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.