Biological membranes are not just cellular envelopes
As originally proposed by J.D.Davson and J.Danielli in 1935, all biological membranes have a bimolecular leaf arrangement of lipids. The amphipathic lipids and cholesterol are oriented so that the hydrophobic portions of the molecules interact, minimizing their contact with water or other polar groups and the polar head groups of lipids are at the interface with the aqueous environment. It was in the early 1970s that S.J.Singer and G.L.Nicholson proposed the mosaic model for membranes in which some proteins(intrinsic) are actually immersed in the lipid bilayer while others(extrinsic) are loosely attached to the surface of membranes.
Membranes are very dynamic structures due to their chemical composition, especially the content of cholesterol. They are an organized sea of lipid in a fluid state in which various components are able to move and interact. Membrane fluidity controls membrane-bound enzymes and receptors, phagocytosis, and cell growth. Abnormal membrane fluidity changes occur in many conditions such as abetalipoproteinemia, anemia associated with chronic liver disease, and also in the intoxicating effects of ethanol.
Cellular membranes control the composition of the space that they enclose by excluding a variety of molecules and by selective protein transport systems allowing the movement of specific molecules from one side to the other. By this translocation, membranes modulate the concentration of components in cellular compartments
Hormones, growth, and metabolic regulators bind to specific protein receptors and transmit the signals thus generated within the cell through second messenger molecules.
Plasma membranes of eukaryotic cells also have a significant role in cell-cell recognition, adhesion, maintenance of cell shape, and cell locomotion
Lipid bilayers can be prepared to close in on themselves forming spherical vesicles called liposomes, separating the external environment from the internal compartment. They are used therapeutically for tissue-specific targeting of drugs, enzymes, DNA for specificity and effectiveness