permeability of membranes
permeability of membranesa theoretical concept of the permeability of a cell membrane, in which the membrane structure is thought to contain pores through which solutes can sometimes pass down a CONCENTRATION GRADIENT by DIFFUSION (see FLUID-MOSAIC MODEL). Many membranes are, however, differentially permeable to certain molecules. For example, SELECTIVE PERMEABILITY enables OSMOSIS to take place. Others will allow movement against a concentration gradient by ACTIVE TRANSPORT. An example is the removal of sodium by cells surrounded by a higher sodium concentration.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005