Lecture 9-10 Flashcards
What is the plasma membrane structure?
Formed by phospholipid bilayer - great example of self assembly with purpose
Allows cells to communicate with external environment
What are the types of channel proteins?
- Resting K+ channel = always open
- Voltage-gated channel = opens transiently in response to changes in membrane potential
- Ligand-gated channel = opens and closes in response to specific extracellular neurotransmitter
- Signal-gated channel = opens and closes in responses to specific intracellular molecule
Resting K+ channels
Generates resting potential across membrane
- Resting potential = voltage difference of plasma membrane before any action potential
- Usually 70 mV
Voltage-gated channels
Propagates action potential along plasma membrane in neurons
Specific voltage (provided by stimulus) is needed to activate these channels
Ligand-gated channels
Found on dendrites and cell bodies - generating electric signals in postsynaptic cells
- Binding site for specific extracellular neurotransmitter – bound activates channel
- Respond to external stimuli
Signal-gated channels
Found on dendrites and cell bodies - generating electric signals in postsynaptic cells
- Respond to intracellular signals resulting from neurotransmitter binding to distant receptor
What can channels be used for?
Action potential = voltage-sensitive channels in neurons that open + close in response to voltage across the membrane
Neurotransmitters = ligand-gated channels open + close in response to binding of ligand molecule
Sensory neurons: ion channels open + close in response to other stimuli (light, temp, pressure)
Modelling plasma membrane: plasma membrane + potential
Membrane potential = electrochemical gradient caused by charged/polar molecules
Ions pass through membrane via facilitated diffusion (voltage-sensitive ion channels) + active transport (pumps)
Modelled as electric circuit - resembles P-N junction
Modelling plasma membrane: properties of electrical circuit
Voltage = difference in electric potential between 2 points (defined as work needed per unit of charge to move test charge between 2 points).
Current = stream of charged particles (electrons/ions) moving through electrical conductor or space. Measured as net rate of flow of electric charge through surface.
Resistance = force that counteracts flow of current + used in electrical circuits to alter current and voltage.
Conductance = measure of how easily electricity flows through electrical components for given voltage difference.
Capacitance = ratio of amount of electric charge stored on conductor to difference in electric potential
Modelling plasma membrane: components of electrical circuit
Battery = converts chemical energy directly into electric charge using redox
Resistor = electrical component that implements electrical resistance as circuit element. Reduces current flow, adjusts signal levels and divides voltages, etc.
Capacitor = stores electrical energy in electric field. Made up of 2 electric conductors with dielectric medium in between
Cell membranes modelled by electrical circuits: Hodgkin-Huxley model
Lipid bilayer = capacitance
Voltage-gated and leak ion channels = non-linear (Gn) and linear (Gl) resistors
Electrochemical gradients = drive flow of ions represented by batteries (E)
Ion pumps + exchangers by current source (Ip)
Hodgkin-Huxley Model: Nernst Potential
Reversal potential of ion in membrane = membrane potential at which there is no net flow of that ion from one side to the other of the membrane
P-N Junction
Boundary between 2 types of semiconductor materials (p-type and n-type)
Created by doping
Electrical current only passes through junction in 1 direction
Plasma membrane + PN junction
Na+ = P-doped (excessive positive charges)
Lipid bilayer = depletion layer (no net charge)
K+ = N-doped (excessive negative charges)
Semiconductors
Group IV elements
Allows them to create 4 bonds with neighbours - stable lattice
Makes hard for electrons to move around - electricity can’t really flow through these materials
Doping
Introducing impurities to intrinsic (pure) lattice structure) can change its electrical properties
n-type = group V (phosphorous, arsenic)
- This electron can move across lattice - creating electric current
p-type = group III materials (boron, gallium)
- This hole can move across the lattice - creating electric current
Increases conductivity of semiconductors
resting membrane potential
-50mV to -90mV
inside of the cell is negative compared to the outside.
difference in charge only exists at the plama membrane
resting K+ channel
always open
voltage-gated channel
open transiently in response to change in the membrane potential
ligand-gated channel
open)closes) in response to a specific extracellular neurotransmitter
signal-gated channel
open(closes) in response to a specific intracellualr molecule
Nernst equation
electric potential across the membrane can be calculated via Nernst equation, if ion concentrations and permeability are knwon
depolarization
depolarizaiton spreads in both directions: passive propagating of depolariation signal
Threshold
action potnetial has to be greater than threshold
