98
UNIT ONE
gather. The M cell binds and takes in a bacterium from the
intestinal side by endocytosis, then transports it through the
cell to the side that faces the immune system cells, where
it is released by exocytosis. The immune system cells bind
parts of the bacterium, and, if they recognize surface fea-
tures of a pathogen, they signal other cells to mature into
antibody-producing cells. The antibodies are then secreted
into the bloodstream and travel back to the small intestine,
where they destroy the infecting bacteria.
HIV, the virus that causes AIDS, uses transcytosis to
cross lining (epithelial) cells such as in the anus, mouth, and
female reproductive tract (F g. 3.34). The virus enters white
blood cells in mucous secretions, and the secretions then
carry the infected cells to an epithelial barrier. Near these
lining cells, viruses rapidly exit the infected white blood cells
and are quickly enveloped by the lining cell membranes in
receptor-mediated endocytosis. HIV particles are ferried, in
vesicles, through the lining cell, without infecting (taking
over) the cell, to exit from the cell membrane on the other
side of the cell. After transcytosis, the HIV particles enter
white blood cells beyond the epithelial barrier. Infection
begins.
Table 3.3
summarizes the types of movement into
and out of the cell, including transcytosis.
PRACTICE
21
How does a cell maintain unequal concentrations of ions on
opposite sides of a cell membrane?
22
How are facilitated diF
usion and active transport similar? How are
they diF
erent?
23
What is the diF
erence between pinocytosis and phagocytosis?
24
Describe receptor-mediated endocytosis.
25
What does transcytosis accomplish?
Receptor-mediated endocytosis is particularly impor-
tant because it allows cells with the appropriate receptors to
remove and process speciF c types of substances from their
surroundings, even when these substances are present in
very low concentrations. In short, receptor-mediated endo-
cytosis provides speciF
city (F g. 3.32).
Exocytosis
Exocytosis
(ex-o-si-to
sis) is essentially the reverse of endo-
cytosis. Substances made in the cell are packaged into a ves-
icle, which then fuses with the cell membrane, releasing its
contents outside the cell. Cells secrete some proteins by this
process. Nerve cells use exocytosis to release the neurotrans-
mitter chemicals that signal other nerve cells, muscle cells,
or glands
(f g. 3.33)
.
Transcytosis
Endocytosis brings a substance into a cell, and exocyto-
sis transports a substance out of a cell. Another process,
trans cytosis
(tranz-si-to
sis), combines endocytosis and
exocytosis to selectively and rapidly transport a substance
or particle from one end of a cell to the other
(f
g. 3.34)
.
Transcytosis moves substances across barriers formed by
tightly connected cells. The process occurs in normal physi-
ology and in disease.
Transcytosis enables the healthy immune system to
monitor pathogens in the small intestine, protecting against
some forms of food poisoning. Scattered among the small
intestinal epithelial cells are rare M cells, so-named because
the cell side that faces into the intestine has microfolds that
maximize surface area. The other side of the M cell appears
punched in, forming a pocket where immune system cells
Receptor
protein
Cell
membrane
Cytoplasm
Molecules
outside cell
Cell
membrane
indenting
Vesicle
(a)
(b)
(c)
(d)
Receptor-ligand
combination
FIGURE 3.32
Receptor-mediated endocytosis. (
a, b
) A speci±
c molecule binds to a receptor protein, forming a receptor-ligand combination.
(
c
) The binding of the ligand to the receptor protein stimulates the cell membrane to indent. (
d
) Continued indentation forms a vesicle, which
transports the molecule into the cytoplasm.
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