Diffusion and Osmosis
Unit 3
Diffusion
• Let’s look at the movement of itsy bitsy
particles that move in liquids or gasses.
Sometimes even moving through the
membrane of cells.
• This is called DIFFUSION!
Real life scenario!
• Your aunt May comes to visit.
• A few minutes after she enters the house can
smell her perfume as you hear her talking to
your mother in the kitchen.
• The gasses from your aunts perfume were in a
high concentration in one area. However, after
the concentrated source of the perfume
particles entered they began to spread out to
be evenly dispersed.
When water molecules do this is it
called Osmosis.
2
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. a Coffee filter
In biology, the important membrane is the
cell membrane
Permeability
The membrane must allow water molecules
to diffuse through. It is permeable to water.
If a concentrated solution is separated from
a dilute solution by a suitable membrane,
water will pass from the dilute to the
concentrated solution.
3
4
Osmosis
water or
dilute solution
concentrated
solution
level falls
level rises
membrane
More water passes from
dilute to concentrated ...
...until concentrations
become equal
5
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
6
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.
7
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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Next slide
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Because the membrane allows only molecules
of a certain size to diffuse through it, it is
called semi or selectively permeable.
The cell membrane functions as a selectively
permeable membrane.
The cell sap and cytoplasm function as
fairly concentrated solutions.
Scale of drawings
The scale of the following drawings is greatly
distorted.
Even if the cells were as large as they appear on
the screen, the molecules would still be invisible
particles
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Osmosis in animal cells
There is a greater concentration of
free water molecules outside the
cell than inside
11
so water diffuses into the cell
by osmosis
and the cell swells up
12
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
Fresh water fish
14
Trout
In fresh water fish, the blood is more concentrated than the surrounding water
As a result the fish takes in water by osmosis
Not so much through the skin, which is thick,
but through the gills which have a very thin
membrane
14
Perch
The fish gets rid of the excess water by means of its kidneys
kidneys expel
excess water ...
blood carries water
to kidneys
gills absorb water
by osmosis
heart
...in the form of
dilute urine
Sea water fish
15
Herring
Sea water is a more concentrated solution than the
fish’s blood. What will be the osmotic effect ?
A sea water fish will lose water by osmosis through
its gills.
The fish drinks sea water and the gills expel the
excess salt from the blood, so keeping its
concentration constant
16
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
The cellulose cell wall cannot stretch, so the
vacuole cannot continue to expand
But the pressure of the vacuole against the cell
wall makes the cell very firm
A plant cell in this condition is said to be turgid
If all the cells in the plant are turgid, the plant will
be firm and upright and the leaves would be
expanded
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Plant cells
cell wall
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vacuole
The cell absorbs water
by osmosis ....
cytoplasm and
cell membrane
....but the cell wall stops the
cell expanding any more
Question
What happens in osmosis?
a.
b.
c.
d.
movement of solute molecules against their concentration gradient
movement of solute molecules down their concentration gradient
movement of water molecules against their concentration gradient
movement of water molecules down their concentration gradient
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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
Limp and turgid tissue
These cells are short of water;
the tissue is limp and the plant
is wilting
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The cells have taken up
water by osmosis; the
cells are turgid and the
tissue is firm
(1) wilting
(2) recovering
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(3) recovered
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In the growing regions of a plant, the cell walls are
not inextensible but are still ‘plastic’
When the vacuole takes in water by osmosis it
expands, pushing out on the ‘plastic’ cell wall
and increasing the size of the cell.
Cell division at a growing point increases the
number of cells but cell expansion causes the
increase in size.
Cell division followed by cell expansion causes
growth
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Cell extension
1
2
3
1 The cell has just divided
2 Vacuoles start to form in one cell
3 The vacuoles absorb water by osmosis and start to extend
the cell
4 The vacuoles begin to join up and continue to absorb water
and extend the cell
4
Growth in a shoot tip
these cells will divide
cell division
continues
vacuoles
forming
cells absorb water by
osmosis and expand
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Osmosis or not?
• My stomach absorbing the water I just drank?
• A paper towel absorbing the mess I just made?
• Our little Hibiscus absorbing water through its roots?
• Excess water leaving your blood and passing
to your kidney?
• Water spreading it self evenly on my patio as I
hose it down?