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.