FIGURE 2.3 • Diagram of a plasma membrane showing the modifed fuid–mosaic model. The plasma membrane is a lipid bilayer consisting primarily of phospholipid molecules, cholesterol, and protein molecules. The hydrophobic fatty-acid chains of phospholipids face each other to form the inner portion of the membrane, whereas the hydrophilic polar heads of the phospholipids form the extracellular and intracellular surfaces of the membrane. Cholesterol molecules are incorporated within the gaps between phospholipids equally on both sides of the membrane. Note the elevated area of the lipid raft that is characterized by the high concentration of glycosphingolipids and cholesterol. It contains large numbers of integral and peripheral membrane proteins. The raft protrudes above the level of asymmetrically distributed phospholipids in the membrane bilayer (indicated by the different colors of the phospholipid heads). Carbohydrate chains attach to both integral and peripheral membrane proteins to form glycoproteins, as well as to polar phospholipid heads to form glycolipids.
enzymes, and structural proteins (Fig. 2.6). The categories are  Pumps serve to transport certain ions, such as Na , actively not mutually exclusive (e.g., a structural membrane protein across membranes. Pumps also transport metabolic precursors may simultaneously serve as a receptor, an enzyme, a pump, or of macromolecules, such as amino acids and sugars, across any combination of these functions). membranes, either by themselves or linked to the Na pump.
FIGURE 2.4 • An image of lipid rafts obtained with tapping-mode atomic force microscopy (AFM). This image shows a 5-nm-thick lipid bilayer spread on the mica support. The bilayer is composed of dioleoylphosphatidylcholine (dioleoyl-PC), sphingomyelin, and cholesterol. Sphingomyelin and cholesterol together form lipid rafts represented on the image by the pink areas; the blue-purple areas are the nonraft background of the bilayer. Because the sphingomyelin molecules are longer than the dioleoyl-PC molecules, the rafts protrude from the nonraft background by about 0.8 nm, and the AFM is sensitive enough to detect this protrusion. The black regions represent the mica support. The image also shows molecules of the Helicobacter pylori toxin VacA (white particles), which preferentially bind to protein receptors on the raft domains. The area depicted in this image is 800 nm square. (Courtesy of Drs. Nicholas A. Geisse, Timothy L. Cover, Robert M. Henderson, and J. Michael Edwardson.) inner leaflet of lipid layer