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specific particles in the extracellular fluidoBinding of particles to receptors concentrates the particles to areas where endocytosis will occuroArea of plasma membrane that forms the vesicle is coated with proteins, called clathrin, on its cytosolic surfaceoMembrane indents in this area, forming what is called a coated pitoThe coated pit becomes a coated vesicle containing the receptors and the particles bound to themoThe protein coat rapidly leaves the vesicle, and the clathrin molecules are recycledoNow uncoated vesicle fuses with a lysosome, forming an endolysosomeoEnzymes in the lysosome will degrade the particles brought into the celloReceptors are often recycled by exocytosisTransport of Molecules Out of Cells by ExocytosisExocytosis is basically endocytosis in reverse: A vesicle inside the cell fuses with the plasma membrane and releases its contents into the extracellular fluidoInvolves complex interactions between calcium and proteins, the process of which will be described for neuronsoExocytosis 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 fluidThe first two functions are related in that both add components to the plasma membrane. oDuring exocytosis, whatever components are present in the vesicle membrane will be added to the plasma membraneoA cell can add certain proteins, phospholipids, or carbohydrates to the plasma membrane, or it can replace the membrane that is lost during endocytosis
oIn fact, endocytosis and exocytosis must be balanced in a cell; otherwise, the sizeof the plasma membrane will changeThe third function of exocytosis, the secretion of materials, serves a number of functionsoCertain white blood cells secrete antibodies to fight infectionsoMost cells—in particular, neurons and endocrine cells—secrete chemical messengers that communicate with other cellsoCells lining certain hollow ducts or passageways, such as the gastrointestinal tractor respiratory airways, secrete a sticky fluid called mucus, which acts as a protective coating4.7 Epithelial Transport: Movement of Molecules Across Two MembranesTransport 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 polarizedEpithelial StructureIn 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 cavityoMembrane on this side is called the apical membraneThe membrane on the opposite side faces the internal environment and is in contact with interstitial fluid, which exchanges materials with the bloodoThis blood-facing membrane or basolateral membrane rests on a basement membrane consisting of noncellular material that is relatively permeable to most substanceso
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Electric charge, Cell Membrane Transport, Ion channel