active transport

Grant R. Cramer cramer at MED.UNR.EDU
Wed Sep 29 12:03:25 EST 1999


The answer is No. Water is not transported actively in a plant. The 
definition of "active transport" is not based upon the use of ATP. It is
defined as the movement of a substance from a lower chemical potential (free
energy) to a region of higher chemical potential. Water always moves
passively down its chemical potential gradient. Secretory glands work by
actively pump ions. The build up of ion concentrations lowers the chemical
potential of water, thus allowing water to move down it chemical potential
gradient. This is not an active transport for water but is for the ions. The
chemical potential of a substance is dependent upon four things: 1)substance
activity or concentration, 2) electrical (e.g. membrane potential), 3)
gravity and 4) pressure. Over short distances (less than a meter) only
pressure and concentration affect water.
--
Grant R. Cramer
Associate Professor
Mail Stop 200
Department of Biochemistry
University of Nevada
Reno, NV 89557
phone: (775) 784-4204
fax: (775) 784-1650
email: cramer at .unr.edu
web page: http://www.ag.unr.edu/cramer/

----------
>From: davehaas at nospamprodigy.net (Dave Haas)
>To: plant-ed at net.bio.net
>Subject: active transport
>Date: Tue, Sep 28, 1999, 4:31 PM
>

>
> Is it possible for plants to "actively transport" water?  Esau makes the
> statement that water may be actively transported in certain secretory
> structures. I had always thought that water was one thing that couldn't
> be actively transported.  Somewhere back in time I had acquired this
> idea.   Now.  I realize that there is not a molecular pump for water like
> there is for various ions,  but if you define active transport as simply
> using ATP to move something against a gradient shouldn't you be able to
> move water out of a cell like contractile vacuoles do in protists.
>
> This brings to mind another question.  Why would a plant want to rid
> itself of water in the first place?  If special cells (hydathodes) have
> evolved to release water in guttation there should be some advantage
> shouldn't there?  Are some plant cell walls unable to hold up under
> excess pressure produced by osmosis.  Anyone have any info in this area?
>
> D. Haas



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