Download Osmosis Notes - Biology Teaching & Learning Resources

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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
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.
In fact, water passes both ways but faster
from the dilute to the concentrated solution.
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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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Next slide
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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 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
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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Fresh water fish
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
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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
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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
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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
The cellulose cell wall cannot stretch, so the
vacuole cannot continue to expand
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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
vacuole
The cell absorbs water
by osmosis ....
cytoplasm and
cell membrane
....but the cell wall stops the
cell expanding any more
Question
What did you notice about the colour of the cell
sap in the two cells?
Can you suggest an explanation for this?
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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
Busy Lizzie
(2) recovering
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(1) wilting
(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
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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
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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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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
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Question 2
Which statements are correct ?
(a) The plant cell wall is permeable to water
(b) The plant cell wall is selectively permeable
(c) The cell membrane is selectively permeable
(d) The vacuole contains a solution of salts and
sugars
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Question 3
Cell sap
concentrated solution
a
c
dilute solution
b
more dilute solution
Water will tend to move by osmosis
(a) from b to a and c, and from c to a
(b) from c to a and b, and from a to b
(c) From a to b and c, and from c to b
(d) From c to b and a, and from b to c
Plant cells
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Question 4
Which statements are correct ?
(a) A fresh water fish tends to absorb water
by osmosis
(b) A sea water fish tends to absorb water
by osmosis
(c) A fresh water fish tends to lose water to
its surroundings
(d) A sea water fish tends to lose water to
its surroundings
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Answer
The colour intensity in the vacuole of the
left hand cell is greater than that in the right
This is because the uptake of water by
osmosis has diluted the cell sap in the
right hand cell
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Correct
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Incorrect