III. MEMRBRAN TRANSPORT
Learning Objectives:
1. Principles of membrane transport;
2. Passive transport and active transport;
3. Two main classes of membrane transport proteins:
Carriers and Channels;
4. The ion transport systems;
5. Endocytosis and Phagocytosis: cellular uptake of
macromolecules and particles.
Transport processes within an eukaryotic cell
1. Principles of membrane transport
A. The plasma membrane is a selectively permeable
barrier. It allows for separation and exchange of
materials across the plasma membrane.
B. The protein-free lipid bilayers are highly
impermeable to ions.
C. The energetics of solute movement:
™Diffusion is the spontaneous movement of material from
a region of high concentration to a region of low
concentration.
™The free-energy change during diffusion of nonelectrolytes
depends on the concentration gradient.
™The free-energy change during diffusion of electrolytes
depends on the electrochemical gradient.
Diffusion -- a spontaneous process that results from the
random motions and collisions of molecules in a region of
high concentration and causes those molecules to disperse
into a region of low concentration.
What happens if we remove
the barrier?
High concentration
Low concentration
Diffusion – fills the space – it is spontaneous, no work is
done
High concentration
Low concentration
Driving forces in diffusion
z Concentration
gradient: uncharged molecules
z Electrochemical
gradient: the net driving force
composed of two distinguished parts, one is due to
the concentration gradient, the other is due to the
voltage across the membrane (electric potential
gradient); electrically charged molecules.
Comparison of two classes of transport.
Figure 11-7 Kinetics of simple diffusion compared to carrier-mediated
diffusion. Whereas the rate of the former is always proportional to the solute
concentration, the rate of the latter reaches a maximum (Vmax) when the carrier
protein is saturated. The solute concentration when transport is at half its
maximal value approximates the binding constant (KM) of the carrier for the
solute and is analogous to the KM of an enzyme for its substrate. The graph
applies to a carrier transporting a single solute; the kinetics of coupled transport
of two or more solutes are more complex but show basically similar phenomena.
Two classes of membrane transport proteins
™Carrier proteins
™Channel proteins
™ Solutes cross membrane by simple diffusion
™If uncharged solutes are small enough,
they can move down their concentration
gradients directly across the lipid
bilayer by simple diffusion.
™Most solutes can cross the membrane
only if there is a membrane transport
protein to transfer them.
™Passive transport, in the same
direction as a concentration gradient.
Diffusion of small
molecules across
phospholipid bilayers
™ Active transport, is mediated by
carrier proteins, against a concentration
gradient, require an input of energy.
2. The Diffusion of ions
through membrane
z The
movement of ion across membranes plays
a critical role in multitude of cellular activities.
z The ion channels are formed by integral
membrane proteins that surround an aqueous
pore.
z All ion channels are members of a small
number of giant superfamilies.
™Characteristics of ion channel
z Most
z Ion
ion channels are highly selective
channels are bi-directional
z Most
ion channels can exist in either an
open or a closed conformation, and such
channels are said to be gated
Some ligand-gated channels are opened (or closed)
following the binding of a molecule to the outer surface
of the channel; other are opened (or closed) following
the binding to the inner surface of the channel
Leaf-closing response in mimosa. (A) resting leaf ( B and C). Successive
responses to touch. A few seconds after the leaf is touched, the leaflets
collapse. The response involves the opening of voltage-gated ion channels,
generating an electric impulse. When the impulse reaches specialized hinge
cells at the base of each leaflet, a rapid loss of water by these cells occurs,
causing the leaflets to collapse suddenly and progressively down the leaf stalk.
How stress-activated ion channel allow us to hear. (A) a section through the
organ of Corti, which runs the length of the cochlea of the inner ear. Each auditory hair
cell has a tuft of processes called stereocilia projecting from its upper surface. The hair
cells are embedded in a sheet of supporting cells, which is sandwiched between the
basilar membrane below and the tectorial membrane above. (B) sound vibrations cause
the basilar membrane to vibrate up and down, causing the stereocilia to tilt. Each tilting
stretches the filaments, which pull open stress-activated ion channels in the stereocilium
membrane, allowing positively charged ions to enter from the surrounding fluid. The
influx of ions activates the hair cells, which stimulate underlying nerve cells that
convey the auditory signal to the brain.
z
z
z
z
The structure of voltage-gated K channel
Four homologous polypeptides (subunits)
Each subunit contains six membrane-spanning segments
A single cell is likely to possess a variety of different K
channels that open and close in response to different voltage
a)Voltage-gated potassium
channel can exist in at least
three distinct conformations:
closed, open, and inactivated.
Open, several thousand
potassium ions can pass
through the channel per
millisecond, it has significant
impact on the electrical
properties of the membrane,
then the channel is
automatically stopped by a
process of inactivation
b)Structural model of the balland-chain inactivated state
The diffusion of water through membrane
z
Osmosis: water moves readily through a
semipermeable membrane from a region of
lower solute concentration to a region of higher
solute concentration
z Plasmolysis: the plasma membrane of a plant
cell pulls away from the surrounding cell wall
Expression of aquaporin by frog oocytes increases their permeability to
water. Frog oocytes, which normally do not express aquaporin, were
microinjected with mRNA encoding aquaporin. These photographs show
control oocytes (bottom cell in each panel) and microinjected oocytes (top cell
in each panel) at the indicated times after transfer from an isotonic salt
solution (0.1 mM) to a hypotonic salt solution (0.035 M). The volume of the
control oocytes remained unchanged because they are poorly permeable to
water. In contrast, the microinjected oocytes expressing aquaporin swelled
because of an osmotic influx of water, indicating that aquaporin is a waterchannel protein. [Courtesy of Gregory M. Preston and PeterAgre, Johns
Hopkins University School of Medicine.]
Structure of the water-channel protein aquaporin
3. Facilitated diffusion
™Facilitated diffusion: Protein-mediated
movement, movement down the gradient
z The
solutes transported in this way need the
assistance of carrier protein, which is
called a facilitative transporter.
z The binding of the solute to the carrier
protein on one side of the membrane leads
to a conformational change in the protein.
™
Carrier proteins bind one or more solute molecules
on one side of the membrane and then undergo a
conformational change that transfer the solute to
the other side of the membrane.
The carrier protein, the Glucose transporter (GluT1 ) in the erythrocyte
PM, alter conformation to facilitate the transport of glucose.
z
GLUT1 to GLUT12
z Insulin
can
regulate the
uptake of glucose
in skeletal muscle
and fat cells.
The characteristics of facilitated diffusion
z
z
z
z
z
The assistance of carrier protein (facilitated
transporter) which can be regulated.
Facilitated transporters mediated the movement of
solutes equally well in both direction.
The direction of movement of solute is down the
gradient
Facilitated transporter are specific for the
molecules they transport, even discriminating
between D and L stereoisomers
The transport rate is relative low.
Ionic differentiation inside and outside cell
4. Active transport
Carrier protein-mediated movement up the gradient
¾ This process differs from facilitated
diffusion in two crucial aspects:
™Active transport maintains the gradients for
potassium, sodium, calcium, and other ions
across the cell membrane. Always moves solutes
up a concentration or electrochemical gradient;
™Active transport couples the movement of
substances against gradients to ATP hydrolysis.
i.e Always requires the input of energy.
¾
Cells carry out active transport in
three main ways
™ Couple the uphill transport of one solute (endergonic) across
membrane to the downhill transport of another (exergonic).
™ Couple uphill transport to the hydrolysis of ATP
™ Mainly in bacteria, couple uphill transport to an input of energy from
light.
Direct active transport depends on four types of
transport ATPases
The four classes of ATP-powered transport proteins:
“P” type stands for phosphorylation;
ABC (ATP-binding Cassette) superfamily, bacteria—humans.
Two transmembrane (T) domains and two cytosolic ATP-binding (A) domains
The Na+-K+ ATPase (pump)
¾ The Na+-K+ ATPase requires K+ outside, Na+ and ATP inside, and is
inhibited by ouabain.
¾ The ratio of Na+:K+ pumped is 3:2 for each ATP hydrolyzed.
¾ The Na+-K+ ATPase is a P-type pump.This ATPase seruentially
phosphorylates and dephosphorylates itself during the pumping cycle.
¾ The Na+-K+ ATPase is found only in animals.
A Model Mechanism
for the Na+/K+ ATPase
The biological function of Na+/K+ pump
¾ The active transport of Na+/K+ ATPase is used to
maintains electrochemical ion gradients, and
thereby maintains cell’s excitability.
¾ The Na+/K+ pump is required to maintain osmotic
balance and stabilize cell volume
¾ forming a phosphorylated protein intermediate
Other ion transport systems
™Other P-type pumps: including H+ and Ca+ ATPases,
and H+/K+ ATPases
¾Plant cells have a H+-transporting plasma membrane pump .
This proton pump plays a key role in the secondary transport of
solutes, in the control of cytosolic pH, and possibly in control of cell
growth by means of acidification of the plant cell wall.
¾Ca2+ pump: Ca2+-ATPase present in both the plasma membrane and
the membranes of the ER. It contains 10 transmembrane α helices.
This Ca2+ pump functions to actively transport Ca2+ out of the
cytosol into either the extracellular space or the lumen of the ER.
¾H+/K+ ATPases (epithelial lining of the stomach): which secretes a solution of
concentrated acid (up to 0.16N HCl) into the stomach chamber.
Acid-secreting parietal cell of stomach
¾H+/K+ ATPases (epithelial lining of the stomach): which secretes a
solution of concentrated acid (up to 0.16N HCl) into the stomach chamber.
Prilosec is a widely prescribed drug that prevents heartburn by inhibiting
the stomach’s H+/K+-ATPase
™The ABC transporters(ATP-binding cassette):
Constitute the largest family of membrane transport
proteins.
¾In bacteria (permease ), ABC
transporter use ATP binding and
hydrolysis to transport molecules
across the bilayer.
¾The eucaryotic ABC transporter
pump hydrophobic drugs out of the
cytosol. MDR(multidrug resistance)
transport protein overexpression in
human cancer cells (>40%).
¾In yeasts, ABC transporter is
responsible for exporting a mating
pheromone.
¾In most vertebrate cells, ABC in ER
membrane actively transports a wide
variety of peptides from the cytosol into
the ER.
4. Indirect active transport is driven by Ion
gradients ----- Cotransport
Gradients created by active ion pumping store energy that can
be coupled to other transport processes.
A. Sugars, amino acids, and other organic
molecules into cells:
™ The inward transport of such
molecules up their
concentration gradients is often
coupled to, and driven by, the
concomitant inward movement
of these ions down their
electrochemical gradients:
¾ Animal cells-----Sodium ions (Na+/K+
ATPase)
¾ Plant, fungi, bacterium-----Protons(H+
ATPase)
The difference between animal and plant cells
to absorb nutrients
Coupled transport (cotransport)
Indirect active transport is driven by Ion gradients
z
z
z
Gradients created by active ion pumping store energy
that can be coupled to other transport processes.
Symport: the transporter moves both solutes in the same
direction across the membrane
Antiport: the transporter moves two kinds of solutes in
opposite direction
Symport and antiport
Na+-linked symporters
import amino acids and
glucose into many animal
cells
Na+-linked antiporter
exports Ca+ from
cardiac muscle cells
Medicine
Ouabain and digoxin
increase the force of
heart muscle contraction
by inhibiting the Na+/K+
ATPase. Fewer Ca+ ions
are exported
5. Endocytosis(胞吞作用):
Large molecules enter into cells
A. Endocytosis imports
extracellular molecules
dissolved or suspended in
fluid by forming vesicles
from the plasma membrane
™Bulk-phase endocytosis does
not require surface membrane
recognition.It is the nonspecific
uptake of extracellular fluids.
™Receptor-mediated
endocytosis(RME) follows the
binding of substances to
membrane receptors.
Receptor-mediated endocytosis
B. Phagocytosis: The uptake of large particles
（吞噬作用）
™Including: macromolecules,
cell debris, even microorganisms
and other cells.
™Phagocytosis is usually
restricted to specialized cells
called Phagocytes (e.g.
marcrophages, neutrophils).
™Phagocytosis is initiated by
cellular contact with an
appropriate target.
™Phagocytosis may be enhanced
by the opsonins in mammals.
(高胆固醇血症)
™Structure of a clathrin –coated vesicle
™Model for the
formation of a clathrincoated pit and the
selective incorporation
of integral membrane
proteins into clathrincoated vesicles
6. Exocytosis
A. Constitutive exocytosis pathway
B. Regulated exocytosis pathway
7. Membrane Potentials and Nerve Impulses
A. K+ gradients maintained by the Na+-K+ ATPase are responsible
for the resting membrane potential.
B. The action potential: The changes in
ion channels and membrane potential.
™Resting state: All Na+ and K+ channels
closed.
™Depolarizing phase: Na+ channels
open,triggering an action potential.
™Repolarizing phase: Na+ channels
inactivated, K+ channels open.
™Hyperpolarizing phase: K+ channels
remain open, Na+ channels inactivated.
™The sequence of events during synaptic transmission:
™Excitable membranes exhibit “all-or-none” behavior.
™Propagation of action potentials as an impulse.