Hence, the only factor that I will vary for each experiment is the concentration of the solution in which the potatoes will be immersed.
Planned method:
I first carried out a prior test to obtain a more accurate idea of what the outcome of my actual experiment would be. This test also served to allow me to make any changes to the procedures I plan to use for my actual experiment, as well as to introduce any precautions that would make my actual test a fair and safe experiment.
Prior test:
Materials:
- One large potato
- Two Petri dishes
- Two labels
- Two beakers
- Two droppers
- One scalpel
- Distilled water
- Sucrose solution of unknown concentration
- One cork borer of size 2
- Ruler
- Graph paper
- Weighing scales
Method:
I used the cork borer to obtain 10 potato cylinders. Then using the scalpel, I cut off both ends of each potato cylinder. I measured the lengths of each potato cylinder and cut it accordingly so that all of them were now 4.5cm in length. Using a tissue paper, I removed any waste matter such as potato skins left on the cylinders. I did so in order to avoid errors when taking down the masses of each potato cylinder. Using a top pan balance, I measured the mass of each potato cylinder.
To form the sucrose solution I placed an unknown amount of sucrose powder in a beaker with distilled water. However, I made sure that all the sucrose was dissolved by continuously stirring the solution. I filled the second beaker with distilled water only.
I then placed 5 potato cylinders in each Petri dish. I used a dropper to place 30cm3 of sucrose solution in the first Petri dish. I replaced its cap and added a label with “Sucrose Solution” written on it. I used the second dropper to place 30cm3 of distilled in the other Petri dish. I replaced its cap, and labeled it “Distilled water”.
After leaving the Petri dishes and their contents for 24 hours, I took my new readings. In order to do so, I removed each potato cylinder from the Petri dish labeled “Sucrose Solution”, gently patted it with a tissue paper to remove any excess solution on it surface and weighed it. I recorded the readings for each potato cylinder. I then repeated the same procedure for the potato cylinders in the Petri dish marked “Distilled Water”.
Precautions:
To ensure that my test was a safe and fair one I took the following precautions:
- I did not hold my hand directly opposite to the place on the potato from which I expected the cork borer to immerge as it could have injured my hand.
- I was exceptionally careful to ensure that I did not cut myself while using the extremely sharp scalpel.
- I had to ensure that my hands were clean and dry in order that I neither contaminated the potato nor passed excess water onto it.
- I ensured that I kept the factors I had mentioned earlier constant.
- I took care to ensure that my cylinders were of identical lengths before the experiment. I did this by carefully measuring their lengths to the nearest millimeter. I measured the length of each cylinder twice to ensure that my measurements were correct. I repeated this process to measure the lengths of the cylinders after having immersed them in the various solutions.
- I used a very sensitive weighing balance to denote the masses of the potato cylinders both before and after the experiment. I reweighed each cylinder to ensure that I have denoted the correct reading.
- I ensured that the potato cylinders were completely immersed in the solutions. Unless all the potato cylinders are completely immersed in the solutions, the rate and amount of osmosis will not be equal and this would make my investigation an unfair one.
- I used cylinders from the same potato to ensure that they were of the same structure and concentration to ensure that any change is due to the effect of osmosis on identical cells.
- As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop his from happening, the beaker and syringe must be washed every time they are used. Better still, I used different syringes and droppers for each solution.
Results obtained during the prior test:
For potato cylinders immersed in distilled water:
The average change in length was an increase of 4.90-4.50= 0.40cm (+0.40cm gain)
The average change in mass was an increase of 0.88-0.69=0.19g (+0.19g gain)
2. For potato cylinders immersed in sucrose solution:
The average change in length was a decrease of 4.50-4.18=0.32cm
(-0.32cm gain)
The average change in mass was a loss of 0.69-0.52=0.17g (-0.17g gain)
Results:
The prior test showed that water does move from places with high water concentration to places with low water concentration. The potato cylinders I had immersed in distilled water swelled, thus increasing in size. They not only gained length and mass, but also became harder and an increase in their cross section became obvious. This shows that water had entered the potato cells by osmosis in order to dilute the cell sap and make the cells turgid.
On the other hand, water had moved out of the potato cylinders that had been immersed in sucrose solution in order to attain equilibrium on both sides of the cell membrane. Since they had lost water by osmosis, the potato cylinders decreased in mass and volume and their cross section was visibly reduced. Thus, the cells became plasmolysed. In conclusion, the prior test showed me that my inferences about the effect of changing the molarity of a solution in which potato cylinders are immersed was reasonable and hence I could proceed to perform my actual experiment in order to find the molarity of cell sap.
The diagram below shows what I think must have happened to the potato cells after I had immersed the potato cylinders in distilled water and sucrose solution:
Planning my actual experiment:
When I conduct my actual experiment, I will use the same procedure as that of my prior test. It is as follows:
Factor that I will keep constant:
- The thickness and permeability of the cell membrane.
- The amount of solution used.
- Temperature at which the experiment is conducted.
- Duration of the experiment. (By changing the amount of time each potato cylinder is immersed in solution, I may obtain erroneous readings since if equilibrium has not been reached by the time I remove them from the solution, osmosis will continue to take place. To prevent this from happening, I will allocate an equal amount of time for each set of solutions.)
- Initial lengths of the potato cylinders used.
Hence, the only factor that I
Will vary for each experiment is the concentration of the solution in which the potatoes will be immersed.
Materials:
- One large potato
- Six Petri dishes
- Twelve labels
- Six beakers
- Six droppers
- One scalpel
- Distilled water
- Sucrose solutions of concentrations 0.125M,0.25M, 0.50M, 0.75 M and 1.00M
- One cork borer of size 2
- Ruler
- Graph paper
- Weighing scales
Method:
I will use the cork borer to obtain 30 potato cylinders. Then using the scalpel, I will cut off both ends of each potato cylinder. I will then measure the lengths of each potato cylinder and cut it accordingly so that all of them will be 5.00cm in length. Using a tissue paper, I will remove any waste left on the cylinders. I will do so in order to avoid errors when taking down the measurements of each potato cylinder. Using a top pan balance, I will measure the mass of each potato cylinder.
I will fill each of the six beakers with a different solution. I will place a label on each denoting its molarity.
I will then place 5 potato cylinders in each Petri dish. I will use a dropper to place 30cm3 of sucrose solution of concentration 0.125M in the first Petri dish. I will replace its cap and add a label with “0.125M” written on it. I will repeat this using the other solutions and labeling the other Petri dishes as “0.25M”, “0.50M”, “0.75M” “1.00M”and “Distilled Water”.
After leaving the Petri dishes and their contents for 24 hours, I will take my new readings. In order to do so, I will remove each potato cylinder from the Petri dish labeled “0.125M”, gently pat it with a tissue paper to remove any excess solution on it surface and weigh it. I will record the readings for each potato cylinder. I will then repeat the same procedure for the potato cylinders in the other Petri dishes.
Precautions:
To ensure that my test is a safe and fair one I will take the following precautions:
- I will not hold my hand directly opposite to the place on the potato from which I expect the cork borer to immerge as it can injure my hand.
- I will be exceptionally careful to ensure that I do not cut myself while using the extremely sharp scalpel.
- I will ensure that my hands are clean and dry in order that I neither contaminate the potato nor pass excess water onto it.
- I will ensure that I keep the factors I had mentioned earlier constant.
- I will use a very sensitive weighing balance to denote the masses of the potato cylinders both before and after the experiment. I will reweigh each cylinder to ensure that I have denoted the correct reading.
- I will ensure that the potato cylinders were completely immersed in the solutions. Unless all the potato cylinders are completely immersed in the solutions, the rate and amount of osmosis will not be equal and this would make my investigation an unfair one.
- I will take care to ensure that my cylinders are of identical lengths before the experiment. I will do this by carefully measuring their lengths to the nearest millimeter. I will measure the length of each cylinder twice to ensure that my measurements are correct. I will repeat this process to measure the lengths of the cylinders after having immersed them in the various solutions.
- I will use cylinders from the same potato to ensure that they are of the same structure and concentration to ensure that any change is due to the effect of osmosis on identical cells.
- As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop his from happening, the beaker and syringe must be washed every time they are used. Better still, I will use different syringes and droppers for each solution.
I will record my results in tables such as those shown below:
Solution concentration: ________
I will then illustrate my results using a graph. The graph I have sketched below represents the expected graph I think that I will obtain after conducting my actual experiment. The expectations are based on my observations during the prior test and my hypothesis that water moves by osmosis from placed with high water concentration to places with low water concentration.
To explain this prediction scientifically, I have to refer to my hypothesis on the movement of water during osmosis. In order for the potato cells to increase in length and mass, water has to diffuse into them. For it to do so, the water concentration outside the cells should be greater than that of the cell sap. On the other hand, if the water concentration of the cell is greater than that of the solution outside the cells, water will diffuse out of the cells- consequently they will shrink and decrease in mass. This will only take place in solutions that are rich in sucrose and contain few free water molecules.
Books I have referred to in order to gain help in planning a safe and fair investigation:
- Biological Science 2
- Advanced Heinemann Biology by Ann Fullock
- Advanced Biology by Michael Robert
Collins Biology by Marcus Barbos
Section 2: Obtaining evidence
Actual experiment:
When I conducted my actual experiment, I used the same procedure as that of my prior test.
Factors that I kept constant were:
- The thickness and permeability of the cell membrane.
- The amount of solution used.
- Temperature at which the experiment is conducted.
- Duration of the experiment. Initial lengths of the potato cylinders used.
- Hence, the only factor that I varied for each experiment was the concentration of the solution in which the potatoes were immersed.
Materials:
- One large potato
- Six Petri dishes
- Twelve labels
- Six beakers
- Six droppers
- One scalpel
- Distilled water
- Sucrose solutions of concentrations 0.125M,0.25M, 0.50M, 0.75 M and 1.00M
- One cork borer of size 2
- Ruler
- Graph paper
- Weighing scales
Method:
I used the cork borer to obtain 30 potato cylinders. Then using the scalpel, I cut off both ends of each potato cylinder. I measured the lengths of each potato cylinder and cut it accordingly so that all of them were 5.00cm in length. Using a tissue paper, I removed any waste left on the cylinders. I did do so in order to avoid errors when taking down the measurements of each potato cylinder. Using a top pan balance, I measured the mass of each potato cylinder.
I filled each of the six beakers with a different solution. I placed a label on each denoting its molarity.
I then placed 5 potato cylinders in each Petri dish. I used a dropper to place 30cm3 of sucrose solution of concentration 0.125M in the first Petri dish. I replaced its cap and added a label with “0.125M” written on it. I repeated this using the other solutions and labeled the other Petri dishes as “0.25M”, “0.50M”, “0.75M” “1.00M”and “Distilled Water”.
After leaving the Petri dishes and their contents for 24 hours, I took my new readings. In order to do so, I removed each potato cylinder from the Petri dish labeled “0.125M”, gently patted it with a tissue paper to remove any excess solution on it surface and weighed it. I recorded the readings for each potato cylinder. I then repeated the same procedure for the potato cylinders in the other Petri dishes.
Precautions:
To ensure that my test is a safe and fair one I took the following precautions:
- I did not hold my hand directly opposite to the place on the potato from which I expected the cork borer to immerge as it could have injured my hand.
- I was exceptionally careful to ensure that I did not cut myself while using the extremely sharp scalpel.
- I ensured that my hands are clean and dry in order that I neither contaminated the potato nor passed excess water onto it.
- I ensured that I kept the factors I had mentioned earlier constant.
- I took care to ensure that my cylinders were of identical lengths before the experiment. I did this by carefully measuring their lengths to the nearest millimeter. I measured the length of each cylinder twice to ensure that my measurements were correct. I repeated this process to measure the lengths of the cylinders after having immersed them in the various solutions.
- I used a very sensitive weighing balance to denote the masses of the potato cylinders both before and after the experiment. I reweighed each cylinder to ensure that I have denoted the correct reading.
- I ensured that the potato cylinders were completely immersed in the solutions. Unless all the potato cylinders are completely immersed in the solutions, the rate and amount of osmosis will not be equal and this would make my investigation an unfair one.
- I used cylinders from the same potato to ensure that they were of the same structure and concentration to ensure that any change is due to the effect of osmosis on identical cells.
- As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop his from happening, the beaker and syringe must be washed every time they are used. Better still, I used different syringes and droppers for each solution.
I recorded my results in tables such as those shown below:
Solution concentration: Distilled water
Solution concentration: 0.125M solution
Solution concentration: 0.25M solution
Solution concentration: 0.5M
Solution concentration: 0.75M
Solution concentration: 1M
Analyzing my results, I think that my experiment was successful as I found out that changing the molarity of a solution does affect the rate of osmosis. I found the results using 0.125M solution as surprising though. This is because I did not expect to see an increase in the turgidity of the potato cylinders immersed in this solution. This is probably due to the fact that the potato’s molarity is greater that 0.125M and the number of water molecules in the solution outside it was greater. Hence, water would have moved on increasing the water consistency of the cell sap.
Increasing the concentration of the solutions, the amount of water lost by the potato cylinders increased since more diffused out. They became flaccid and shrank. However, my results also showed me that the difference between the percentage loss in mass and length after 0.75M was less than when I used weaker solutions. This is probably because the cells had become almost fully plasmolysed.
I have illustrated my results on a graph.
SECTION 3: ANALYZING EVIDENCE
Analyzing the results of change in mass and length, I obtained after immersing the potato cylinders in various sugar solutions, I made several observations.
I had expected that when the concentration of the sucrose solution increases, the potato cylinders would also undergo an increase in water loss. This is so because increasing the concentration of the solutions causes water from the potato cells to diffuse into the solution, resulting in the concentration on both sides to be equal. They would have shrunk, resulting in a decrease of length and mass when immersed in the solutions. As expected, they increased in mass and length when immersed in distilled water. Comparing the results I obtained for each solution, I was surprised to find the potato cylinders immersed in 0.125M solution also gained mass and length. The percentage change was less than that for the distilled water. Nevertheless, it shows that the concentration of cell sap is above 0.125M. The fact that the percentage change was less than that for distilled can be explained by the fact that less water had to diffuse into the potato cells to equalize the concentration of the solution on both sides of the cells wall. Since the concentration gradient was smaller than that between distilled water and the potato cells, the amount of water required to dilute the cell sap of the potato cells in the 0.125M solution was far less than that for distilled water.
When the potato cylinders were immersed in 0.25M solution, they lost mass and length. This shows the concentration of cell sap is less than 0.25M and water had diffused out of the potato cells in order to equalize the concentration on both sides of the cell sap. Considering the fact that water diffused into the plant cells, when the sucrose solution had a concentration of 0.25M, we can conclude the cell sap has a concentration between 0.125M and 0.25M
As the concentration of the sucrose was further increased, the percentage decrease in mass and length also increased. Thus, a 1M solution had the highest percentage decrease in mass and length.
I have included the observation table below to illustrate the percentage change for the various solutions.
I obtained the above lengths and masses using the following calculations:
Solution concentration: Distilled water
For mass:
[(1.02+0.99+1.02+ 1.04+1.00)/5]-[(0.95+0.96+0.94+0.95+0.97)/5]
=0.06g
Therefore 0.06/0.95 x 100%
=+6.32%
For length = [(5.40+5.30+5.35+5.40+5.40)/5]+[(5.00+5.00+5.00+5.00+5.00)/5.00]
Therefore 0.37/5.00x100
=+7.40%
I repeated this method of calculation for all my other solutions.
Observing the pattern obtained from the table, I have come to the conclusion that the percentage change in mass and length are not directly proportional. However, there is a pattern in percentage change showing that as concentration of the solution increases, both decrease. It also shows that the percentage change in mass and length is inversely proportional to the gradient. This was showed to be true when I plotted the following graphs.
I have used the diagrams below to illustrate the process of osmosis, which has taken place after the potato cylinders were immersed in the various solutions.
Using my graphs, I calculated the molarity of cell sap. I did this by denoting the point on the x-axis where the graphs for change in mass and change in length intersected. At this point, the potato cylinders did not gain neither mass nor length. This meant that the isotonic point of the potato sap had been attained and the concentration of water at both sides was equal.
This observation matched the prediction I had made earlier as the isotonic point of the cell sap is greater than 0.125M. I found it to be around 0.13M.
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 0.19 M, 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 molar concentration. The next point, 0.25 M looses approximately 1% mass in and 3.6% in length. This shows that the water potential of the sucrose solution in the beaker is weaker than that of the potato chip. The next, 0.50 M, looses approximately 11.6 % in mass and 4.8% in length. This shows that osmosis took place in and that its rate was higher than for the previous solution. I noticed that this pattern carries on through the graph, and that even more mass is lost, as more water moves out of the potato into the solution for the higher sucrose concentrations
My graph matched my predictions as it showed that with increase in the molarity of the sucrose solutions, an increase in the movement of water out of the cells also increased. I have used the diagrams below to illustrate what I believe took place in the cells when they underwent osmosis:
SECTION 4:EVALUATION
Since I followed my planning precisely, I believe that the results I obtained in order to show that my hypothesis was quite reliable. Due to my precautions, I tried to minimize the possibility of human errors. However, I would make several improvements to my investigation if I were to repeat it again. These include:
- I would use more types of molar sucrose solutions to obtain a greater range of values. This will mean that even if there are mistakes in my values, I will be able to obtain values closer to the actual that if I took the readings over a smaller range. I can also use more solutions with smaller differences in concentration (0.05M, 0.1M, 0.15M, 0.2M, 0.25, 0.3M etc).
- I would keep the potato cylinders immersed in the sucrose solution for a longer period of time. In this way, I will allow the maximum effect of the difference in concentration to take place.
- I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. In this way, I will ensure that the same amount of water left the cells before the experiment. Not all of my results were on the linear graphs. One reason for this may have been that when the potato chips were removed from the Petri dishes and dried I may have dried some potatoes more thoroughly than others and thus some would have had more excess water than others would. This would have added to their mass.
-
I may conduct the same experiment at different temperatures (for example at 15oC, 20oC,25oC,30oC and 35oC). In this way I will still obtain results for 0.125M.0.25M, 0.5M, 0.75M, 1.00M and distilled water. I expect the results to be the same as I think that temperature will only affect the sped at which osmosis takes place, and not the final amount of osmosis, which has taken place. I expect to obtain the same isotonic point for the concentration of cell sap no matter what temperature I carry the experiment at, since I intend to leave it for a longer period of time, thus allowing maximum osmosis to take place for all the solutions.
- I can also use onions instead of potatoes to investigate the rate of osmosis. Here again I expect to observe more water loss when they are immersed in more concentrated solutions than in weaker ones.
- I found the accurate cutting of the potato cylinders to be among the most difficult parts of the experiment. If I am to repeat the experiment I will try and use a device, which cuts the cylinders to the reset length?
- I can also try and compare the rate of osmosis in different species of potatoes or in young and old potatoes. I expect to obtain similar isotonic point for each potato since they have almost identical structures and hence their cell saps have similar concentrations.
In conclusion, I find my experiment quite successful. My procedures were shown to be safe and reliable and my method resulted in me obtaining a range of values I had expected. The experiment did not require the use of apparatus that is difficult or expensive to obtain and the materials were easily available. It was not time consuming and did not require extensive reparation in order to be carried out.