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Determining the cytoplasmic osmolarity of onion cells using interpolation

Yezi Cho




The cytoplasmic osmolarity of onion epidermis were determined by allowing osmosis to occur when leaving onion cells in 0.0, 0.25, 0.50, 1.00, 1.50, 2.00 M NaCl solution for two minutes. The number of plasmolyzed cells were counted in three fields of views in order to obtain three trials for each concentration of NaCl solution. Using interpolation in the graph of NaCl solution concentration vs. Average percentage of plasmolyzed cells, the osmolarity of the epidermis cytoplasm was estimated.




Raw Data


Table 1. Number of total onion cells and plasmolyzed onion cells in 0.0, 0.25, 0.50, 1.00, 1.50, 2.00 M NaCl solution





Trial 1

Trial 2

Trial 3

NaCl solution concentration (M)

Total

Plasmolyzed

Total

Plasmolyzed

Total

Plasmolyzed

0.0

68

0

89

0

72

0

0.25

80

0

76

0

69

0

0.50

78

43

79

45

74

41

1.0

109

109

144

109

106

90

1.5

86

78

75

66

91

79

2.0

66

66

68

68

94

94




Qualitative Observations

NaCl solution concentration (M)

Observations

0.0

No cells are plasmolyzed in all fields of view.

0.25

No cells are plasmolyzed in all fields of view.

0.50

Approximately half of the cells are plasmolyzed in all fields of view. In plasmolyzed cells, the cell membrane separated from the cell wall.

1.0

One field of view shows all of the cells to have been plasmolyzed, while two other fields of view shows most of the cells to have been plasmolyzed.

1.5

Most of the cells are plasmolyzed in all fields of view.

2.0

All of the cells are plasmolyzed in all fields of view.





Figure 1. Micrographs of onion cells in 0.0 M NaCl solution after two minutes




Figure 2. Micrographs of onion cells in 0.25 M NaCl solution after two minutes




Figure 3. Micrographs of onion cells in 0.50 M NaCl solution after two minutes




Figure 4. Micrographs of onion cells in 1.0 M NaCl solution after two minutes




Figure 5. Micrographs of onion cells in 1.5 M NaCl solution after two minutes




Figure 6. Micrographs of onion cells in 2.0 M NaCl solution after two minutes




Processed Data


Table 2. Percentage of plasmolyzed onion cells in 0.0, 0.25, 0.50, 1.00, 1.50, 2.00 M NaCl solution (%)






Percentage of plasmolyzed onion cells (%)

NaCl solution concentration (M)

Trial 1

Trial 2

Trial 3

Average

Standard Deviation

0.0

0

0

0

0

0

0.25

0

0

0

0

0

0.50

55

57

55

56

1.2

1.0

100

76

85

87

12

1.5

91

88

87

89

2.1

2.0

100

100

100

100

0




Sample Calculations


1. 

Percentage (%) of plasmolyzed onion cells

{(Number of plasmolyzed onion cells)/(Total number of onion cells)}*100


Percentage (%) of plasmolyzed onion cells in 0.50 M NaCl solution for trial 1

(43/78)*100 = 55.128 %


2. 

Average number of plasmolyzed onion cells

(Sum of data set)/(Sample size)


Average number of plasmolyzed onion cells in 0.50 M NaCl solution

(55+57+55)/3 = 55.667


3. 

Standard deviation of percentage of plasmolyzed cells






Graph


It is estimated that the solute concentration of NaCl solution is equal to that of the onion cell cytoplasm when 50% of the cells are plasmolyzed at the isotonic point. According to the line of best fit, 50% percent of the cells would be plasmolyzed when the NaCl solution concentration is 0.76 M. Therefore the solute concentration of onion cells is estimated to be 0.76 M.




Conclusion 


A limitation of this experimental method is that the onion epidermal tissue used for the different NaCl solute concentrations was not consistent. A different onion was used for the 1.5 M NaCl solution trials, and while the same onion was used for the 0.0, 0.25, 0.50, 1.0, 2.0 M trials, different parts of the onion epidermis were used. The cytoplasm concentration may differ for each onion as well as between parts of an onion, leading to less accuracy. Some onion cells may be more likely to plasmolyze, resulting in a greater number of plasmolyzed cells. To account for this, the same region of the same onion can be used to ensure that the number of plasmolyzed cells is based on the independent variable - the concentration of NaCl solution.


Another limitation is found in the process of counting the number of plasmolyzed cells. While most of the cells could be easily determined whether they were plasmolyzed, a few cells were slightly plasmolyzed on the sides but not all the way through. In this analysis, onion cells whose cell membranes were partially separated from the cell walls were also considered to be plasmolyzed.

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