21 - Osmosis and Regulation Of Cell Volume

Abstract

This chapter aims to provide the basis for understanding regulation of cell volume through the exchange of water and solutes across the plasma membrane. The chapter explains water movement across model membranes in detail. Osmosis refers to the movement of fluid across a membrane in response to differing concentrations of solutes on the two sides of the membrane. The ultimate cause of osmosis is the reduction of the chemical potential of water in a solution. The chapter discusses three examples of isosmotic regulation of cell volume that illustrate some of the principles. Gibbs-Donnan equilibrium is discussed in more depth. The primary mechanism of regulatory volume decrease (RVD) in a number of cell types is activation of conductive pathways, is discussed in detail. The chapter discusses anisosmotic media, membrane potential, cytoskeleton, calcium, phosphorylation, mass action model, and macromolecular crowding under signaling pathways underlying RVD and regulatory volume increase (RVI). Although principles of the pump-leak or double-Donnan model are correct and still relevant to the regulation of cell volume, it has become apparent that neither the leak nor the pump is constant. Not only is the control of these fluxes more complex than originally envisioned, but a myriad of other transport processes also contribute to cell volume regulation. The regulation of cell volume under anisosmotic conditions is also discussed in the chapter. From the perspective of regulation of ion transport, much remains to be understood about how cells sense swelling and shrinking and how a cell decides on its optimal volume. There are unanswered questions concerning the regulation of volume regulatory ion transporters by cellular messengers, metabolic demands, and pathological states.