active transport

Bob Vickery vickery at MPX.COM.AU
Sat Oct 9 00:09:30 EST 1999


>We'll have to define active transport to have any sense of this discussion.
> Is it, then, your contention that they flood the stream or lake with actively
>transported ions, and lower the osmotic pressure in their environment?  Of
>course not. Do they pump ions into and out of vacuoles, filling them with water,
>and then expelling the water?  But that isn't active transport?
>And plants with the opposite problems of sea water or the deserts? Do many of
>them take up salt ridden water and then secrete the salts?  That isn't a way to
>actively transport the water either?
>
>And then there are the trees that are too tall for both the water column and
>capillary action to explain their hydrated heights.
>
>I think that it's the definition of active transport that's at fault.  YES
>plants use energy to control
>water. The fact that the active control of water isn't a direct action on the
>water molecules is not relevant to the query of active transport. Any movement
>of water which goes against it's osmotic pressure MUST , by my definition be
>active.
>
>
<SNIP> (discussion of water transport)

Steve, I think that your definition of 'active transport' is too general to be useful.  You seem to imply that because plants 'use energy to control water'  therefore the movement of water is active.  On your argument everything that plants do is active and there is no such thing as a passive process.   This loses the valuable distinction between active (apparently against the electrochemical potential gradient) and passive (with the electrochemical potential gradient).  

In the thermodynamic sense, the whole plant is a _passive_ system.  It takes energy from the sun (at an equivalent of over 1000 K) and loses it to the environment (at about 300 K).  Labelling a process as 'active' implies that there must be somewhere a linked passive system driving the movement.  This linkage is then a good topic for further investigation.

Cheers

Bob Vickery
vickery at mpx.com.au



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