Osmosis
Osmosis
• Osmosis is a special case of diffusion
• Osmosis involves the diffusion of water
through a membrane
• Water moves from a high concentration
of water to a low concentration of
water
Osmosis Example
dilute solution
concentrated
solution
level falls
level rises
membrane
More water passes from
dilute to concentrated ...
...until concentrations
become equal
• There are microscopic pores in the
membrane.
• Molecules below a certain size can
diffuse through the pores
• Water molecules can easily diffuse
though the pores
In the next slides
represents a water molecule
and
represents a sugar molecule
water
membrane
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.
Because there are more freely moving water molecules on
the left, more water molecule will diffuse through the pores
of the membrane from left to right than from right to left.
Osmosis and animal cells
• If there is a greater
concentration of free water
molecules outside the cell
than inside
• Then water will diffuse
into the cell by osmosis
• The cell will swell up
Osmosis and animal cells
• If osmosis continued the animal cell
would burst
• This would be bad news for animals
• In this picture a red blood cell is
put in a glass of distilled
water. Because there is a higher
concentration of water outside
the cell, water enters the cell by
OSMOSIS. In this case too much
water enters and the cell swells
to the point of bursting open.
Osmosis and plant cells
• 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
Osmosis and plant cells
• The cellulose cell wall cannot stretch, so
the vacuole cannot continue to expand
• But the pressure of the vacuole against
the cell makes the cell very firm
• A plant cell in this condition is said to
be turgid
Plant cells
cell wall
18
vacuole
The cell absorbs water
by osmosis ....
cytoplasm and
cell membrane
....but the cell wall stops the
cell expanding any more
26
Question 1
In osmosis, water will diffuse through a membrane
(a) From water to a concentrated solution
(b) From a concentrated solution to a dilute
solution
(c) From a dilute solution to a concentrated
solution
(d) From a dilute solution to a more
dilute solution
Question 2
Cell sap
28
concentrated solution
a
c
dilute solution
more dilute solution
Water will tend to move by osmosis
(a) from b to a and c
(b) from c to a and b
(c) From a to b and c
(d) From c to b and a
b
Plant cells
Tonicity
• The measurement of osmotic
pressure preventing the inward flow
of water across the semipermeable
membrane
• Solute: A solute is a substance
dissolved in another substances
e.g. Salt
• Solvent: A substance that dissolves a
solute
e.g. Water
Tonicity is a relative term
• Hypotonic Solution - One solution has
a lower concentration of solute (e.g
salt) than the other
• Hypertonic Solution - One solution
has a higher concentration of solute
(e.g. salt) than the other
• Isotonic Solution - Both solutions
have same concentrations of solute
Plant and Animal Cells put into
various solutions