Specific particles in the extracellular fluid o

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

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