UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 23/05/2023 [SEARCH]

Transport: 6. Simple demonstrations of osmosis - investigations using semi-permeable membrane bags and potato cylinders

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(6) Simple demonstrations of osmosis

In the phenomenon osmosis, the relative water potentials decides the direction of net transfer of water through a semi-permeably (partially permeable) membrane e.g. a cell membrane.

(i) Partially permeable membrane bags osmosis experiment

osmosis experiment with partially permeable membrane bags and sugar solution (semi-permeable membrane)

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!


(II) Potato cylinders osmosis experimental investigation

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.


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