I have chosen to take 5 different concentration so that I have a wide range to make a conclusion from.
From my experiment I will find the % change in mass.
%change = average difference in mass
average initial mass
I will show the % change of each solution on a best fit line graph.
Dependant Variable
I am going to weigh the mass of the potato before and after the experiment, we will use a weighing scale to do so. We will leave the potato in the solution for 48 hours.
Control Variable
During the experiment I will keep the following factors the same:
- The surface are of the tissue – I will so this by keeping the diameter of the potato the same throughout the experiment.
- Volume of the external solution – I will do this by measuring the same amount of solution each time by using a measuring cylinder.
- The type of external solution – I will use the salt solution throughout the experiment.
- The type of tissue – I will keep the potato the same throughout the experiment.
- The mass of the tissue – I will keep the mass of the potato the same throughout the solution. I will keep the quantity the same.
prediction
If the external solution is more concentrated than the potato cells, the potato will loose water. Therefore there will be a decrease in mass. If the external solution is less concentrated than the potato cells, then the potato will take in water. Therefore, there will be an increase in mass. If the external solution is the same concentration as the potato cells there will be no change in mass and no percentage increase or decrease.
Method
- Measure out 5 different solutions of water and salt into 5 different boiling tubes and label each one.
- Cut 15 cylinders of potato using the same potato borer to cut each one. Then weigh each one and record which weigh of potato is going into each solution.
- Put three potato cylinders into each boiling tube and place coloured drawing pins on to two of them so they can be identified, stopper on top and leave for 48 hours.
- After 48 hours take each potato out of its boiling tube one at a time and weigh it and record its mass beside which it was before the experiment.
- Work out the calculations to find the average difference in mass.
Safety
- Tie hair back
- Tuck in stool
- Careful when handling solutions
- Use apparatus safely
- Place bags under desk
- Wash hands after using solutions
- Be careful when handling blade
Equipment
- 5 boiling tubes
- potato
- boiling tube rack
- 5 stoppers
- weighing scales
- water
- salt solution
- potato borer
- measuring cylinder
- coloured drawing pins
Results
See previous page
Conclusion
The evidence obtained from this investigation supports the prediction I made.
I found that as you increase the concentration of the salt solution the change in mass decreases. Therefore if you decrease the concentration of the salt solution, the change in mass increases. For example the mass of the potato in the 0.8 salt solution had an average of –0.05 decrease, whereas the mass of the potato in the 0.2 salt solution had an average increase of 0.06 after 48 hours. For unknown reasons we were unable to take results from the 100% water solution because the potatoes had rotted away in the boiling tube, I predicted what the % change in mass would be on my graph, I think it would have the greatest increase in mass.
From the graph I can see the isotonic point where the concentration of water inside and outside of the potato are equal. At this point there will be no change in length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred. From my graph I can this happened when the potato was placed in the 10% salt solution.
When the potato is placed in a solution greater than 20% it loses water, it moves from an area of high concentration to an area of lower concentration.
Turgid Cell Normal Plasmolysed Cell
The cell membrane in cells is semi-permeable and the vacuole contains sugar/salt solution. So when a cell is placed in high water concentration e.g. distilled water, the water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid.
If potato cells were placed in a solution with a low water concentration, the water would move out of the cells and into the solution. As the cytoplasm shrinks, it stops pushing outwards on the cell wall. The cell becomes floppy this is said to be flaccid. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed.
My prediction was correct as I predicted that if the external solution was more concentrated than the potato cells that the potato would lose water. Therefore there would be a loss in mass. I also predicted that if the external solution was less concentrated than the potato cells, the potato would take in water. So therefore the greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater amount of water that leaves the cell.
Evaluation
During my experiment I used coloured drawing pins to identify each piece of potato, this was so I knew which potato I put in first to make it a fair test.
If I were to do the experiment again I would use pipettes which consist of smaller volumes, instead of measuring cylinders to measure out the solutions as this would give me more accurate results.
I think my results are quite accurate but I do think there is room for improvement. I could improve my results by repeating the experiment. I may repeat it four or five times more and this would then give me a more accurate average. I would also again use pipettes instead of measuring cylinders. Also instead of my percentages of salt solution going up in twenty’s, I would go up in five’s. There is a smaller range so therefore I could more accurately pinpoint the exact position where the potato starts to lose water. For example my isotonic point is 20% but if I used a smaller range of percentages I could actually find that it could be 17%
From my graph I can see that I have two anomalies. This may have been due to the fact that the pieces of potato were not thoroughly dried out. Finally I would repeat the experiment to see if I were to find the same results.
