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.