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  • Two 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.

  • So let me draw a cross section of a cell membrane.

  • And let me draw the sodium potassium pump right here.

  • We saw it pumps out three sodiums for every two

  • potassiums that it pumps in.

  • It definitely doesn't look like that, but

  • it gives the idea.

  • And we're pumping potassium ions in-- so 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 pumps

  • 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 near 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.

  • Two potassium ions get pumped in for every three sodium ions

  • that get pumped out.

  • So I don't want-- even thought I drew a K plus, 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 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 thing to

  • think about in general.

  • So here's what he wrote and 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 flux is 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 permeabilities is how easy it is for these

  • ions to go through the membrane.

  • And we'll talk more about that in a second.

  • And the resting permeabilities of the membrane to sodium,

  • potassium, chloride, et cetera.

  • 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 ratios.

  • So they change it from 3:2 to 2:2.

  • So what that means is, they have a simulation program that

  • says, well, what if the sodium potassium pump, instead of

  • pumping three sodiums out for every two potassium it pumps

  • in, what if it was even?

  • What if it was two sodiums and two potassiums?

  • 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 to the potassium being pumped in.

  • But he goes on to say: They could change it to 2:2 in the

  • simulation.

  • As a result of this maneuver, the membrane potential changes

  • from its normal value of about -80 millivolts-- and they

  • measure that.

  • 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 -80 millivolts to about -78

  • millivolts.

  • So what he's saying is, if you change this from three and

  • two-- three sodiums for every two potassiums that get pumped

  • in-- if you change that to 2:2, it actually doesn't

  • change the potential that much.

  • You still have a more positive environment outside than you

  • have inside.

  • So that leads to the question-- then why do we have

  • the potential if the stoichiometry of this ratio is

  • not the main cause?

  • So it says, it changes a little bit.

  • The potential difference becomes a little bit less.

  • 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 stoichiometry of the pump does have 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 in the last video-- or the first video on the

  • sodium potassium pump-- we said there were channels that

  • the sodium could go through and there's also channels 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

  • 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 2:2 ratio-- it's

  • actually a 3:2, but even if this was a 2:2 ratio, even

  • though this environment is more positive, you're just

  • more likely to have to 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 than 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-- than 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 than sodium coming inside-- and 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.

Two corrections I want to make to the video on the sodium

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ナトリウムとカリウムのポンプ動画の訂正 (Correction to Sodium and Potassium Pump Video)

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    Cheng-Hong Liu に公開 2021 年 01 月 14 日
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