78
UNIT 1
Organization of the Body
3
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.
Table 3.2
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
v-SNAREs (
v
for vesicle), recognize certain plasma membrane
proteins, called t-SNAREs (
t
for target), and bind with them.
Table 3.2
Active Membrane Transport Processes
PROCESS
ENERGY SOURCE
DESCRIPTION
EXAMPLES
Active Transport
Primary active transport
ATP
Transport of substances against a concentration
(or electrochemical) gradient. Performed across
the plasma membrane by a solute pump, directly
using energy of ATP hydrolysis.
Ions (Na
1
, K
1
, H
1
, Ca
2
1
, and
others)
Secondary active transport
Ion concentration
gradient maintained
with ATP
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
2
1
, H
1
(out
of cells via antiporters)
Vesicular Transport
Endocytosis
 
 
Via clathrin-coated
vesicles
 
 
 
Phagocytosis
ATP
“Cell eating”: A large external particle (proteins,
bacteria, dead cell debris) is surrounded by a
“seizing foot” and becomes enclosed in a vesicle
(phagosome).
In the human body, occurs
primarily in protective
phagocytes (some white blood
cells and macrophages)
Pinocytosis (fluid-phase
endocytosis)
ATP
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
and intestine
Receptor-mediated
endocytosis
ATP
Selective endocytosis and transcytosis. External
substance binds to membrane receptors.
Means of intake of some
hormones, cholesterol, iron,
and most macromolecules
Via caveolin-coated
vesicles (caveolae)
ATP
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
Vesicular trafficking
 
 
 
Via coatomer-coated
vesicles
ATP
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
clathrin-coated.
Exocytosis
ATP
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
exterior.
Secretion of neurotransmitters,
hormones, mucus, etc.; ejection
of cell wastes
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