Osmosis - this is a special case of diffusion for water


Osmosis is the movement of water particles across a semi-permeable membrane (partially permeable membrane PPM)  from an area of high water concentration to an area of low water concentration.  

Cell membranes are partially permeable because they will only permit some things to pass through, but not others.  The reason for this is that PPM has extremely small pores in it.  They are so small that only water particles can pass through them, and larger particle like glucose and proteins can not.  

  • The water particles in fact pass both ways through the visking tubing. Just like!  Traffic  
  • As there are more water particles on one side than the other, there is a steady net flow into the area with fewer water particles, i.e. more water particles moving into the glucose side, than other way round.
  • This results in water filling up the glucose rich side i.e. solution of glucose becomes diluted.  The movement of water particles carries on until both sides of the membrane has equal number.
  • Water enters the plant by osmosis through root hairs.  It make plant cells swell up and they become TURGID- (swollen).  TURGIDITY is important for plants as it is very helpful in opening STOMATAL  GUARD CELLS and giving support to green plant TISSUES.
  • Turgidity is not useful in the case of animal cells as they do not have a cell wall.  For this reason, animal cells can easily burst open if placed into pure water since they take in so much water by osmosis.

Experiment to demonstrate osmosis!

You can demonstrate this effect with VISKING TUBING, which is partially permeable membrane.  It is sometimes called dialysis tubing as it is used in kidney dialysis machines.  




From the animation it can be seen that water level in the glass tube begins to rise or fall because water either enters or leaves through the visking tubing by osmosis.


Remember: Potato Slices

It can be seen from the animation that potato slices, in pure water, swell up as water enters their cells by osmosis. 

However, in concentrated sugar solution the potato slices shrink as water leaves their cells as a result of osmosis.

Plant Structure and Function


Tags:Osmosis, Movement of water particles, Semi-permeable membrane, Cells




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