Download Chapter 9 Membrane Transport

Chapter 9
Membrane Transport
Molecules in the cell membrane
(1) Transport of molecules → Signaling
(2) Adhesion to the ECM → Cell recognition
Specialized membrane transport proteins are responsible
for transferring small water-soluble molecules
across cell membranes
Impermeable to most
water-soluble molecules
Transfer a particular type
of molecule
HCO3-, PO43-,proteins, nucleic acids, metabolites…
Simple diffusion
The diffusion of water is known as osmosis
Osmotic pressure
aquaporins
Cell use different tactics to avoid osmotic swelling
Na+-K+
pump
Turgor pressure
Stomata open on the underside of a leaf
Stomata
A Venus flytrap uses electrical signaling
to capture its prey
Mechanical stimulation on hairs in the central of each leaf

Electrical signal

Rapid change in turgor pressure
The rate at which a molecule diffuses across a synthetic
lipid bilayer depends on its size and solubility
(1) Smaller molecule
(2) Oil solubility
(hydrophobic,
nonpolar)
Rate of diffusuin
Small molecules and ions can enter the cell through
a transporter or a channel
Fit into a binding site on the transporter
Size & electric charge
Greater rate than transporters
Most solutes cross cell membrane
by passive or active transport
small,
uncharged,
fat-soluble
Facilitated diffusion
Some transporters carry a single solute across the
membrane (uniports); others couple the uphill transport
of one solute across to the downhill transport of another
Both passive & active transport
Each cell membrane has its own characteristic set
of transporter
A conformational change in a transporter could mediate
the passive transporter of a solute such as glucose
uncharged
molecule
Facilitated transport via transporter
Glucose transporter
An electrochemical gradient has two components
Concentration gradient
+
Membrane potential
Electrochemical gradient
(net driving force)
Cells drive active transport in three main ways
The Na+-K+ pump plays a central role in
membrane transport in animal cells
X3
X2
Na+-K+ ATPase (Na+-K+ pump)
The Na+-K+ pump transporter ions in a cyclic manner
Na+-K+ pump
(10 ms/cycle)
Ouabain inhibits the pump by preventing K+ binding
Primary active transport
Na-K pump
Na-K exchanger
Na-K ATPase
Na+ outside the cell is like water behind a high dam
Potential energy
Distribution of Ca2+ in the cell
A Ca2+ pump returns Ca2+ to the sarcoplasmic reticulum
in a skeletal muscle cell
10  helices
Sarco/Endoplasmic reticulum Ca2+ ATPase (SERCA)
Secondary active transport
The glucose-Na+ symport protein uses the electrochemical
Na+ gradient to drive the import of glucose
The binding is cooperate, if one of the two solutes is missing,
the other will fail to bind o the pump
Two types of glucose transporters enable gut epithelial
cells to transfer glucose across the gut lining
from diet
Examples of secondary active transport
Na/glucose cotranspoter
Na/Ca exchanger
Two types of glucose transporters enable gut epithelial
cells to transfer glucose across the gut lining
Cooperation of primary and secondary active transport systems
There are similarities and differences in transporter
-mediated solute movement in animal and plant cells
fungi, bacteria
Plant cells, fungi (including yeast) and bacteria
do not have Na+-K+ pumps
Na+-glucose symport
A K+ channel possesses a selectivity filter that controls
which ion it will transport across the membrane
Ion channel
(1) ion selectivity: a. diameter and shape; b.distribution of the charged amino acids
(2) Ion channels are not continuously open: most ion channels are gated
A typical ion channel fluctuates between
closed and open conformations
Hydrophilic pore
(1) faster than the transfer rate of transporter
(2) cannot couple the ion flow to an energy source to carry out active transport
Properties of ion channel
Selectivity:
(1) Ion size or charge
(2) Binding to the protein surface
(3) Stabilization of the nonhydrated ion
Rate:
Simple diffusion
Ion channel
(Fast)

Facilitated transporter
Active transporter
(Slow)
Different types of patch-clamp recording
Patch-clamp recording is used to monitor
ion channel activity
Current can enter or leave the
microelectrode only by passing
through the channels in the patch
of membrane covering its tip
Easy to alter the composition
of the solution on either side
of the membrane
Patch-clamp recording is used to monitor
ion channel activity
The voltage (membrane potential) across the isolated patch
of membrane is held constant during the recording
Gated ion channels respond to different types of stimuli
(Mechanical force)
Voltage sensor:
sensitive to changes
in the membrane potential
Stress-gated ion channels allows us to hear
K+
Stereocilia on the organ of Corti in cochlea of the inner ear
Stress-gated ion channel
Voltage-gated ion channels underlie the leaf-closing
response in mimosa
The leaf is touched

The opening of voltage-gated ion channel

Generating an electric impulse

Loss of water

Leaflets to fold closed suddenly
The distribution of ions on either side of the lipid bilayer
gives rise to the membrane potential
ion channels → membrane potential
How the resting potential is generated
Neuron
Axon
Plasma
membrane
Outside of neuron
Na
Na
Na
Na
K
K
Na
channel
Na
Na
Na
Na
K
Na
Na
Na
Na-K
pump
K
K
Plasma
membrane
Na
Na
Na
Na
Na
Na+ is high
k+ is low
ATP
K channel
K
Na
K
K
Na
Na
K
K
Inside of neuron
K
K
K
K
K
K
K
K+ is high
Na+ is low
K+ leak channel play a major role in generating the
membrane potential across the plasma membrane
The K+ leak channel
The Nernst equation can be used to calculate the
resting potential of a membrane
A typical neuron has a cell body, a single axon,
and multiple dendrites
Up to 100 m/sec
Neurons
Muscle cells
Gland cells
How stimuli trigger signals in a living neuron?
An electrode can be inserted into the squid giant axon (A)
to measure action potentials (B)
100X the diameter
of a mammalian axon
10 cm
The cytoplasm in an axon can be removed and replaced
with an artificial solution of pure ions
Na+, K+, Cl-, SO42-
The shape of the action potential depends on the
concentration of Na+ outside the membrane
The Nobel Prize in Physiology or Medicine 1963
The ionic mechanisms involved in excitation and inhibition
in the peripheral and central portions of the nerve cell membrane
The action potential propagates itself along the axon
 Action potentials are
- self-propagated in a one-way chain reaction along a neuron.
- all-or-none events.
 The frequency of action potentials (but not their strength)
changes with the strength of the stimulus.
Action potentials are all-or-none events
絕對不反應期
相對不反應期
去極化
再極化
過極化
全有全無律
Ion flows dictate the rise and fall of an action potential
Resistant/refractory
to stimulation
Opening of voltage gated K+ channels
An action potential is triggered by a rapid change
in membrane potential
With voltage-gated ion channels
Self-amplifying
depolarization
of neuron
20 mV
Without voltage-gated ion channels
A voltage-gated Na+ channel can adopt at least
three conformations
highly polarized
repolarized
depolarized
depolarized
The action potential
Na
Na
Na
Na
K
Additional Na channels
open, K channels are
closed; interior of cell
becomes more positive.
Na
50
Membrane potential
(mV)
3
Na
K
2
Na
Sodium Potassium
channel channel
Action
potential
3
0
閥值
4
Na channels close
and inactivate; K
channels open, and
K rushes out;
interior of cell is more
negative than outside.
5
The K channels
close relatively
slowly, causing a
brief undershoot.
4
2
50 Threshold
1
100
Resting potential
5
1
Time (msec)
A stimulus opens some Na
channels; if threshold is reached,
an action potential is triggered.
Na
K
Outside
of neuron
Na
Na
Plasma membrane
K
1
Resting state: Voltage-gated Na
and K channels are closed;
resting potential is maintained by
ungated channels (not shown).
K
Inside of neuron
1
Return to resting
state.
A action potential can be propagated
along the length of an axon (1)
Propagation of the action potential along the axon
Axon
Na
Plasma
membrane
Action
potential
Axon
segment
1
Na
Action potential
Na
K
2
K
Na
Action potential
Na
K
3
K
Na
A action potential can be propagated
along the length of an axon (2)
Depolarized
membrane
Neurons transmit chemical signals across synapses
Synapse
20 nm
(neurotransmitters inside)
An electrical signal is converted into a chemical signal
at a nerve terminal
Electrical signal
Chemical signal
exocytosis
An chemical signal is converted into an electrical signal
by transmitter-gated ion channels at a synapse
Chemical signal
Electrical signal
The acetylcholine receptor, present in the plasma
membrane of muscle cells, opens when it binds to the
neurotransmitter acetylcholine released by a nerve
5 transmembrane subunits
Aqueous pore
Acetylcholine receptor
(transmitter/ligand-gated ion channel)
Neuromuscular junction
Synapses can be excitatory or inhibitory
Acetylcholine
Glutamate
or Ca2+
GABA
Glycine
Thousands of synapses form on the cell body and
dendrites of a motor neuron in the spinal cord
Axon terminals
Synapses
Cell body
Dendrites
Actions of excitatory and inhibitory neurotransmitters