1. Osmosis

2. 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

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

4. 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

5. 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.

6. 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 water molecule will diffuse through the pores of the membrane from left to right than from right to left.
Molecular movement

7. Osmosis and animal cells
Then water will diffuse into the cell by osmosis
If there is a greater concentration of free water molecules outside the cell than inside
The cell will swell up

8. 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. 

9. 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

10. 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

11. Plant cells cell wall vacuole cytoplasm and cell membrane18
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

12. Question 1 In osmosis, water will diffuse through a membrane26
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

13. Question 2 Cell sap concentrated solution a c dilute solution b28
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
(b) from c to a and b
(c) From a to b and c
(d) From c to b and a

14. 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

15. 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

16. Plant and Animal Cells put into
various solutions