Osmosis is the movement of water molecules from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane. This movement is caused by difference in water potential. ‘Water potential of a solution is the term given to the tendency for water particles to leave that solution by osmosis, (…) water diffuses from a region of high water potential to a region of lower water potential’ (Clegg and Mackean 2000).
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When the sucrose solution has a low concentration and a high water potential (higher than that of the potato), water will diffuse into the potato and so cause its mass to increase. When the sucrose solution has a high concentration and a low water potential (lower than that of the potato), water will diffuse out of the potato into the sucrose solution and the mass of the potato will decrease. When the water potential of the sucrose solution and the potato are equal, the mass of the potato will remain the same – so from this value the water potential of the potato tuber cells can be derived.
Explanation
Data Collection and Processing
Table 1 – The initial masses of the potato cylinders for different concentrations of sucrose solution
Concentration of sucrose solution [mol dm-3]
Initial mass [g ±0.01g]
group 1
group 2
group 3
group 4
0.0
2.48
2.60
2.50
2.42
0.1
2.60
2.46
2.52
2.56
0.2
2.58
2.50
2.65
2.44
0.3
2.46
2.34
2.45
2.54
0.4
2.52
2.62
2.60
2.46
0.5
2.52
2.56
2.63
2.44
0.6
2.56
2.62
2.58
2.52
* The results in bold were taken by me
Table 2 – The final masses of the potato cylinders for different concentrations of sucrose solution
Concentration of sucrose solution [mol dm-3]
Final mass [g ±0.01g]
group 1
group 2
group 3
group 4
0.00
2.72
2.62
2.84
2.64
0.10
2.72
2.56
2.66
2.66
0.20
2.62
2.44
2.65
2.46
0.30
2.16
2.16
2.28
2.34
0.40
2.12
2.24
2.23
2.04
0.50
2.03
2.06
2.17
1.88
0.60
1.89
1.98
1.95
1.82
* The results in bold were taken by me
The following formula was used to calculate the percentage change in mass of the potato cylinders:
An example is shown below for 0.1 molarity concentration for group 4:
Table 3 – The percentage change in mass of the potato cylinders, the mean percentage change in mass and the standard deviation for different concentrations of sucrose solution
Concentration of sucrose solution [mol dm-3]
Percentage change in mass [%]
Mean [%]
Standard deviation
group 1
group 2
group 3
group 4
0.00
9.68
*0.77
13.60
9.09
10.79
2.45
0.10
4.62
4.07
5.56
3.91
4.54
0.74
0.20
1.55
-2.40
0.00
0.82
-0.01
1.72
0.30
-12.20
-7.69
-6.94
-7.87
-8.68
2.38
0.40
-15.87
-14.50
-14.23
-17.07
-15.42
1.32
0.50
-19.44
-19.53
-17.49
-22.95
-19.85
2.27
0.60
-26.17
-24.43
-24.42
-27.78
-25.70
1.61
*The is anomalous and was excluded in the calculation of the mean and the standard deviation
The mean of the results was calculated using the formula =AVERAGE in Microsoft Office Excel. The standard deviation of the results was calculated using the formula =STDEV, also using Microsoft Office Excel.
Figure 1 – graph showing the relationship between the concentration of sucrose solution and the mass of the potato cylinders (error bars show the standard deviations)
Table 3 – Solute potential of different sucrose solutions at 20°C
h increa
Solute potential [kPa]
0.05
-130
0.10
-260
0.15
-410
0.20
-540
0.25
-680
0.30
-860
0.35
-970
0.40
-1120
0.45
-1280
0.50
-1450
0.55
-1620
0.60
-1800
0.65
-1980
0.70
-2180
0.75
-2370
0.80
-2580
0.85
-2790
0.90
-3000
0.95
-3250
1.00
-3500
Source: the instruction sheet of the experiment
Figure 2 – Graph showing the relationship between the concentration of sucrose solution and its water potential
The solute potential of the potato tuber cells is the same as the solute potential of a sucrose solution in which the potato cylinder did not change in mass. The concentration of this sucrose solution (where ψsolution = ψcell) mass of the potato cylinder will occur can be determined using the trendline in figure 1.
Figure 1 indicated that the value of sucrose concentration at which there is no mass change should be 0.17mol dm-3 and not 8.5mol dm-3. For the sucrose concentration of 0.17mol dm-3 the trendline intersects the x-axis; this is the point where there is no mass change of the potato cylinder.
This value can then be substituted into the equation of trendline in figure 2 to determine the solute potential of the potato tuber cells. In this equation ‘y’ represents the solute potential and ‘x’ the sucrose concentration, so ‘x’ is substituted by 0.17mol dm-3.
Thus the solute potential (ψs) of potato tuber cells is approximately -450kPa
Conclusion
Using the trendline in figure 1 and figure 2, the solute potential of potato tuber cells was calculated as approximately -450kPa. A potato cylinder placed in a solution of this solute potential should not change in mass. The concentration of the sucrose solution in which no mass change of the potato cylinder would occur is 0.17mol dm-3.
Evaluation
The trendline which was used to model the relationship between the sucrose concentration in which the potato cylinder was placed, and the change in mass of the potato cylinder, is polynomial. This trendline has the best fit however a linear function could also act as a best fit line. The final result does not change whether the equation of a linear or polynomial best fit line is used to derive the concentration of sucrose solution in which the potato cylinder did not change mass. In order to find which best fit line is better, the experiment should be repeated with higher concentrations of sucrose solution. This may be difficult to carry out because after a certain concentration the sucrose solution will become saturated the temperature at which the experiment is carried out.
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A better improvement would be to reduce the intervals between the sucrose solution concentration, especially around 0.1 and 0.2mol dm-2 (the concentration of sucrose solution when the potato cylinder did not change in mass). This would make it easier to choose the best fit line and more importantly, improve the accuracy of the final result. In the case of figure 2 it is clear that a polynomial trendline provides the best fit.
The experiment was carried out only one type of potato. The different trials used different individual potatoes which varied in size. The different individual potatoes may have varying solute potentials due to slightly different content of nutrients. This may be a reason why the results of the trials varied slightly.
The fact that the results varied slightly is unlikely to be due to the uncertainty of measurement which was very small and almost insignificant to the results: the uncertainty of measurement caused by the imprecision of the electronic balance is much smaller than the standard deviation of the results. As the percentage change in mass was calculated rather than the change in mass in grams, the small differences in mass between the samples of potatoes are not a relevant source of error. In order to get a more accurate average, more trials should be carried out.
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