Electrochemical Equilibria

  • Created by: LBCW0502
  • Created on: 25-09-18 11:41
What are the three ways electrical events can be measured in cells?
Intracellular (electrode inside cell), extracellular (electrode outside cell) and patch clamping (electrode sealed to cell surface)
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What is a microelectrode?
A piece of glass tubing pulled to a very fine point (1 micron diameter) filled with good electrical conductor (concentrated salt solution e.g. potassium dichromate)
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At rest, what is the electrical potential of the inner membrane?
Negatively charged with respect to the outside
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When cells become active, what happens to the potential of the inner membrane?
Becomes positively charged (depolarised)
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What is the purpose of the Na-K ion pump?
Hydrolysis ATP to release energy. Allows 3 Na ions to leave and 2 K ions to enter. Formation of concentration gradients
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Describe the model to two compartments where compartment 1 has a higher concentration of KCl compared to compartment 2 and the membrane is selectively permeable to K ions
There is movement of K ions down its concentration gradient (from 1 to 2), leaving a more negative charge in compartment 1. Movement of K ions leads to attraction of Cl ions (negative charge stacks up on membrane)
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Which two gradients are present in the compartment model?
Concentration gradient (moving K ions out of cell) and electrical gradient (pulling K ions into cell) - analogy of ball and spring on steep level
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What is an electrical capacitor?
E.g. membrane stores electrical charge
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Why do the compartments reach equilibrium?
The force of the concentration gradient pushing out K ions is equal to the electrical gradient pulling in K ions. No net movement. Electrical force is equal to osmotic force (equilibrium potential)
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How can the equilibrium potential be determined given the concentration gradient?
Using the Nernst equation
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Describe features of the Nernst equation
The equilibrium potential of any ion (x). Gas constant (R), absolute temperature (T, in Kelvin). z is valence (electrical charge of permeant ion) and F is Faraday's constant
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What is Faraday's constant?
The amount of electrical charge contained in one mole of a univalent ion
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How does osmotic work relate to concentration gradient?
Pump derives energy from hydrolysis of ATP and uses this to make concentration gradients (conc. gradient is a store of potential energy to perform osmotic work)
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How do you calculate the concentration gradient?
[C] out / [C] in (for positively charged ion). [C] in / [C] out (for negatively charged ions)
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What does osmotic potential depend on?
Number of particles in solution and their kinetic energy
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What is voltage a measure of?
Work done in separating the charges
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As R and F are constants, how is the Nernst equation written for monovalent ions?
E = 58 (mV) . log [C] out / log [C] in
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What is the value for the resting potential?
-70 mV
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Which ion principally determine the resting potential?
Sodium and potassium ions
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What is the equilibrium potential for an ion?
The membrane voltage that a cell needs to prevent movement of that ion down its concentration gradient
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What would happen to potassium ions if the inner membrane was very negative?
Potassium ions would stop leaving (at -90 mV)
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What would happen to sodium ions if the inner membrane was very positive?
Sodium ions would stop entering (at + 60 mV)
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Why is Vm closer to Ek than ENa?
Membrane is about 50 time more permeable to K+ than Na+
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At constant Vm why is the net flow of ions is zero?
The passive leak of K+ out is matched by leak of Na+ in
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What happens when a cell becomes permeable to an ion?
That ion will move down its electrochemical gradient and will drive Vm towards the equilibrium potential for that ion
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Does EK or ENa dominate Vm?
EK
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How does permeability affect conductance?
The more permeable a membrane is to K+ ions (more potassium ion channels), the more conductance takes place (more K+ ions move out of cell)
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What are the six properties of the action potential?
Triggered by depolarisation, threshold of depolarisation required for AP, all or none, propagates without decrement, at peak Vm approaches ENa, after AP membrane is inexcitable during refractory period
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The action potential is due to which type of channels?
Sodium and Potassium voltage-gated channels. Channels are open or closed. Probability of opening/closing is determined by voltage across channel. Channels are highly ion-selective
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Describe the structure of a potassium ion channel
The ion channel is surrounded by four sub units (helices) which cause the conformational change in shape of the channel to allow opening and closing of channel
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During the action potential the membrane becomes permeable to which ion first?
Sodium (and then potassium) - membrane is driven to ENa and then EK
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Give an example of positive feedback
Vm to ENa. Na influx, depolarisation, opening of Na channels quickly, prolonged depolarisation causes sodium inactivation
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Describe the steps for Vm to EK
Depolarisation, opening of K channels (slowly), K efflux, repolarisation
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Does Na or K have higher conductance?
Na
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Outline the process for generating an action potential
Membrane becomes depolarised, voltage-gated Na+ channels open, influx of Na+, at ~ + 30 mV voltage-gated Na+ channels close and K+ channels open, efflux of K+, ~-75 mV K+ channels close and hyperpolarisation occurs (back to -70mV) - graph
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What happens when sodium or potassium ion channels open?
They are pulled out of the cell but remain lined up on the membrane due to electrical attraction (electrostatic forces of attraction)
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Other cards in this set

Card 2

Front

What is a microelectrode?

Back

A piece of glass tubing pulled to a very fine point (1 micron diameter) filled with good electrical conductor (concentrated salt solution e.g. potassium dichromate)

Card 3

Front

At rest, what is the electrical potential of the inner membrane?

Back

Preview of the front of card 3

Card 4

Front

When cells become active, what happens to the potential of the inner membrane?

Back

Preview of the front of card 4

Card 5

Front

What is the purpose of the Na-K ion pump?

Back

Preview of the front of card 5
View more cards

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