Correction to sodium-potassium pump video

WEBVTT
Kind: captions
Language: en

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

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potassium pump.

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One very minor one-- and I don't
think it would trip too

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many of you guys up, but near
the end of the video, as we

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learned, we have potassium
getting pumped into the cell

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by the sodium potassium pump.

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Let me draw the membrane.

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It'll actually be useful
in the more significant

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correction I'd like to make.

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

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And let me draw the sodium
potassium pump right here.

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We saw it pumps out three
sodiums for every two

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potassiums that it pumps in.

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It definitely doesn't
look like that, but

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it gives the idea.

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And we're pumping potassium
ions in-- so K plus-- and

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we're pumping sodium ions out--
and that's what the

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whole point of that video was.

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When this thing changes shape
with ATP, it pumps

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the sodium ions out.

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Now the minor correction I want
to make-- and I don't

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think it would have tripped you
up too much-- is near the

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end of that video, I drew the
potassium ions-- and I wrote a

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K plus, but a few times near
the end of the video, I

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referred to them as sodium
ions-- and I don't want that

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to confuse you at all.

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It is potassium ions that
are getting pumped in.

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Two potassium ions get pumped in
for every three sodium ions

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that get pumped out.

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So I don't want-- even thought
I drew a K plus, sometimes I

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said sodium by accident.

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Don't want that to
confuse you.

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That is the minor error.

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The more significant error is
that I said that the main

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reason that we had this
potential difference-- why it

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is more positive on the outside
than the inside-- so

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this is less positive.

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I said that the main reason
was because of this ratio.

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We're pumping out three sodium
ions for every two potassium

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ions that we pump in.

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And I just got a very nice
letter from a professor of

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physiology, Steven Baylor at
University of Pennsylvania,

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and he wrote a very interesting
email and it

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corrects me.

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And it's a very interesting
thing to

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think about in general.

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So here's what he wrote
and let's think

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about what he's saying.

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He says: Here at Penn Medical
School, we have a nice

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teaching program that stimulates
the ion fluxes

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across a generic cell, --So
the ion flux is just the

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movement of the ions across the
membrane-- including that

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due to the sodium potassium
pump and that which arises

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from the resting

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permeabilities of the membrane.

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So the resting permeabilities
is how easy it is for these

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ions to go through
the membrane.

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

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And the resting permeabilities
of the membrane to sodium,

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potassium, chloride,
et cetera.

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One option our program gives
students is to change the pump

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stoichiometry from
three to two.

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So when he's talking about pump
stoichiometry from three

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to two, he's just talking
about they're

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changing the ratios.

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

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

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

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pumping three sodiums out for
every two potassium it pumps

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in, what if it was even?

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What if it was two sodiums
and two potassiums?

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And based on my explanation of
why we have this potential

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difference, that should not lead
to a potential difference

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if the main reason was the
stoichiometry-- the ratio of

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sodium being pumped to the
potassium being pumped in.

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

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simulation.

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As a result of this maneuver,
the membrane potential changes

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

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measure that.

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They take the voltage here minus
the voltage there so

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that you get a negative
number.

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This is more positive.

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It's a larger number.

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So it changes from -80
millivolts to about -78

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millivolts.

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

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

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

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change the potential
that much.

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You still have a more positive
environment outside than you

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have inside.

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

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the potential if the
stoichiometry of this ratio is

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not the main cause?

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So it says, it changes
a little bit.

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The potential difference becomes
a little bit less.

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The cell swells a few percentage
and then everything

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stabilizes.

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So then he goes on to write: So
while it is true that the

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normal stoichiometry of the
pump does have a slight

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negative influence on the
membrane potential-- that's

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just the membrane potential,
the voltage across the

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membrane-- the imbalance in the
pump stoichiometry is not

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the main reason for the large
negative membrane

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potential of the cell.

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Rather, the main-- let me
underline this-- the main

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reason is the concentration
gradients established by the

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pump in combination with the
fact that the resting cell

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membrane is highly permeable to
potassium and only slightly

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permeable to sodium.

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So we said in the last video--
or the first video on the

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sodium potassium pump-- we said
there were channels that

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

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the potassium could
go through.

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And now what he's saying is
that the main cause of the

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potential difference isn't this
ratio, it's the fact that

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the membrane is highly permeable
to potassium.

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So this is very permeable.

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Potassium can get out if it
wants to, much easier than it

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is for sodium to get in.

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So what that happens-- even if
this was a 2:2 ratio-- it's

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

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though this environment is more
positive, you're just

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more likely to have to potassium
ions down here bump

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in just the right way to get
across and get to the other

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side, go against its chemical
gradient, right, because you

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have a higher concentration of
potassium here than over here.

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So you're more likely to have
a potassium bump in just the

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right way to get through this
channel and get out-- than you

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are to have a sodium be able to
go the opposite direction.

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And that's what makes
this environment.

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So you have more potassium
coming outside because of this

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permeability than sodium coming
inside-- and that's the

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main cause of the potential
difference between the outside

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and the inside.

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And so thank you, Steven Baylor,
for that correction.

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Very interesting.