Endocytosis brings a substance into a cell, and exocytosis transports a substance out of a cell. Another process, transcytosis (tranz-si-toossis), combines endocytosis and exocytosis (fig. 3.34). Transcytosis is the selective and rapid transport of a substance or particle from one end of a cell to the other. It enables substances to cross barriers formed by tightly connected cells.

HIV, the virus that causes AIDS, may initially infect a human body by using transcytosis to cross lining (epithelial) cells in the anus and female reproductive tract. Experiments using tissues growing in laboratory culture show that HIV enters white blood cells in mu cous secretions, and the secretions then carry the infected cells to an epithelial barrier. Near these lining cells, viruses rapidly exit the infected white blood cells and are quickly enveloped by the lining cell membranes in receptor-mediated endocytosis. HIV particles are ferried, in vesicles, through the lining cell, without actually infecting (taking over) the cell, to exit from the cell membrane at the other end of the cell—in as little as thirty minutes! After transcytosis, the HIV particles infect white blood cells beyond the epithelial barrier. Table 3.3 summarizes the types of movement into and out of the cell.

99 What type of mechanism maintains unequal concentrations of ions on opposite sides of a cell membrane?

^9 How are facilitated diffusion and active transport similar? How are they different?

^9 What is the difference between pinocytosis and phagocytosis?

□ Describe receptor-mediated endocytosis.

Q What does transcytosis accomplish?

Shier-Butler-Lewis: I I. Levels of Organization I 3. Cells I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

HIV - infected white blood cells

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