Channel structure Flashcards
What effect do the contributions of Na+, K+, Ca+ and Cl- have on the resting membrane potential of a typical cell
Change membrane potential.
What is channel gating?
GATING is a reversible process by which a channel opens and closes and therefore limits ion flux in or out.
It is regulated by the conformation of the transporter.
Permembility has no effect
Can be voltage (charged ions) or ligand binding
What is channel inactivation?
channel driven by maintained depolarisation to different non-conducting other than closed state.
This is mediated by an INTRACELLULAR GATE that bind the the mouth of the pore and acts as a blocker.
The gate is made of THREE HYDROPHOBIC AMINO ACIDS (I, F, M) that “flop over pore”
What is channel deactivation?
Channel can return to closed state after opening
Explain the reason for channel state regulation.
x
Describe the structural adaptations that enable control of channel gating, channel inactivation, and ion selectivity for a voltage-gated ion channel
Sodium, and calcium channels are very similar; potassium channels are one quanter of the other chnnel types (smaller)
The fours loops from each subunit of sodium channel come together in the central pore and determine FLOW and SELECTIVITY
Describe ion channels and their function.
The proteins of the cell membrane which mediate ion transfer are ion channels.
The cell plasma membrane acts as a barrier separating the contents of the cell from outside.
The ionic concentrations of salts on either side of the membrane are kept at a different level inside cells compared to outside.
This distribution of ions is maintained by ionic pumps and exchangers and so consumes energy (ATP).
The result of this ionic imbalance is an ELECTRIC POTENTIAL DIFFERENCE (and ELECTROCHEMICAL GRADIENT) between the cytoplasm and the external medium.
electrochemical gradients allows ion flow along the concentration gradient; as long as the channel is permeable (to that ion)
What is the equation for conductance.
conductance (G) : the on influence of electrical CURRENT on ion flow.
Resistance = 1/ conductance(g)
Ohm’s law:
CURRENT (I) = CONDUCTANCE X VOLTAGE
How are electrochemical gradients produced?
Active transport (Na+ pump has selective membrane permeability to certain ions and molecules.
1) At rest sodium will tend to LEAK IN and potassium to leak out. The balance between these determines the Resting Membrane Potential (RMP).
2) The RMP is driven MOSLY by potassium efflux giving an overall negative (inside cell) potential
Imagine that the membrane allows only K+ ions to diffuse across it (cell is said to be PERMEABLE only to K+).
Potassium moves from inside to outside; sodium in side, the electrical gradient acts to slow movement of ions until the concentration and electrical gradients balance and ion reach EQUILIBRIUM.
Higher lower concentration outside the cell (potassium of 5mM) than inside the cell (100mM) will give a negative gradient of -80
What is Equilibrium potential of potassium?
Membrane potential difference at which movement DOWN concentration gradient equals movement DOWN electrical gradient.
where the Electrical gradient EQUAL AND OPPOSITE the concentration gradient
It value is given by the NEARST equation (Ek)
Why is the Ek of potassium not exactly the resting membrane potential?
the equilibrium potential from the Nernst equation is -80 mV.
However, most mammalian cells have RMPs MORE POSITIVE (-60 mV) as there is always some LEAKAGE pathway(s) for Na+ (sometimes Cl– ) that contribute
The RMP therefore cannot reach its Nersterian K+potential
What is relationship between the M.P and membrane permeability
the extent to which each ion gradient influences the membrane potential is defined by the permeability of the membrane to that ion (Pi).
If Pi value is small, even with a large concentration gradient, an ion has little influence on M.P
Depolarisation of MP is caused by increasing Na permeability.
But K+ has a bigger influence; if K+concentration gradient is small, LESS K+ will leave and cell will still be quite positive (not as big a depolarisation/hyperpolarisation amplitude? )
Permeability is regulated in order to control cell functions.
What is the Nearst Equation?
Nernst equation displays the theoretical balance
between electrical and concentration gradients
What is the Goldman Equation?
Goldman equation reflects the REAL situation
of finite and variable ionic permeabilities; the gradients are stable though
Describe the gradients in a normal resting cell
At -65 mV the outward chemical gradient IS GREATER THAN inward electrical gradient hence K+ current is outwards so K+ will travel OUTWARDS.
As K+ leave cell, cell becomes positive and depolarises.
If cell made negative, K+ will follow electrically gradient INWARDs.
