FIGURE 2.10 • Phagocytosis. a. This drawing shows the steps in the phagocytosis of a large particle, such as a bacterium that has been killed as a result of an immune response. The bacterium is surrounded by antibodies attached to the bacterial surface antigens. Fc receptors on the surface of the plasma membrane of the phagocytotic cells recognize the Fc portion of the antibodies. This interaction triggers rearrangement of actin cytoskeleton. Depolymerizations and repolymerizations of actin flaments produce temporary projections of the plasma membrane called pseudopodia. They surround phagocytosed particle and lead to the formation of phagosomes. By targeted delivery of lysosomal enzymes, a phagosome matures into a lysosome that digests its phagocytosed content. b. Nonbiologic materials such as inhaled carbon particles, inorganic dusts, and asbestos fbers, as well as cellular debris resulting from infammation, are internalized without involvement of antibodies and Fc receptors. These particles are bound to multiple receptors on the plasma membrane.
regions’ appearance in the electron microscope (EM) as an accumulation of electron-dense material that represents aggregation of clathrin molecules on the cytoplasmic surface of the plasma membrane. Cargo receptors recognize and bind to specific molecules that come in contact with the plasma membrane. Clathrin molecules then assemble into a basketlike cage that helps change the shape of the plasma membrane into a vesiclelike invagination (Fig. 2.11b). Clathrin interacts with the cargo receptor via another coating-protein complex, adaptin, which is instrumental in selecting appropriate cargo molecules for transport into the cells. Thus, selected cargo proteins and their receptors are pulled from the extracellular space into the lumen of a forming vesicle. The large (100kilodalton) mechanoenzyme GTPase called dynamin mediates the liberation of forming clathrin-coated vesicles from the plasma membrane during receptor-mediated endocytosis. The type of vesicle formed as a result of receptor-mediated endocytosis is referred to as a coated vesicle, and the process itself is known as clathrindependent endocytosis. Clathrin-coated vesicles are also involved in the movement of the cargo material from the plasma membrane to early endosomes and from the Golgi apparatus to the early and late endosomes.
Exocytosis is the process by which a vesicle moves from the cytoplasm to the plasma membrane, where it discharges its contents to the extracellular space.
A variety of molecules produced by the cell for export are initially delivered from the site of their formation to the Golgi apparatus. The next step involves sorting and packaging the secretory product into transport vesicles that are  destined to fuse with the plasma membrane in a process known as exocytosis. Intracellular traffic of these vesicles is achieved by the presence of specific proteins on their surface (coatomers such as COP-I and COP-II) that mediate their movements (see page 48). The molecules that travel this route are often chemically modified (e.g., glycosylated, sulfated) as they pass through different cellular compartments. The membrane that is added to the plasma membrane by exocytosis is recovered into the cytoplasmic compartment by an endocytotic process. There are two general pathways of exocytosis:  In the constitutive pathway, substances designated for export are continuously delivered in transport vesicles to the plasma membrane. Proteins that leave the cell by this process are secreted immediately after their synthesis and exit from the Golgi apparatus, as seen in the secretion of