Vesicular Transport

So far, we have considered processes that move from one to a few ions or molecules through the plasma membrane at a time. Vesicular transport processes, by contrast, move large particles, droplets of fluid, or numerous molecules at once through the membrane, contained in bubblelike vesicles of membrane. Vesicular processes that bring matter into a cell are called endocytosis23 (EN-doe-sy-TOE-sis) and those that release material from a cell are called exocyto-sis24 (EC-so-sy-TOE-sis).

There are two basic forms of endocytosis: phagocytosis and pinocytosis. Phagocytosis25 (FAG-oh-sy-TOE-sis), or "cell eating," is the process of engulfing particles such as bacteria, dust, and cellular debris—particles large enough to be seen with a microscope. Neutrophils (a class of white blood cells), for example, protect the body from infection by phagocytizing and killing bacteria. A neu-trophil spends most of its life crawling about in the con-

'3endo = into + cyt = cell + osis = process Aexo = out of + cyt = cell + osis = process '5phago = eating + cyt = cell + osis = process

Phagocytosis Digestion

Figure 3.21 Phagocytosis, Intracellular Digestion, and Exocytosis. (1) A phagocytic cell encounters a particle of foreign matter. (2) The cell surrounds the particle with its pseudopods. (3) The particle is phagocytized and becomes incorporated into a phagosome.

(4) A lysosome fuses with the phagosome and forms a phagolysosome.

(5) Enzymes from the lysosome digest the foreign matter. (6) The phagolysosome fuses with the plasma membrane. (7) The indigestible residue is voided by exocytosis.

Figure 3.21 Phagocytosis, Intracellular Digestion, and Exocytosis. (1) A phagocytic cell encounters a particle of foreign matter. (2) The cell surrounds the particle with its pseudopods. (3) The particle is phagocytized and becomes incorporated into a phagosome.

(4) A lysosome fuses with the phagosome and forms a phagolysosome.

(5) Enzymes from the lysosome digest the foreign matter. (6) The phagolysosome fuses with the plasma membrane. (7) The indigestible residue is voided by exocytosis.

nective tissues by means of blunt footlike extensions called pseudopods26 (SOO-doe-pods). When a neutrophil encounters a bacterium, it surrounds it with its pseudopods and traps it in a phagosome27—a vesicle in the cytoplasm surrounded by a unit membrane (fig. 3.21). A lysosome merges with the phagosome, converting it to a phagolysosome, and contributes enzymes that destroy the invader. Several other kinds of phagocytic cells are described in chapter 21. In general, phagocytosis is a way of keeping the tissues free of debris and infectious microorganisms. Some cells called macrophages (literally "big eaters") phagocytize the equivalent of 25% of their own volume per hour.

Pinocytosis28 (PIN-oh-sy-TOE-sis), or "cell drinking," is the process of taking in droplets of ECF containing molecules of some use to the cell. While phagocytosis occurs in only a few specialized cells, pinocytosis occurs in all human cells. The process begins as the plasma membrane becomes dimpled, or caved in, at points. These pits soon separate from the surface membrane and form small membrane-bounded pinocytotic vesicles in the cytoplasm. The vesicles contain droplets of the ECF with whatever molecules happened to be there.

Receptor-mediated endocytosis29 (fig. 3.22) is a more selective form of either phagocytosis or pinocyto-sis. It enables a cell to take in specific molecules from the ECF with a minimum of unnecessary fluid. Particles in the ECF bind to specific receptors on the plasma membrane. The receptors then cluster together and the membrane sinks in at this point, creating a pit coated with a peripheral membrane protein called clathrin.30 The pit soon pinches off to form a clathrin-coated vesicle in the cytoplasm. Clathrin may serve as an "address label" on the coated vesicle that directs it to an appropriate destination in the cell, or it may inform other structures in the cell what to do with the vesicle.

One example of receptor-mediated endocytosis is the uptake of low-density lipoproteins (LDLs)—protein-coated droplets of cholesterol and other lipids in the blood (see chapter 26). The thin endothelial cells that line our blood vessels have LDL receptors on their surfaces and absorb LDLs in clathrin-coated vesicles. Inside the cell, the LDL is freed from the vesicle and metabolized, and the membrane with its receptors is recycled to the cell surface. Much of what we know about receptor-mediated endocy-tosis comes from studies of a hereditary disease called familial hypercholesterolemia, which dramatically illustrates the significance of this process to our cardiovascular health (see insight 3.3).

pseudo = false + pod = foot '7phago = eating + some = body epino = drinking + cyt = cell + osis = process [9endo = into + cyt = cell + osis = process ,0clathr = lattice + in = protein

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 3 Cellular Form and Function 113

Chapter 3 Cellular Form and Function 113

Receptor Mediated Endocytosis

Figure 3.22 Receptor-Mediated Endocytosis. (1) Low-density lipoproteins (LDLs) are suspended in the blood. The endothelial cells of blood vessels have LDL receptor proteins in their plasma membranes. (2) LDLs bind to their receptors. (3) LDL-bearing receptors become clustered together. (4) The plasma membrane sinks inward at that point and forms a clathrin-coated pit. (5) The pit separates from the membrane and becomes a clathrin-coated vesicle containing concentrated LDLs. The electron micrographs show stages 3 to 5 of this process.

Figure 3.22 Receptor-Mediated Endocytosis. (1) Low-density lipoproteins (LDLs) are suspended in the blood. The endothelial cells of blood vessels have LDL receptor proteins in their plasma membranes. (2) LDLs bind to their receptors. (3) LDL-bearing receptors become clustered together. (4) The plasma membrane sinks inward at that point and forms a clathrin-coated pit. (5) The pit separates from the membrane and becomes a clathrin-coated vesicle containing concentrated LDLs. The electron micrographs show stages 3 to 5 of this process.

Insight 3.3 Clinical Application

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  • miranda
    What is a vesicular process that releases material from a cell?
    8 years ago

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