Hypothesis: I have chosen to investigate the effect of changing salt concentration on osmosis in potato cells. Osmosis is the movement of water molecules across a selectively-permeable membrane from a region of high water concentration to a region of low water concentration. This means that the tiny holes in the membrane of the potatoes will allow the water molecules to pass through in and out of the solution. In osmosis only the water molecules move, solutes do not. The water molecules can pass both ways (as shown in the diagram below). However because there are more water molecules on one side than the other there is a steady net flow into the region with fewer water molecules, (into the stronger solution of glucose). Osmosis make the potato cell swell up if it is surrounded in a weak solution and it becomes turgid.
This is a diagram to show the movement of water molecules across a selectively permeable membrane.
(Source copied from ‘CGP revision book)
In the distilled water, I predict that the water is more concentrated than in the potato cell, and therefore the water should transfer to the potato making it bigger in size. This is because of the extra water in the cell.
Solutions of salt should make a difference to the length, as it is more concentrated. Therefore, with the potato in 0.5 and 1 molar solutions of salt, I do not think there will be much of a difference of length. This is because there is not a big difference between the solute concentrations inside and outside of the cell membrane. They are both similar. Therefore I predict that the size of the potato should be decreased by a fraction.
The 1.5 and 2 molar the length should decrease at least noticeable for us to notice.
Finally, the difference between the water concentration in the potato and the 2.5 molar solution of salt is big. As a result it will probably become smaller and shrink. This is because more water goes into to the cell than out, as the concentration of water molecules is lower.
My theory in this experiment is that the more the solution is concentrated (salt molecule concentration), the more the potato cell will decrease.
I have researched to find out further background information. The information was found from: www.chadevans.co.uk/gcsebiology/
A graph to show how different concentration of sugar solutions affects the mass of potato chips.
At point A, the graph shows that no osmosis has occurred. The concentration of water inside the cell is equal to the solution outside.
At point B (high water concentration), there is no indication that the cell is increasing further in size. This is because the cell is fully turgid and no more water can enter.
At point C (low water concentrations), there is no indication that the cell is decreasing further in size. This is because no more water can leave the cell.
Apparatus
-6 Petri dishes
-18 potato chips (all the same length)
-measuring cylinder
-timer
-salt solutions of (0.5, 1, 1.5, 2, 2.5 moles)
-distilled water
-white tile
-knife
-ruler
-tweezers
Diagram
Method
-Collect 6 Petri dishes and place the on the table.
-Measure 100mls of distilled water with a measuring cylinder and put it in a Petri dish.
-Now repeat it with the other salt solutions.
-Make sure you wash the measuring cylinder out thoroughly, before trying to measure the next solution otherwise it could affect your results and it will not be a fair test.
-Now put your potato chips on a white tile. With a ruler measure them to 5cm and cut them with a knife.
-Put in 3 potato chips in the distilled water and time them for ½ an hour.
-5 minutes after putting the first three potato chips in a solution put the next three in the next solution (0.5 molar).
-Repeat this for all the solutions.
-After the potato chip has been in the solution for ½ an hour. Take them out with tweezers and put them on a white tile.
-Measure each potato chip with a ruler and record results in a table.
-Repeat this until all the potato chips are measured.
-The reason to why we used the potato chips for each solution is to make the experiment a fair test. An average result can be obtained from this.
Results
Graph
Conclusion
From having carried out this experiment, I have found out that the solute concentration of salts does affect the length of the potato chips. This is because there will be a big difference between the solute concentrations inside and outside of the cell membrane The tiny holes in the membrane of the potatoes will allow the water molecules to pass through in and out of the solution. My theory in this experiment is that the more the solution is concentrated (salt molecule concentration), the more the potato cell will decrease. This prediction was correct. The graph also shows this. As the salt concentration increased the length of the potato chips decreased. For example, for the 1.5 molar salt concentrations there was a difference of –0.7 and for the 2.0 molar salt concentration there was a difference of –0.8. Although there was not a big difference in the lengths, it was still noticeable suggesting the experiment did work. The potato chips shrank, as the salt solution was more concentrated because more water went into to the cell than out, as the concentration of water molecules is lower.
From looking at the graph, I can see anomalous results. These are circled on the graph and numbered.
- The potato chips in the distilled water were supposed to increase. I predicted that that the water is more concentrated than in the potato cell, and therefore the water should transfer to the potato making it bigger in size. This is because of the extra water in the cell. However, this was not the case. Instead of expanding, they shrank. This may have occurred because the distilled water may have been mixed with the salt solutions
- There was an anomalous result with the 0.5 molar salt concentration. From looking at the graph, there is a big difference between the length of chips in the distilled water and that of the 0.5 molar salt concentration. This may have been due to inaccurate measuring of the chips.
Evaluation
After evaluating my work I think my experiment did answer the original question, as you could see the salt concentrations got more concentrated the potato cells expanded. I carefully planned my procedure and made sure that the experiment was a fair test. I repeated the experiment three times to obtain an average. Most of the evidence obtained was accurate. However, some of it was not. The anomalous results may have occurred due to inaccurate measuring. If I were to carry out the experiment again, I would make sure the petri dishes were thoroughly clean before using them. This may have also affected the results when testing the length of potato chips in the distilled water.
I had made sure that the experiment was as accurate as it possibly could have been. I used a ruler to make sure the potato chips were the same size. I also used a stopwatch to time how long the potato hips were in the salt concentrations. However, it is not possible to get 100% accurate results as human errors such as measuring and timing could occur.
If I were to do this experiment again, I would have to use precision equipment to improve the reliability of my results. I would also carry out extra experiments such as investigating the percentage mass change of the potato cells and testing different variety of potatoes. This would be useful; as it would show us the effect of osmosis when different; potatoes have different water concentration in them.