Investigation of the point of incipient plasmolysis of onion cells (Allium cepa) using NaCl (Sodium Chloride) concentrations of 0. 1M, 0. 2M, 0. 3M, 0. 4M, 0. 5M, 0. 6M Design Research Question (Aim): The aim of this lab was to determine the point of incipient plasmolysis of onion (Allium cepa) cells using Sodium Chloride (NaCl) concentrations of 0. 1M, 0. 2M, 0. 3M, 0. 4M, 0. 5M, 0. 6M. Hypothesis: When the water concentration of a solution outside the cell is lower than the concentration inside the cell, water will move from the inside to the outside of the cell due to osmosis.
As we increase the concentration of the NaCl solutions we have used (0. 1M to 0. 6M), more moles of NaCl are dissolved in the solution. Thus, the solutions increases in solute concentration but decreases in water concentration. We can therefore assume; the higher the concentration of the NaCl solution, the higher the number of plasmolysed cells as water moves outside the cell in order to dilute the NaCl concentration. Data Collection and Processing Table 1: The number of plasmolysed onion cells (out of 30) ±1 for each of the 6 NaCl concentrations (0. 1M, 0. 2M, 0. 3M, 0. 4M, 0. 5M, 0. M) for three trials |Concentration (in M) |Trial 1 (Number of Plasmolysed |Trial 2 (Number of Plasmolysed |Trial 3 (Number of Plasmolysed | | |Cells ±1) |Cells ±1) |Cells ±1) | |0. 1 |0 out of 30 |0 out of 30 |0 out of 30 | |0. 2 |2 out of 30 |0 out of 30 |0 out of 30 | |0. |6 out of 30 |4 out of 30 |3 out of 30 | |0. 4 |6 out of 30 |12 out of 30 |13 out of 30 | |0. 5 |6 out of 30 |16 out of 30 |19 out of 30 | |0. 6 |6 out of 30 |30 out of 30 |30 out of 30 | Qualitative Data:
In general, it was hard to keep an overview of the cells one has counted yet and one has not as one only counted the cells at random. To this, it was hard to determine visually whether a cell was plasmolysed or not so that one could have assumed some cells to be plasmolysed although they were not. Finally, as the results of our first trial show, we did not allow enough time for the cells to plasmolyse so that the results became inaccurate. Table 2: The percentage of plasmolysed onion cells ±3. 33% for all of the six solute concentration (0. 1M, 0. 2M, 0. 3M, 0. 4M, 0. 5M, 0. M) for three trials, including the average percentage of plasmolysed cells for the second and third trial only*, as well as for all three trials together |Concentration (in M) |Trial 1 (Percentage of |Trial 2 (Percentage of |Trial 3 (Percentage of |Average Percentage of |Average Percentage of | | |Plasmolysed Cells |Plasmolysed Cells |Plasmolysed Cells |Plasmolysed Cells for |Plasmolysed Cells for | | |±3. 33%) |±3. 33%) |±3. 33%) |second and third trial |all three trials ±3. 3%| | | | | |±3. 33% | | |0. 1 |0. 00% |0. 00% |0. 00% |0. 00% |0. 00% | |0. 2 |6. 67% |0. 00% |0. 00% |2. 22% |0. 00% | |0. 3 |20. 00% |13. 33% |10. 00% |14. 44% |11. 67 % | |0. 4 |20. 00% |40. 00% |43. 3% |34. 44% |41. 67% | |0. 5 |20. 00% |53. 33% |63. 33% |45. 56% |58. 33% | |0. 6 |20. 00% |100. 00% |100. 00% |73. 33% |100. 00% | * = I have calculated the average for the second and third trial only in addition to the overall average so that I can draw another graph of the averages of the second and third trial only since the results of our first trial seemed to be inaccurate. Sample Calculations:
Percentage: In order to determine the percentage of plasmolysed cells for each solute concentration, one can use the following formula; (N:T) x 100, where N stands for the numerical value (in this case the number of plasmolysed cells we have counted) and T stands for the total (in this case 30). For example, if one would want to calculate the percentage of 2 out of 30, this would result in the following formula; (2:30) x 100, since 2 is the numerical value (the number of plasmolysed cells we have counted) and 30 is the total. Average: To calculate the average, one simply adds the values and then divides it by the number of values.
One could also use the formula (? x) : n, where x are the individual values of plasmolysed cells for each trial and n is the number of values. For example, if one would want to determine the average for the number of plasmolysed cells for the concentration of 0. 6 M, one would simply add 20, 100 and 100 and then divide it by 3, since the number of plasmolysed cells is the x-values and 3 in this case is the n value. Figure 1: The average percentage of plasmolysed cells ±3. 33% of all three trials determined for each of the six NaCl concentration. The graph shows a linear trend-line in order to determine the point of incipient plasmolysis. pic] Figure 2: The average percentage of plasmolysed cells ±3. 33% of only the second and third trial determined for each of the six NaCl concentration. The graph shows a linear trend-line in order to determine the point of incipient plasmolysis. [pic] Conclusion & Evaluation Conclusion: As the NaCl concentration outside is increased, more NaCl molecules are dissolved in the solution causing the solution to have a higher solute concentration but a lower water concentration. The water from the plant cell thus has a higher concentration than the outside diffuses (through osmosis) in order to dilute the NaCl concentration.
The turgor pressure that maintains the shape of the cell by pushing the plasma membrane against the cell wall is then lowered causing the cells to shrink. This is known as plasmolysis. Our hypothesis; the higher the concentration of the NaCl solution, the higher the number of plasmolysed cells as water moves outside the cell in order to dilute the NaCl concentration was therefore correct. Furthermore, we have used Figure 2 in order to determine the point of incipient plasmolysis as the results are more representable of the whole since the first trial was not included.
Figure 2 indicates that the point of incipient plasmolysis lies at approx. 0. 42M. Supporting my findings, a similar experiment was done however with saccharose solution instead of NaCl. The results show that the point of incipient plasmolysis lies of this experiment lies approx. 0. 38M which is fairly close to my results (Stadelmann, 156). In general, one can assume that the higher the concentration of the outside solution, the higher the number of plasmolysed cells as water moves outside the cell in order to dilute the outside oncentration. Evaluation: |Limitation |Significance |Improvement | |It was hard to determine the number of |As there were generally more than 30 cells |We could have used the method of a | |plasmolysed cells visually as we just counted |visible in the eye piece, it was not too hard |hemocytometer instead so that we could have | |30 visible cells at random and did not have an |to count 30 individual cells.
However, we could|counted the number of plasmolysed cells per | |overview of the cells we have already counted. |have still counted one cell twice and assumed |square. | |In addition, it was hard to determine if a cell|that it was two different cells. This | | |was plasmolysed or not. limitation therefore causes an overall | | | |inaccuracy. Furthermore, we could have assumed | | | |some cells to be plasmolysed although they were| | | |not plasmolysed. | |We only estimated what the point of incipient |This only has a slight significance on the |We should have done the lab with the NaCl | |plasmolysis approximately would approximately |exact accuracy of the point of incipient |concentrations we were given and then determine| |be basing it on our graph (figure 1) and only |plasmolysis. |where the point of incipient plasmolysis | |using concentrations of 0. 1M, 0. 2M, 0. 3M, 0. 4M,| |approximately lies.
Then, we could have done | |0. 5M, 0. 6M. | |the experiment with more accurate solutions | | | |such as 0. 45, 0. 475 in order to find the exact | | | |point of incipient plasmolysis. | |For our first trial we did not allow enough |Due to this limitation, our overall average was|We should have allowed more time for the cells | |time for the cells to plasmolyse. lowered leading to a higher point of incipient |of the first trial to plasmolyse. We could have| | |plasmolysis. In general, those results were |also simply repeated this trial. | | |outliers which affected the accuracy of our | | | |processed data. | | |We have only done three trials of the |This is very significant in our case as the |Instead, we should have firstly determined | |experiment. results of the first trial therefore had a |where the point of incipient plasmolysis | | |great effect on the accuracy of our processed |approximately lies and then repeated the | | |data. |experiment with these NaCl concentrations at | | | |least 5 times. | |We did not agree on an overall method of the |The more NaCl was used, the more cells were |Agree on an overall amount of drops of NaCl | |amount of drops of NaCl we used. likely to plasmolyze as more water would |such as 1 drop or agree on an amount such as | | |diffuse outside the cell in order to dilute the|1mL and then use a pipette and a graduated | | |solution. |cylinder in order to measure that amount. | Works Cited Stadelmann, E. J. Methods in Cell Physiology. Ed. David M. Prescott. New York: Academic, 1966. Print.
