Chapter 6 Notes

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Cell and Systems Biology
Melanie Woodin

CSB332H1S Ch 6 ReadingK diffusion out is selflimiting due to membrane potElectrical grad slow K efflux eventually stops it Ionic Basis of the Resting PotentialAt rest neuronwrt outsideat K eqm potEK conc grad effects exactly KHigh K inside Low NaCl insidebalance pot gradMembrane pot stops them from moving w conc Indiv K still enterleave but no net movementgradsIon flux itself produces the required membrane Changing outside Kchanges K eqm potchanges membrane potpotWhereas is change outside Clequivalent Eqm in model cell inn automaticinevitablechange in inner ClCl eqm potmembrane Rmbr Nernstpot unchangedE25ln KK 58log KK 58 KoioiPassive At rest Na constantly moves into celllog13085mVrecuding membrane K no longer in eqmK E58log ClCl58log ClClCloiioleaks out 58 log 13085mVIf no compensation then changes in inner KNaz 1 since anionActive Compensated by NaK pump 3Na out 2K balances Cl conc grad exactly for its intendency to move into cellOverall transports moreoutsince KCl only 2 ions that can move acrossboth contributes several mV to membrane potat eqm at 85mVmodel cell can exist indefinitely Resting membrane pot depends onwo any net gain or loss of ionsK eqm potElectrical NeutralityNa eqm pot charge separation across membraneexcess ofRelative permeability of membrane to NaKanions inside cations outsidePump ratiodoes not violate principles of electrical neutralityCl permeability important for electrical stabilityK diffusing out collect as excess cations against Cl eqm pot can beor wrt resting membrane outer membraneexcess anions closesly pot depends on Cl transport processesattracted to inner surfacePlays little role in resting membrane potThese Kanions in effect removed from intracellular bulk solnleaving it neutralElectrical signals generated in nerve cellsmuscle Cl diffusing in add to anions at inner surfacefibres primarily by changes in permeability of leave counterions in outer laterextracellular membrane to ions K Nasoln still neutralIncreases in permeability allows more movement Held to membrane by mutual attraction down electrochem gradsDue to activation of ion channelsbalancedIons in charged layer not stuck interchange freely w bulk soln identities change amt A Model Cellstays constantCaMg dont contribute to resting membrane pot membrane acts as capacitor separatingSomewhat higher concs in birdmammals than storing chargefrogs much higher in marine invertebrates exof ions at charged layer does not represent squidsignificant fraction of total ions in cell on insideon outsideIn cell at membrane pot 85mV approx Permeable to K ClK conc gradK leaves cell1100000 of the cells ions are at the inner membranePot gradK comes back inThese 2 grads in exact balance at restThe Effect of Extracellular KCl on Membrane Reverse grads for ClPotentialImpermeable to Na internal anions none outside Infinitely more extracellular fluid than volume of cellex protsMovement of ionwater inout dont affect To remain stable1Intracellularextracellular soln s must be extracellular concselectrically neutralIf double outside Koutside Na reduces to keep aavoid repulsionosmolarity unchanged reduces conc grad for K bbalancew outward initially electrical grad unchangednet K 2Cell must be in osmotic balance avoiding inward accumulates insidemembrane swellingshrinkagedepolarizesCl no longer in eqmCl move awhen total solute conc same on inside as inwardsKCl entry until new eqm in the outsideexample 68mV3Must be NO net movement of any particular ion KCl entry accompanied by water to maintain into or out of cellosmotic balanceslight increase in cell volume in our example by 35Ionic EqmKCl concs entered actually greater than More K inside squid axon more ClNaCa outside indicated values by 35 since the water in bloodseawater envt equilibriatedcompensated for this
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