Endocytosis

Endocytosis is a process in which the plasma membrane furrows inward, instead of extending outward with pseudopods. One form of endocytosis, pinocytosis, is a nonspecific process performed by many cells. The plasma membrane invaginates to produce a deep, narrow furrow. The membrane near the surface of this furrow then fuses, and a small vesicle containing the extracellular fluid is pinched off and enters the cell. Pinocytosis allows a cell to engulf large molecules such as proteins, as well as any other molecules that may be present in the extracellular fluid.

Another type of endocytosis involves a smaller area of plasma membrane, and it occurs only in response to specific molecules in

Extracellular environment

Plasma membrane (pit forming)

Extracellular environment

Endocytosi Plasma Membrane Step
(1)
Endocytosi Plasma Membrane Step

Membrane pouching inward

Membrane pouching inward

Extracellular environment

Cytoplasm

Vesicle forming

Extracellular environment

Cytoplasm

Vesicle forming

Vesicle within cell

Vesicle within cell

■ Figure 3.4 Receptor-mediated endocytosis. In stages 1 through 4 shown here, specific bonding of extracellular particles with membrane receptor proteins results in the formation of endocytotic vesicles.

the extracellular environment. Since the extracellular molecules must bind to very specific receptor proteins in the plasma membrane, this process is known as receptor-mediated endocytosis.

In receptor-mediated endocytosis, the interaction of specific molecules in the extracellular fluid with specific membrane receptor proteins causes the membrane to invaginate, fuse, and pinch off to form a vesicle (fig. 3.4). Vesicles formed in this way contain extracellular fluid and molecules that could not have passed by other means into the cell. Cholesterol attached to specific proteins, for example, is taken up into artery cells by receptor-mediated endo-cytosis. This is in part responsible for atherosclerosis, as described in chapter 13. Hepatitis, polio, and AIDS viruses also exploit the process of receptor-mediated endocytosis to invade cells.

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Essentials of Human Physiology

Essentials of Human Physiology

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