Membrane Transport of Small Molecules and electrical properties of membranes'

1
Membrane Transport of Small Molecules and
electrical properties of membranes. Lecture 9,
Fall 2004
2
The phospholipid bilayer is a barrier that
controls the transport of molecules in and out of
the cell.
Gases diffuse freely, no proteins required.
Water diffuses fast enough that proteins arent
required for transport
Sugars diffuse very slowly so proteins are
involved in transport.
Charged molecules are virtually impermeable.
Studies of synthetic lipid bilayers help define
which types of transport will require the
activity of a protein. Hence, transport of an ion
should require a protein.
3
Why charged compounds dont permeate the bilayer?
They are strongly attracted to the water.
Panel 2-2, page 112
4
Transport of molecules that are impermeable to
the lipid bilayer is achieved by two classes of
membrane transport proteins.
Conformational change carries the solute across
the membrane.
Aqueous pore provides passage way so solute can
diffuse through the membrane.
5
Two types of transport are defined by whether
metabolic energy is expended to move a solute
across the membrane.

• Passive transport no metabolic energy is needed
  because the solute is moving down its
  concentration gradient.
• In the case of an uncharged solute, the
  concentration of the solute on each side of the
  membrane dictates the direction of passive
  transport.
• Active transport metabolic energy is used to
  transport a solute from the side of low
  concentration to the side of high concentration.

6
When considering an uncharged solute, one only
considers the concentration gradient. If the
solute is charged, movement across the membrane
is affected by both the chemical gradient and the
electrical gradient. The combination of the the
two is called the electrochemical gradient.
Passive transport occurs when a solute moves down
the electrochemical gradient. Active transport
occurs when a charged solute moves up the
electrochemical gradient.
7
Possible scenarios for a charged solute Passive
transport charged solute moving from region of
high concentration to low concentration and
moving towards the side of the membrane with
opposite charge. Active transport charged
solute moving towards the side of the membrane
having the same charge. Sometimes the electrical
potential acts in the opposite direction of the
concentration gradient. Whether transport is
active or passive depends on which dominates.
?GcRTlnNain/Naout ?GmzFV
8
Carrier proteins and active membrane transport.
9
A carrier protein binds solute on one side of a
membrane, undergoes a conformational change, and
releases the solute on the other side of the
membrane.
10
A carrier protein resembles an enzyme.
11
By coupling the conformational change to a source
of energy, a carrier protein can perform active
transport.
12
Depending on how many solute molecules are
transported and in what direction, carrier
proteins are dubbed uniporters, symporters, or
antiporters.
13
Three carrier proteins, appropriately positioned
in the plasma membrane, function to transport
glucose across the intestinal epithelium.
14
Na-K pump (aka Na-K ATPase) in the plasma
membrane is an antiporter that performs active
transport.
This protein establishes a concentration
gradients with Na low inside the cell and high
outside, and K high inside the cell and low
outside.
15
The Na gradient generated by the Na - K ATPase
powers the transport of glucose into the cell by
a Na -driven glucose symporter.
The energetically favorable movement of Na down
its electrochemical gradient is coupled to the
energetically unfavorable transport of glucose up
its concentration gradient. Hence, glucose is
being subjected to active transport.
16
Glucose uniporter binds glucose inside the cell,
undergoes a conformational change, and releases
the glucose into the fluid where it then enters
the blood. Since glucose is moving from high to
low concentration, this is passive transport.
17
Concentration gradients of Na, K, and Ca in
animal cells are maintained by structurally
similar proteins the Na - K ATPase and the
Ca ATPase.
Maintaining the low concentration of Ca inside
the cell is critical for controlling many
cellular processes because the responses are
trigger by sudden increases in intracellular
Ca. The Na gradient is essential for uptake
of many metabolites. As you will learn shortly,
the Na and K gradients are also essential for
generating nerve impulses.
18

• Ion channels
• Multipass transmembrane proteins with
  alpha-helical transmembrane domains organized
  into a hydrophilic channel.
• Selects ions by size and charge. The selectivity
  arises because the channel narrows and forces the
  ions to come in contact with charged and polar
  portions of the peptide chain lining the channel.
• Transport of ions is 100,000 times faster than
  the fastest carrier protein.
• Limited to passive transport.

19
Ion channels are ion-selective because of narrow
region called the selectivity filter.
side view
R
R
R
top view - each circle represents an end-on view
of an alpha-helix and the R-groups radiating
outwards are amino acid side chains.
R
R
R
R
R
R
R
R
R
R
R
R
20
We can anticipate that the red R-groups are
hydrophobic and the blue R-groups are
hydrophilic.
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
21
How can a K channel discriminate between K and
Na since they both have the same charge and Na
is smaller than K?
The answer as the ion passes through the
selectivity filter, the ion must shed water.
Carbonyl oxygens with partial negative charge can
take the place of water for K but Na is too
small. Hence, Na favors remaining associated
with water and hydrated ion is too large to fit
through the selectivity filter.
22
The carbonyl oxygens that line the selectivity
filter are presented as parts of loops. They
are available because they are not tied up in
hydrogen bonding with other nitrogens in the
peptide backbone.
23
Ion channels fluctuate between open and closed
state to regulate ion flow.

• Voltage-gated channels respond to the membrane
  potential.
• Ligand-gated channels respond to association of
  small molecules called ligands.
• Mechanically gated channels respond to movement.

24
Understanding the electrical properties of
membranes.
25
Establishing a resting membrane potential in
animal cells results from the coordinated action
of carrier proteins and ion channels.
1. Na-K ATPase concentrates K inside the cell
and Na outside (active transport). 2. K leak
channels allow K to diffuse out of the cells,
down the concentration gradient (passive
transport). 3. Negative charge left behind in the
cytoplasm counteracts the efflux of K so only a
very small amount (1/100,000) K leak out. 4. The
efflux of the K is sufficient to generate a
membrane potential of approximately -100 mV -
positive outside and negative inside.
The resting potential is defined as the membrane
potential occurring when there is no net flow of
ions. Cells typically have resting potentials
between -20 and -200 mV. The major contributor
is the K leak channel, but other channels
account for the range of potentials observed.
26
Nerve cell.
Dendrites - cell protrusions that receive signals
from axons. Cell body - location of nucleus. Axon
- single long protrusion that sends signal away
from the cell body.
A nerve impulse results from electrical
disturbances in the plasma membrane that spread
from one part of the cell to another. The
electrical disturbance is called an action
potential and it consists of a wave of membrane
depolarization that moves down the axon.
27
If you measure the membrane potential at a given
point on the axon, a rapid depolarization
followed by repolarization of the membrane
represents an action potential.
28
Voltage-gated channels are the key to an action
potential.
Depolarization to a specific membrane potential
causes the channels to open. This specific
membrane potential can be thought of as the
threshold that must be reached to get the channel
to open.
29
A threshold depolarization initiates the action
potential.
When a nerve receives a signal, a modest
depolarization of the membrane occurs. If this
depolarization reaches the threshold, all the
voltage-gated Na channels experiencing this
threshold depolarization will open
simultaneously. Na rushes into the cell causing
a rapid depolarization of the membrane.
30
The voltage gated channels stay open for only a
brief period and then close.
31
The channel opens briefly but then automatically
becomes inactive. It will remain inactive (and
closed) until it is reset by the resting
potential. Only after resetting, can the voltage
gated channel respond again to a threshold
potential.
32
Repolarization occurs because the voltage-gated
Na channels quickly inactivate and because
voltage-gated K channels. The voltage gated K
channels open more slowly so there is time for
the influx of Na to depolarize the membrane
before the efflux of K begins to repolarize the
membrane.
33
The voltage-gated Na channel will not respond to
another threshold potential until it has been
subjected to the original negative membrane
potential.
34
The channel cycle of closed, opened, inactivated
along the axon results in propagation of the
action potential.
35
The changes in membrane potential are achieved by
the movement of very few ions. Hence, the
overall concentrations of potassium and sodium
inside and outside the cell do not change when
various channels open and close. Sodium stays
high outside and potassium stays high inside.
Small fluxes of ions are sufficient to change
potentials.
36
Initiation of an action potential requires that
something cause the membrane to depolarize to the
threshold potential. The process begins at the
synapse.
presynaptic cell
postsynaptic cell
37
Depolarizing the postsynaptic membrane so an
action potential can be initiated. 1.
Neurotransmitter is released from the presynaptic
cell when the action potential arrives at the end
of the terminal branches. 2. Neurotransmitter
diffuses across the synaptic cleft and associates
with ligand-gated Na channels (a.k.a.
transmitter gated Na channels). 4.
Ligand-gated Na channels open and the influx of
Na depolarizes the membrane. 5. If enough
channels open, the threshold potential will be
reached and the action potential will begin as a
result of the opening of voltage-gated Na
channels.
38

• The threshold potential required to initiate an
  action potential is affected by inhibitory and
  excitatory neurotransmitters acting upon distinct
  transmitter-gated (ligand-gated) ion channels.
• Excitatory neurotransmitters open ligand-gated
  ion channels that depolarize the membrane towards
  the threshold potential.
• Acetylcholine, glutamate and serotonin open
  distinct cation channels that allow influx of
  Na.
• Inhibitory neurotransmitters open ligand-gated
  ion channels that resist depolarization of the
  membrane towards the threshold potential.
• Gamma-aminobutyric acid (GABA) and glycine open
  distinct Cl- channels.
• Psychoactive Drugs alter specific ligand-gated
  ion channels.

39
To understand the action of neurotransmitters,
begin by considering the resting state of a cell.
40
The graph shows the impact that opening different
channels will have on the membrane potential.
Recall that channels transport ions much faster
than carrier proteins so the ATPases below are
insignificant when considering what happens when
channels are opened.
41
The keys to understanding the behavior of the
nerve is to remember 1. Relative distributions
of Na, K, Cl-, and Ca. 2. The resting
potential is negative on the inside and positive
on the outside. 3. Will the movement of a
particular ion promote or inhibit depolarization
of the membrane to the threshold required for an
action potential?