Concept 2 Review

Phagocytosis and Pinocytosis

In endocytosis, membranes invaginate, or pinch in, to form a vesicle, moving the enclosed materials inside the cell. This process can take different forms, each involving its own specific cell machinery:

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In phagocytosis, or "cell eating," the cell engulfs debris, bacteria, or other sizable objects. Phagocytosis occurs in specialized cells called phagocytes, which include macrophages, neutrophils, and other white blood cells. Invagination produces a vesicle called a phagosome, which usually fuses with one or more lysosomes containing hydrolytic enzymes. Materials in the phagosome are broken down by these enzymes and degraded.

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In pinocytosis, or "cell drinking," the cell engulfs extracellular fluid, including molecules such as sugars and proteins. These materials enter the cell inside a vesicle, although they do not mix with cytoplasm. Epithelial cells in capillaries use pinocytosis to engulf the liquid portion of blood at the capillary surface. The resulting vesicles travel across the capillary cells and release their contents to surrounding tissues, while blood cells remain in the blood.