Biology: Concepts and Investigations 2nd Edition by Mariëlle Hoefnagels

Exercise 8

Seawater contains about 35 grams of salt per liter, whereas a liter of fresh water contains 0.5 g of salt or less. The blood of a fish has about 10 g of dissolved salt per liter. Cells in a fish's gills have transport proteins that pump salts across their membranes. In what direction would a saltwater fish pump ions? What about a freshwater fish?