Potato Osmosis
Biology SL - ATh
INVESTIGATION
“Potato Osmosis”
INTRODUCTION
Osmosis is a process that occurs at a cellular level that entails the spontaneous net
movement of water through a semi-permeable membrane from a region of low solute concentration
to an area of high solute concentration in order to equalize the level of water in each region.
Involved in this process are hypotonic, hypertonic and isotonic solutions. A hypotonic solution is one
with a lower osmotic pressure, indicating that the net movement of water moves into the said
solution whereas a hypertonic solution is one with a higher osmotic pressure, thus the net
movement of water will be leaving the hypertonic solution. Lastly, an isotonic solution entails no net
movement of water across a semi-permeable membrane as the two substances involved display
osmotic equilibrium.
AIM
To observe the effect of solutions different levels of NaCl concentration on potatoes, considering the
process of osmosis
METHOD
(see ‘Potato Osmosis’ – exercise document)
Table of NaCl-H2O Dilutions
NaCl Solution / ml ±1
100
75
50
25
0
Distilled H2O/ ml ±1
0
25
50
75
100
NaCl Concentration/M
1.00
0.75
0.50
0.25
0.00
Official Timeframe of Experiment
Insertion Time: 29 November, 2007 – 8:54 am
Extraction Time: 30 November, 2007 – 9:40 am
Legend for Potato Differentiation
Potato
Mark
Potato 1
Vertical line
Potato 2
Horizontal Line
Potato 3
Notch
DATA COLLECTION
(Fig. 1) Diagram demonstrating the
correct preparation of a potato –
cylindrical strips marked according to
the legend
Potato Osmosis
Biology SL - ATh
Raw Data Table – Mass and Lengths of Potato Strips – Pre- & Post-Experiment
NaCl
Potato
Initial Mass/
Mass after
Initial Length
Concentration/M Samples
g±0.01
Solution /
/mm±1
g±0.01
1.00
Potato 1
1.93
1.42
40
Potato 2
1.88
1.31
40
Potato 3
2.02
1.56
40
0.75
Potato 1
1.98
1.42
40
Potato 2
2.09
1.46
40
Potato 3
2.01
1.36
40
0.50
Potato 1
2.02
1.38
40
Potato 2
2.00
1.47
40
Potato 3
2.00
1.45
40
0.25
Potato 1
2.01
1.98
40
Potato 2
1.98
1.86
40
Potato 3
2.01
1.98
40
0.00
Potato 1
2.04
2.48
40
Potato 2
1.98
2.46
40
Potato 3
1.98
2.17
40
Length after
Solution/
mm±1
35
34
38
36
37
37
38
36
38
40
39
41
44
45
44
Observations – Pre-Insertion
- Generally rigid in structure
although slightly bendy
- Pale yellow in colour
- Moist
- All strips appear the same/similar in
structure and size at this point
Observations – Post-Extraction
- Strips immersed in 1.0M NaCl
Solution are very soggy, soft and
appear shrunken
- Strips immersed in 100% H2O are
very rigid, swollen, turgid and
appear larger/longer - they are
slightly bent and cannot be
straightened due to their rigidity
- Strips become progressively soggier
as the solutions they are immersed
in are higher in concentration of
NaCl
(Fig. 2) Potato strips from the same potato arranged in
descending order of concentration to demonstrate the
differences in structure post-extraction
Potato Osmosis
Biology SL - ATh
DATA PROCESSING
Calculating Percentage Change
*Required in order to calculate percentage change in mass and in length
Processed Data Table – Average Percentage Change in Mass and Length of Potato Strips
NaCl
% Change in
Average %
% Change in
Average %
Concentration/M Mass/%
change/%
Length/%
change/%
-26.42
-12.50
1.0
-26.51
-10.83
-30.32
-15.00
-22.77
-5.00
-28.28
-10.00
0.75
-30.25
-8.33
-30.14
-7.50
-32.14
-7.50
-31.68
-5.00
0.50
-28.56
-6.67
-26.50
-10.00
-27.50
-5.00
-1.49
0.00
0.25
-3.02
0.00
-6.06
-2.50
-1.49
2.50
21.57
10.00
0.00
18.47
10.83
24.24
12.50
9.60
10.00
Processed Data Table – Difference between % Change in Mass and Length and Average % Change
NaCl
Concentration/M
1.0
0.75
0.50
% Change Average %
% Change
Average %
in Mass
Change - Mass Difference
in Length
Chang - Length Difference
-26.42
-12.50
0.09
-1.67
-26.51
-10.83
-30.32
-15.00
-3.81
-4.17
-22.77
-5.00
3.74
5.83
-28.28
-10.00
1.97
-1.67
-30.25
-8.33
-30.14
-7.50
0.11
0.83
-32.14
-7.50
-1.89
0.83
-31.68
-26.5
-28.56
-27.5
0.25
-1.49
-6.06
-3.02
-1.49
0.00
21.57
24.24
9.60
18.47
-3.12
-5.00
2.06
-10.00
1.06
-5.00
1.53
0.00
-3.04
-2.50
1.53
2.50
3.1
10.00
5.77
12.50
-8.87
10.00
-6.67
1.67
-3.33
1.67
0.00
0
-2.5
2.5
10.83
-0.83
1.67
-0.83
Potato Osmosis
Biology SL - ATh
Average Percentage Change in Mass and Length of Potato Strips at each NaCl Concentration
Average Percentage Change in Mass and Length of Potato
Strips at each NaCl Concentration
30
Average Percentage Change/%
20
10
0
0
0.25
0.5
0.75
1
1.25
R² = 0.989
-10
-20
R² = 0.978
-30
-40
NaCl Concentration/M
% change in mass
% change in length
CONCLUSION & EVALUATION
As can be seen from the above graph and observations, we can ascertain that as NaCl
concentration in the solution decreases, the mass and the length exemplified after an approximate
24-hour period increases. Thus, we can state that there appears to be a negative correlation
between NaCl concentration and the mass and length of the potato strips, clearly evident in the
above graph which shows an exponential decrease in both mass and length. This can also be initially
seen in the post-extraction observations where it is evident that the potato strips immersed in lower
NaCl concentration were far more turgid than those immersed in 100% NaCl solution which were
flacid and fragile (see strip-comparison in Fig. 2).
This occurrence can be explained through the process of osmosis. As mentioned in the
introduction, a hypertonic solution is one with higher osmotic pressure meaning that the net
movement of water leaves the solution. This would explain the physical changes – the increase in
mass and length as well as the increase in turgidity - in the potato strips immersed in 100% H2O
solutions or low NaCl-concentration solutions. Since the solution it is submerged in is higher in
concentration in water molecules, or hypertonic, the water molecules will diffuse into the area of
lower H2O-concentration (the potato strip) in order to achieve equilibrium. Alternatively, the
decrease in mass and length in the potato strips submerged in highly concentrated NaCl solutions
can be explained by its immersion in a hypotonic solution. Hypertonic solutions, as mentioned
Potato Osmosis
Biology SL - ATh
before, are described as those with lower osmotic pressure, indicating that the net movement of
water moves into the solution. Therefore, as NaCl solution is less concentrated in H2O molecules
than the potato strips, the decrease in mass and length and loss of turgidity results from the net
movement of water leaving the potato strips, which is higher in osmotic pressure, and diffusing into
the solution.
Nevertheless, there are several possible sources of error that could have greatly or
negligibly affected the outcome of the experiment. First, we must note the varying external factors
resulting from an uncontrolled environment – the biology classroom. Primarily, these would include
varying temperatures and humidity which could potentially affect the rate of osmosis as increased
temperature results in increased diffusion while increased humidity results in an increased number
of water molecules. Secondly, we must note the human errors involved, for example, miscalculations
in experimental preparations. These would include the miscalculation of solutions leading to an
inaccurate concentration of NaCl as well as the possibility of impurities in the NaCl concoction in the
first place while imprecise cutting of the potato strips could’ve affected the surface area and thus
the rate of osmosis. This leads us to the errors resulting from variances in the substances used. As
already discussed previously, differences in surface area of each potato strip caused by imprecise
cutting as well as the marks (lines and notches) imprinted would’ve affected the rate of osmosis
while the concentration gradient between each potato strip is likely to differ as well. This stems from
the differences in water content of each potato, as, for example, a potato with high water
concentration in highly concentrated NaCl solution would have a faster rate of erosion. Further
affecting factors could include barriers to diffusion such as the size of pores which would also
determine the rate of osmosis. All the mentioned errors above hold the possibility of skewing the
data.
Subsequently, such errors could have an effect on the reliability of the results. The level of
accuracy which has been used throughout this investigation would come into question as a
combination of these errors would not permit such precision. Values of percentage change have
been taken at two decimal places corresponding with the actual values of mass and length, however,
this could be seen as far too precise. A better option would have been to take percentage change as
whole numbers or at one decimal place. Nevertheless, we attempted to reduce the potential errors
through several measures. With surface area, a cork borer was used in order to uniform the size of
the potato strips while the varying concentration gradients were controlled through the completion
of several trials (three trials with three potatoes) in order to limit error. Furthermore, to control the
effects of the external environment, foil was secured over the beaker containing the submerged
potato strips. However, if we refer to the graph, we can see the minimum and maximum spread for
each data-point is generally close-set while the R2 value, which calculates the spread of the datapoints from the line of best fit, are both relatively high – both around 0.9. This demonstrable trend
indicates a limiting of the amount of error, and thus fairly reliable results despite possible errors.
Overall, the results ultimately seem reliable although it might’ve been even more reliable by
reducing the level of precision (decimal places) when recording it.
Ultimately, potential improvements will stem from attempting to reduce the amount of
error in this investigation, particularly involving controlling the external environment and the
miscalculations. To control the external affecting factors, the solution containing the potato strips
can be kept overnight instead in a controlled environment with consistent temperatures and
humidity. Limiting the human error would be difficult and time-consuming as this would involve
Potato Osmosis
Biology SL - ATh
highly-precise instruments or even more focus dedication from the experimenter during preparation.
Finally, nothing can be done to uniform the response of the materials used, thus the completion of
even more trials limits the potential error and allows the formation of generalizations. Despite the
improvements proposed, those relating to limiting human error and completing more trials may
prove to be futile as they are not only time-consuming, but the demonstrable trends resulting from
this experiment indicate that no further improvements are necessary to reach the desired
conclusion.
Having established that there is no real need to pursue drastic improvements for the initial
experiment, we can now proceed to discuss possible extensions to the investigation. While we
already know the results of osmosis on a potato, we may now wish to better understand it. This can
be done by recording the progress of the potato’s transformation either (a) over a period of time
(perhaps 24 hours) or (b) until it has reached the point of equilibrium. The mapping of this progress
would involve the periodic removal of the samples in order to measure its mass and length, after
which it can be compiled into a graph to chart the transformation under osmosis. Alternatively, we
could compare the progress of a potato to another type of vegetable or fruit in order to ascertain
water content of each. Lastly, the submerged potato strips may be subjected to different kinds of
environment, particularly, varying humidity and temperature, without the protection of a foil cap.
This would reveal how much of an impact environmental factors would have on the osmotic process
and how would the effects manifest. In relation to the question of the sailor, this could represent the
life-span one would expect when trapped in certain climates.