1.6.3 FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE
Although the basic structure of biological membranes is provided by the lipid bilayer, membrane proteins perform most of the specific functions of membranes. It is the proteins, therefore, that give each type of membrane in the cell its characteristic functional properties. Amount of protein in membrane varies depending on function (nerve 25%, those involved in ATP production 75%). Like membrane lipids, membrane proteins often have oligosaccharide chains attached to them that face the cell exterior. Thus, the surface that the cell presents to the exterior is rich in carbohydrate, which forms a cell coat.

Membrane Proteins

Transmembrane Proteins (integral proteins)
Extend through the lipid bilayer. They are amphipathic (both polar and non-polar regions). Hydrophobic cytosolic regions pass through the membrane and interact with the hydrophobic tails of lipid molecules in interior bilayer. Hydrophilic regions are exposed to water. Hydrophobicity of inner regions can be increased by covalent attachment of fatty acid chains. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet (barrel) protein motifs. Examples include: ion channels, G protein coupled receptors and protein pumps.

Lipid “anchored” proteins
Covalently bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. Examples: G proteins
Some of these are anchored to the cytosolic surface by an amphipathic α helix that partitions into the cytosolic monolayer of the lipid bilayer through the hydrophobic face of the helix. Others are attached to the bilayer solely by a covalently attached lipid chain—either a fatty acid chain or a prenyl group—in the cytosolic monolayer or, via an oligosaccharide linker, to phosphatidylinositol in the noncytosolic monolayer.

Surface proteins (peripheral)
Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and once reacted, the molecule dissociates to carry on its work in the cytoplasm. Bind by non-covalent interaction with other membrane proteins. Many of the proteins of this type can be released from the membrane by relatively gentle extraction procedures, such as exposure to solutions of very high or low ionic strength or of extreme pH, which interfere with protein-protein interactions but leave the lipid bilayer intact. Transmembrane proteins, many proteins held in the bilayer by lipid groups, and some proteins held on the membrane by unusually tight binding to other proteins cannot be released in these ways. These proteins are called integral membrane proteins.

Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it. Cell-surface receptors are transmembrane proteins that bind signal molecules in the extracellular space and generate different intracellular signals on the opposite side of the plasma membrane. Proteins that function on only one side of the lipid bilayer, by contrast, are often associated exclusively with either the lipid monolayer or a protein domain on that side. Some of the proteins involved in intracellular signaling, for example, are bound to the cytosolic half of the plasma membrane by one or more covalently attached lipid groups.

Motion of membrane components
A remarkable feature of several biological membrane systems is that their phospholipids are asymmetrically distributed across the lipid bilayer. Most of our knowledge on phospholipid asymmetry of lipids has come from studies on human erythrocytes. Sphingomyelin and glycosphingolipids residing predominantly on the non-cytosolic (luminal) side and the anionic phospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE) enriched in the cytosolic leaflet. The discovery that