1. Living Cells
Diffusion and Osmosis
Mr G Davidson

2. Diffusion
Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient.
Molecules of liquids and gases are constantly moving, and they will always try to spread out until there is an equal distribution.
Saturday, December 08, 2018
Mr G Davidson

3. Diffusion
A purple crystal of potassium permanganate is added to a beaker of water.
It is left for a day.
The particles of the crystal begin to diffuse into the water.
Water
Crystal
Saturday, December 08, 2018
Mr G Davidson

4. Diffusion
The molecules from the crystal have spread out from the crystal (high conc.) to the water (low conc.)
This continues until the molecules are equally spread out.
Saturday, December 08, 2018
Mr G Davidson

5. Importance of Diffusion
Diffusion is important to all living organisms as it allows important substances to pass into and out cells.
In unicellular organisms oxygen is constantly diffusing into the cell as it uses it up, and carbon dioxide is constantly diffusing out.
Saturday, December 08, 2018
Mr G Davidson

6. Diffusion in Unicellular Organisms
Respiration in the cell
uses up oxygen and
produces carbon dioxide
High concentration
of oxygen
Low concentration
of carbon dioxide
Oxygen diffuses
into the cell
Carbon dioxide diffuses
out of the cell
Saturday, December 08, 2018
Mr G Davidson

7. Diffusion in Multicellular Organisms
Diffusion is also important to multicellular organisms.
Blood is used to carry gases around the body.
Saturday, December 08, 2018
Mr G Davidson

8. Diffusion in Multicellular Organisms
This blood picks up and loses the gases in the lungs of mammals.
In the lungs oxygen diffuses into the blood.
When this blood reaches the tissues, the oxygen diffuses into the tissue cells from the blood.
Saturday, December 08, 2018
Mr G Davidson

9. Diffusion in Multicellular Organisms
Air sac
in lungs
High oxygen
concentration
Low oxygen
concentration
High oxygen
concentration
Low oxygen
concentration
Saturday, December 08, 2018
Mr G Davidson

10. Diffusion in Multicellular Organisms
In the tissues the oxygen is used up in respiration and carbon dioxide is produced.
In the tissues carbon dioxide diffuses from the tissue cells into the blood.
When this blood reaches the lungs, the carbon dioxide diffuses into the air sacs and this air is then breathed out.
Saturday, December 08, 2018
Mr G Davidson

11. Diffusion in Multicellular Organisms
Air sac
in lungs
Low CO2
concentration
High CO2
concentration
Low CO2
concentration
High CO2
concentration
Saturday, December 08, 2018
Mr G Davidson

12. Osmosis Osmosis is a special type of diffusion.
Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a selectively permeable membrane.
A selectively permeable membrane only allows small molecules to pass through it.
Saturday, December 08, 2018
Mr G Davidson

13. Osmosis Sugar molecule Selectively permeable membrane Water molecule
Low water concentration
High water concentration
Saturday, December 08, 2018
Mr G Davidson

14. diffuse from high water through the pores in the
Osmosis
The sugar molecules
try to diffuse through
the membrane but can’t
get through the pores
The water molecules
diffuse from high water
conc.to low water conc.
through the pores in the
membrane.
Saturday, December 08, 2018
Mr G Davidson

15. Osmosis The following shows a model cell made of visking tubing. Water
2% sugar
solution
H2O
H2O
98% water
H2O
H2O
In this case
the bag will
gain mass
H2O
H2O
Saturday, December 08, 2018
Mr G Davidson

16. Osmosis
If we always think of the water concentration, we can easily explain what happens in terms of osmosis.
A hypotonic solution will always lose water.
A hypertonic solution will always gain water.
An isotonic solution will neither gain nor lose water.
Saturday, December 08, 2018
Mr G Davidson

17. In this case, water enters the cell and the cell becomes TURGID.
Water Concentrations
Hypotonic solution (water)
High water conc.
Low water conc.
Plant cell
In this case, water enters the cell and the cell becomes TURGID.
Saturday, December 08, 2018
Mr G Davidson

18. In this case, water leaves the cell and the cell becomes FLACCID.
Water Concentrations
Hypertonic solution (5% salt solution)
Low water conc.
High water conc.
Plant cell
In this case, water leaves the cell and the cell becomes FLACCID.
Saturday, December 08, 2018
Mr G Davidson

19. Water Concentrations Isotonic solution
Plant cell
In this case, water neither leaves nor enters the cell and the cell remains unchanged.
Saturday, December 08, 2018
Mr G Davidson

20. Water Concentrations
If a cylinder of potato tissue is placed in a hypertonic solution the cells lose weight and the tissue feels soft.
If a cylinder of potato tissue is placed in a hypotonic solution the cells gain weight and the tissue feels firm.
HYPOTONIC
SOLUTION
HYPERTONIC
SOLUTION
Saturday, December 08, 2018
Mr G Davidson

21. Plant Cells When plant cells are in a hypotonic solution they
Water
When plant cells are in a
hypotonic solution they
gain water by osmosis. This
fills the large vacuole which
forces the cell membrane
out against the cell wall.
This is what makes the
cells go turgid and allows
plants to stand upright.
Water
Saturday, December 08, 2018
Mr G Davidson

22. Plant Cells When plant cells are in a hypertonic solution they
Water
When plant cells are in a
hypertonic solution they
lose water by osmosis. This
empties the large vacuole
and pulls the cell membrane
away from the cell wall.
This is called PLASMOLYSIS
and the cell is soft and said
to be PLASMOLYSED.
Water
Saturday, December 08, 2018
Mr G Davidson

23. Animal Cells
Water
When animal cells are placed in a hypotonic solution, they too gain water. However, there is no strong cell wall and the cell membrane eventually ruptures.
Water
Saturday, December 08, 2018
Mr G Davidson

24. Animal Cells
Water
When animal cells are placed in a hypertonic solution, they too lose water by osmosis and the cell shrinks.
Water
Saturday, December 08, 2018
Mr G Davidson

25. Animal Cells
Some unicellular animals, such as Paramecium, have a problem because they live in fresh water which is hypotonic. This means they constantly gain water by osmosis. In order to prevent them from bursting they have special “contractile vacuoles” which they use to expel water. They do this by filling up and then going to the cell membrane where they burst, “spitting” out the excess water.
Contractile
vacuoles
Paramecium
Saturday, December 08, 2018
Mr G Davidson