MadSci Network: Biochemistry

Re: What is the exact reason that snails and slugs die when salt is applied?

Date: Fri Nov 3 18:59:30 2000
Posted By: Andrew D. Brabban, Faculty, Biology, The Evergreen State College
Area of science: Biochemistry
ID: 972892102.Bc

You are right in that this is a sort of a chemical reaction, but not 
really, in that no actual physical change is occurring to the chemicals. 
The reason that they foam is a process called osmosis. Osmosis is a 
fundamental process that is involved in many branches of biology as well 
as chemistry. The term osmosis describes a type of diffusion (diffusion is 
when a chemical moves from an area of high concentration to a low 
concentration, such as adding dye to water, it spreads out) involving the 
net movement of solvent (the liquid part) molecules, as opposed to solute 
(the things dissolved in the liquid) molecules, across a semi-permeable 
(that lets something's but not others through) membrane from an area of 
high water/low solute concentration to an area of low water/high solute 
concentration (hypotonic). In this case the semi-permeable part lets water 
across the membrane but not the dissolved chemicals. All organisms are in 
state of osmotic balanced; you are at the moment. The reason you get 
thirsty is that you lack water and your cells are depleted of it and 
shrivel up. So how come you don't foam if you put salt on your skin. Well 
you skin is unlike a slug. One of the critical steps in evolution was the 
development of our skin, which is flexible, hard wearing and prevents 
water from escaping. But slugs didn't evolve this far. Their skin does not 
prevent water from passing across it; it acts as a semi-permeable 
membrane. So if you add salt to a slug you change the osmotic balance 
across the membrane. Now the slug has lots of salt (solute) on the outside 
in the water. So effectively that water is less concentrated (it has other 
things in it) relative to the water inside the cell. So the water inside 
the cell rushes out across the membrane to make things equal. This makes 
it look like the slug is foaming.

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