always swimming up

"Donny" wrote in message
...

Osmostic pressure (as defined in Biology I think, the Chemists'
definition is different) is the water pushing on the membrane trying to
get in. The amount and rate the water diffuses depends on how much
osmostic pressure the membrane can stand.

From what I remember from high school, the amount and rate actually
depends only on the difference in ion concentration of the liquids on
either side of the membrane. For a given concentration difference,
osmotic pressure will raise the liquid level by a defined height
in a U-shaped tube with the membrane at the bottom, dividing
the left and right half of the U. Double the concentration on one
side, and you get a difference in liquid level twice as large.

Sal****er fish are used
to living in an environment that the osmotic pressure on their cells is
low. So, in freshwater, a sal****er fish can't remove water as well and
its cells swell with water and explode (kablooey ), which of course
kills the fish.

Sal****er fish have to drink sea water to make up for the loss of liquid
that is caused by osmotic pressure. The ocean around them is at a
higher salt concentration than their blood, so water continuously escapes
through the fish's skin trying to dilute the ocean. The fish has to drink
water to compensate for the loss (and sal****er fish have mechanisms
to get rid of the extra salt they take in that way). For freshwater fish,
the opposite is true: the water around them is at a lower concentration
than their blood, so water continuously intrudes trying to dilute the fish.
The fish copes with that by excreting the excess water. In other words,
sal****er fish **** has a lot of salt in it, and freshwater fish **** has
little
salt in it :-)

While I'm rambling, it turns out that the salt concentration of most animals
is in quite a narrow band. In particular, animals living in sal****er have
very closely the same salt concentration in their blood as animals living
in freshwater. This gives rise to the theory that oceans were once less
salty than they are now, and that the salt concentration of animal blood
closely approximates the salt concentration of ancient oceans. The idea
is that animals started out with a salt concentration that matched that of
the surrounding ocean and then evolved mechanisms to cope with
different concentrations in their environment later.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com