active transport

Steve Hinkson sphinkson at worldnet.att.net
Fri Oct 8 05:59:14 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.


Grant R. Cramer wrote:

> 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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