Lecture 9: Electrical properties of biological membranes Flashcards
What are the different channel proteins present in the plasma membrane ?
- Resting K+ Channels
- Voltage-gated channels
- Ligand- gated channels
- Signal-Gated Channels
How do the resting K+ channels work?
Generate the resting potential across the
membrane
● The resting potential is the voltage difference
of the plasma membrane before any action
potential
● The resting potential is usually around ~70
mV
How do the voltage-gated channels work ?
● Responsible for propagating action potential
along the plasma membrane in neurons
● A specific voltage (provided by a stimulus) is
needed to activate these channels
How do the ligand-gated channels work?
Have a binding site for a specific
extracellular neurotransmitter, which
when bound activates the channel
● Respond to external stimuli
How do the signal-gated channels work?
Respond to intracellular signals
resulting from a neurotransmitter
binding to a distant receptor
What can the channels be used for?
● Action potential: Voltage-sensitive channels in neurons and open and
close in response to the voltage across the membrane.
● Neurotransmitters: Ligand-gated channels open and close in response to
the binding of a ligand molecule.
● Sensory Neurons: Ion channels open and close in response to other
stimuli, such as light, temperature or pressure.
How can we model the plasma membrane ?
● Membrane potential: electrochemical
gradient caused by charged or polar
molecules
●Ions pass through the membrane via
facilitated diffusion (voltage –sensitive
ion channels) and active transport
(pumps)
● Can be modelled as an electric circuit,
and closely resembles a P-N junction
What are the properties of an electric circuit?
- Voltage (V, in volts): The difference in electric potential between two
points which is defined as the work needed per unit of charge to move a
test charge between two points. - Current (I, in amperes): A stream of charged particles (such as electrons
or ions) moving through an electrical conductor or space. It is measured
as the net rate of flow of electric charge through a surface. - Resistance (R, in ohms): The force that counteracts the flow of current.
Resistors can be used in electrical circuits to alter the current and voltage. - Conductance (G, in siemens): The measure of how easily electricity flows
through electrical components for a given voltage difference. - Capacitance (C, in farad): The ratio of the amount of electric charge
stored on a conductor to the difference in electric potential.
What are the components of an electrical circuit?
Battery: Device that converts chemical energy contained
within its active materials directly into electric charge,
using an electrochemical oxidation-reduction (redox)
reaction.
Resistor: Electrical component that implements electrical
resistance as a circuit element. In electric circuits,
resistors are used to reduce current flow, adjust signal
levels, divide voltages, etc.
Capacitor: Electrical component that stores electrical
energy in an electric field. Most are made up of 2 electric
conductors, with a dielectric medium between them.
How can cell membranes are modelled by electric circuits ?
● Hodgkin-Huxley model
○ Uses conductance to describe how action potentials are
initiated and propagated, i.e. the biophysical
characteristics of cell membranes.
● The lipid bilayer is represented as capacitance
(Cm)
● Voltage-gated and leak ion channels are
represented by non-linear (Gn) and linear (Gl)
resistors
●Electrochemical gradients driving the flow of
ions is represented by batteries (E)
●Ion pumps and exchangers by current source
(Ip)
Describe the Hodgkin-Huxley Model.
Nernst Potential:
● In a biological membrane, the reversal potential of an ion is the
membrane potential at which there is no net flow of that particular ion
from one side of the membrane to the other
Using a series of voltage clamp experiments and by varying extracellular
Na+ and K+ concentrations, Hodgkin and Huxley developed a model in which
the properties of an excitable cell are described by a set of four ordinary
differential equations.
● However, it is difficult to study because it is a nonlinear system which
can not be solved analytically.
● This system is based on probabilities of channel open/closed states
What’s a positive-negative (P-N) Junction ?
Boundary between two types of semiconductor materials (p-type and
n-type)
● The P-N junction is created by doping
● Electrical current only passes through the junction in one direction
What are the semiconductors?
●Group IV elements of the periodic table have 4
electrons in their valence shells.
● This allows them to create 4 bonds with their
neighbors, resulting in a stable lattice.
● However, this arrangement makes it really hard
for electrons to move around, and thus
electricity can’t really flow through these
materials.
○ A silicon crystal, for example, is nearly an insulator
What’s the concept of doping ?
●Introducing impurities to the intrinsic (pure) lattice structure (i.e.,
semiconductor group IV element) can change its electrical properties.
●There exist two types of doping, n-type and p-type
●In n-type group V materials such as arsenic or phosphorus are added in
small quantities
○ These have 5 valence electrons, one free electron is left after bonding with Si. This electron
can move across the lattice, creating electric current.
●In p-type group III materials such as boron or gallium are added.
○ These have 3 valence electrons, one free hole after bonding with Si. This hole can move
across the lattice, creating electric current.
●This increases the conductivity of the semiconductors!
How are values for ion concentration experimentally obtained ?
The dynamics of the ion channel
can be studied using the patch
clamp technique. This technique
involves a glass micropipette
which forms a tight gigaohm seal
with the cell membrane. The
micropipette contains a wire
bathed in an electrolytic solution
to conduct ions. To measure
single ion channels, a “patch” of
membrane is pulled away from
the cell after forming a gigaohm
seal.
