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Current time:0:00Total duration:8:45

Video transcript

so in the last video we talked about how you have a higher concentration of potassium on the inside on 150 milli moles per liter and then you do on the outside let's say around five million moles per liter so just to just to recap some of the kind of important points we brought up we said that essentially what happens is that you have these potassium that are bound to little anions that are the green dots there and because the concentration gradient is going to want to make the potassium leave the cell it will and so it'll leave the cell let's say this little fellow will leave the cell here and he'll end up on the outside so by doing that he leaves that anion all by itself and if this continues to happen then these anions create this negative charge and we can actually figure out exactly what that negative charges it turns out that that negative charge is going to attract back the potassium right we said that this potassium then is going to want to kind of swim back inside to be closer to that negative charge and and this is kind of that interesting idea the idea that K leaves behind a negative charge and then comes right back and wants to be by that negative charge and the amount of negative charge that's going to offset the concentration gradient is around negative 92 so let me let me write that in now so negative 92 is the amount we know that we need to kind of offset the concentration gradient so that's where we left off and now I want to do a little thought experiment let's say that we come at this cell with a little injection full of let's say some positive charge and try to ignore the the kind of ridiculousness of what I'm saying just for the moment just focus on the positive charge the fact that I'm going to pour a bunch of positive charge into this cell and let's assume that we don't know exactly where this is coming from but that this positive charge is essentially what it's going to do is it's going to make my cell not negative 92 anymore right it's going to make it more positive than it is let's say I make it let's say halfway back to zero so instead of negative 92 it's negative 46 volts so this is the new membrane potential right and our cell is still just permeable to potassium and that's really important it's only permeable to one ion that's potassium so what's going to happen well these potassium these little guys right here they're going to notice that the charge is actually not drawing them back as as strong as it was before so this potassium might see that and it might leave and so more potassium basically starts leaving the cell and if more potassium is leaving and kind of going on the outside then you have more of these little anions that are left behind and the process continues right so these anions say well you know if we're we're all by ourselves we're going to contribute to this negative charge we're going to add to it just as just as it did before and that negative 46 is quickly going to go down again it's going to slide back down and the question is how far does it slide down well goes back to the equilibrium point so if we said negative 92 is what you need to make this yellow squiggly attraction the membrane potential equal the concentration gradient if that's what's needed then it will slide back down tonight negative 92 so think about that for a second it's pretty pretty powerful stuff I mean you can do all sorts of kind of funky things to the cell you can add positive charge or negative charge and as long as you maintain two things two important things one of them being the concentration gradient so one is this concentration gradient of 150 versus just five that's one thing and the other is the permeability to only potassium as long as you maintain the permeability you'll get back to negative 92 so actually let me even hammer this point harder by showing you a little little diagram so let's say we have a concentration gradient over here and I also have permeability over here permeability and this is permeability to potassium okay and assuming that we only have permeability so assume I'll write that very clearly only the cell is only permeable to one ion only one ion so assume only one ion for this permeability so if you have let's say permeability yes and permeability no and you have concentration gradient yes and concentration gradient no then what do you get exactly so let's say you have four possibilities here right we have four possibilities and let's say we have no concentration gradient and no permeability would we get a membrane potential well no because the potassium would never have a way of leaving in the first place and would have no desire to leave now what if you have concentration gradient so you have the desire to leave but you don't have a way for that potassium to actually leave the cell well again you don't actually have any membrane potential and the same is true if you have you know a permeability but you have no concentration gradient then the potassium again has no desire to actually leave and then finally if you have permeability and a concentration gradient then you actually get down to negative 92 millivolts so a concentration gradient is kind of when I use the word desire does the does the potassium have a desire to leave and permeability is does it have a means does it have a way to leave so these are the two things to kind of think about when you're when you're thinking about whether you would create a membrane potential or not so if we have that set up let's actually move down a little bit make some space and as you talk about how you actually get your negative 92 where where in the world is that number exactly come from so there is a formula and that formula is I'm going to write it out over here it's V m and all that means is membrane potential now if you're like me the first thing you notice is that there's no V there's no letter V in the word membrane or potential so how do they come up with that and I don't know the answer that I don't know where the V comes from exactly but V M stands for membrane potential and the formula is actually surprisingly simple hits 61.5 and this is kind of a simplified version because there lot of constants in here that get kind of thrown together that's 61.5 and you just take the log of the concentration of potassium on the outside i'll say k out potassium on the outside over the concentration of potassium on the inside of the cell you take these two concentrations and you get this fantastic little formula and you can actually now I can write for you potassium over here we said was equal to negative 92 millivolts so that would be the membrane potential this is a membrane potential and I can even walk through a few other ones some other key ones so there's sodium there's let's do chloride and calcium so a few of them and all of them have the same kind of formula you just take their concentrations on the inside and outside and plug it into the formula and you get positive 67 for sodium you get negative 86 for chloride and you get positive 123 for calcium and now keep in mind calcium has a two plus charge so for calcium this 61.5 actually gets changed to 30 point seven five and that's rounded off but that's because of that to positive charge and all you do is as I said throw in the the concentrations on the inside and outside so actually let's let's even write that down so concentration gradient and keep in mind exactly which way things are moving so concentration gradient for potassium I mean really you're looking at a positive ion right that's moving out of the cell and for sodium you have a positive ion but it's moving into the cell for chloride you have a negative ion moving into the cell and for calcium you have a positive ion moving into the cell really just like sodium so this is how you kind of can think about the four major ions that contribute to our cells membrane potentials