Organization of the Body
Tis binding causes the membranes to “corkscrew” together
and fuse, rearranging the lipid monolayers without mixing
them (Figure 3.14a). As described, membrane material added
by exocytosis is removed by endocytosis—the reverse process.
summarizes active membrane transport processes.
Exocytosis, like other mechanisms in which vesicles are
targeted to their destinations, involves a “docking” process in
which transmembrane proteins on the vesicles, fancifully called
for vesicle), recognize certain plasma membrane
proteins, called t-SNAREs (
for target), and bind with them.
Active Membrane Transport Processes
Primary active transport
Transport of substances against a concentration
(or electrochemical) gradient. Performed across
the plasma membrane by a solute pump, directly
using energy of ATP hydrolysis.
Secondary active transport
Cotransport (coupled transport) of two solutes
across the membrane. Energy is supplied
indirectly by the ion gradient created by
primary active transport. Symporters move the
transported substances in the same direction;
antiporters move transported substances in
opposite directions across the membrane.
Movement of polar or charged
solutes, e.g., amino acids (into
cell by symporters); Ca
of cells via antiporters)
“Cell eating”: A large external particle (proteins,
bacteria, dead cell debris) is surrounded by a
“seizing foot” and becomes enclosed in a vesicle
In the human body, occurs
primarily in protective
phagocytes (some white blood
cells and macrophages)
Plasma membrane sinks beneath an external
ﬂuid droplet containing small solutes. Membrane
edges fuse, forming a ﬂuid-ﬁlled vesicle.
Occurs in most cells; important
for taking in dissolved solutes
by absorptive cells of the kidney
Selective endocytosis and transcytosis. External
substance binds to membrane receptors.
Means of intake of some
hormones, cholesterol, iron,
and most macromolecules
Selective endocytosis (and transcytosis). External
substance binds to membrane receptors (often
associated with lipid rafts).
Roles not fully known; proposed
roles include cholesterol
regulation and trafﬁcking, and
platforms for signal transduction
Vesicles pinch off from organelles and travel to
other organelles to deliver their cargo.
Accounts for nearly all
intracellular trafﬁcking between
certain organelles (endoplasmic
reticulum and Golgi apparatus).
Exceptions include vesicles
budding from the trans face of
the Golgi apparatus, which are
Secretion or ejection of substances from a cell.
The substance is enclosed in a membranous
vesicle, which fuses with the plasma membrane
and ruptures, releasing the substance to the
Secretion of neurotransmitters,
hormones, mucus, etc.; ejection
of cell wastes