active transport

Grant R. Cramer cramer at MED.UNR.EDU
Fri Oct 8 17:07:37 EST 1999


I am sorry Steve, but I am having trouble understanding your stream of 
thought. Perhaps you could break this down to one point at a time and I
could respond to it.
--
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: Steve Hinkson <sphinkson at worldnet.att.net>
>To: "Grant R. Cramer" <cramer at MED.UNR.EDU>
>Newsgroups: bionet.plants.education
>Subject: Re: active transport
>Date: Fri, Oct 8, 1999, 3:59 AM
>

> 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
>
>
>
> --
> Drop by and see me at :
>  http://www.geocities.com/HotSprings/Spa/6811
> 



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