Electrochemical Driving Force PDF

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Summary

Lecture 2 in Week 3, on electrochemical driving force (with diagrams)...

Description

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To know the concentrations of sodium, potassium, chloride and calcium ion concentrations inside and outside cells To understand the Nernst Equation and use it in calculations To understand the Electrochemical driving force (ECDF) and the net movements of ions across the membrane To calculate the magnitude and direction of the ECDF

Membrane potential 

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Caused by the separation of charges across the lipid membrane o Positive charge extracellularly o Negative charge intracellular Membrane potential is perpetuated by active transport mechanisms and carriers/ion channels Ion concentrations Extracellular (mM)

Intracellular (mM)

Na

142

10

K

4

140

Ca

2.4

0.0001

Cl

103

4

Nernst Equation 

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Diffusion of ions in influenced by concentration and electrical potential o No net diffusional force o Assumes membrane is only permeable to one ion o Potential of membrane at which the chemical driving force = the electrical driving force (and the net flux = 0) is called the Reversal Potential Em is proportional to log (Co / Ci) Em = RT / F log (Co / Ci) o RT / F = 61 at 37° and 58 at 20° o For negative ions use a - sign at the front (for chloride) o Em = RT / zF ln (Co / Ci)

ECDF 

Movement of charged particles depends on the Electrochemical Driving Force (ECDF) o Sum of force generated by the chemical gradient and electrical field

Chemical driving force is the movement down the concentration gradient  The greater the gradient the greater the force  Electrical driving force is the result of unequal distribution of charges across a membrane  The direction of the force is the result of an ion moving towards a region where the opposite charge exists  For a cell, a -ve electrical driving force is moving outside, whereas a +ve driving force is going inwards  The magnitude of the force depends on the size of the membrane potential and the charge of the ion  The greater the membrane potential/charge of the ion the greater the electrical driving force  Ca2+ will have a greater electrical force than Na+ ECDF = EK where the difference is how far it is from resting membrane potential o Comparison of where the membrane is to where an ion wants to be Direction of driving force is away from the resting membrane potential Membrane potential (Vm) is the separation of charge across a membrane o ECDF  Chemical driving force (CDF) is from the concnetration gradient  Calculated from Nernst Equation  Electrical driving force (EDF) comes from the interaction between the charge of an ion and Vm  The driving force is the net electromotive force that acts on the ion  Magnitude of the force indicates how far Vm is from the electrochemical equilibrium (Ep)  How far na ion is from its equilibrum  ECDF = (Vm - Ep)  Ep = Nernst potential  -ve means inward  +ve means outward 

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