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.