Final answer:
Pumping Cl- out of the cell helps maintain the electrochemical gradient across the cell membrane due to the Na+/K+ ATPase activity, creating a negative charge inside the cell which repels Cl-, and allowing Cl- reabsorption and paracellular movement via symporters and tight junctions respectively.
Step-by-step explanation:
When chloride ions (Cl-) are pumped out of a cell, it contributes to the maintenance of the electrochemical gradient across the cell membrane. Typically, Cl- accumulates outside because it is repelled by negatively-charged proteins within the cytoplasm. The Na+/K+ ATPase pump plays a critical role in this process, as it pumps three Na+ ions out of the cell while bringing in two K+ ions per ATP consumed, contributing to the inside of the cell being negatively charged compared to the extracellular fluid. This negative charge inside the cell aids in the expulsion of Cl- due to the repulsion between like charges.
The Na+/Cl- symporters and the leaky tight junctions in cells, such as those in the ascending loop of Henle in the kidney, also facilitate the movement of Cl- in response to the Na+ gradient created by the Na+/K+ ATPase. The symporters allow Na+ and Cl- to reenter the cell together, while the tight junctions enable Cl- ions to move paracellularly, maintaining ionic balance and contributing to the osmotic balance necessary for fluid reabsorption.