Plasma membranes are subcellular structures, approximately 10nm thick, that form a protective boundary around the cell as well as the cell’s organelles. They serve to both impede foreign material from entering the cell, and prevent the cellular contents from leaking out. With the structural makeup of the lipid bilayer conferring membranes unique physical and chemical properties, these structures also contribute to diverse and critical cellular functions.
Representative structure of the plasma membrane. Two leaflets of phospholipid form the fluid lipid bilayer. Interspersed throughout the bilayer are proteins which carry out specific functions, and sterols which enhance membrane rigidity.
Membranes are composed of lipids and proteins, balanced in equal proportions by mass. The current views on membrane structure are derived from the Fluid-Mosaic Membrane Model (F-MMM) , which depicts them as two-dimensional fluids made up of lipid-bilayers interspersed with proteins. The fluidic nature of membranes is due to the constant rotational or lateral motion of both lipids and proteins. While lipids provide the basic structure of membranes, the proteins carry out a vast array of specialized functions, from ion and small molecule transport to the regulation of signalling pathways.