Osmosis In Potato Tuber Cell The Weighting Method Biology Essay

Aim:

To determine solute potential of a plant tissue at 20 ºC.

Research question:

What is the solute potential of a potato tuber cell at 20 ºC?

Data collection and processing

Table 1. Initial and final masses of potato tuber cells for different concentrations of sucrose solution.

Concentration of sucrose solution [mol dm-3]

Trial 1

Trial 2

Trial 3

Trial 4

Initial mass [g]

± 0.01

Final mass [g]

± 0.01

Initial mass [g]

± 0.01

Final mass [g]

± 0.01

Initial mass [g]

± 0.01

Final mass [g]

± 0.01

Initial mass [g]

± 0.01

Final mass [g]

± 0.01

0.00

2.10

2.17

*1.84

*0.80

2.26

2.34

2.68

2.70

0.25

2.08

1.70

1.82

1.80

2.28

2.02

2.66

2.40

0.50

2.10

1.66

1.84

1.38

2.24

1.72

2.68

2.12

0.75

2.06

1.56

1.78

1.34

2.24

1.64

2.68

1.86

1.00

2.00

1.52

1.82

1.20

2.26

1.68

2.60

1.74

*These results will not be considered in further processing of the data as the sample was destroyed and there was a high probability that they would disrupt the reliability of conclusions.

Table 2. Observations of changes to samples for each sucrose solution concentration.

Concentration of sucrose solution

[mol dm-3]

Observations made during the experiment

at the beginning

at the end

0.00

Samples sank immediately.

Samples swollen.

0.25

Samples sank almost immediately.

The texture of samples did not change much.

0.50

Samples initially floated on the middle of beaker’s height and sank.

The texture of samples did not change much.

0.75

Samples floated on the middle of beaker’s height.

Samples did not sink; they still floated on the middle of beaker’s height. Samples became softer.

1.00

Samples floated near the top of the beaker.

Samples did not sink; they still floated near the top of the beaker. Samples became softer.

The mass change of potato tuber cells for different sucrose solutions was calculated according to the formula

Example calculation for trial 1 for the sucrose solution concentration 0 mol dm-3:

The formula for mean mass change of potato tuber cells for different sucrose concentrations:

Example calculation for sucrose solution concentration 0 mol dm-3:

Table 3. Mass changes of potato tuber cells for different sucrose solutions.

Concentration of sucrose solution

[mol dm-3]

Trial 1

Trial 2

Trial 3

Trial 4

Mean

Mass change [g]

± 0.02

Mass change [g]

± 0.02

Mass change [g]

± 0.02

Mass change [g]

± 0.02

Mass change [g]

±0.02

0.00

0.07

excluded

0.08

0.02*

0.07

0.25

-0.38

-0.02

-0.26

-0.26

-0.23

0.50

-0.44

-0.46

-0.52

-0.56

-0.50

0.75

-0.50

-0.44

-0.60

-0.82

-0.59

1.00

-0.48

-0.62

-0.58

-0.86

-0.64

*This result will be excluded in further calculations as its magnitude is equal to the uncertainty of the measurement and using it calculate mean might lead to significant systematic errors.

Collected data indicates that potato tuber cells samples gained mass only when placed in 0 mol dm-3 sucrose solution, however this change was very small (0.07 g on average, which is only three and a half times greater than the uncertainty of results). For higher concentrations of sucrose solution potato tuber cells samples decreased in mass. Hence, directly proportional relationship between the concentration of sucrose solution and mass change of the potato tuber cell can be noticed (for 0.25. 0.50. 0.75 and 1 mol dm-3 sucrose solutions mean mass losses were 0.23; 0.50; 0.59 and 0.64 g respectively).

Graph 1. Relationship between sucrose solution concentration and mean mass change of potato tuber cells.

Error bars indicate uncertainty of measurements equal to ± 0.02. Note that negative values indicate the loss of mass, but the relationship between mean mass change of potato samples and sucrose solution concentration remains directly proportional.

However, as initial masses and mass changes in samples differed significantly. calculating the percentage mass change of potato tuber cells for different sucrose solutions will be much more convenient in terms of results analysis. These values will be calculated according to the formula

Note. In percentage mass changes results will be rounded to two decimal places, as it provides descent certainty of results for further analysis.

Percentage errors will be calculated according to the formula:

Example calculations for trial 1 for the sucrose solution concentration 0 mol dm-3:

Percentage mass change:

Percentage error:

The formula for mean percentage mass change of potato tuber cells for different sucrose concentrations:

Formula for mean percentage error for different sucrose concentrations:

Example calculation for sucrose solution concentration 0 mol dm-3:

Mean percentage mass change:

Mean percentage error:

To realize the magnitude of percentage errors of mass change, absolute errors of mass change will be calculated for mean mass changes of potato tuber cells placed in sucrose solutions of different concentrations. The formula used:

Example calculation for sucrose solution concentration equal to 0.00 mol dm-3:

Table 4. Percentage mass changes of potato tuber cells for different sucrose solutions and percentage and absolute errors.

Concentration of solution [mol dm-3]

Trial 1

Trial 2

Trial 3

Trial 4

Mean

Mass change [%]

Error of mass change [%]

Mass change [%]

Error of mass change [%]

Mass change [%]

Error of mass change [%]

Mass change [%]

Error of mass change [%]

Mass change [%]

Error of mass change [%]

Absolute error of mass change [g]

0.00

3.33

0.94

excluded

excluded

3.51

0.87

excluded

excluded

3.42

0.91

0.0006

0.25

-18.27

1.07

-1.10

1.11

-11.40

0.93

-9.77

0.79

-10.14

0.98

0.0023

0.50

-20.95

1.08

-25.00

1.27

-23.21

1.03

-20.90

0.84

-22.52

1.05

0.0053

0.75

-24.27

1.13

-24.72

1.31

-26.79

1.06

-30.60

0.91

-26.59

1.10

0.0065

1.00

-24.00

1.16

-34.07

1.38

-25.66

1.04

-33.08

0.96

-29.20

1.13

0.0072

Mean percentage mass changes of potato tuber cell samples reflect the trend found when calculating mass changes of these samples in grams. Here, it can be clearly seen that the increase in samples’ mass when placed in 0 mol dm-3 sucrose solution was very small (3.42 %). For higher sucrose solution concentrations the magnitude of mass loss was more significant (from 10.14 % for 0.25 mol dm-3 sucrose solution, up to almost 30% for 1 mol dm-3 solution).

Mean percentage and mean absolute errors of measurements and calculations are low (approximately 1% and no more than 0.0072 g respectively, in all sucrose concentrations) and a directly proportional relationship can be seen between the magnitude of these errors and the concentration of each sucrose solution.

Graph 2. The relationship between the sucrose solution concentration and mean percentage mass change of potato tuber cells.

Error bars representing mean absolute errors of measurements are too small to be displayed on the graph. The equation of the best fit line was found using Microsoft Office Excel. Note that negative values on the y ─ axis indicate the loss of mass of potato tuber cells, but the relationship between mean mass change of samples and sucrose solution concentrations remains directly proportional.

As the relationship between mean percentage mass change of potato samples and sucrose solution concentration is not normally described by polynomial equation, the best fit line would be linear. However, linear line would not pass through the x ─ axis and thus it would not be possible to derive potato’s water potential from it. Therefore, a polynomial functions, which crosses the x ─ axis has been used.

Table 5. Sucrose solution concentrations and corresponding solution potential at 20ºC (source: the instruction sheet for the experiment).

Sucrose solution concentration [mol dm-3]

Sucrose solution 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

Graph 3. The relationship between the sucrose solution concentration and sucrose solution potential at 20ºC.

The line on the graph indicates best fit line and its equation was found using Microsoft Office Excel.

In case of data presented on graph 3, normally the best fit line would be linear, however such line would not cross the x ─ axis and therefore determination of water potential of potato tuber cell would be impossible. Thus, a polynomial best fit line has been used here, as it passes the x ─ axis.

Determining potato tuber cell’s solute potential

Knowing that sucrose solution potential equals the potato tuber cell solute potential when no mass change of the potato sample occurs. the potato tuber cell potential can be determined using graphs 1 and 2. Using the best fit line from graph 2 to calculate the sucrose solution concentration when the mass of the potato sample does not change:

by the means of Texas Instruments – 83 Plus Graphic Display Calculator

On the graph 2 the best fit line intersects the x – axis at x= 0.05, which indicates the point where potato tuber cell sample does not change its mass and therefore, meaning that sucrose solution potential must be equal to potato cell potential.

In order to determine the solute potential of the potato tuber cell that value will be substituted into the best fit line equation from the graph 3 (y will indicate the plant cell potential and x sucrose solution potential. hence x = 0.05).

by the means of Texas Instruments – 83 Plus Graphic Display Calculator

Therefore, the solute potential of the potato tuber cell is approximately -156.36 kPa.

Conclusions

Results gained in the experiment indicate the increase in potato tuber cell’s mass when placing samples in sucrose solutions of concentrations between 0.00 ─ 5.00 mol dm-3 (in the experiment, mean increase in mass of the sample was 0.07 g and 3.42% when placed in sucrose solution of concentration 0.00 mol dm-3 ). 5.00 mol dm-3 is the concentration when no mass changes of samples occur. Samples of potato tuber cells decreased in mass when placed in sucrose solutions of concentrations higher than 5 mol dm-3 (in the experiment, mean mass losses, absolute and percentage, were 0.23 g and 10.14%; 0.50 g and 22.52 %; 0.59 g and 26.59 %; 0.64 g and 29.20 % for 0.25, 0.50, 0.75 and 1 mol dm-3 sucrose solutions respectively).

Mass changes in samples can be explained by osmosis ─ the passive process enabling cells to exchange water with their external environment. It is defined as ‘the net movement of freely moving water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane’ (Kent, 2000). Osmosis is caused by the water potential – the pressure that freely moving water molecules exert in a system, measured in kilopascals (kPa). The potential of the pure water is assigned the value of 0 kPa and it falls down as solutes are added, because water molecules gather around particles of the solute (Kent, 2000). When the solute potential of sucrose is lower than the solute potential of the potato tuber cell and the concentration of the sucrose solution is high, water molecules diffuse from the potato cell into the sucrose solution, causing the decrease in potato’s mass. Hence, the sucrose solution is hypertonic to the cytoplasm of the potato. This happened in case of samples placed in 0.25, 0.5, 0.75 and 1 mol dm -3. The softening of potato samples placed in sucrose solutions of 0.75 and 1 mol dm-3 concentrations might be connected with the loss of turgidity of cells. Sucrose solution hypotonic to the cytoplasm of potato sample, hence of high water potential and low concentration causes water molecules to diffuse into the potato tuber cell and thus, increase its mass. This happened when potato samples were placed in 0 mol dm -3 sucrose solutions. Additionally, swelling of potato tuber cells samples might be connected with more water entering them and thus, the increase in cells’ turgidity. If water potential of the sucrose solution equals the water potential of the potato tuber cell, water molecules still move between these two environments, but no net gain of water molecules occurs and thus, the mass of potato tuber cell does not change. Thus, the sucrose solution is isotonic with the cytoplasm of the potato tuber cell. In this case, the values of water potential of the sucrose solution and potato tuber cell and the sucrose solution were assigned by using trend lines produced from processing obtained results.

Calculations made indicate that no mass change in potato tuber cell should occur if placing it in a sucrose solution of concentration of 0.05 mol dm -3 at 20ºC. Thus, a potato tuber cell sample in solution of such potential should not change its mass. Therefore, the solute potential of the potato tuber cell has been determined to have the approximate value of -156.36 kPa.

Evaluation

Table 6. Sources of errors and inaccuracies, their possible influences on data obtained and possible improvements of the method to eradicate them.

Error/ inaccuracy source

Possible influences on data obtained

Possible improvements

Trials were carried out for samples varying in mass and size.

Great diversity among samples may result in biased mean, vulnerable to changes from single large values, resulting in more approximations being made in the experiment and thus, results becoming less reliable.

To eradicate the influence of the error on results percentage mass changes of the samples were calculated.

Samples should be prepared more precisely, for example using slide rule instead of ruler to prepare samples of equal sizes. More trials should be done.

Differences between chemical compositions of samples deriving from different potatoes.

Differences in composition between samples may cause differences in water potential among potato tuber cell samples, thus increasing random error connected with final calculation of potato tuber cell’s water potential.

All samples should derive from the same potato.

Table 7. Sources of errors and inaccuracies, their possible influences on results and possible improvements of data processing to eradicate them.

Error/ inaccuracy source

Possible influences on results

Possible improvements

Using polynomial trend lines to derive results instead of linear ones.

Polynomial trend lines do not reflect the nature of the correlation between mass changes of potato tuber cell and sucrose solution concentration, thus they are only approximations that can further decrease reliability of obtained results.

Perform more trials for more sucrose solution concentrations (the interval between concentrations should be decreased to 0.1 mol dm-3, experiment can be performed for higher sucrose solution concentrations) to gain more results and thus, increase the possibility of linear trend line passing the x ─ axis.

It should be stressed that performing the tests for high concentrations of sucrose solution might be difficult, as at some point the concentration will become saturated in fixed temperature – at 20ºC the sucrose solution becomes saturated when 2.0047 grams of sucrose are dissolved in a gram of water (Mathlouthi. Reiser. 1995).