FIGURE 7-1 Fluid mosaic model of biomembranes. A bilayer of phospholipids about 3 nm thick provides the basic architecture of all cellular membranes; membrane proteins give each cellular membrane its unique set of functions. Individual phospholipids can move laterally and spin within the plane of the membrane, giving the membrane a fluidlike consistency similar to that of olive oil. Noncovalent interactions between phospholipids, and between phospholipids and proteins, lend strength and resilience to the membrane, while the hydrophobic core of the bilayer prevents the unassisted movement of water-soluble substances from one side to the other. Integral membrane proteins (transmembrane proteins) span the bilayer and often form dimers and higher-order oligomers. Lipid-anchored proteins are tethered to one leaflet by a covalently attached hydrocarbon chain. Peripheral proteins associate with the membrane primarily by specific noncovalent interactions with integral membrane proteins or membrane lipids. Proteins in the plasma membrane also make extensive contact with the cytoskeleton. See D. Engelman, 2005, Nature 438:578–580.