2. 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)

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

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

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

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

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

8. Cation Distribution Cations may become more concentrated inside than outside inward flow along electrical gradient countered by outward flow along concentration gradient

9. 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

10. Cation Distributions 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 Typical Concentrations in ECF and ICF

11. 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

12. Equilibrium Potential Equilibrium potential is calculated for a particular ion using the Nernst Equation –E x = 61/z log [X o ]/[X i ] –Equilibrium potential for K + = -90 mV –Equilibrium potential for Na + = +60 mV

13. 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 E K, but higher since some Na + enters the cell –K + tends to leak out of the cell under normal conditions

14. Resting Potentials Concentrations of Na + and K + inside the cell are maintained using Na + /K + pumps

15. 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