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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 The membrane must allow water molecules to diffuse through. It is permeable to water.

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

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 (inside vacuole) and cytoplasm function as fairly concentrated solutions. 9

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 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

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

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

Inquiry into Osmosis! You are now going to design an experiment to investigate the process of osmosis!