The Experiment
Aim
-To find out what happens when potatoes have been left in sugar solutions of different quantities and concentrates, over a 24 hour period o time.
-To measure the mass of the potato before and after the experiment, has it increased or decreased in mass?
Preliminary Work
I researched some of the work of the internet on the BBC learning website and some other websites with existing essays involving the process of osmosis, and how it is used in human cells as well as the plant cells. In class we discussed turgid and flaccid cells. Turgid cells are when the cell has become firm and rigid (like a fully blown up tyre). As water moves into the cytoplasm, the vacuole swells up, and because the cell wall is strong it will not burst. The cytoplasm presses on the cell wall, but the cell wall resists and presses back on the cell contents. The turgidity of a plants cell helps a plant that has no wood in it to stay upright, and keeps the leaves firm. Flaccid cells are when the cells in plants lose water in osmosis. As the cytoplasm shrinks, it stops pushing outwards on the cell wall, and becomes floppy (like a tyre when the air is let out). This way the plant loses its firmness and begins to wilt. As more water leaves the cells, the cytoplasm starts to peel away from the cell wall, they are plasmolysed. I will compare the results of my experiment with the results of someone else’s to make my results more reliable OR I will repeat the experiment myself.
Apparatus
- 5 pieces of potato (each piece must measure to 5mm)
- 5 test tubes
- Sugar solution, different quantities (to see amounts please review ‘solutions’)
- Test tube rack
- 24 hr Stopwatch
- Scales
- Syringe
- Results table
- Cling-film
The Solutions & Variables
For my experiment I am using 5 different solutions with a large range of concentrations. Each test tube will hold a different solution and each of the solutions will be mixed with water. I will use a syringe to extract the amounts.
The 1st tube will contain 4mm³ of sugar solution and 16mm³ of water (20% solution).
The 2nd tube will contain 8mm³ of sugar solution and 12mm³ of water (40% solution).
The 3rd tube will contain 12mm³ of sugar solution and 8mm³ of water (60% solution).
The 4th tube will contain 16mm³ of sugar solution and 4mm³ of water (40% solution).
The 5th tube will contain 20m³ of sugar solution and no water (100% solution).
There are several variables and we can control them. The temperature will be at room temperature and fairly cool. They will be kept in the same place with the same amount of sunlight, just in case this affects the process. They will all be put in the solutions at the same time; there will be the same amount of liquid in each test tube. The cling film will be on tight.
Method
- Put each of the solutions into a different tube for all of them, clearly marking them with a pen, writing on the test tubes which tube contains what percentage of solution.
- Weigh out the pieces of potato (5mm) in turn and record their masses onto the results table. The masses should be relatively the same. Then carefully put one piece of potato into each of the test tubes, at the same time and then start the stop watch to record a time of 24 hours. Cover the top of the tubes with cling-film and leave somewhere for 24 hours, either on a workbench out of sunlight or somewhere where the temperature is in controlled surroundings.
- After 24 hours remove the cling-film and carefully drain away the solutions, careful not to lose the pieces of potato. Then place each of the pieces of the potato in turn onto the scales and record the results, be sure to do this quickly but accurately.
- When you have recorded your results, workout the percentage change and plot a graph. Below are what my outcomes of the experiment are and the line graph. I think this was a very hard experiment to predict a conclusion for and to actually perform.
Results
Results Table
Line Graph to show percentage change in concentration
Conclusion
In my experiment I discovered that my prediction was wrong. Instead of the mass increasing, it decreased which means that the high concentration of water has passed through the semi-permeable membrane of the potato cell and into the weaker concentration of water in the sugar solution. I thought that it was the high concentration of sugar solution that would pass through the semi-permeable membrane. I therefore have got the idea of osmosis mixed up and did not understand the outcomes of osmosis. But I did understand that the concentrations have travelled after taking part in the experiment.
I did retake the experiment and I managed to get results which came very close to my last lot. But this time I understood what I was doing and what was happening and why it was happening. I have understood the process of osmosis more after actually doing the experiment and recorded what happened in the procedure.
I think I could have made the test fairer by keeping the solutions the same temperature before AND during the experiment (and actually measuring the temperature to make sure of this) to see if this input variable affected the results, and by making sure the potato not only measured the same but weighed the same too. I should make these improvements if I am to do this experiment for a third time. I could extend it further by doing more sets of concentrations (in a bigger range) and to make the length of time (24 hours) into something a bit longer or shorter. I could also try an experiment involving osmosis in the form of animal cells and compare my results from this experiment, with ones from an animal cell experiment. I could see if there are any similarities between the two or if there are any big changes.
Vikki Stabler 10J 02/05/2007 Page of