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Osmosis is a special case of diffusion

Osmosis involves the diffusion of water through a membrane

The membrane may be artificial and non-living e.g. Cellophane

In biology, the important membrane is the cell membrane

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The membrane must allow water molecules to diffuse through. It is permeable to water.

Osmosis

water ordilute solution

concentratedsolution

membrane

More water passes fromdilute to concentrated ...

...until concentrationsbecome equal

level riseslevel falls

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There are microscopic pores in the membrane.

Molecules below a certain size can diffusethrough the pores.

Water molecules can easily diffuse throughthe pores.

In the next slides represents a water molecule

and represents a sugar molecule

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membranewater sugar solution

There are as many water molecules on the right as there are on the left but many of them are attached to sugar molecules and are not free to move.

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Because there are more freely moving water molecules onthe left, more diffuse through the pores of the membrane from left to right than from right to left.

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Molecular movement

Because the membrane allows only molecules of a certain size to diffuse through it, it is called selectively permeable.

The cell membrane functions as a selectivelypermeable membrane.

The cell sap (inside vacuole) and cytoplasm function as fairly concentrated solutions.

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There is a greater concentration of free water molecules outside the cell than inside

so water diffuses into the cellby osmosis

and the cell swells up

Osmosis in animal cells 11

If osmosis continued the animal cell would burst

This would be bad news for animals

Consequently there are processes in the animal’s body which control osmosis

Mainly, this is done by keeping the concentration of body fluids outside the cellthe same as it is inside

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In a plant cell, the cell membrane acts as a selectively permeable membrane

The vacuole contains a solution of salts and sugars

If there is water outside the cell, it will diffuseby osmosis into the vacuole

The cell wall is freely permeable to water

The vacuole will expand, pushing the cytoplasmoutwards against the cell wall

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cell wall cytoplasm and cell membrane

vacuole

The cell absorbs waterby osmosis ....

....but the cell wall stops the cell expanding any more

Plant cells 18

Osmosis between cells

If the concentration of the cell sap is greater in one cell than in its neighbour, water will pass by osmosis from the less concentrated to the more concentrated.

cell sap more concentrated

cell sap less concentrated

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Inquiry into Osmosis!

You are now going to design an experiment to investigate the process

of osmosis!