the same piece of potato will be used for all the experiments.
3. The mass of the potato is a dependent variable, and this means that it will be measured
throughout the experiment. I will measure the mass in grams. The potato chip will be
measured before it is put in the solution, and after. This will allow us to see whether
osmosis has taken place, and to what extent.
4. The volume of the solution that the potato chips are kept in must be fair. They must be
totally covered in the solution, and the amount of solution will be kept the same because
all the potato chips are the same size.
5. I am also going to use the same balance to weigh my potato chips. This is because the measurements may slightly vary between scales.
Method
A range of sucrose sugar solutions will be prepared with concentrations of 0% sucrose (water), 5% sucrose, 10%, 15% and 20%. Sections of potato will be cut using an apple corer, and a sharp knife (see safety), care taken to ensure the slices are as equal as humanly possible, as a change in the surface area may allow more or less osmosis to occur. The mass of each chip will also be measured and recorded. Five chips will be placed in each petri dish each time so that and average may be taken from each dish and thus, solution. 20 ml of each concentration of sugar solution will be added to each petri dish (preliminary experiment showed this was the optimum amount) and each dish carefully labelled. The potato pieces will then be added to the dishes, care being taken to ensure the pieces may easily be identified by their respective weights. The dishes will then be moved to conditions best suited to minimise evaporation, and other situations that may prove problematic; they are then left for two hours (again, preliminaries proved this was an optimum period). Then the potato pieces will be removed, any solution remaining on the surface of the pieces removed using paper towels before the pieces are reweighed.
- Potato to be used was checked prior to the experiment to ensure it is in fact a good, healthy, firm potato.
- Using an apple corer, a cylinder of flesh is removed from the potato. (See safety)
- The cylinder is then cut using a sharp knife into thin (1-2mm) circular pieces. (See safety)
- Petri dishes were labelled 0%, 5%, 10%, 15% and 20%; and were filled with the various solution concentration respective to their labels.
- Each potato piece is weighed, the values recorded before the pieces were swiftly transferred into their respective dishes.
- After the allotted time period has passed, the chips are removed, dried and weighed in such a fashion as to ensure no mistakes are made with solutions and initial weights. The final weights are then recorded.
Results
The attached results show that potato cell increase in mass in solutions in high water concentrations, proving that osmosis must have taken place. Also, when the concentration of the solution grows to around 15%, there seems to be no more loss of mass beyond this point; suggesting the cell has become fully plasmolysed. From the graph, an estimation of the concentration of sucrose originally present in the potato is shown to be around %, as at this point, the potato is nor increasing, nor decreasing in mass. This value is known as the isotonic point; where no osmosis is taking place due to the concentration of the solution equalling that of inside the potato. The graph of the change in mass helps prove the point of complete plasmolysis, whereby the potato cannot expand and take in any more water. As can be seen, as the concentration of the solution increases the change in mass decreases.
Evaluation
In my opinion, the experiment proved to be very successful. Large quantities of results were obtained, allowing me to come to an accurate conclusion. I believe the number of pieces of potato used was more than sufficient to produce an average that fits with my expectations; however, if I were to repeat the experiment, I would execute it using a larger number of concentrations. This will allow me to produce a much better graph from which accurate predictions and information can be obtained. This would also have allowed me to find the isotonic point far more accurately as the one that I estimated is very approximate.
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. I encountered no striking anomalies, however, I did find that some values were not as close to best fit than others. This may have been caused by human error, e.g. when the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. 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.