Membranes are highly fluid, dynamic structures consisting of a lipid bilayer and associated proteins. Plasma membranes form closed compartments around the cytoplasm to define cell boundaries. The plasma membrane has selective permeabilities and acts as a barrier, thereby maintaining differences in composition between the inside and outside of the cell. Selective membrane molecular permeability is generated through the action of specific transporters and ion channels. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, and there are special areas of membrane structure—g
