Several varieties of endocytosis can be identified. When the endocytotic vesicle simply encloses a small volume of extracellular fluid, as described above, the process is known as fluid endocytosis. In other cases, certain molecules in the extracellular fluid bind to specific proteins on the outer surface of the plasma membrane and are carried into the cell along with the extracellular fluid when the membrane invaginates. This is known as adsorptive endocytosis. In addition to taking in trapped extracellular fluid, adsorptive endocy-tosis leads to a selective concentration in the vesicle of the material bound to the membrane. Both fluid and adsorptive endocytosis are often referred to as pinocy-tosis (cell drinking). A third type of endocytosis occurs when large particles, such as bacteria and debris from damaged tissues, are engulfed by cells. In this form of endocytosis, known as phagocytosis (cell eating), the membrane folds around the surface of the particle so that little extracellular fluid is enclosed within the vesicle. While most cells undergo pinocytosis, only a few special cells carry out phagocytosis (Chapter 20).
Movement of Molecules Across Cell Membranes CHAPTER SIX
Endocytosis of any kind requires metabolic energy and is associated with the binding of specific "coating" proteins that form a shell around the newly forming vesicle on its cytoplasmic surface. After the vesicle separates from the plasma membrane, the coating proteins that helped to form it are removed, and the vesicle membrane now fuses with the membranes of intracel-lular organelles, adding the contents of the vesicle to the lumen of that organelle. The passage of material from one membrane-bound organelle to another involves the formation of vesicles from one organelle and the fusion with the second. These processes of intra-cellular budding and fusion are similar to endo- and exocytotic events occurring at the plasma membrane and involve some of the same proteins to mediate vesicle formation and fusion with other membranes.
What is the fate of most endocytotic vesicles once they enter the cell? After separating from the plasma membrane, they fuse with a series of vesicles and tubular elements known as endosomes, which lie between the plasma membrane and the Golgi apparatus (Figure 6-22). Like the Golgi apparatus, the endo-somes perform a sorting function, distributing the contents of the vesicle and its membrane to various locations. Most of the contents of endocytotic vesicles are passed from the endosomes to lysosomes, organelles that contain digestive enzymes that break down large molecules such as proteins, polysaccharides, and nucleic acids. The fusion of endosomal vesicles with the lysosomal membrane exposes the contents of the vesicle to these digestive enzymes. The phagocytosis of bacteria and their destruction by the lysosomal digestive enzymes is one of the body's major defense mechanisms against germs (Chapter 20).
Some endocytotic vesicles pass through the cytoplasm and fuse with the plasma membrane on the opposite side of the cell, releasing their contents to the extracellular space. This provides a pathway for the transfer of large molecules, such as proteins, across the epithelial cells that separate two compartments (for example, blood and interstitial fluid). A similar process allows small amounts of protein to be moved across the intestinal epithelium.
Each episode of endocytosis removes a small portion of the membrane from the cell surface. In cells that have a great deal of endocytotic activity, more than 100 percent of the plasma membrane may be internalized in an hour, yet the membrane surface area remains constant. This is because the membrane is replaced at about the same rate by vesicle membrane that fuses with the plasma membrane during exocyto-sis. Some of the plasma-membrane proteins taken into the cell during endocytosis are stored in the membranes of endosomes, and upon receiving the appropriate signal can be returned to the plasma membrane
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.