1. Osmosis is a special case of diffusion Osmosis involves the diffusion of water through a membrane...
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Transcript of 1. Osmosis is a special case of diffusion Osmosis involves the diffusion of water through a membrane...
1
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!