6.1 Nernst Derivation Flashcards
What 2 sets of energy need to be measured? (2 answers)
1) electrical gradient
2) concentration gradient
Volt
If potential is 1 volt it takes 1 joule of work to move 1 coulomb of charge
How many coulombs?
There are f coulombs of charge in 1 mole of univalent ions (Faradays constant: 96500)
Energy work due to electrical gradient
To move 1 mole of z-valent ions through a membrane potential of Vm Volts takes:
z x F x Vm
z= valence
F= Faradays constant
Vm= potential difference
(in joules)
Energy work due to concentration gradient
To move 1 mole of substance from a concentration ci (inside cell) to co (outside cell) takes:
R x T x ln(ci/c0)
R= gas constant
T= Temperature (K)
(in joules)
ln 1= 0 so if ci=co no work needed)
Total work
sum of electrical and concentration gradient:
z x F x Vm + R x T x ln(ci/c0)
When work greater than 0
energy needed to move ion across membrane (active transport)
When work is less than 0
energy is released when ions move across membrane (occurs spontaneously downhill)
When work is 0
no energy required or released (equilibrium)
Total work at equilibrium
= 0
Vm= R x T x ln(co/ci)/ z x F
ln(1/a)= -ln(a)
Convert to log 10
Vm= 2.303 x R x T x log10(co/ci)/ z x F
(answer in volts)
Charge for K+
+1
Charge for Na+
+1
Charge for Cl-
-1
Charge for Ca2+
+2
What does Nernst equation tell us?
equilibrium potential for specific ion
0 mV Potassium (2 answers)
K+ leaves down concentration gradient (no electric gradient)
K+ exit makes inside cell more -ve
-30 mV Potassium (2 answers)
K+ still leaves down concentration gradient but electrical gradient opposes this and slows it
Further K+ exit increases electrical gradient
-80 mV Potassium (2 answers)
Electrical gradient balances concentration gradient= equillibrium
No net K+ movement
Equilibrium potential
voltage at which membrane potential balances the concentration gradient
at equilibrium potential there is no net movement of ions
