AGconc = RT In [X]2 [X] AGvolt = ZFEm LECTURE 3 Potential energy in the concentration gradient Potential energy in the electrical gradient The sum of these is the change in free energy from moving an ion across the mem- brane NERNST EQUATION Calculates the equilibrium potential for one ion (Eion) or for the whole membrane (Em) How much energy is needed to create a electrical/concentration gradient? · RT is the thermal energy; R = 8.315 J/K mol What happens to diffusion time when you increase temperature? · It decreases; time to dissipate gradient goes down · Energy to create the gradient increases Ion influence on membrane potential To influence membrane potential, there must be a concentration gradient across the membrane and some permeability for that ion. Higher permeability -> higher influ- ence on membrane potential Clicker: why do we only consider Cl-, K+, and Na+ in the Goldman equation because the permeability of other ions is very low under resting con- ditions GOLDMAN EQUAITON Nernst equation but with multiple ions RT In PR[K+] +PNA[Na+ ]. +Pa[Cl ] Em = F PR[K+ ]; + PNA[Na+ ]; + Par[Cl ]. All valences are 1 or -1, so can get rid of the z term, but must inverse the Cl- term P = permeability of that ion F is in joules/Volt mol three factors contribute to membrane potential
1. concentration gradient of ions across the membrane 2. relative permeability of these ions 3. charges of ions Polarity of the other membrane potential is set by the most permeable ion Most of membrane potential magnitude is set by the ion of highest permeability