Figure 1.

The model cell in normal conditions. Its membrane contains three types of channels, Cl, K, and Na, which mediate passive movement of these ions. Ion concentrations are in mM. Permeability of the resting cell is P_K 1.0, P_Na 0.02, and P_Cl 2.0 (in arbitrary units). An ATP-driven pump extrudes three Na⁺ ions and imports two K⁺ ions per cycle. The resting potential is −80 mV, and the resting volume is 1.0 (arbitrary units). Internal [Cl⁻] is only 7 mM, making osmotic room for internally sequestered substances, S⁻, at a concentration of 143 mM.