Membrane transport and action potentials
Have bilipid membrane for transport. Very hostile to polar substances.
Different methods of transport.
Particles will diffuse until there is an equal amount on both sides. Equalising the
concentration gradient.
The diffusion rate is determined by the diffusion coefficient (depended by the
viscosity of the surrounding medium, thickness of the membrane and the
concentration gradient across the membrane). Big molecules will be slower, also
if they have to cross further.
If a particle is lipophilic it will cross the bilipid membrane easily.
Most of the molecules in our body are polar. No matter how high the rate of
diffusion, they cannot cross the bilipid membrane.
Still a passive process, but is using proteins embedded inside the membrane to
open up a channel for polar molecules to travel through. Still high conc. to low,
but there are now transporters to pass through.
Occurs in tow ways – channels and carriers.
Channels facilitate area that is hydrophobic. They form a hole in the membrane.
For smaller things. they only facilitate diffusion, passive and don’t go against
concentration gradient.
Carrier is for larger molecules and can move against the concentration gradient.
Uses energy to bring things into the membrane. Need this to restore what has
been spent. Utilises ATP to transport sodium/potassium ions across.
P type pumps phosphorylate something to cause a change in shape which can
then push ions across the membrane.
4) Secondary Active Transport
These transporters use an existing ion gradient (usually Na+ in
mammals) to fuel the movement of another solute against its gradient
Co-Transport (symport)
solutes
l
transported
d in same direction
d
+
Na /glucose
Na+/amino acids
Neurotransmitter transporters
K+/Cl- transporters
p
Na+/S transporters (cysteinuria)
Counter-transport
(exchanger, antiport)
solutes transported in opposite
direction
Na+/Ca2+ exchanger
Na+/H+ exchanger
Simpl
Protein
-
As a consequen
interactions, all
transport
p can be
drugs
- proza
- digito
- local
Uses the gradient of ions to push materials out eg. sodium/calcium.
This is using a gradient that already exists. Fuelling the movement of something
Membrane Transporters can contribute to three
else against it’s gradient. Usually involves carriers.
sorts of membrane transport
Symport – when something comes in with something else. Antiport – is when
ions exchange
places. Diffusion
Facilitated
Solute can only travel down its chemical or
electrochemical gradient
Active
ct e Transport
a spo t
Solute can be "pumped" against its chemical or
electrochemical gradient using an energy source
Sodium Pu
is the most
Is only selected in active transport. Proteins that equip cells create different
characteristics. This is mediation by protein. Different transport.
High flux rate – with facilitated diffusion there can be a high flux rate. (high flux
rate is when things can be moves fast across a membrane, rather than waiting for
concentration gradients.
Carrier can be saturated however in facilitated diffusion – only so many things
they can move at once.
Specificity – is it specific to sodium?
Selectivity – discrimination between ions. Does not occur in simple diffusion.
Carriers allow you to select for one.
Drugs exploit these mechanisms.
in
gradient
irection
a)
e
hree
Na /Ca exchanger
Na+/H+ exchanger
Simple diffusion vs facilitated diffusion
ProteinTransporters
mediated diffusion
(facilitated
diffusion):
Membrane
can
contribute
to three
sorts of membrane transport
- high flux rates
- saturation
Facilitated Diffusion
- specificity
Solute can only travel down its chemical or
- selectivitygradient
electrochemical
Active
ct e Transport
a spo t
As a consequence of protein - solute
Solute
can be "pumped" against its chemical or
interactions,
all protein-mediated
electrochemical
transport
p can be selectively
ygradient
blocked by
yusing an energy source
drugs
- prozac
primary
- directly coupled to energy use
-secondary
digitoxin
- indirectly coupled to energy use by a
- local anaesthetics
membrane gradient;
Levy et al., 2006
- can be co-transport (symport) or
counter-transport (antiport)
AE1-2A.S.Alma.L.Channels
Sodium Pump (Na+/K+/ATPase; Na+/K+ pump)
is the most well-known
well known primary active transporter
Ubiquitous
Uses up to 50% of body’s
energy
Establishes and maintains
concentration gradients
Electrogenic
Prevents swelling
g
Blocked by ouabain and
cardiac glycosides (e.g.
(e g
digitoxin)
For each 3 Sodium out, 2 potassium enter.
3
This is against the concentration gradient due to salty extracellular fluid. Needs
to use ATP. This stops cells from swelling.
Sodium Pum
is the most w
Complex conformational changes in the transporter
protein are involved in active transport
Inside
One role
concen
+
2K
+
3Na
sites prefer Na+
-
E1
-
Outside
Inside
-
-
-
Outside
-
E2 (2K+)
Inside
E1P (3Na+)
Outside
Inside
sites prefer K+
-
based on Fig 16 in Glynn (1993)
E2P
+
2K
+
3Na
model:
1) ATP bound,
b
d binds
bi d Na
N +
2) phosphoryl & Na+ trapping
3) Na+ released, K+ bound
4) dephosphoryl & K+ trapping
5) ATP binds, K+ released
Outside
Don’t need to know specific details.
Interplay of Multiple Transporters
When ATP and sodium bind there is a conformational change. Causes
example 1.
1 Epithelial Membranes
phosphorylation and sodium is released, then potassium is trapped and then
released.
Epithelial NaCl and H2O excretion in shark rectal gland
Carriers go through changes.
Channels are always open or closed.
CFTR
CFTR: cystic fibrosis transmembrane
AE1-2A.S.Alma.L.Channels
conductance
regulator; a Cl channel
based on Fig 1 of Riordan et al., 1994
Na is all outside and K inside.
Sodium gradient is used to retain glucose and amino acids and pump out calcium
and other things we don’t need. The sodium gradient can be used for energy and
to create an electrical gradient.
Ex. 2: Mult
This slide illustrates h
together to mediate th
are shown. Which are
many different channe
Back to Channels - recap what are they?
Top view
Sid view
Side
i
IIon channels
h
l are facilitated
f ilit t d
diffusion proteins that
traverse the membranes of all
excitable cells. By carefully
controlled opening and
closing (“gating”) of these
selective ion pores, cells
across all
ll species
i are able
bl to
t
produce and transduce
electrical signals.
signals
A more compl
p
extracellular
intracellular
Gramicidin A channel
"leak"
rest
est
Gating of Ion Channels
- changing between non-conducting to conducting conformations
Main modes of gating:
- extracellular ligand (A)
(e.g. neurotransmitter-gated channels at
synapses)
1.
2.
3
3.
- intracellular ligand (B)
(e.g. cyclic nucleotide channels,
ATP sensitive
ATPiti K+ channels)
h
l)
4.
- voltage (C)
(Vdep Na+, K+, Ca2+ and Cl channels)
5
5.
- stretch
(cation, Ca2+, Cl-)
- background or leak channels
(typically K+ or Cl-)
AE1-2A.S.Alma.L.Channels
You need something to bind to the channel for it to open – this is the extracellular ligand. (like and agonist.)
This can also occur intracellularly.
Voltage gated – charge change in the cell will open or close the gate.
Stretch.
Leak channels – always open. Allowing things to diffuse back and forth. Like K.
Leaky K channels cause negative resting membrane potential.
The Na+/K
and a high
Normally
K+ to cros
The efflux
respect to
The efflux
the K+ is
trying to d
This poin
electrical
electrical
intracellular
High sodium concentration
outside,
Doyle et al., Science 1998;
280: 69-77 high potassium inside. Not enough to make
gate
an electrical gradient de to chloride ion channels which offsets the difference.
"leak" K+ channels result in a negative
resting
est g membrane
e b a e pote
potential
ta (
(Vm)
)
ons
els at
els)
1.
The Na+/K+ pump establishes a high intracellular concentration of K+
+/K+ pump establishes a high intracellular concentration of K+
1. aThe
and
highNaextracellular
concentration of Na+.
+
and
a high extracellular
concentration
NaK
Because
Na is pumped
out andofthe
pumped in.
+
2.
Normally
only
leak
channels
including
K
are
theseopen,
allowsthese
some allows some K+
2.
Normally only leak channels includingopen,
K+ are
+ to cross the membrane
K
to cross the membrane.
Lots of potassium wants to get out.
3 The efflux of K+ causes the inside of the cell to become negative with
3.
3.
The efflux of K+ causes the inside of the cell to become negative with
respect to the outside (because positive charges are leaving the cell)
respect
to the outside (because positive charges are leaving the cell)
4. The efflux of K+ continues until the electrical force trying to hold onto
negative that
the inside.
theKKis+ positive,
is equal to so
the outside
chemical the
forcecell
dueistomore
the concentration
gradient
4.
The
efflux
of
K+
continues
until
the
electrical
force
trying
to hold onto the
trying to drive it out
K+5.
equal
to the
chemical
due to theequilibrium,
concentration
gradient
trying to
5 is This
point
of balance
is theforce
electrochemical
equilibrium
where
the
drive electrical
it out force is equal and opposite to the chemical force. The
electrical
forceof
is balance
usually described
by the membrane potential
(Vm) where the
5.
This point
is the electrochemical
equilibrium,
electrical force is equal and opposite to the chemical force. The electrical force is
usually described by the membrane potential (Vm).
5
Equilibrium Potential / Nernst Potential
The membrane potential at which the electrical and chemical driving forces for
ion flux across the membrane are exactly equal
Electrical force
z
F
EX
R
T
[X]o
[X]i
at 37°C:
=
chemical force
zFEX
=
RT ln ([X]o/[X]i)
Ex
=
RT/zF ln ([X]o/[X]i)
= valence
= Faraday's
F d ' constant
t t (96,500
(96 500 coulombs
l b mole
l -11).
)
= Nernst potential for ion X
= Gas constant (8.314 J deg-1 mole-1)
= Absolute temperature (Kelvins, 0 C = 273K)
= concentration of ion X outside (molar)
= concentration of ion X inside (molar)
RT/F ln [[X]]
= 26.7 mV ln [[X]]
- If rest
permea
Vm =
+
K+
-
K
3Na+
2K+
- If the cell is permeable t
(typically at rest the mem
Will be in a prac.
Resting Vm is close to EK but not exactly equal
Vm is a weighted av
of the mem
This can be formula
Data
points
Data fit to the GHK equation
with PNa+:PK+ = 0.01
Vm
=-
P = (µK/a)(RT/F) and µ=
for
ual
K equation
01
ing
on
ws EK,
lity
utes.
n Aidley.
- If resting
g cell is only
y
permeable to K+ then
Vm = EK+ (-98 mV)
-
+
+
K+
- If resting
g cell is only
y
permeable to Na+ then
Vm = ENa+ (+67 mV)
-
K+
-
K+
Na+
-
+
K+
-
-
+
+67 mV
Vm = ENa
K+
-98 mV
Vm = EK
3Na+
2K+
-
+
Na+
+
+
Na+
+
3Na+
2K+
Na+
- If the cell is permeable to both, Vm will be somewhere in between
(typically at rest the membrane is about 100 times more permeable to K+ than Na+)
Na wants to go inside the cell. The only way this won’t happen is when the inside
is positive – which K makes it.
is an equal and opposite chemical force.
VThis
m is a weighted average of the driving force and permeability
of the membrane to the different ions present
p
This can be formulated as the Goldman-Hodgkin Katz (GHK) equation:
Vm
RT
= ----F
ln
+
PK (K+)o + PNa
N (Na )o + PCl(Cl )i
-------------------------------------PK (K+)i + PNa(Na+)i + PCl(Cl-)o
P = (µK/a)(RT/F) and µ=mobility; K=partition coefficient; a = thickness of membrane
As Cl- doesn’t contribute much to setting Vm, and using relative
permeability, this equation can be simplified to:
RT
(K+)o + (Na+)o
Vm = ----- ln ------------------F
(K+)i + (Na+)I
where
= PNa+/PK+
typically = about 0.01
0 01
6