Two experiments to demonstrate a
partially permeable membrane is a two-way system are illustrated above..
All you need are partially permeable
membrane bags and a sugar solution of e.g. sucrose and pure water.
(pure water can distilled water or
deionised water - its all the same!)
The bags are fixed on to a vertical piece
of capillary tubing enabling you to observe the direction of water.
Just use the same beakers, same volumes
of solution and do all the experiments in the same temperature.
Experiment A
In experiment A, you fill the bag
with the sugar solution and place it in a beaker of pure water.
Observation: You find the bag
swells up and the water moves up the tube (picture A1 to A2).
Conclusion: Water has moved
from the surrounding pure water into the bag of sugar solution through
the partially permeable membrane.
Explanation:
The solution in the bag is more
concentrated with solute than the water surrounding the membrane.
The water concentration
(potential) is higher in the water around the bag than in the bag,
therefore the water must diffuse into the bag through the partially
permeable membrane.
The water diffusion gradient is
from inside the bag into the surrounding solution, again, as the
water tries to dilute the solution, but this time in the opposite
direction through the partially permeable membrane.
Experiment B
In experiment B, you fill the bag
with pure water and place it in the beaker of sugar solution.
Observation: The bag shrinks
and the water moves down the tube (B1 to B2).
Conclusion: Pure water has
moved out of the bag through the partially permeable membrane into the
surrounding sugar solution.
Explanation:
The solution in the beaker is
more concentrated with solute than the water in the bag.
The water concentration
(potential) is higher in the bag than the surrounding water,
therefore the water must diffuse out the bag through the partially
permeable membrane.
The water diffusion gradient is
from around the bag into the bag as the water tries to dilute the
solution.
Remember: In osmosis, only the
water moves through the partially permeable membrane, NOT the sugar
molecules!
Set-up and method
You can do a simple experiment to
demonstrate osmosis by placing blocks or cylinders of potato into pure water
and then a series of sugar solutions (e.g. glucose/sucrose) of increasing in
concentration (increasingly higher molarity e.g. from 0.0 to 1.0 mol/dm3) i.e.
from pure water, and a dilute to a very concentrated sugar solution.
The dependent variable is the potato
mass and the independent variable is the concentration of the sugar
solution.
The different concentrations of sugar
represent different water potentials - the more concentrated, the lower
the water potential.
The water moves by diffusion from a higher concentration
to a lower concentration through the cell membrane.
In the diagram the
sugar
molecules are purple and the
water
molecules blue.
All the other variables should be
kept constant - so make sure:
the original potato blocks are identical in
size and mass - with no peel left on, this ensures the cells have the
same surface area exposure to the water or solution,
same temperature - so rate is
unaffected,
same time left to change -
another rate factor,
same sugar and same volume of liquid,
these are all about a 'fair test'
and controlling variables and just changing the concentration of
sugar.
You measure and record the initial mass of
the potato blocks and place them individually in pure water and the
range of sugar solutions in a series of beakers.
Leave the beakers for as longer times
as possible and allowing time at the end to make the necessary
measurements - best for class if it can be done in a lesson.
Carefully remove the potato blocks
from the liquid, dry them with a paper towel and re-weigh them.
Different pupil groups can use one
particular concentration and use 3 lots of potato and submit an average
to the whole class results.
Results
From the weighings work out the mass
gain/loss from each potato block.
You can convert the weighing into %
mass gain/loss.
% change in mass = 100 x (final
mass - initial mass) / initial mass
You can plot a graph of mass gain/loss (g
or %) versus the sugar concentration (mol/dm3), and the graph
might not be quite what you expect!
Osmosis is taking place with water
and most of the sugar solutions - but not always in the same direction!
from measurements (a) to (e) etc.
Typical results from an osmotic
experiment using potato tubes or blocks.
Initially the potato blocks gain
mass, then there is little change in mass and then the blocks lose mass.
Explanation
Diagram (a) to (c): With the potato cylinder in pure
water, initially the concentration of
the sugar in the water is less than the solute concentration in the
potato cells of starch.
You can say the water
concentration in the external sugar solution is greater than that in
the potato cells - pure water has a greater water potential than the
cell fluids.
Therefore when osmosis takes
place with pure water, or very dilute solutions of sugar, the potato
cells absorb water by osmosis giving a percentage mass increase.
The water will diffuse through the partially permeable membranes
of the potato cells to try and dilute the internal solute solution of the
potato cells - osmosis is happening.
On diagram equal to (a) At a 'medium' concentration of
sugar, its concentration matches the concentration of the solutes in the
potato cells and there is no net osmosis.
You can say the water
concentration in the external sugar solution is the same as in the
potato cell - no osmosis - both the potato cells and sugar solution
have the same water potential.
Or you can say the solute
concentrations are the same.
When there is no change in mass,
the two solutions have the same water/solute concentration and the
solutions are said to be isotonic - identical water
potentials.
On the graph, this corresponds to
~0.3 mol/dm3 sugar concentration and equivalent to (b) on
the diagram.
Diagram (d) to (e): At higher external sugar
concentrations (above 0.3 mol/dm3 in this case), the osmotic effect reverses direction and water will
diffuse through the partially permeable membranes out of the potato cells to try and dilute the
external more concentrated sugar solution.
You can say the water
concentration in the external sugar solution is less than that in
the potato cell, so the water diffusion gradient is out of the
potato cells giving a net mass loss for the potato cylinders. In other words, the water
potential of the potato cell fluids is greater than the external
sugar solution water potential.
Ultimately the greater the concentration, the
greater the osmotic effect - a higher concentration of sugar will draw
out more water - a greater rate of diffusion and osmosis - the more concentrated the sugar
solution, the greater the mass loss of the potato block.
Sources of error
Obviously, all experiment can be
repeated e.g. groups of students in the same class, this reduces errors
- some groups of pupils might be more careful than others. You can then
use mean values in your numerical analysis.
If the potato blocks are not
completely dried, your will record a smaller mass of water loss than
actually happened.
Especially if the room is warm,
evaporation would increase the concentration of the sugar solution, this
would increase the mass loss. This error can be eliminated by putting
'lids' over the beaker - paper covers held with an elastic bands will
do.
Other similar experiment
You can repeat the experiments using
common salt (sodium chloride, NaCl) and you should get a similar pattern
of results.
Drinks and hydration
Most soft drinks contain water, sugar and ions.
Sports drinks contain sugars to replace the sugar
used in energy release during the activity.
They
also contain water and ions to replace the water
and ions lost during sweating.
The sports drinks are supposed to be
isotonic.
Know and understand that if water and ions are not replaced, the ion / water
balance of the body is disturbed and the cells do
not work as efficiently.
