In 1972 S. Singer and G. Nicolson advocated a revised membrane model. According to them a bilayer of phospholipids is not coated with solid sheets of proteins rather the proteins are dispersed and individually inserted into the phospholipids bilayer.
Only the hydrophilic portions of the proteins protrude far enough from the bilayer to be associated with water. This arrangement views membrane as a mosaic of protein molecules bubbling in a fluid bi-layer of lipids.
This molecular arrangement would be a stable one because it would maximize the contact of hydrophilic regions of proteins and phospholipids with water while providing the hydrophobic parts with a non aqueous environment.
A method of preparing cells for electron microscopy called Freeze- fracture technique has provided the most compelling evidence that proteins are embedded in lipids. In this preparation the interior of membrane under electron microscope gave a cobblc-stoned or bumpy appearance.
Other experiments proved that the bumps are proteins in lipid bilayer. The lipids are arranged in bilayer so that their hydrophilic head groups are towards the surface and the non-polar tails of both layers face each other at the core of the membrane.
Depending on their arrangement the proteins found in the membrane are of two broad categories. The extrinsic or peripheral proteins are those which do not interact directly with the hydrophobic core of the membrane.
They are usually bound to the membrane either indirectly by interactions with integral membrane proteins or directly by interactions with lipid polar head groups. Most Peripheral proteins are soluble in aqueous solutions and are bound to internal membrane proteins by ionic and weak interactions.
They can be easily removed from the membrane either by solutions of high ionic strength or by chemicals that bind bivalent cations such as Mg++. Most peripheral proteins are not solubilised by detergents since they are not bound directly to the hydrophobic core.
The proteins spectrin and ankyrin, the cytoskeleton proteins that are bound to inner face of erythrocyte cell membrane are some examples of extrinsic proteins. Other peripheral proteins are localized to outer surface of plasma membrane, such as certain proteins of glycocalyx (discussed later).
The integral membrane proteins are internal membrane proteins which pass into the lipid bilayer to different depths. They are not soluble in water and contain at least one very hydrophobic segments of 10 to 20 amino acid long.
A few intrinsic proteins are anchored to the membrane mainly by glycophospholipids that is attached covalently to the carboxyl terminus of the protein. They are held to the membrane by three types of interactions like hydrophobic interactions with lipid interior, ionic interactions with polar heads of lipids or specific interactions with defined structures of lipid.
Some intrinsic proteins can span across the membrane from outer face to inner face. These membrane spanning proteins may span the membrane only once so that only one segment of the protein is within the membrane.
These proteins are called single pass proteins, e.g., Glycophorin of RBC. Multi pass proteins span the membrane many times so that more than one segment of the proteins is within the membrane. Bacterial Rhodesian spans the membrane seven times.
Integral membrane proteins can be removed from the membrane by the action of detergents which displaces the lipid bound to hydrophobic side chains of proteins. The transmembrane or membrane spanning proteins either singly or in groups function as tunnel proteins providing channels for diffusion of water and water soluble substances. Some of them behave as permeases allowing facilitated diffusion.
There are transmembrane proteins involved in active transport known as carrier proteins. Some cell surface membrane proteins act as signal receptors and on the inner side of the membrane there are proteins which anchor the membrane to cytoskeleton.
The two faces of the membrane (the exterior and the cytoplasm face) can be studied separately by freeze-fracture technique. This reveals that the two faces of the membrane are not the same. The amount and type of the proteins found on two faces are different.
The cytoskeleton anchor proteins are always towards the cytoplasm face. The lipids also vary in their amount and type on both the faces. On the erythrocytes membrane sphingomyelin, phosphatydylcholine are more on outer face than on inner face. While phosphatidyl serine is only found on the inner face.
The oligosaccharides attached to glycolipids and glycoproteins are found only on the outer surface. The proteins found on one face never flip-flop across the membrane as such movement would be energetically unfavorable
(1) Forms the boundary of cells enclosing the semi fluid contents of the cells.
(2) Sub cellular membranes help in compartmentalization of cells in eukaryotes.
(3) In folding of plasma lemma in bacteria form mesosomes required for nucleoid replication and cell division.
(4) Grows over cilia and flagella, forming sheaths.
(5) Form functional complexes like tight junction in epithelial cells, gap junction between adjacent cells for connections, plasmodesmata in plant cells, and desmosomes in epithelial cells.
(6) Form microvilli or striated border or brush border on the free surfaces of absorbing cells, e.g., intestinal cells, hepatic cells etc.
(7) Membrane proteins transport materials in and out of the cells
(8) Signal receptor proteins receive signals from hormones and such other chemicals and transmit those signals to the interior of cells.
(9) Membrane proteins also act as anchor for cytoskeleton components and extra cellular matrix.
(10) Membrane proteins on the outer face of the membrane endow the cells with individuality to allow them to assort appropriately during differentiation.
(11) Various enzymes associated with cell and sub cellular membranes allow different chemical reactions to be catalyzed in different parts of the cell.