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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 2

If a concentrated solution is separated from a dilute solution by a suitable membrane, water will pass from the dilute to the concentrated solution. The membrane must allow water molecules to diffuse through. It is permeable to water. In fact, water passes both ways but faster from the dilute to the concentrated solution. Permeability 3

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

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 5

membrane watersugar 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 the left, more diffuse through the pores of the membrane from left to right than from right to left. 7 Molecular movement

Next slide 8 Animation

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. 9

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 10 Scale of drawings

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 Osmosis in animal cells 11

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 12

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 Trout Fresh water fish 14

The fish gets rid of the excess water by means of its kidneys Perch gills absorb water by osmosis blood carries water to kidneys kidneys expel excess water......in the form of dilute urine heart 14

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 Herring Sea water fish 15

In a plant cell, the cell membrane acts as a selectively permeable membrane 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 cell wall is freely permeable to water The vacuole will expand, pushing the cytoplasm outwards against the cell wall 16

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 17

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

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

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 20

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 Limp and turgid tissue 21

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

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 23

1 1 The cell has just divided 2 2 Vacuoles start to form in one cell 3 3 The vacuoles absorb water by osmosis and start to extend the cell 4 4 The vacuoles begin to join up and continue to absorb water and extend the cell Cell extension 24

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

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 26

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 27

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

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 29

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 30

Correct 31

Incorrect 32