Measuring The Water Potential Of A Potato Cell Students’

measuring the water potential of a potato cell students’ sheet instructions – chardakov’s method 1. prepare 10cm

Measuring the Water Potential
of a Potato Cell
Students’ Sheet
Instructions – Chardakov’s Method
1.
Prepare 10cm3 of each of 0.8M, 0.4M, 0.2M, 0.15M and 0.075M
sucrose solutions, each in a separate tube. Be careful to mix the
water and the more dense strong sugar solutions thoroughly. Add a
sixth tube of water to complete the series.
2.
Label each solution and another tube to match eg. 2 tubes labelled
0.8M, 2 tubes labelled 0.4M etc..
3.
Cut up about 10g potato tissue into small cubes about 2mm square.
Weigh accurately about 0.75g potato tissue into each empty tube.
Blot the tissue before weighing and be sure the balance is
carefully zeroed. Record the weight in each tube accurately.
4.
When all the weighings are complete, cover the tissue in each tube
with the corresponding solution (5cm3 is sufficient) and note the
time. ie. cover the tissue in the 0.2M tube with 0.2M sucrose etc.
Note how the potato looks when this is complete.
5.
Leave the tubes for 15 minutes, swirling occasionally. (Time to
make a diagram of the technique so far.)
6.
Add 1 drop of Methylene Blue solution to each tube with potato
tissue in and mix it with the contents of the tube.
7.
Arrange your eye level with the tubes and with a Pasteur pipette
carefully withdraw some of the coloured solution from each tissue
tube in turn and add it VERY CAREFULLY to the corresponding
sucrose solution tube by immersing the pipette to about half the
depth of the solution in the tube and gently expel one drop. ie.
Remove some blue solution from the 0.8M solution with potato
tissue and add it to the 0.8M stock tube. Watch the drop carefully
and record your observations in each case.
8.
Remove the tissue from each tube, blot it on a paper towel and
weigh each amount separately; making sure that the balance is
carefully zeroed before each weighing. (Six weighings, one from
each tube.)
9.
Make a table of Initial weight; Final weight; and Final
weight/Initial weight for each concentration.
10.
Plot a graph of Final weight/Initial weight against concentration.
11.
Using your graph, predict the concentration of sucrose which is in
equilibrium with the potato tissue and hence calculate the Water
Potential of the tissue in MPa by consulting the table at the foot
of this page.
Instructions – by Weighing
1.
Prepare 10cm3 of each of 0.8M, 0.4M, 0.2M, 0.15M and 0.075M
sucrose solutions, each in a separate tube. Be careful to mix the
water and the more dense strong sugar solutions thoroughly. Add a
sixth tube of water to complete the series.
2.
Label each solution.
3.
Cut up about 10g potato tissue into small cubes about 2mm square.
Weigh accurately about 0.75g potato tissue into each empty tube.
Blot the tissue before weighing and be sure the balance is
carefully zeroed. Record the weight in each tube accurately.
4.
When all the weighings are complete, cover the tissue in each tube
with the corresponding solution (5cm3 is sufficient) and note the
time. ie. cover the tissue in the 0.2M tube with 0.2M sucrose etc.
Note how the potato looks when this is complete.
5.
Leave the tubes for 15 minutes, swirling occasionally. (Time to
make a diagram of the technique so far.)
6.
Remove the tissue from each tube, blot it on a paper towel and
weigh each amount separately; making sure that the balance is
carefully zeroed before each weighing. (Six weighings, one from
each tube.)
7.
Make a table of Initial weight; Final weight; and Final
weight/Initial weight for each concentration.
8.
Plot a graph of Final weight/Initial weight against concentration.
9.
Using your graph, predict the concentration of sucrose which is in
equilibrium with the potato tissue and hence calculate the Water
Potential of the tissue in MPa by consulting the table at the foot
of this page.

Molarity
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Osmotic Potential
-2.7
-5.4
-8.2
-11.2
-14.5
-18.0
-21.7
-25.8
-30.1
-35.1
(atmos.)
-0.27
-0.55
-0.83
-1.13
-1.46
-1.82
-2.20
-2.61
-3.05
-3.56
(MPa)

Questions – Chardakov method
*
Explain the behaviour of the drops in part 7
*
Explain how that method can be used to measure the Water Potential
of the tissue.
*
Note any errors in this method and think especially of what
happened to the blue drop in the pure water tube.
Science & Plants for Schools: www.saps.org.uk
Measuring the Water Potential of a Potato Cell: p. 3
This document may be photocopied for educational use in any
institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2010.