Chapter 3
Cells: The Living Units
77
3
proteins in general is that they play a significant role in all forms
of endocytosis.
Exocytosis
Vesicular transport processes that eject substances
from the cell interior into the extracellular fluid are called
exo-
cytosis
(ek
0
so-si-to
9
sis; “out of the cell”). Typically stimulated
by a cell-surface signal such as binding of a hormone to a mem-
brane receptor or a change in membrane voltage, exocytosis ac-
counts for hormone secretion, neurotransmitter release, mucus
secretion, and in some cases, ejection of wastes. ±e substance
to be removed from the cell is first enclosed in a protein-coated
membranous sac called a
secretory vesicle
. In most cases, the
vesicle migrates to the plasma membrane, fuses with it, and then
ruptures, spilling the sac contents out of the cell
(Figure 3.14)
.
1
The membrane-
bound vesicle migrates
to the plasma
membrane.
2
There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins).
3
The vesicle and
plasma membrane fuse
and a pore opens up.
4
Vesicle contents are
released to the cell
exterior.
(a) The process of
exocytosis
(b) Photomicrograph
of
a secretory
vesicle releasing
its contents
by exocytosis
(100,000
m
)
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
Secretory
vesicle
Vesicle SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
Fusion pore formed
Phagosome
Receptors
Vesicle
Vesicle
(a) Phagocytosis
The cell engulfs a large
particle by forming pro-
jecting pseudopods (”false
feet”) around it and en-
closing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be protein-
coated but has receptors
capable of binding to
microorganisms or solid
particles.
(b) Pinocytosis
The cell “gulps” a drop of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Figure 3.13
Comparison of three types of endocytosis.
Figure 3.14
Exocytosis.
