"r norman"  wrote in message
news:ci4jtj$12s4$1{at}darwin.ediacara.org...
> On Sun, 12 Sep 2004 18:23:23 +0000 (UTC), Tim Tyler 
> wrote:
> >Brett Aubrey  wrote or quoted:
> >> 
> >Quite a few OOL scenarios put the OOL on the ocean floor.
> >Others invoke evaporative tidal pools.
> >I'd say that the odds were that the water was pretty salty.

I was aware of thermal vents and tidal pools; but thought there might
fresh water scenarios based of my "stunning logic"  ;-)  that I hadn't
heard about. Thanks, Tim.

> Fresh water is an enormously difficult environment for cells to live
> in.  Sea water is easy!
>
> The problem is maintaining osmotic balance across the cell membrane.
> Cells must contain a certain internal concentration of ions and
> organic chemicals to sustain life.  Fresh water is extremely dilute --
> for osmotic purposes it is essentially equivalent to distilled water.
> As a result, cells in fresh water are constantly taking in water
> osmotically.  Without a rigid cell wall to support the resulting
> pressure (turgor pressure), the cell will burst.  Protistan animal
> cells (protozoans) without cell walls have elaborate systems like
> contractile vacuoles to bail out the water.
>
> Maintaining an internal ion composition sufficiently concentrated to
> balance the concentration of salt water and thus remain in water
> balance is trivial.  It is simple enough to maintain some membrane
> pumps so the specific ion concentrations intracellularly can differ
> from the external environment without disturbing the osmotic balance.
>
> Another problem with fresh water is the extreme variability of
> chemical composition of the environment with the effect of rainwater
> or tidal wash.  The oceans are extremely stable environments.
>
> Go with salt  water.

 Thanks for yet another helpful response, r. norman.  Freshwater's toast.
Regards,  Brett.
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