Biology Reference
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a major mechanism used by the immune system to remove pathogens and cell debris. In
fact very early studies of the immune system led Elie Metchnikoff to discover phagocytosis
in 1882. For this work Metchnikoff shared the 1908 Nobel Prize in Medicine with Paul
Ehrlich.
Exocytosis is the process by which cells excrete waste and other large molecules from the
cytoplasm to the cell exterior [55] and therefore is the opposite of endocytosis. Exocytosis
generates vesicles referred to as secretory or transport vesicles. In exocytosis intracellular
vesicles fuse with the plasma membrane and release their aqueous sequestered contents to
the outside at the same time that the vesicular membrane hydrophobic components (mostly
lipids and proteins) are added to the plasma membrane ( Figure 14.26 , [56] ). Steady state
composition of the plasma membrane results from a balance between endocytosis and exocy-
tosis. The resultant process of plasma membrane recycling is amazingly fast. For example,
pancreatic secretory cells recycle an amount of membrane equal to the whole surface of
the cell in ~90 min. Even faster are macrophages that can recycle the contents of the plasma
membrane in only 30 min.
Before approaching the plasma membrane for fusion, exocytosis vesicles have a prior life
that will not be considered here. The vesicles must first dock with the plasma membrane,
a process that keeps the two membranes separated at
10 nm. During docking, complex
molecular rearrangements occur to prepare the membranes for fusion. The process of vesicle
fusion and release of aqueous compartment components is driven by SNARE proteins (see
Chapter 10) [57] . Therefore by the process of exocytosis:
<
5
e
The surface of the plasma membrane increases by the size of the fused vesicular
membrane. This is particularly important if the cell is growing.
￿
Extracellular
fluid
Molecules to
be secreted
Cytoplasm
Secretory vesicle
FIGURE 14.26 Exocytosis. Intra-cellular secretory vesicles fuse with the plasma membrane releasing their
water-soluble contents to the outside and adding membrane material to the plasma membrane [56] .
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