Boheler 2 Flashcards
Where does Em(battery) of a cell originate?
two requirements: ion concentration gradient, ion-selective permeability of ion channels
by adding a charge difference (Q) across the lipid bilayer, a potential difference can be generated
Diffusion
a passive motion of molecules or ions from regions or higher concentration to regions of lower concentration
Rate of diffusion
depends on the concentration difference, the surface area, volume, and the permeability
Concentration gradient is
important for movement of molecules according to Fick’s Law
Electrical gradient
the electrical potential that acts on an ion to drive the movement of the ion in one or another direction
How does a cell establish concentration gradients?
Passive transport = the exergonic movement of substances across a membrane
Active transport = the endergonic movement of substances across a membrane that is couple to a exergonic reaction
The Na/K ATPase
maintains the electrochemical gradient in living cells
primary active transporter located on the plasma membrane
directly uses chemical energy in the form of ATP hydrolysis to move 3 ions of Na+ to the outside of a cell and 2 ions of K+ to the inside of the cell
it’s electrogenic because the charge movements and concentrations are not equal, and it establishes both a diffusion gradient and an electrical gradient across the membrane
ion channels
proteinaceous ‘holes’ in the membrane that ‘selectively’ allow ions to pass
Na and Ca channels
Four homologous domans (I-IV)
K channels
Most Vm-dependent K channels have the same overall structure except that each of the four domains is a separate protein, which assembles into a tetramer that is analogous to the Na and Ca channel
Permeation
the act of spreading through something, the movement of molecules through a membrane or interface
Conductivity
how well do ions pass through, it relates to the amount of current through a channel, given a specific transmembrane voltage and specific concentration of ions. Ion channels can pass ~10^6 ions/sec
Selectivity
how picky channels are for certain kinds of ions, is usually measured in a relative sense with PNa/PK for Na channels being 50-100:1
Gating
how the channels opens/closes
voltage-gated channels open in response to voltage with transitions at the single channel level that are very rapid («1 ms)
there are also ligand-gated channels that respond to ligands (not important to electrical signals)
Nernst Potential
the transmembrane voltage where there is no net current, the potential at which the electrical force acting on ion species X is equilibrated with the diffusional force
A master equation
Cm*dVm/dt = Icap = I1 + I2 +I3 + …
G
stands for conductance and is always positive
Goldman Hodgkin Katz equation
helps us calculate the Diffusion Potential when the membrane is permeable to multiple ions
The resting potential
the weighted sum of the Nernst potentials for Na, K, and Cl channels, the weights are the values of the conductances Gx at rest
In most cells
the dominant conductance at rest is GK
Vm, rest = GKVK/GK = Vk
Voltage clamp
potentiostatic circuit where a feedback amplifier compares a command voltage to the actual voltage against ground and delivers current to cancel any deviation
HH gating gates
independent, channel is open when all gates are open, probability that all n gates being open is the product of the probabilities that each individual gate is open, the gates are identical – thus the probability for each gate being open is the same
For each given voltage, the change of n(t) represents
the opening of the gate (activation), with a time constant tau_n
Total channel conductance is
GK = gKn^4
The K current through the channel is
IK = gK(Vm-VK)
gK is a function of
Vm, t
The rates of opening and closing (alphas and betas)
determine the time constant of n gate opening and the n steady-state value. They are functions of voltage
Na channel gating - like K channels but with a twist
superposition of activation and inactivation, Na channels activate much faster (open), Na channels inactivation (close) occurs with voltage and time, not a simple two-state model
gNa
= gNam^3h
m gates
similar to K channel activation gate (n gates)
h fate
the inactivation gate – has a different votlage-dependence than m gate
both m and h gates
have to be open to allow ion flux through the channel
