Thursday, October 2, 2008

Curious about the Apoplastic vs. Symplastic Routes?

So the question posed today was, "Why or How does the water and minerals choose the apoplastic route over the symplastic route AND why in mid transport would it switch routes?"

So I have a few thoughts:

1. The root cortex outer membrane is quite porous, thus allowing easy access for the water and minerals to enter for transport to the stele.
2. The water and minerals that travel the apoplastic route initially, must transition to the symplastic route in order to enter the endodermal cell which will then select which and how many minerals will transport into the vascular bundle. Don't forget that the casparian strip interupts the cell wall outer membrane transport.
3. The apoplastic route is sometimes called the path of least resistance as it does not have to traverse across membranes or junctions (plasmodesmas) as in the symplastic route.
4. In the bundle sheath and mesophyll layers (where phloem will transport the sugar to a sink) actually prefer to "load" their sugars in the apoplastic route as it is easy to travel through the membrane/wall than to travel through the cytoplasm.

Don't forget to review Water Potential:

Note: Distilled Water=no ions=no solute= solute potential =0 and open air so no pressure!

Next picture: No pressure still, but since the plant cell is made of different solutes the solute potential is defined as -2. Over all water potential is negative.
Last Picture: After the cell has had time for all the water to rush in (high water potential to a low water potential) then the pressure is positive as this will simulate root pressure which exerts a positive pressure. The solute is still the same as we are only discussing osmosis and no other type of diffusion. There for the water potential is =0 which is the ideal state of the plant!

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