What would happen if a saltwater shark was placed in freshwater?

Well, a shark, when in salt water is in an isotonic environment and its biological structure allows it to osmoregulate in this environment, due to the fact that salt and water are equally concentrated between both the shark and its surroundings. When placed in fresh water, the shark would be in a hypotonic environment [lower solute [salt] in the water in comparison to the sharks cells] thus there will be a net osmotic movement into the cells of the shark b/c there is a higher concentration of free water molecules outside in the fresh water in comparison to the inside of the sharks cells [the water is moving down its concentration gradient] and the cells of the shark will most likely lyse [burst] causing the shark to die b/c the cell cannot handle such a great movement of water into the cell in comparison to out. In short, the shark cannot osmoregulate in the fresh water.