Steady state (biochemistry)

In ionic steady state, cells maintain different internal and external concentrations of various ionic species. Cell membranes are permeable to sodium and various other ions, so in order to maintain a constant ionic concentration the cell must expend energy to actively transport these ions against the electrochemical gradient, out of the cell, at the same rate as they diffuse inward. Conversely, membranes are not permeable to potassium, so this ion must be actively pumped into the cell. Sodium and potassium pumps in the membrane are thus responsible for the differences in the intracellular and extracellular concentrations of these ions. The unequal distribution of ions represents a steady state (and not an equilibrium) as it requires the continual expenditure of cellular energy. Ionic steady state is a form of homeostasis.