6.021/Notes/2006-10-16

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  • Rest is not equal to equilibrium
    • eventually all ions would equilibrate inside and outside and there would be no membrane potential
    • For this to not happen, must have pumps (primary active transport)
    • Pumps allow us to go from rest to equilibrium
    • treat as current source
    • for all where is the active current (pump) and is the passive current (electro-diffusion).
    • Define quasi-equilibrium: no net flux but requires energy
      • The first term is the "indirect effect" whereas the second term is the "direct effect" of the pump
      • Both terms depend on the pump as without the pump, both would be 0
    • All pumps have indirect effect but only some pumps have direct effect
    • A non-electrogenic pump has no net charge change (e.g. trade one sodium ion for one potassium ion)
    • An electrogeneic pump such as 3 sodium for 2 potassium would have net active current
  • Active transport gets energy from glucose metabolism (namely ATP)
  • Experiment shows that sodium and potassium transport are linked
    • Sodium is needed for potassium transport and vice-versa
    • Sodium is pumped out, potassium pumped in
    • The direct effect on membrane potential is negative