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Membrane Potentials

Membrane Potentials. All cell membranes are electrically polarized Unequal distribution of charges Membrane potential (mV) = difference in charge across the membrane Interior of the cell contains negatively charged proteins and phosphate groups Cannot pass through membrane (fixed anions).

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Membrane Potentials

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  1. Membrane Potentials • All cell membranes are electrically polarized • Unequal distribution of charges • Membrane potential (mV) = difference in charge across the membrane • Interior of the cell contains negatively charged proteins and phosphate groups • Cannot pass through membrane (fixed anions)

  2. Membrane Potentials • Tend to attract cations (Na+, K+, Ca2+, etc.) to extracellular surface of membrane • Some cations can enter through channel proteins in the membrane

  3. When is equilibrium achieved?Let + move into the cell…

  4. When is equilibrium achieved?Not when membrane potential = 0

  5. When is equilibrium achieved?Not when concentration of (+) is equal

  6. When is equilibrium achieved?When the electrical and concentration gradients balance one another

  7. Cation Distribution • Cations may become more concentrated inside than outside • inward flow along electrical gradient countered by outward flow along concentration gradient See Fig 6.21

  8. Example: Potassium • Membrane is more permeable to K+ than other ions • more K+ ion channels than any other type • K+ enters cell • reaches equilibrium point between concentration gradient and electrical gradient • not enough K+ in the cell to balance negative charges • Cell is more negative inside than outside See Fig 6.21

  9. Cation Distributions Typical Concentrations in ECF and ICF • For K+ • Concentration gradient draws ion toward ECF • Electrical gradient draws ion toward ICF • For Na+ • both concentration and electrical gradients draw ion toward ICF

  10. Equilibrium Potential • Equilibrium (no net movement) will be reached when a particular electrical potential is reached • Equilibrium potential = electrical potential at which the net flow of ions across the membrane is 0 • balance between EG and CG is achieved See Figs 6.21, 6.22

  11. Equilibrium Potential • Equilibrium potential is calculated for a particular ion using the Nernst Equation • Ex = 61/z log [Xo]/[Xi] • Equilibrium potential for K+ = -90 mV • Equilibrium potential for Na+ = +60 mV

  12. Resting Potentials • Movements of different ions across the membrane influence each other • Typical membrane potential for cells (resting potential) = -65 to -85 mV • close to EK, but higher since some Na+ enters the cell • K+ tends to leak out of the cell under normal conditions See Figs 6.23, 6.24

  13. Resting Potentials • Concentrations of Na+ and K+ inside the cell are maintained using Na+/K+ pumps See Fig 6.25

  14. Resting Potentials • NOTE: Resting Potential  Equilibrium Potential for either Na+ nor K+ • Gradients used for coupled transport in most cells • Nerve and muscle cells = Excitable Membranes • Can rapidly change membrane permeability to Na+ and K+by opening gated ion channels • Membrane potential can undergo rapid changes away from resting level = electrical signal

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