1. Osmosis
In the left hand picture the plant is wilting from lack of water. On the right it has recovered largely due to the process of osmosis. The presentation explains how this happens

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. Cellophane
In biology, the important membrane is the
cell membrane

3. 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.

4. Osmosis water or dilute solution concentrated solution level falls
level rises
...until concentrations
become equal
membrane
More water passes from
dilute to concentrated ...

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
represents a water molecule
and
represents a sugar molecule

6. membrane water 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
Sugar molecules can pass through the membrane but, being surrounded by a cloud of water molecules, they move more slowly.
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.
Molecular movement

8. Next slide
Animation

9. 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.

10. Solution Differences & Cells
solvent + solute = solution
Cell in a Hypotonic solution
Solutes in cell more than outside
Outside solvent will flow into cell
In a Isotonic
Solutes equal inside & out of cell
In a Hypertonic
Solutes greater outside cell
Fluid will flow out of cell

11. 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

12. Osmosis in animal cells
so water diffuses into the cell
by osmosis
There is a greater concentration of free water molecules outside the cell than inside
and the cell swells up

13. 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

15. As a result the fish takes in water by osmosis
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

16. 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
...in the form of
dilute urine
heart

17. Sea water is a more concentrated solution than the
Sea water fish
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.
Sea water fish related to sharks, e.g. dogfish, avoid losing water to their surroundings by maintaining their blood at the same concentration as sea water
The fish drinks sea water and the gills expel the
excess salt from the blood, so keeping its
concentration constant

18. The vacuole will expand, pushing the cytoplasm
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

19. 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

20. Plant cells cell wall vacuole cytoplasm and cell membrane
A useful analogy is the bicycle tyre. The outer casing represents the cell wall; the inner tube represents the cytoplasm and the air inside represents the vacuole. When the tyre is pumped up, the air pressure pushes the inner tube against the outer casing. The outer casing cannot expand and so the pressure in the tube builds up until the tyre is firm
The cell absorbs water
by osmosis ....
....but the cell wall stops the
cell expanding any more

21. Question
What did you notice about the colour of the cell sap in the two cells?
Can you suggest an explanation for this?

22. 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
Osmosis between one cell and the next plays a part in the movement of water throughout the tissues of the plant.

23. Limp and turgid tissue
These cells are short of water; the tissue is limp and the plant is wilting
The cells have taken up
water by osmosis; the
cells are turgid and the
tissue is firm

24. Busy Lizzie (1) wilting (2) recovering (3) recovered
(1) Wilting. The plant is short of water. The leaf cells are no longer turgid and the leaves curl up and droop.
(3) Recovered. Water is once more available. The leaf cells take up water by osmosis and become turgid. The leaves are now firm and are held horizontally.
Woody plants such as trees and shrubs do not collapse when they wilt but their leaves become limp.

25. 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

26. Cell extension 1 The cell has just divided4
4 The vacuoles begin to join up and continue to absorb water
and extend the cell 2
2 Vacuoles start to form in one cell 3
3 The vacuoles absorb water by osmosis and start to extend
the cell 1
1 The cell has just divided
Notice that the cell expands in length rather than width. This is due to the arrangement of cellulose fibres in the cell wall

27. Growth in a shoot tip cell division continues these cells will divide
vacuoles
forming
cells absorb water by osmosis and expand
The osmotic forces generated by the cells and tissues are very large. This why you get weeds pushing up through tarmac.

28. 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

29. 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

30. Question 3 Cell sap concentrated solution a c dilute solution b
more dilute solution
Water will tend to move by osmosis
Plant cells
(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

31. 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

32. 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

33. Correct

34. Incorrect