Lecture Two A

• Membrane Permeability:

  • Smaller non-charged particles can pass through relatively freely
  • Charged particles cannot pass through as freely
  • Some transmembrane proteins are ion channels
• Resting Membrane Potential:

  • Resting membrane potential is -70mV (more negative intracellular)
  • Ions tend to move because of electrical and concentration gradients
  • Diffusion occurs through 'leak channels'
  • Restored by Na/K pump which brings 2 K ions back into the cell in exchange for 3 Na ions
  • Cl- donnan equilibrium: Cl wants to move into the cell down the gradient, but the net (-) charge carried by the large protein anions keeps them out
• • The Donnan effect is the phenomenon of predictable and unequal distribution of permeant charge ions on either side of a semi-permeable membrane, in the presence of impermeant charge ions
• Equilibrium Potential:

  • Membrane potential at which there is no net passive movement of a permeant ion species into or out of the cell
  • Dependent on the intra- and extracellular concentrations of the ions
  • Difference between the membrane and the equilibrium potentials is known as the driving force
• Potential for Na:

  • Na tends to move into the cell down the gradient, also because of the net negative charge within the cell
  • When net movement is zero, the membrane reaches equilibrium and has an equilibrium potential of +60mV
• Potential for Cl:

  • Cl tends to move into the cell down the gradient, but the net negative charge carried by the anions keeps them out
  • Cl has a potential of -61mV
• Potential for K:

  • K has a potential of -88mV
  • Driving force: K+ will tend to diffuse out, Na+ will tend to diffuse in, Cl- is relatively happy
• Na+ K+ Pump:

  • Membrane is more permeable to K+ than Na+
  • The K+ leak channel is more leaky than the Na+ leak channel
• Membrane Potential:

  • You can change the membrane potential by changing the permeability to any of the ions
  • High Na+ low K+ permeability leads to depolarization
  • High K+, low Na+ permeability leads to hyperpolarization
  • Electrogenic Pump: transports unequal quantities of charge across the membrane, hyperpolarizing the cell relative to that predicted by the Goldman Equation