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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
The membrane must allow water molecules
to diffuse through. It is permeable to water.
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Osmosis
water or
dilute solution
concentrated
solution
level falls
level rises
membrane
More water passes from
dilute to concentrated ...
...until concentrations
become equal
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There are microscopic pores in the membrane.
Molecules below a certain size can diffuse
through the pores.
Water molecules can easily diffuse through
the pores.
In the next slides
and
represents a water molecule
represents a sugar molecule
water
membrane
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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 on
the left, more diffuse through the pores of the membrane from
left to right than from right to left.
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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 selectively
permeable membrane.
The cell sap (inside vacuole) and cytoplasm
function as fairly concentrated solutions.
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Osmosis in animal cells
There is a greater concentration
of free water molecules outside
the cell than inside
so water diffuses into the cell
by osmosis
and the cell swells up
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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 cell
the same as it is inside
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In a plant cell, the cell membrane acts as a
selectively permeable membrane
The cell wall is freely permeable to water
The vacuole contains a solution of salts and sugars
If there is water outside the cell, it will diffuse
by osmosis into the vacuole
The vacuole will expand, pushing the cytoplasm
outwards against the cell wall
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Plant cells
cell wall
vacuole
The cell absorbs water
by osmosis ....
cytoplasm and
cell membrane
....but the cell wall stops the
cell expanding any more
Osmosis between cells
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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
Inquiry into
Osmosis!
You are now going to design an
experiment to investigate the process
of osmosis!