Final answer:
The Nerst potential for Cl- when there is no net pumping into or out of the cell depends on the established electrochemical gradient. Without net pumping, and if the ion is at equilibrium, the Nerst potential for Cl- would be zero. However, this can be influenced by active transport processes, paracellular pathways, and concentration gradients established by the Na+/K+ ATPase.
Step-by-step explanation:
The Nerst equilibrium potential for Cl- when there is no net pumping of Cl- into or out of the cell can be analyzed in the context of the electrochemical gradients established by the Na+/K+ ATPase pumps
Nerst equilibrium potential is determined by the ratio of the concentrations of the ion on the inside and outside of the cell. In the absence of net pumping of Cl-, the distribution of Cl- ions will be governed by the electrochemical gradient that is established partly by the Na+/K+ ATPase, which pumps sodium out of the cell and potassium into the cell. The pump contributes to a net negative charge inside the cell which establishes a gradient Cl- ions can follow. If the concentration of Cl- ions is at equilibrium across the membrane, then the Nerst potential for Cl- will be zero, meaning that there is no net movement of Cl- across the cell membrane.
However, in tissues where there are additional active transport processes for Cl- such as reabsorption in the kidneys, the Nerst potential will reflect the different concentrations established by these processes. When Cl- is passively following Na+ through a paracellular route or through symporters as described, its distribution is also impacted by leaky channels and tight junctions in the ascending loop of Henle.
The sodium-potassium pump is crucial for maintaining this electrochemical gradient as it drives the transport of Na+ out of the cell and K+ into the cell, using ATP in the process; creating a negative charge inside the cell membrane. As more cations are expelled from the cell than taken in, this contributes to the interior of the cell maintaining its negative charge. Consequently, Cl- ions, which are negatively charged, are influenced by these gradients and can either accumulate outside the cell or follow the positive ions into the cell depending on the specific conditions within a given tissue.