tosis (en'-do-si-TO-sis;
endo-
= into) involves ingesting
material by forming a vesicle from the plasma mem-
brane. The sac of ingested material buds off inside the
cell and usually fuses with lysosomes. There are three
types of endocytosis:
Phagocytosis (fag'-o-si-TO-sis;
phago-
= eat). The
cell “eats” large particles such as bacteria, viruses,
and
dead
cells.
The
plasma
membrane
forms
projections
called
pseudopods
(SOO-do-pods)
that enclose the material to form a vesicle called a
phagosome
(Figure
3.16). The phagosome later
fuses with lysosomes. White blood cells kill invading
bacteria by phagocytosis.
Pinocytosis
(pm'-o-si-TO-sis;
pino-
=
drink). The
cell periodically “drinks” by forming small vesicles
around droplets of extracellular fluid; these droplets
may have small particles dissolved in them as well.
These vesicles fuse with lysosomes and release their
contents.
Receptor-mediated endocytosis. When hormones
bind to receptors on the plasma
membrane,
the
hormone-
receptor
complex
is
often
ingested by endocytosis after
the hormone has produced its
effect. The patch of membrane
containing
the
hormone-
receptor
complex
pinches
off to form a vesicle, which
then merges with one or more
lysosomes. The complex gets
broken down; new receptors are made and packaged
into vesicles, which later merge again with the plasma
membrane. When cells are exposed to constant levels of
hormones or drugs, they may become less sensitive to
them. This desensitization can be explained by receptor-
mediated
endocytosis.
The
hormone-drug
receptor
complexes are taken inside the cell, thereby leaving
fewer receptors available to bind to the hormone or drug.
hormone
(HOR-mon)
A chemical message
by which one cell
produces an effect in
another cell.
receptor
A specific
molecule or cluster
of molecules that
recognize and bind a
hormone or drug.
P hag ocy to sis • Figure 3.16
Lysosome
Phagosome
Digestive
enzymes
Fusion of lysosome
and phagosome
Digestion by
lysosomal enzymes
Residual body
Phagocytosis is a type of endocytosis in which
large particles, such as large proteins, bacteria,
viruses, and dead cells, are brought inside a cell
and digested.
Pseudopods
Plasma membrane
Microbe
Receptor
Cells Carry Out Many Processes 63
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