READ 5 1 The Structure of Membranes The membranes that encase all living cells are two phospholipid sheets that are only 5 10 nm thick more than 10 000 of these sheets piled on one another would just equal the thickness of this sheet of paper Biologists established the components of membranes not only lipids but also proteins and other molecules through biochemical assays but the organization of the membrane components remained elusive The fluid mosaic model shows proteins embedded in a fluid lipid bilayer The lipid layer that forms the foundation of a cell s membranes is a bilayer formed of phospholipids These phospholipids include primary the glycerol phospholipids and the sphingolipids such as sphingomyelin Note that although these look superficially similar they are built on a different carbon skeleton In 1972 S Jonathan Singer and Garth J Nicolson revised the model in a simple but profound way They proposed that the globular proteins are inserted into the lipid bilayer with their nonpolar segments in contact with the nonpolar interior of the bilayer and their polar portions protruding out from the membrane surface In this model called the fluid mosaic model a mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond We now recognize two categories of membrane proteins based on their association with the membrane Integral membrane proteins are embedded in the membrane and peripheral proteins are associated with the surface of the membrane Cellular membranes consist of four component groups 1 Phospholipid bilayer Every cell membrane is composed of phospholipids in a bilayer The other components of the membrane are emebedded within the bilayer which provides a flesxible matrix and at the same time imposes a barrier to permeability Animal cell membranes also contain cholesterol a steroid with a polar hydroxyl group OH Plant cells have other sterols but little or no cholesterol 2 Transmembrane proteins A major component of every membrane is a collection of proteins that float in the lipid bilayer These proteins have a variety of functions including transport and communication across the membrane Many integral membrane proteins are not fixed in position They can move about just as the phospholipid molecules do Some membranes are crowded with proteins but in others the proteins are more sparsely distributed 3 Interior protein network Membranes are structurally supported by intracellular proteins that reinforce the membrane s shape For example a red blood cell has a characteristic biconcave shape because a scaffold made of a protein called spectrin links proteins in the plasma membrane with actin filaments in the cell s cytoskeleton Membranes use networks of other proteins to control the lateral movements of some key membrane proteins anchoring them to specific sites 4 Cell surface markers As you learned in the preceding chapter membrane sections assemble in the endoplasmic reticulum transfer to the Golgi apparatus and then are transported to the plasma membrane The ER adds chains of sugar molecules to membrane proteins and lipids converting them into glycoproteins and glycolipids Different cell types exhibit different varieties of these glycoproteins and glycolipids on their surfaces which act as cell identity markers Cellular membranes have an organized substructure Originally it was believed that because of its fluidity the plasma membrane was uniform with lipids and proteins free to diffuse rapidly iin the plane of the membrane However in the last decade evidence has accumulated suggesting the plasma membrane is not homogeneous and contains microdomains with distinct lipid and protein composition This was first observed in epithelial cells in which the lipid composition of the apical and basal membranes was shown to be distinctly different Theoretical work also showed that lipids can exist in either a disordered or an ordered phase within a bilayer This led to the idea of lipid microdomains called lipid rafts that are heavily enriched in cholesterol and sphingolipids These lipids appear to interact with each other and with raft associated proteins together forming an ordered structure This is now technically defined as dynamic nanometer sized sterol and sphingolipid enriched protein assemblies There is evidence that signaling molecules such as the B and T cell receptors associate with lipid rafts and that this association affects their function In addition to these horizontal structures there is also vertical structure to the plasma membrane That is the distribution of membrane lipids in the plasma membrane is asymmetrical with the outer leaflet enriched in the glycerol phospholipid phosphatidylcholine and in sphingolipids This is despite being symmetrically distributed in the ER where they are synthesized Some of this sorting occurs in the Golgi and is also affected by enzymes that transport lipids across the bilayer from one face to the other Electron microscopy has provided structural evidence Electron microscopy allows biologists to examine the delicate filmy structure of a cell membrane When examining cell membranes with electron microscopy specimens must be prepared for viewing In one method of preparing a specimen the tissue of choice is embedded in a hard epoxy matrix The epoxy block is then cut with a microtome a machine with a very sharp blade that makes incredibly thin transparent epoxy shavings less than 1 m thick that peel away from the block of tissue These shavings are placed on a grid and a beam of electrons is directed through the grid with the TEM At the high magnification an electron microscope membrane liquid between pits pores pull apart membrane provides resolution is good enough to reveal the double layers of a Flase color can be added to the micrograph to enhance detail Freeze fracturing a specimen in another way to visualize the inside of the membrane The tissue is embedded in a medium and quick frozen with nitrogen The frozen tissue is then tapped with a knife causing a crack the phospholipid layers of membranes proteins carbohydrates channels or any other structure affiliated with the membrane will whole usually and stick with one or the other side of the split 5 2 Phospholipids The Membrane s Foundation Like the fat molecules triglycerides glycerol phospholipids have a backbone derived from the three carbon polyalcohol glycerol Attached to this backbone are one to three fatty acids long chains of carbon atoms ending in a