Aim:
To determine the isotonic point of a potato.
Research Question:
To determine the isotonic point of a potato by placing it in different concentrations of salt (NaCl) solution
and measuring the change in mass after a fixed time period.
Background Information
Osmosis is defined as the movement of water molecules from a region of low solute concentration, to a
region of high solute concentration, through a semi permeable membrane, which is one that allows only
certain small particles to pass through. A solution with a high water concentration, or low solute
concentration is also called a hypotonic solution, and one with a low water concentration is known as a
hypertonic solution.
When a cell is placed inside a hypertonic solution, water molecules diffuse out of the cell (which has a
higher water concentration, or is hypotonic) through the cell membrane, due to the high solute
concentration outside the cell. This process is called exosmosis, and it makes the cell flaccid, as it loses
water. The very theory can be applied to plants, which causes wilting.
When a cell is placed in a hypotonic solution, water molecules diffuse from the solution into the cell, as
the solvent concentration of the cell is lower. This causes the cell to become turgid, and it is called
endosmosis.
Source - http://biosimplicity.webs.com/
A solution which has the same concentration as that of the cell has no concentration gradient as both the
cell and its environment have the same water potential. Thus the net movement of water is zero and the
system is in equilibrium. Water molecules flow in and out at an equal rate by osmosis, causing the cell
size to stay the same. Such a solution is called an isotonic solution, and the concentration at which this
occurs is the isotonic point of the cell. Thus when cells are placed in such a solution, there should be no
change in the mass.
Brief Outline Method (Provided in instructional sheet):
In this experiment the isotonic point of potato will be found.
The potato slices will be kept in salt (sodium chloride) solutions of different concentrations.
Some of the solutions would be relatively hypertonic, and some would be hypotonic,
assuming constant potato concentration.
After a period of time, the change in mass of the potato slices will be obtained. A graph
would be plotted of concentration against percentage change in mass.
From the graph the concentration where there is no change in mass can be obtained, hence the
isotonic point of the potato can be determined.
Apparatus:
● Potato Slicer
● Two potatoes
● Digital balance (±0.01g)
● A few tissue papers
● 5 beakers (100ml)
● A Measuring cylinder
● A marker
● A Scalpel
● Sodium Chloride (salt) solutions of the following molar concentration – 0.1M, 0.2M, 0.3M,
0.4M and 0.5M
Variables
How they will be controlled
Why is this variable chosen?
Constant
Surrounding conditions
like temperature and
pressure
Volume of each solution
used
The trials will all be conducted in the
same surrounding conditions
This will be measured using a
measuring cylinder
Time for which potato All the trials will be kept for 24 hours
chips are kept in each
solution
Temperature and pressure both
influence the rate of diffusion
hence they have to be kept
constant
a not
fair
Even
thoughtothe ensure
volume does
comparison.
influence
the concentration, the
volume should be kept constant in
order
maintain
a fair
If
the to
potato
chips
aretest.
left for an
indefinite time, diffusion would
continue till both solutions inside
and outside the cell have the same
concentration. Thus a uniform time
frame has to be kept
Approximate
size
of Since a cork borer is used the width This is to ensure a uniform rate of
potato (area of cross should be the same. The length will be diffusion across the potato cell.
section)
kept constant by cutting the potato
cylinders of the same length using a
ruler. Any major difference in the
length would be cut using a knife.
Independent
Concentration of solution. Sodium chloride solutions of different
concentrations would be provided
Varied concentrations have to be
used with some of the solutions
being hypertonic and some
hypotonic, so that the isotonic
point can be identified
Dependent
Change in mass of the
potato slices
This is measured using a digital balance
The mass of the potato would
change based on the volume of
water absorbed or expelled by the
cell, hence this is the dependent
variable
Procedure:
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
Cut out 25 small cylinders of potato using the cork borer.
Using a ruler make sure that the potato cylinders are of the same length. Cut the potato
cylinders to the same length using a knife.
Measure 50ml of each solution into five different beakers
Label the beakers with a marker to know which beaker has which solution
Using a marker, label each of the five potato cylinders (from numbers 1-5) that are to be
placed in each beaker. Thus five different trials would be obtained for each concentration.
Then measure the mass of each potato slice or cylinder using the digital balance. Record
these values in a table.
Place the five potato cylinders in each of the sodium chloride solutions
Keep all the beakers in the same place, and leave them for about 24 hours.
After this period of time, measure the mass of the potato cylinders again, and record all the
values.
Work out the difference in the mass of the potato cylinders before and after keeping them
in the different solutions.
Plot a graph using the average values of the change in mass and hence find the isotonic
point of potato (where the change in mass is zero)
Fair test:
▪
▪
▪
▪
▪
The potato cylinders in each concentration should be of approximately the same length
Cling film should be wrapped on the beakers to prevent evaporation.
The potato cylinders should not directly be kept on the digital balance as they might be
slightly wet. A tissue paper must first be used to wipe off the water from the potato
cylinders.
Make sure all test tubes are kept together so that the surrounding conditions are the same
The same potato should be used for the experiment as different types of potato may have a
different isotonic point
Results:
Qualitative observations:
Observations before placing the potato cylinders in the different solutions:
▪
▪
▪
▪
All the potato cylinders look similar. This is obvious as they are cut from the same potato
The potato cylinders are a bright yellow colour
The texture of the cylinders is neither very rough nor very smooth.
The potato pieces are moderately hard
Observations after placing the potato cylinders in different solutions:
M ▪
The potato cylinders looked slightly thicker and longer
▪ They were very hard and did not bend easily, showing an increased turgidity and rigidness
▪ The texture of the potato was quite smooth
▪ The colour of the potato changed from bright yellow to pale
yellow. 0.2M –
▪ The potato cylinders looked slightly longer than before
▪ They were hard but not as much as those of 0.1M solution.
▪ They did not bend very easily.
▪ The colour was again a slightly lighter shade of yellow
0.3M –
▪ The potato cylinders were quite soft.
▪ They looked slightly smaller and shrunken then before
▪ The colour was a light yellow
▪ The potato cylinders could bend easily
0.4M –
▪ The potato cylinders looked very small and thin
▪ They were very soft as well, and seemed to be softer in the centre
▪ The colour of the potato cylinders had become very light yellow
▪ The potato could bend very easily
0.5M –
▪ The potato cylinders were very small and extremely thin
▪ They had become very soft and completely flaccid
▪ They could bend very easily and became very delicate, thus had to be held carefully
▪
The colour had become very pale, and the cylinders appeared slightly translucent.
▪ The cylinders seemed to be thinner from the centre.
Table showing change in mass and percentage change in mass of the potato for each trial of the
different concentrations
Percentage change = (change in mass/mass before) * 100
Concentration
of
NaCl
solution (M)
Trial
Mass
befor
e (±0.01g)
Mass after
(±0.01g)
Change
i
n mass(±0.02g)
Percentage
change
i
n mass (%)
0.1
1
2
3
4
5
1.53
1.46
1.53
1.58
1.56
1.80
1.77
1.81
1.90
1.87
0.27
0.31
0.28
0.32
0.31
17.6
21.2
18.3
20.3
19.9
1
2
3
4
5
1.53
1.53
1.51
1.41
1.42
1.71
1.72
1.67
1.62
1.57
0.18
0.19
0.16
0.21
0.15
11.8
12.4
10.6
14.9
10.6
1
2
3
4
5
1.16
1.15
1.14
1.09
1.05
1.22
1.20
1.19
1.18
1.12
0.06
0.05
0.05
0.09
0.07
5.2
4.3
4.4
8.3
6.7
1
2
3
4
5
0.89
0.90
0.82
0.91
0.86
0.84
0.85
0.79
0.88
0.83
-0.05
-0.05
-0.03
-0.03
-0.03
-5.6
-5.5
-3.7
-3.3
-3.5
1
2
3
4
5
1.17
1.13
1.11
1.16
1.12
0.93
0.92
0.95
0.94
0.92
-0.24
-0.21
-0.16
-0.22
-0.20
-20.5
-18.6
-14.4
-19.0
-17.9
0.2
0.3
0.4
0.5
Calculation of average percentage change in mass of the potato
1
Formula used: x = )xi - Where x is the percentage change in mass; n is the number of trials and
is the
n
mean
x
Concentration (M)
0.1
0.2
Average or mean % change in mass
17.6+21.2+18.3+20.3+19.9/5 = 19.5
11.8+12.4+10.6+14.9+10.6/5 = 12.1
0.3
0.4
0.5
5.2+4.3+4.4+8.3+6.7/5 = 5.78
-5.6-5.5-3.7-3.3-3.5/5 = -4.32
-20.5-18.6-14.4-19.0-17.9/5 = -18.1
Table showing calculations for standard deviation:
Concentration Trial
%
change
in mass
Deviation (d) d2
0.1
1
2
3
4
5
17.6
21.2
18.3
20.3
19.9
1.9
-1.7
1.2
-0.8
-0.4
3.61
2.89
1.44
0.64
0.16
1
2
3
4
5
11.8
12.4
10.6
14.9
10.6
0.3
-0.3
1.5
-2.8
1.5
0.09
0.09
2.25
7.84
2.25
1
2
3
4
5
5.2
4.3
4.4
8.3
6.7
0.58
1.48
1.38
-2.52
-0.92
0.34
2.19
1.90
6.35
0.85
1
2
3
4
5
-5.6
-5.5
-3.7
-3.3
-3.5
1.28
1.18
-0.62
-1.02
-0.82
1.64
1.39
0.38
1.04
0.67
1
2
3
4
5
-20.5
-18.6
-14.4
-19
-17.9
2.4
0.5
-3.7
0.9
-0.2
5.76
0.25
13.69
0.81
0.04
0.2
0.3
0.4
0.5
Standard deviation can be calculated using the following formula:
Formula used: o x =
1
2
n )d
where o is the standard deviation, n is the number of trials and d2 is the
square x
of the deviation. In this case the value of n = 5
Table showing calculation of standard deviation
Concentration of solution (M)
0.1
0.2
0.3
0.4
0.5
Sum of d2 I 5
1.75
2.50
2.33
1.02
4.11
Standard deviation
1.32
1.58
1.52
1.01
2.02
Graph Analysis:
The graph shows the percentage change in mass for the different concentrations of the glucose
solutions. The graph clearly shows an inverse relationship between the concentration of the solution
and the change in mass.
With the concentration of 0.1M, there was a large increase in the mass, of about 19.5%. This shows
that the solution was hypotonic and had a higher water potential than the potato cell. Hence water
diffused into the cell. With higher concentrations of 0.2M and 0.3M, there was a lower increase in the
mass (12.1% and 5.78%), showing that the solutions were less hypotonic. It is thus evident from the
results that when the potato cylinder is in a hypotonic solution, there is endosmosis leading to increase
in mass and turgidity.
As the concentration further increased, there was a negative change in the mass, or a percentage
decrease in the mass of the potato. This shows that the solutions above 0.4M were hypertonic, or
more concentrated than the potato cell. Hence water diffuses out of the potato cell, resulting in a fall
in mass and turgidity. Thus it is evident that in a hypertonic solution, the potato cells lose water and
exosmosis takes place resulting in the potato cells becoming flaccid.
From the graph one can see that the isotonic point of the potato was obtained as 0.33M. This is an
isotonic concentration, where there is no net movement of water thus there is no change in the mass
of the potato. This is the point where the line of best fit cuts the x- axis, as there is a 0% change in the
mass of the potato.
It is also evident from the line of best fit that there are no major anomalies in the results, as most of
values are close to the line of best fit. The standard deviation is also low for all the results, showing
that the results were fairly precise.
Percentage error:
Literature value of isotonic point of potato = 0.25M (source
- Experimental value = 0.33M
% Error:
0.33-0.25/0.25 * 100 = 32%
Conclusion:
It can be concluded from the above results and graph that the isotonic point of potato from this
experiment is 0.33 moles/dm3 with an error of ±32% (when compared to the literature value). This is
the concentration where there is no net diffusion of water between the potato cylinders or the sodium
chloride solution, as the concentration in the potato cells is equal to the concentration of the solution
(0.33 moles as per these results). Since the concentrations are equal, there is no concentration
gradient and thus even though water molecules move through the partially permeable membrane of
the potato cells there is no net movement of water. At concentrations above the isotonic point of the
potato, the concentration of sodium chloride inside the cells is lower than the concentration outside
cells. As a result, there is a concentration gradient and thus water moves from a region of a high water
potential to
a region of low water potential. The effect is that water diffuses out of the potato cells, thus leading to
a decrease in the mass of these cells. However at lower concentrations of the salt solution, the water
potential outside the potato cells is greater than that inside the potato cells (which has a comparatively
higher concentration of sodium chloride ions). Water thus diffuses into the potato cells down the
concentration gradient, leading to an increase in the mass of the potato cells. Thus there is an inverse
relationship between the concentration of the solution and the percentage increase in the mass of the
potato cells – as the solution becomes more concentrated, the percentage increase in the mass of the
potato significantly decreases as there is a change from net movement of water into the potato cells to
net movement of water outside the potato cells.
Even though there were no major anomalies in the results, as a line of best fit could be obtained, there
may have been some errors while conducting the experiment. These could have been random or
systematic errors, and have been identified below:
Possible sources of error:
●
The mass may not have been measured accurately as the air conditioner was on, which resulted
in the continuous fluctuation of the reading on the balance. This would result in inaccurate
recording of data and thus the difference in the mass of the potato calculated would not have
been correct.
●
●
●
●
●
●
The solutions were not left for exactly 24 hours. However all the solutions were left for the
same period of time thus this should not have led to any major experimental errors.
The beakers were left in open air and were not covered. Thus the solutions may have
evaporated resulting in an increase in the concentration of the solutions. This could have led to
major errors in the experimental results as the volume of the solution that evaporated from
each beaker would not have been the same.
The temperature or pressure were simply monitored and not regulated, hence a change in
temperature or pressure would result in a change in the rate of diffusion, which would in turn
affect the final mass.
The surface area of the potato cylinders may not have been equal for all trials as the cork borer
may have been pierced in the potato at different angles.
If the potato pieces were not completely dried before weighing then the mass of the residual
water on the surface of the pieces (which had not diffused into the potato) would be taken into
account. Consequently this would increase the mass of the potato, thus resulting in errors in
the results.
Two different potatoes were used as the first potato was not enough to obtain 25 potato
cylinders. As a result the isotonic points of both potatoes may have been slightly different.
Improvements to the experiment:
▪
▪
▪
▪
The air conditioner and fans should have been switched off while taking readings of mass
Instead to keeping the beakers out, they could have been kept inside a fridge. This would
prevent evaporation of water. The beakers could also have been covered with cling film to
prevent evaporation of the water. This would ensure that the concentration of the solutions
remained constant, and thus the rate of diffusion would not get affected.
In order to make the experiment more accurate, concentrations between 0.1M, 0.2M, 0.3M
and 0.4M could have been taken, such as 0.15m, 0.25m etc. This would make it easier to find
the exact isotonic point of the potato, and thus the experiment would have been much more
accurate.
One potato should have been used in order to maintain a fair test