specific particles in the extracellular fluid o Binding of particles to

Specific particles in the extracellular fluid o

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specific particles in the extracellular fluid o Binding of particles to receptors concentrates the particles to areas where endocytosis will occur o Area of plasma membrane that forms the vesicle is coated with proteins, called clathrin, on its cytosolic surface o Membrane indents in this area, forming what is called a coated pit o The coated pit becomes a coated vesicle containing the receptors and the particles bound to them o The protein coat rapidly leaves the vesicle, and the clathrin molecules are recycled o Now uncoated vesicle fuses with a lysosome, forming an endolysosome o Enzymes in the lysosome will degrade the particles brought into the cell o Receptors are often recycled by exocytosis Transport of Molecules Out of Cells by Exocytosis Exocytosis is basically endocytosis in reverse: A vesicle inside the cell fuses with the plasma membrane and releases its contents into the extracellular fluid o Involves complex interactions between calcium and proteins, the process of which will be described for neurons o Exocytosis has three functions: to add components to the plasma membrane, to recycle receptors removed from the plasma membrane by endocytosis, and to secrete specific substances out of the cell and into the extracellular fluid The first two functions are related in that both add components to the plasma membrane. o During exocytosis, whatever components are present in the vesicle membrane will be added to the plasma membrane o A cell can add certain proteins, phospholipids, or carbohydrates to the plasma membrane, or it can replace the membrane that is lost during endocytosis
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o In fact, endocytosis and exocytosis must be balanced in a cell; otherwise, the size of the plasma membrane will change The third function of exocytosis, the secretion of materials, serves a number of functions o Certain white blood cells secrete antibodies to fight infections o Most cells—in particular, neurons and endocrine cells—secrete chemical messengers that communicate with other cells o Cells lining certain hollow ducts or passageways, such as the gastrointestinal tract or respiratory airways, secrete a sticky fluid called mucus, which acts as a protective coating 4.7 Epithelial Transport: Movement of Molecules Across Two Membranes Transport materials into the internal environment from outside (a process called absorption) or from the internal environment to the outside (secretion) Membranes on the two sides are distinctly different (in both structure and function), epithelial cells are said to be polarized Epithelial Structure In an epithelial cell layer of the type that is specialized for absorption or secretion, one side of an epithelial cell faces the lumen of a body cavity o Membrane on this side is called the apical membrane The membrane on the opposite side faces the internal environment and is in contact with interstitial fluid, which exchanges materials with the blood o This blood-facing membrane or basolateral membrane rests on a basement membrane consisting of noncellular material that is relatively permeable to most substances o
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  • Fall '08
  • BRINN
  • Electric charge, Cell Membrane Transport, Ion channel

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