FIGURE 2.5 • Freeze fracture examination of the plasma membrane. a. View of the plasma membrane seen on edge, with arrow indicating the preferential plane of splitting of the lipid bilayer through the hydrophobic portion of the membrane. When the membrane splits, some proteins are carried with the outer leaflet, though most are retained within the inner leaflet. b. View of the plasma membrane with the leaflets separating along the cleavage plane. The surfaces of the cleaved membrane are coated, forming replicas; the replicas are separated from the tissue and examined with the TEM. Proteins appear as bumps. The replica of the inner leaflet is called the P-face; it is backed by cytoplasm (protoplasm). A view of the outer leaflet is called the E-face; it is backed by extracellular space. c. Electron micrograph of a freeze fracture replica shows the E-face of the membrane of one epithelial cell and the P-face of the membrane of the adjoining cell. The cleavage plane has jumped from the membrane of one cell to the membrane of the other cell, as indicated by the clear space (intercellular space) across the middle of the figure. Note the paucity of particles in the E-face compared with the P-face, from which the majority of the integral membrane proteins project. (Courtesy of Dr. Giuseppina d’Elia Raviola.)  Channels allow the passage of small ions, molecules, and include the family of integrins that link cytoplasmic actin water across the plasma membrane in either direction (i.e., filaments to an extracellular matrix protein (fibronectin). passive diffusion). Gap junctions formed by aligned chan- Enzymes have a variety of roles. ATPases have specific nels in the membranes of adjacent cells permit passage of roles in ion pumping: ATP synthase is the major protein of ions and small molecules from the cytoplasm of one cell to the inner mitochondrial membrane, and digestive enzymes the cytoplasm of the adjacent cells. such as disaccharidases and dipeptidases are integral mem  Receptor proteins allow recognition and localized brane proteins. binding of ligands (molecules that bind to the extracellu- Structural proteins are visualized by the freeze fracture lar surface of the plasma membrane) in processes such as method, especially where they form junctions with neigh-hormonal stimulation, coated-vesicle endocytosis, and boring cells. Often, certain proteins and lipids are concenantibody reactions. trated in localized regions of the plasma membrane to  Linker proteins anchor the intracellular cytoskeleton to carry out specific functions. Examples of such regions can the extracellular matrix. Examples of linker proteins be recognized in polarized cells such as epithelial cells.
FIGURE 2.6 • Different functions of integral membrane proteins. The six major categories of integral membrane proteins are shown in this diagram: pumps, channels, receptors, linkers, enzymes, and structural proteins. These categories are not mutually exclu-sive. A structural membrane protein involved in cell-to-cell junctions might simultaneously serve as a receptor, enzyme, linker, or a com-bination of these functions. pumps channels receptors linkers enzymes structural proteins actin NaHK
Integral membrane proteins move within the lipid bilayer of the membrane.
Particles bound to the membrane can move on the surface of a cell; even integral membrane proteins, such as enzymes, may move from one cell surface to another (e.g., from apical to lateral) when barriers to ﬂow, such as cell junctions, are disrupted. The ﬂuidity of the membrane is a function of the types of phospholipids in the membrane and variations in their local concentrations.