Phagocytosis

Phagocytosis literally means "cell eating" and can be compared to pinocytosis, which means "cell drinking." Certain cell types, such as macrophages and polymorphonuclear leukocytes, are specialized for incorporating and removing foreign bacteria, protozoa, fungi, damaged cells, and unneeded extracellular constituents. For example, after a bacterium becomes bound to the surface of a macrophage, cytoplasmic processes of the macrophage are extended and ultimately surround the bacterium. The edges of these processes fuse, enclosing the bacterium in an intracellular phagosome.

Exocytosis is the term used to describe the fusion of a membrane-limited structure with the plasma membrane, resulting in the release of its contents into the extracellular space without compromising the integrity of the plasma membrane. A typical example is the release of stored products from secretory cells, such as those of the exocrine pancreas and the salivary glands (Figure 4–27). The fusion of membranes in exocytosis is a complex process. Because cell membranes exhibit a high density of negative charges (phosphate residues of the phospholipids), membrane-covered structures coming close to each other will not fuse but will rather repel each other, unless specific interactions facilitate the fusion process. Consequently, exocytosis is mediated by a number of specific fusogenic proteins. Usually, Ca²⁺ regulates the process. An increase in cytosolic Ca²⁺ often triggers exocytosis.

During endocytosis, portions of the cell membrane become an endocytotic vesicle; during exocytosis, the membrane is returned to the cell surface. This phenomenon is called membrane trafficking (Figures 2–7 and 2–8). In several systems, membranes are conserved and reused several times during repeated cycles of endocytosis.

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Figure 2-7 & Figure 2-8

 

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