Most of the movement of molecules and ions between the intra-cellular and extracellular compartments involves passage through the plasma membrane (see chapter 6). However, the plasma membrane also participates in the bulk transport of larger portions of the extracellular environment. Bulk transport includes the processes of phagocytosis and endocytosis.
Some body cells—including certain white blood cells and macrophages in connective tissues—are able to move in the manner of an amoeba (a single-celled organism). They perform this amoeboid movement by extending parts of their cytoplasm to form pseudopods, which attach to a substrate and pull the cell along. This process depends on the bonding of membrane-spanning proteins called integrins with proteins outside the membrane in the extracellular matrix (generally, an extracellular gel of proteins and carbohydrates).
Cells that exhibit amoeboid motion—as well as certain liver cells, which are not mobile—use pseudopods to surround and engulf particles of organic matter (such as bacteria). This process is a type of cellular "eating" called phagocytosis. It serves to protect the body from invading microorganisms and to remove extracellular debris.
Phagocytic cells surround their victim with pseudopods, which join together and fuse (fig. 3.3). After the inner membrane of the pseudopods has become a continuous membrane surrounding the ingested particle, it pinches off from the plasma membrane. The ingested particle is now contained in an organelle called a food vacuole within the cell. The food vacuole will subsequently fuse with an organelle called a lysosome (described later), and the particle will be digested by lysosomal enzymes.
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