Endocytosis is a
general term for a group of processes that bring
macromolecules, large particles, small
molecules, and even small cells into the
eukaryotic cell. There are three types of
endocytosis: phagocytosis, pinocytosis, and
receptor-mediated endocytosis. In all three, the
plasma membrane invaginates (folds inward)
around materials from the environment, forming a
small pocket. The pocket deepens, forming a
vesicle. This vesicle separates from the plasma
membrane and migrates with its contents to the
cell’s interior.
Exocytosis is the
process by which materials packaged in vesicles
are secreted from a cell when the vesicle
membrane fuses with the plasma membrane. The
initial event in this process is the binding of
a membrane protein protruding from the
cytoplasmic side of the vesicle with a membrane
protein on the cytoplasmic side of the target
site on the plasma membrane. The phospholipid
regions of the two membranes merge, and an
opening to the outside of the cell develops. The
contents of the vesicle are released to the
environment, and the vesicle membrane is
smoothly incorporated into the plasma membrane.
