Correction to Sodium and Potassium Pump Video Correction to Sodium and Potassium Pump Video
Correction to Sodium and Potassium Pump Video
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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
- 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
- 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
- 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.
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