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So far, we have considered processes that move one or a few ions or molecules at a time through the plasma membrane. Vesicular transport processes, by contrast, move large particles, droplets of fluid, or numerous molecules at once through the membrane, contained in bubble-like vesicles of membrane.
Vesicular processes that bring matter into a cell are called endocytosis and those that release material from a cell are called exocytosis. These processes employ motor proteins whose movements are energized by ATP.
There are three forms of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis, 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.
For example, neutrophils (a class of white blood cells) protect the body from infection by phagocytizing and killing bacteria. A neutrophil spends most of its life crawling about in the connective tissues by means of its pseudopods. When a neutrophil encounters a bacterium, it surrounds it with pseudopods and traps it in a vesicle called a phagosome.
A ----- is a vesicle in the cytoplasm surrounded by a membrane. A lysosome merges with the phagosome, converting it to a phagolysosome, and contributes enzymes that destroy the invader. phagosome
In general, phagocytosis is a way of keeping the tissues free of debris and infectious microbes. Some cells called macrophages (literally, “big eaters”) phagocytize the equivalent of 25% of their own volume per hour.
Pinocytosis, 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 (pinocytosis) process begins as the plasma membrane dimples, or caves 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 happen to be there.
Receptor-mediated endocytosis (fig. 3.21) is a more selective form of either phagocytosis or pinocytosis. It enables a cell to take in specific molecules from the ECF with a minimum of unnecessary matter.
Particles in the ECF bind to specific receptors on the plasma membrane. The receptors then cluster and the membrane sinks in at this point, creating a pit coated with a peripheral membrane protein called clathrin. 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. 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.
Endothelial cells also imbibe insulin by receptor-mediated endocytosis. Insulin is too large to pass through channels in the plasma membrane, yet it must somehow get out of the blood and reach the surrounding cells if it is to have any effect.
Endothelial cells take up insulin by receptor-mediated endocytosis, transport the vesicles across the cell, and release the insulin on the other side, where tissue cells await it. Such transport of material across a cell (capture on one side and release on the other) is called transcytosis (fig. 3.22). This process is especially active in muscle capillaries and transfers a significant amount of blood albumin into the tissue fluid.
Why isn’t transcytosis listed as a separate means of membrane transport, in addition to pinocytosis and the others? Answer: Transcytosis is a combination of endocytosis and exocytosis.
Receptor-mediated endocytosis isn’t always to our benefit; hepatitis, polio, and AIDS viruses trick our cells into engulfing them by receptor-mediated endocytosis, thus exploiting this mechanism to establish infection.
Exocytosis is a process of discharging material from a cell. It occurs, for example, when endothelial cells release insulin to the tissue fluid, sperm cells release enzymes for penetrating an egg, mammary gland cells secrete milk sugar, and other gland cells release hormones. It
Exocytosis It bears a superficial resemblance to endocytosis in reverse.
Exocytosis A secretory vesicle in the cell migrates to the surface and “docks” on peripheral proteins of the plasma membrane.
Exocytosis These proteins pull the membrane inward and create a dimple that eventually fuses with the vesicle and allows it to release its contents.
The question might occur to you, If endocytosis continually takes away bits of plasma membrane to form intracellular vesicles, why doesn’t the membrane grow smaller and smaller? Another purpose of exocytosis, however, is to replace plasma membrane that has been removed by endocytosis or that has become damaged or worn out.
Empty vesicles are transported to the inner surface of the plasma membrane and fuse with it. Exocytosis must keep pace with the rate of membrane removal by endocytosis, or else a cell would shrivel and die. Plasma membrane is continually recycled from the cell surface into the cytoplasm and back to the surface.
Transport Without Carriers Filtration, Simple diffusion, Osmosis
Filtration Movement of water and solutes through a selectively permeable membrane as a result of hydrostatic pressure.
Simple diffusion Diffusion of particles through water or air or through a living or artificial membrane, down their concentration gradient, without the aid of membrane carriers.
Osmosis Net flow of water through a selectively permeable membrane, driven by either a difference in solute concentration or a mechanical force.
Created by: Russells3709
 

 



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