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Lecture 2

Lecture 2 - Membrane Potential

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University of Toronto St. George
Michelle French

PSY300 L2Membrane Potential step 1An enzyme ion pump must work to create aconcentration gradient of ion species acrossthe cell membrane 2semipermeable membrane allows one ionspecies to diffuse across more than others 3diffusion of that ion species down its concgradient creates electrical gradientNaK Pump NaKdependent ATPase is enzyme thatmoves Na out of cell and K into cell for each ATP molecule broken down 3 NaPore loops create the speicificity filter just for K not even ions are pumped out and 2 K pumped inwater consumes 13 of energy needs of bodyneurons used 23 since membrane pot more importantNaK inequalitygenerates potential difference of 10 Equilibrium potentialmV of the inside of the membrane relative to the outsideAt equilibrium electrical work to repelof the membrane which is 10mV since more cations on outward cation diffusion equals chemicaloutside not too importantuseful thowork of diffusion down conc gradientThe pump isnt fully responsible it just creates themembrane potential at equilibrium isconcentration gradientsdetermined by the concentration gradient can be calculated using Nernst EquationResting Potential resting membrane is most permeable to KNernst Equationion ERTF lnKo Ki90 mVK semipermeability to 1 ion species K diffuses out of cell Major part is concentration ratio for outside and inside down concentrationRTF is constant Rgas temp T absolute temp so 37C gradient via 2pore family of K channel proteins side by but in Kelvin since fairly constant body temp F is charge side typical for K familiesper mole of univalent ion would be double in bivalent ion cations accumulate outside membrane the equation gives the potential differenceleaving a net negativity inside membraneacross the membrane inside with respect tooutside at equilibrium the result is valid if and only if one ionspecies K in this case is diffusing acrossthe membrane ex For muscle cells like the heart where only K goes acrossBut not valid for neurons where not only K moves thru so 65 or 70 which means it isnt the only ion diffusing thruso its due to Cl and Na due to leaky membrane which cant exclude everything K is selected ion diffuses outward other ions generally perfectlystuckCl Na concentrated on outdise Cl due to transporters pushing it out inside the cell are anions Goldman equation for actually membrane potentiallproteins which cant move across the membrane orMembrane is most permeable to K but Naexocytosed so are stuck most of the time inside then tend and Cl ions are also diffusing somewhatto repel oter anions to outside the membrane electrostaticactual membrane potential can be calculatedrepulsionfrom an expanded nernst equation containing aK diffuses out until it reaches a state of electrostatic term for each diffusable ion speciesrepulsion once enoughcharge on the outside so can nofrom Goldman equation Em70 mV approx across longer diffuse them out therefore eqmso chemical work the membrane from inside to outsidebalanced by the electrical work pushing the cation back insidethis balance is described by the nerst eqnno need to memorize itTaking natural log of all the conc ratios K ratio is biggest contributing factor some permeability for Na going in the opposite direction note conc ratio for Cl is REVERSED
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