many soluble proteins, ions and metabolites.
(the PF). Where they have been identified, clusters of particles usually
represent channels for the transmembrane passage of ions or molecules
Plasma membrane
between adjacent cells (gap junctions).
Biophysical measurements show the lipid bilayer to be highly fluid,
Cells are enclosed by a distinct plasma membrane, which shares fea- allowing diffusion in the plane of the membrane. Thus proteins are able
tures with the cytomembrane system that compartmentalizes the cyto- to move freely in such planes unless anchored from within the cell.
plasm and surrounds the nucleus. All membranes are composed of Membranes in general, and the plasma membrane in particular, form
lipids (mainly phospholipids, cholesterol and glycolipids) and pro- boundaries selectively limiting diffusion and creating physiologically
teins, in approximately equal ratios. Plasma membrane lipids form a distinct compartments. Lipid bilayers are impermeable to hydrophilic
lipid bilayer, a layer two molecules thick. The hydrophobic ends of each solutes and ions, and so membranes actively control the passage of ions