Sunday 25 August 2013

Science: Osmosis in plants

Do you remember my blog post: How do plants drink water? - This post follows on from that nicely. 

What is osmosis?

It is the word we use to describe the tendency of molecules of a solvent to pass through a semipermeable (partly waterproof, partly not) membrane from a less concentrated solution into a more concentrated solution.

Where do we find the osmosis process in plants?
Water enters the plant through its roots by osmosis.  Osmosis is a diffusion of water across the semipermeable cell walls (of the roots in this case), the diffusion happens because water passes through the cell wall from the (outside) area of high concentration to the (inside) area of low concentration of dissolved substances. 
Diffusion is the movement of substances from areas of high concentration to low concentration.
The water is drawn up the roots in the same way as a wet spillage is by a piece of kitchen roll.  
You can try this with a strip of kitchen roll and some water yourself...
It then continues to climb the kitchen roll, we call this process capillary action which is exactly the same process plants use to take the water all the way through the plant right to the top.
How plants Transport water through osmosis.  Another fun way to see how plants transport water through Osmosis is this little experiment here...
What you need:
  • Two glasses or see though cups
  • Water
  • Food colouring (optional)
  • kitchen roll
What to do...
Put a 50ml of water in a glass and add a couple of drops of food colouring to it.  Mix it in well.
Cut a strip of kitchen roll.  we made ours 5cm thick and then cut it in half nearly all the way but not quite so it looks like this...
You can ask the children to make a hypothesis in their science journals or verbally.
Put it in your two glasses like this.  One glass should have your water in and the other none. 
Leave it overnight.
What has happened? There is blue water in the other glass too?
This is a great example how plants use osmosis and diffusion to transport water.
This is how it works in plants, even great big trees; the water is taken up through the roots right up to the top to the leaves.   But that is not the end of it because transpiration works with osmosis.  
Transpiration works is when water evaporates through the stomata into the air.  This transpiration process is caused because the cells, veins and vascular bundles in the leaves creates a lower pressure, it causes suction, this is what promotes osmosis, a bit like when you suck your drink up through a straw I suppose - this is what draws the water up through the soil and feeds the plant with the water a nutrients it needs from the ground.
  • osmosis
  • diffusion
  • transpiration.

Other science posts on here you might be interested in:

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