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Current time:0:00Total duration:6:48

Video transcript

whew - Corrections I want to make to the video on the sodium potassium pump one very minor one and I don't think it would trip too many of you guys up but near the end of the video as we learned we have potassium getting pumped into the cell by the sodium potassium pump let me draw the membrane it'll actually be useful in the more significant correction I'd like to make I'd like to make so let me draw a cell membrane a cross-section of a cell membrane and let me draw the sodium potassium pump right here the sodium potassium pump and we saw it pumps out three sodium's for every two potassium that it pumps in so it might look somewhere it definitely doesn't look like that but it gives the idea and we're pumping we're pumping potassium ions in so k plus k plus and we're pumping sodium ions out and that's what the whole point of that video was when this thing changes shape with ATP it pump it pumps out the sodium the sodium ions out now the minor correction I want to make and I don't think it would have tripped you up too much is near the end of that video I drew the potassium ions and I wrote a k plus but a few times you're the end of the video I referred to them as sodium ions and I don't want that to confuse you at all it is potassium ions that are getting pumped in to potassium ions get pumped in for every three sodium ions that get pumped out so I don't want you know I even drew k+ sometimes I said sodium by accident don't want that to confuse you that is the minor error the more significant error is that I said that the main reason that we had this potential difference why it is more positive on the outside why it is more positive on the outside than the inside so this is less positive I said that the main reason was because of this ratio we're pumping out three sodium ions for every two potassium ions that we pump in and I just got a very nice letter from a professor of physiology Steven Baylor at University of Pennsylvania and he wrote a very interesting email and it corrects me and it's a very interesting well it's an interesting thing to think about in general so here's what let's think about what he's saying he says here at Penn medical school we have a nice teaching program that stimulates the ion fluxes across a generic cell so the ion fluxes it's just the movement of the ions across the membrane including that due to the sodium potassium pump and that which arises from the resting permeabilities of the membrane so the resting permeability is how easy it is for these ions to go through to go through the membrane we'll talk more about that in a second and the resting permeabilities of the membrane to sodium potassium chlorine chloride etc our option one option our program gives students is to change the pump stoichiometry from three to two so when he's talking about pump stoichiometry from three to two he's just talking about they're changing the ratio so they change it from 3 to 2 to 2 to 2 so what that means is they have a simulation program that says well what if the sodium potassium pump instead of pumping three sodium's out for every 2 potassium it pumps in what if it was even what if it was two sodium's and two potassium and based on my explanation of why we have this potential difference that should not lead to a potential difference if the main reason was the stoichiometry the ratio of sodium being pumped out to potassium being pumped in but he goes on to say so you could change it to two to two in the simulation as a result of this maneuver the potential the membrane potential changes from its normal value of about negative eighty millivolts and they measure that they take they take the voltage here minus the voltage there so that you get a negative number this is more positive it's a larger number so it changes from negative eighty millivolts to about negative 78 millivolts so what he's saying is if you change this from three and to three sodium's for every two potassiums that get pumped in if you change that to two to two it actually doesn't change the potential that much you still have a more a more positive environment outside than you have inside so that's it that that leads to the question then why do we have the potential if this if the stoichiometry if this ratio is not the main cause and then so it says it's changes a little bit it becomes a little bit let the potential difference becomes a little bit less this well the cell swells a few percentage and then everything stabilizes so then he goes on to write so while it is true that the normal Stokey AMA tree of the pump does have a slight negative a slight negative influence on the membrane potential that's just the membrane potential the voltage across the membrane the imbalance in the pump stoichiometry is not the main reason for the large negative membrane potential of the cell rather the main let me underline this the main reason is the concentration gradients established by the pump in combination with the fact that the resting cell membrane is highly permeable to potassium and only slightly permeable to sodium so we said and we when we in the last video or the first video on the sodium potassium pump we said that there were channels there were channels that the sodium could go through that the sodium could go through and there's also channels that the potassium could go through and that the potassium could go through that the potassium could go through and now what he's saying is that the main cause of the potential difference isn't this ratio it's the fact that it's the membrane is highly permeable to potassium so this is very permeable potassium can get out if it wants to much easier than it is for sodium to get in so what that happens even if this was a 2 to 2 ratio it's actually a 3 to 2 but even if this was a 2 to 2 ratio even though this environment is more positive you're just more likely to have potassium ions down here bump in just the right way to get across and get to the other side go against its chemical gradient right because you have a higher concentration of potassium here then over here so you're more likely to have a potassium bump in just the right way to get through this channel and get out then you are to have a sodium be able to go the opposite direction and that's what makes this environment so you have more potassium coming outside because of this permeability then sodium coming inside and that is what causes or that's the main cause of the potential difference between the outside and the inside and so Thank You Steven Baylor for that correction very interesting