Why and how sea animals die if they placed into sweet water?

Question

Is it related to salt concentration in the cells?

Answer 1

When animals are put in water that is less concentrated than their bodies, they tend to absorb water and lose salts, often through their gills and or mucus membranes in the digestive system.  If the animal comes naturally from an environment that does not usually change its salt concentration (like the ocean), the animal will not usually have any mechanism to prevent this (water gain / salt loss).  However, animals that come from more variable environments (like estuaries) will have a mechanism to prevent it, kind of like the nephron in the human kidney.

Answer 2

Do mean sugar water?  Sugar does not actually dissolve in water like salt does so it would just settle in the cells and build up faster then the lobsters could process and use it up eventually killing them.

Answer 3

The term you're looking for is osmoregulation. I've included some links on osmoregulation in lobsters and the consequences of disruption of osmoregulation in these animals.

You can find lots of information (some of it is quite technical) by searching for "lobster osmoregulation."