Correction to Sodium and Potassium Pump Video

Correction to Sodium and Potassium Pump Video

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.