Download Membrane Domains and Membrane Potential

Membrane Domains and Membrane Potential
Extracellular Domain
Transmembrane Domain
Intracellular Domain
Body Fluids
•  Two compartments
–  Intracellular (~67% of body’s H20)
–  Extracellular (~33% of body’s H20)
•  Blood plasma: about 20% of BF
u  It is mostly water (93% by volume) and contains dissolved proteins (major
proteins are fibrinogens, globulins and albumins), glucose, clotting factors,
mineral ions (Na+, Ca++, Mg++, HCO3- , Cl- etc.), hormones, and carbon dioxide
(plasma being the main medium for excretory product transportation).
•  Tissue fluid (or interstitial fluid)
–  Includes extracellular matrix
u  In humans, the normal glucose concentration of extracellular fluid that is
regulated by homeostasis is approximately 5 mM. The pH of extracellular fluid
is tightly regulated by buffers and maintained around 7.4. The volume of ECF
is typically 15L (of which 12L is interstitial fluid and 3L is plasma).
•  Lymph
Extracellular Environment
•  Includes all parts outside of cells
–  Cells receive nourishment
–  Cells release waste
–  Cells interact (through chemical mediators)
Transport across cell membrane
•  Plasma (cell) membrane
•  Generally not permeable to
–  Proteins
–  Nucleic acids
•  Selectively permeable to
–  Ions
–  Nutrients
–  Waste
•  It is a biological interface between the two compartments
Transport across cell membrane
•  Plasma (cell) membrane
–  Recognition factors: allow for cellular adhesion
–  Site of chemical reactions
•  Enzymes located in it
•  Receptors: can bond to molecular signals
Based on structure
•  Transporter molecules
Based on energy
divided in various categories
requirements
Transport across cell membrane
•  Transport categories
–  Based on structure
•  Carrier-mediated
–  Facilitated diffusion
–  Active transport
•  Non-carrier mediated
–  Diffusion
–  Osmosis
–  Bulk flow
(pressure gradients)
•  Vesicle mediated
–  Exocytosis
–  Endocytosis
»  Pinocytosis
»  phagocytosis
- Saturation (trasport maximum, Tm; similar to
Michaelis- Menten enzyme kinetics: involve
proteins with limited number of binding sites)
- Stereospecificity (solute bind in stereospecific
manner the transport proteins)
- Competition (Antagonists i.e. D-glucose/Dgalactose)
Transport across cell membrane
•  Transport categories
•  Based on energy requirements
–  Passive transport
•  Based on concentration gradient
•  Does not use metabolic energy
• 
Active transport
• 
• 
• 
Against a gradient
Uses metabolic energy
Involves specific carriers
Diffusion
Non-carrier mediated
Passive transport
•  Physical process that occurs:
–  Concentration difference across the membrane
–  Membrane is permeable to the diffusing substance.
•  Molecules/ions are in constant state of random motion due to their
thermal energy.
–  Eliminates a concentration gradient and distributes the
molecules uniformly.
Concentration gradient àThe side of the membrane with a higher
concentration of a certain molecule/ion will lose them down the concentration
gradient to the side with the lower concentration of those molecules.
Diffusion Through Cell Membrane
•  Cell membrane permeable to:
–  Non-polar molecules (02)
–  Lipid soluble molecules (steroids)
–  Small polar covalent bonds (C02)
–  H20 (small size, lack charge)
•  Cell membrane impermeable to:
–  Large polar molecules (glucose)
–  Charged inorganic ions (Ca2+)
Rate of Diffusion
•  Dependent upon:
–  The magnitude of concentration gradient (CA - CB).
•  Driving force of diffusion.
–  Permeability and thickness (Δx) of the membrane.
•  Neuronal cell membrane 20 x more permeable to K+
than Na+.
•  The thicker the cell membrane the lower the rate of
diffusion
–  Temperature.
•  Higher temperature, faster diffusion rate.
–  Surface area of the membrane.
•  Microvilli increase surface area.
-  Partition Coefficient (K)
Ÿ Describes the solubility of a solute in oil relative
to its solubility in water
The greater solubility in oil the more easily the solute
can dissolve in the cell membrane’s lipid bilayer
-  Diffusion Coefficient (D)
Ÿ Correlates inversely with the solute dimension and
the viscosity of the medium. Small solutes in
nonviscous solutions have the largest diffusion
coefficient and diffuse most readily
D: diffusion coefficient
k: Boltzmann’s constant
T: absolute temperature
R: molecular radius
η: Viscosity of the medium
Osmosis
Non-carrier mediated
transport
•  Flow of H20 across a selectively permeable membrane.
•  Requirements for osmosis:
–  Must be difference in solute concentration on the 2 sides of the
membrane.
–  Membrane must be impermeable to the solute.
–  Osmotically active solutes: solutes that cannot pass freely through the
membrane.
Osmosis
The driving force for osmosis is a difference in osmotic pressure
caused by the presence of solute.
If two solutions have different solute concentrations then their osmotic and
hydrostatic pressures are also different, and the difference in pressure causes water
flow across the membrane.
•  Movement of H20 from high concentration of H20 to lower
concentration of H20.
þNote: Osmosis of H2O is not diffusion of H2O.
Osmosis occurs because of a pressure difference.
Diffusion occurs because of a concentration difference of H2O.
Osmolarity
•  The osmolarity of a solution is its concentration of
osmotically active particles.
•  Its necessary to know the concentration of solute and
whether the solute dissociates in solution.
- Example: NaCl dissociates into 2 particles (g=2)
CaCl2 dissociates into 3 particles (g=3)
Osmolarity = g C
g = number of particles per mole
in solution (Osm/mol)
C = molar concentration of the
solute (mmol/L)
Osmotic pressure
•  The force that would have to be exerted to prevent osmosis.
•  Indicates how strongly the solution “draws” H20 into it by osmosis.
•  The osmotic pressure (π) depends on two factors
- the concentration of osmotically active factors
- whether the solute can cross the membrane or not
van’t Hoff equation
π=gCσRT
Converts the concentration of particles
to a pressure, taking into account
whether the solute is retained in the
original solution
π  = Osmotic pressure (atm/mol)
g = Number of particles per mole in solution
C = Concentration (mmol/L)
σ = Reflection coefficient (varies from 0 to 1)
R = Gas constant (0.082L-atm/mol-K)
T = Absolute temperature (K)
Osmosis -Tonicity
•  The effect of a solution on the osmotic
movement of H20.
•  Isotonic:
–  Equal tension to plasma.
–  Same effective osmotic pressure.
•  Hypotonic:
–  Osmotically active solutes in a
lower
osmolality and osmotic pressure
than plasma.
–  RBC will hemolyse.
•  Hypertonic:
–  Osmotically active solutes in a
higher osmolality and osmotic
pressure than plasma.
–  RBC will crenate.
Facilitated Diffusion
Carrier mediated
Passive transport
•  Facilitated diffusion: carrier-mediated transport
•  Passive:
–  ATP not needed. Occurs down an electrochemical potential
gradient
–  Involves transport of substance through cell membrane from
higher to lower concentration (carrier-mediated transport).
Note that…
When solute concentration is
low, facilitated diffusion
proceeds faster than simple
diffusion because of carrier.
Active Transport
Carrier mediated
Active transport
•  Movement of molecules and ions against their
concentration gradients.
–  From lower to higher concentrations.
•  Requires energy - ATP.
•  2 Types of Active Transport:
–  Primary
Na+-K+ ATPase (cell membranes)
Ca2+ ATPase (sarcoplasmic and endoplasmic reticulum)
H+-K+ ATPase (gastric parietal cells)
–  Secondary
glucose, aminoacids, K+ or Cl-
Primary Active Transport
Na+/K+ ATPase Pump
•  Primary active transport.
•  It pumps 3Na+ from ICF
to ECF and 2K+ from
ECF to ICF
•  For each cycle of the Na
+-K+ ATPase more
positive charge is
pumped out of the cell.
•  Carrier protein is also an
ATP enzyme that
converts ATP to ADP
and Pi.
Electrogenic process: It creates a
charge separation and a potential
difference
Na+/K+ ATPase Pump
–are powered by ATP
–are active forces across the membrane
–carries 3 Na+ out and 2 K+ in
–balances passive forces of diffusion
–maintains resting potential (—70 mV)
But what maintains the extracellular
concentrations of K+ and Na+
(organ level) ?
•Two organ-systems are responsible for
maintaining the low extracellular K+
concentration and the high extracellular Na+ concentration.
The kidneys (urinary system) working in conjunction with the
endocrine system are responsible for extracellular K+ and Na+
homeostasis
Ca2+ ATPase Pump
One Ca2+ ion is extruded for each ATP hydrolized.
The sarcoplasmic reticulum and the ER contain variants of Ca2+
ATPase that transport 2 Ca2+ ions/ATP from intracellular fluid to the
sarcoplasmic or endoplasmic reticulum
H+ K+ ATPase Pump
H+ K+ ATPase is found in the parietal cells of the gastric mucosa and
in the α-intercalated cells of the renal collecting duct.
In the stomach, it pumps H+ from the ICF of the parietal cells into the
lumen
Secondary Active Transport
•  The ion pump builds up a difference in charge, or electrical
potential, across the membrane.
•  One side of the membrane gains a more positive or negative
charge compared to the other side due to the accumulation of
positive or negative ions
•  The combination of concentration gradient and electrical potential
is called an electrochemical gradient
•  This gradient stores potential energy that can be used by the cell
•  This energy is used by another protein to transport other
molecules across a membrane à Secondary active transport
Secondary Active Transport
•  Cotransport (symport):
–  Molecule or ion moving in the same direction.
•  Countertransport (antiport):
–  Molecule or ion is moved in the opposite direction.
Secondary Active Transport
•  Cotransport (symport)
Solutes are transported in the same direction across the cell
membrane.
Na+ gradient established by the Na+ K+ ATPase is used to
transport solutes such as glucose, aminoacids, K+ or Cl- against
electrochemical gradients
•  Glucose transport is an example of:
–  Cotransport
–  Primary active transport
–  Facilitated diffusion
Secondary Active Transport
•  Countertransport (antiport):
–  Molecule or ion is moved in the opposite direction across the
cell membrane.
–  Example: Ca2+/Na+ exchange (maintain the intracellular Ca2+
concentration at very low levels 10-7M
–  Example: Na+/H+ exchange
As with contrasport, each process uses the Na+ gradient
established by the Na+/K+ ATPase as an energy source
Cell Membrane Potential –
General Concepts
+++++++++++
Ion channels
Ions Cannot Diffuse Across the Hydrophobic Barrier of
the Lipid Bilayer
Ion Channels that are integral membrane proteins Provide a Polar
Environment for Diffusion of Ions Across the Membrane
Passive transporters
-Ions flow from high to low concentration
-No energy is used
Ion channels
• 
• 
• 
• 
• 
Small highly selective pores in the cell membrane
Move ions or H2O
Fast rate of ions transport
Transport is always down the gradient
Cannot be coupled to an energy source
Ion channels are controlled by gates
• 
• 
• 
Two discrete states;
open (conducting)
closed (nonconducting)
Some channels have also
inactivated state (open but non
conducting)
Part of the channel structure or
external particle blocks
otherwise open channel
Ion channels
Ligand-gated channels à gates
controlled by neurotrasmitters,
hormones and second messengers.
Nicotinic acetylcholine receptor
Ion channels
Mechanical stimulus, heat
(thermal fluctuations)
Voltage-gated channels have
gates that are controlled by
changes in membrane
potential.
Diffusion potential
Diffusion potential à potential difference generated across the
membrane when an ion diffuses down its concentration gradient.
Thus a diffusion potential is caused by diffusion of ions.
Cell Membrane Potential –
General Concepts
•  Ion Concentrations (millimoles/liter)
Eion at 37° C Permeability
Ion
ECF
ICF
Na+
150
15
+60mV
.04
K+
5
150
-90mV
1
Ca2+
1
.0001
+122mV
negligible
Cl-
108
10
-63mV
negligible
Cell Membrane Potential –
General Concepts
All cells have a potential difference across their plasma membrane.
The potential or chemical charge inside of the cell is different to
that of the solution outside of the cell.
This potential difference is referred to as the membrane potential.
•  All cell membranes produce electrical signals by ion movements,
but transmembrane potential is particularly important to neurons,
because rapid changes in the membrane potential of neurons bring
about the nervous impulse, which is the basis of neuronal
signaling.
•  In muscle cells, changes in the membrane potential bring about
contraction.
•  In endocrine cells, changes in the membrane potential bring about
release of hormones.
Cell Membrane Potential –
General Concepts
It is important to keep in mind that the potential difference occurs at
the level of the cell membrane.
•This is because biological membranes can act to allow separation
of electrical charge, by separating solutions in two compartments
by the very short-distance, non-conducting, hydrophobic core of the
membrane (=3 nm).
•Charge separation across the membrane leads to an electric field
across the membrane.
•This electric field gives rise to the measured membrane potential.
Cell Membrane Potential –
General Concepts
Membrane Potential
A membrane potential results from separation of positive and
negative charges across the cell membrane (potential difference).
In order for a potential difference to be present across a membrane, two
conditions must be met:
①  There must be an unequal distribution of ions of one or more species
across the membrane (concentration gradient)
②  The membrane must be permeable to one or more of these ion species.
The permeability is provided by the existence of channels or pores
Cell Membrane Potential –
General Concepts
There is a difference in the concentration of K+ between the inside
of the cell and the outside of the cell.
The membrane is highly permeable to K+. This means that K+ can
get through the membrane with ease.
The majority of the negative charges
(anions, A) inside the cell are proteins
so that for every K+ inside the cell there
is a negative charge on a protein
(not totally true, but nearly true).
Cell Membrane Potential –
General Concepts
Why do not all of the intracellular K+ ions leak out of the cell?
•The negative charge that has built up holds K+ ions in. After the first
few potassium ions leave, the inside of the cell becomes more negative.
•At this time two opposing forces act on K+ ions.
v One force tends to make K+ ion leave the cell, and that force is the
difference in K+ concentration (chemical gradient).
v The other force results from the
accumulated negative charge inside
of the cell that tends to prevent K+ from
leaving the cell (electrical gradient).
K+ is a positively charged ion and the
inside of the cell is now negative.
Cell Membrane Potential –
General Concepts
The cell membranes of nerves, like those of other cells, contain
many different types of ion channels.
Some of these are voltage-gated and others are ligand-gated.
It is the behavior of these channels and particularly Na+ and K+
channels, which explain the electrical events in neurons.
Cell Membrane Potential –
General Concepts
Membrane potentials are established primarily by three factors which
act on ions:
①  the concentration of ions on the inside and outside of the cell, and
their asymmetric distribution across the membrane to form a
concentration gradient (Na+, K+).
②  by the activity of electrogenic pumps (e.g., Na+/K+-ATPase and
Ca2+ transport pumps). These are special proteins in the membrane
that maintain the ion concentrations across the membrane by
moving ions.
③  the selective permeability of the cell membrane to those ions (i.e.,
ion conductance or electrical force) through specific ion channels
(K+ channels and Na+ channels).
•These parameters maintains a charge difference across the membrane
(i.e resting potential of -70 mV in nerve cells)
Equilibrium potential
Equilibrium potential is the diffusion potential that balances or
opposes the tendency for diffusion down the concentration
difference.
At electrochemical equilibrium, the chemical and electrical
driving forces acting on an ion are ugual and opposite.
"Balance" means that the electrical force that acts to move the ions
tends to increase until it is equal in magnitude but opposite in direction
to the tendency for net movement of K+ due to diffusion.
Equilibrium potential
The Nernst equation is used to calculate the equilibrium potential
for an ion at a given concentration difference across a membrane
considering that the membrane is permeable to ion.
E = equilibrium potential (mV)
2.3RT/F = constant (60 mV at 37o C)
z = charge on the ion (+1 for Na+; +2 for Ca2+
Ci = Intracellular concentration (mmol/L)
Ce= Extracellular concentration (mmol/L)
T = absolute temperature
R = gas constant
F = Faraday’s constant
Nernst equation converts a concentration difference for an ion into a
voltage. Membrane potential is expressed as intracellular potential
relative to extracellular potential.
Equilibrium potential
Tipical concentration gradients across the membrane: ENa+ = +65mV
ECa2+ = +120mV
EK+ ≈ -85mV
•  Each permanent ion attempts to drive the membrane
potential toward its own equilibrium potential.
Equilibrium potential
“The membrane potential can change over time, allowing signals to be
transmitted. These changes in membrane potential are caused by particular ion
channels opening and closing, and thereby changing the conductance of the
membrane to the ions. You can understand the changes in membrane potential
by using the Nernst potential for each ion”
THREE STANDARD NERNST POTENTIALS:
Nernst potential for Na+ = +55mV,
Nernst potential for Cl- = -65mV,
Nernst potential for K+ = -90mV.
The Nernst potential is the voltage which would balance out the unequal
concentration across the membrane for that ion.
For example, a positive voltage (+55) inside the neuron would keep the high
concentration of positive Na+ ions outside the cell.
A large negative voltage (-90mV) would hold the positive K+ ions inside the cell.
Opposites attract, similar charges repel each other.
Equilibrium potential
REMEMBER THE FOLLOWING: When the membrane conductance increases
for a particular ion, the membrane potential will move toward the Nernst
potential for that ion.
For example, if the neuron is at resting potential (-70mV) and the conductance to
Na+ increases, the membrane potential will be depolarized (it will move toward
+55mV). When the conductance to Na+ goes back to its original value, the
membrane potential will return to the resting potential. If the neuron is at resting
potential (-70mV) and the conductance to K+ increases, the membrane potential
will be hyperpolarized (it will move toward -90mV).
Transmission along the axon of a neuron occurs due to sequential activation of
voltage-sensitive sodium and potassium channels. These cause large but brief
changes in membrane potential which are referred to as action potentials (spikes).
Neurons communicate with other neurons by releasing transmitters which change
the conduc- tance of other neurons to various ions. The changes induced by this
synaptic input are called synaptic potentials.
Cell Membrane Potential –
General Concepts
The force established across the membrane creates a
resting potential
Ø  Potential means a separation of charge. In this case the
separation is across the membrane
Ø  Resting means that no current is flowing across the membrane
Resting Potential à The potential difference between the two sides
of the membrane of a nerve cell when the cell is not conducting an
impulse
There is an excess of positive charges outside and negative charges inside the
membrane.This gives rise to an electrical potential difference, which ranges from
about 60 to 70 mV. This potential difference is maintained because the lipid
bilayer acts as a barrier to the diffusion of ions.
Resting membrane potential
² It is the potential that would be maintained if there were
no action potentials, synaptic potentials, or other active changes in
the membrane potential.
² The resting membrane potential is dominated by the ionic species in
the system that has the greatest conductance across the membrane.
For most cells this is potassium (K+).
In neurons the resting membrane potential is usually about -70mV, which is close to
the equilibrium potential for K+. Because there are more open K+ channels than Na+
channels at rest, the membrane permeability to K+ is greater. Consequently, the IC
and EC K+ concentrations are the prime determinants of the resting membrane
potential, which is therefore close to the equilibrium potential for K+. Steady ion leaks
cannot continue forever without dissipating the ion gradients. This is prevented by the
Na, K ATPase.