Osmosis

1. 1 Osmosis

2. 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 3 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. 4 Osmosis ...until concentrations become equal water or dilute solution concentrated solution level falls level rises membrane More water passes from dilute to concentrated ...

5. 5 In the next slides represents a water molecule represents a sugar molecule and 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.

6. membrane 6 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. 7 Molecular movement 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.

8. Animation Next slide 8

9. 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. Scale of drawings 10 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

11. Osmosis in animal cells 11 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

12. 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

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

15. 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. 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

17. 17 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

18. Plant cells 18 cell wall vacuole cytoplasm and cell membrane The cell absorbs water by osmosis .... ....but the cell wall stops the cell expanding any more

19. Question 19 What did you notice about the colour of the cell sap in the two cells? Can you suggest an explanation for this?

20. Osmosis between cells 20 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

21. Limp and turgid tissue 21 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

22. Busy Lizzie (1) wilting 22 (2) recovering (3) recovered

23. 23 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

24. Cell extension 3 4 2 1 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 24

25. Growth in a shoot tip 25 cell division continues these cells will divide vacuoles forming cells absorb water by osmosis and expand

26. 26 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

27. 27 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

28. 28 Cell sap Question 3 concentrated solution a c dilute solution b more dilute solution Plant cells 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

29. 29 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

30. 30 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

31. 31 Correct

32. 32 Incorrect