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Current time:0:00Total duration:8:22

Video transcript

I'm going to draw a little cell here for us this cell is going to be a typical cell and it's going to be full of potassium we know that cells love to hold on to potassium so let's draw lots of potassium in here and the concentration of potassium let's say is something like 150 milli moles per liter that's a lot of potassium right now I'm going to put brackets because brackets indicate concentration and of course there's some potassium on the outside too let's say the concentration here is something like 5 millimoles per liter and I have to also show you how this concentration gradient gets set up right it's not like it just happens to be set up it's it's something that we put a lot of energy into into creating so you get 2 potassium is pumped in and you actually kick out 2 or sorry three sodium's so that's how you get all those potassium Zin there in the first place so now that they're in there are they hanging out by themselves the answer is definitely no they are finding anions little negatively charged molecules or atoms to sit next to and so then the net charge is going to be neutral right because every cation has an anion and usually these anions are things like proteins something that has maybe like a negative sidechain like a protein could be a chloride could be phosphate could be a number of things so any one of these anions would be fine and actually let me draw a little a couple of anions here as well for these two potassiums that just got welcomed into our cell and so this is how things look you know if things are nice and static this is how they look and actually to be quite honest there's also a little anion hanging out here as well for this potassium so now that the truth is that we have little gaps in our cell little holes where we allow potassium to actually leak out so let's actually show how that would look and how that would affect what's going on so we have these little channels and they only allow potassium through so these channels are actually very specific for potassium they're not going to allow any anion through or any other thing out the protein certainly can't get out and so these potassium czar kind of looking at these channels that are there and they're thinking huh this is interesting there's a lot of potassium in here we're going to want to just you know slip out and so these potassium just kind of bail on the cell and they they just get right outside now when they do that an interesting thing happens most of them move outside but you know there are some potassium outside as well I said that there was this one little fellow over here and he could theoretically kind of make his way in over here you could you know come into this cell if you wanted to but the truth is overall on on the whole on net you have more movement outside than you do inside and so I'll just for the time being erased that path just because I want you to remember that overall we have more potassium that's going to move outside because of the concentration gradient in fact that's point number one so actually let me write that down here concentration gradient is going to make the potassium move outside and that's on net so the potassium starts moving out right so k out and what happens next well when it moves outside let me actually draw it moving outside so this K is now over here and this K is over here and what it's left behind is an anion in fact this guy's left behind anion as well and those anions they are all by them look all all by their lonesome they start generating a negative charge a big big negative charge actually just a few ions moving back and forth will create a negative charge and these potassium is on the outside they're thinking to themselves huh that's interesting there's a negative charge in there and if there's a negative charge in there they're attracted to it because they're thinking well I'm positive this is a negative charge I want to go back inside and so on the one hand think about it you have a concentration gradient driving potassium out but on the other hand you have this what we call membrane potential in this case a negative one a membrane potential that gets set up because the potassium is left behind an anion that's actually going to drive the potassium to want to be back inside so you have one force the concentration driving K out and another force the membrane potential that gets created by its absence that's going to drive it back in so I'm going to actually make a little Spacey I'm going to show you something that's kind of interesting so let's create two curves let's say we have actually I don't want to lose everything on this slide the me actually just set this up here so you can see the last little bit of it so let's set up two curves one will be for the concentration gradient and one will be for the membrane potential so this is let's say K out and you actually if you followed it over time this is time you'd actually see that you actually have something like that K is actually going to move out over time and it's going to at some point get to an equilibrium and if we did the exact same thing with time on this axis right here now let's say this is membrane potential membrane potential and we start at time zero and this is also negative access so this is going more and more negative this way and we start at zero for the membrane potential and this is at the point where you start letting the K kind of wander out you get something like this basically looks the same but is kind of a parallel of what's going on with the concentration gradient and when the two equal each other when the amount of K moving out equals the amount of K moving in we get to this kind of plateau and turns out it's about ninety negative 92 millivolts so that's the point where you really have almost no difference in terms of the net movement of K it's equal and in fact we even call that term we call that the equilibrium potential for potassium so when you get to that negative 92 and it differs depending on the ion but when you get to the negative 92 for potassium you've hit its equilibrium potential so let me just write that out for K is negative 92 and again this is assuming that the cell is only permeable to one thing which is potassium now this actually might still bring up a certain question your head you might be thinking and I want to make sure I address this well wait a second if potassium ions are moving out and that's what I said is happening then at some point don't we have a lower concentration in here because the potassium is actually left and a higher concentration out here because you know potassium is moving outside and technically that is correct I mean of course you have more potassium ions on the outside and no I haven't said the volume has changed so yes you would have a higher concentration and the same is true for the cell you'd have a lower concentration technically but realistically I haven't changed the numbers and the reason I haven't changed the numbers is because if you look at the numbers these are moles I mean this is a huge number right 6.02 times 10 to the 23rd that's not a small number and if you multiply it by five then you get something you know this kind of works out to about I'm going to quickly do the math six times five is about 30 and then you got millimoles here to consider so about 10 to the 20 moles right I mean that's an enormous number of potassium ions and really you just need a handful of ions to create this negative charge so if only a handful of ions are moving back and forth you're not going to really make a difference to that enormous number 10 to the 20th so that's why we don't really think of the concentrations as changing very much at all