Final answer:
When a cell is placed in seawater, the WNK system activates to maintain osmotic balance by excreting salt and producing concentrated urine. In freshwater, the WNK system is inhibited, and fish actively take in salt and excrete diluted urine to prevent excess water uptake.
Step-by-step explanation:
When a cell is placed in seawater, the WNK system is activated to help regulate ion concentrations and maintain osmotic balance. The WNK system stimulates the cells in the gills to excrete excess salt, and it also triggers the production of highly concentrated urine to eliminate the excess water. This allows the cells to maintain the proper internal balance in the hypertonic seawater environment.
In contrast, when a cell is placed in freshwater, the WNK system is inhibited. Freshwater is hypotonic compared to the cells, causing water to enter the cells through osmosis. Freshwater fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water, helping to maintain the proper balance in the hypotonic environment.