Osmoregulation: Some principles of water and solute transport

Living cells have developed the ability to persist in the face of a fundamental contradiction. On the one hand, they preserve an internal composition that is an optimal milieu for metabolic processes that are essential to the maintenance of the living state and maintain the ability to regulate that composition as a defense against external perturbations. On the other hand, the maintenance of this environment and the nature of the associated regulatory processes demand that matter be continuously shuttled in and out of the cell. In other words, cells must maintain an internal composition that is constant but is also not in equilibrium with its environment. Cellular composition is maintained in a so-called “steady state” in the face of constant in and out traffic across the cell membrane.