Correction to sodium-potassium pump video | Health & Medicine | Khan Academy

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.


  1. @FungusyHam No, you still need them to set up the concentration difference so that the potassium leaks out (and the pumping out of the Na+ helps as well)

  2. @FungusyHam Sodium leaks into the cell via sodium channels, following its concentration gradient. Likewise potassium leaks out of the cell via potassium channels, following the concentration gradient for potassium. The purpose of the sodium-potassium pump is to pump the sodium that has leaked into the cell back outside the cell, and to pump potassium that has leaked out, back into the cell. The na+-k+ pump maintains the concentration levels so that gradient remains and process can continue.

  3. Hi Khan How R u? I have a question that is a little bit off topic but i hope you can refresh my memory. Im writing my thesis about the information content of quarterly earnings announcements in the Netherlands. But i would like to know the reason why scientists in Finance but also in Accounting research always exclude Financial firms and Utilities? Best regards..

  4. @1fuseemerun1 Good question. My thinking is that if the pump is designed to maintain concentration gradients, and it pumps out 3 Na ions for every 2 K ions it pumps in. If maintaining gradient, more Na must have leaked into the cell than K leaked out, else over time wouldn't an imbalance be created unless there are other stabalizing mechanisms present to maintain gradient. to your ? maybe combination of factors: number of channels, membrane potential, and concentration gradient.

  5. how is the membrane more permeable to K+ than Na+?

    i don't understand because K+ is bigger according to period trends, and both ions are equally +1 charged

  6. @fire89zhao Based on what he said (and I would need to review my biology text here, but assuming remember) since they are channel proteins, they molecules CAN go one way, but in practice they do no, due to the electro-potential difference.

  7. Furthermore, I was taught that the main reason for this voltage difference (beyond all of the reasons mentioned) is the "negatively charged proteins inside of the cell". Is this information provided to me correct?

  8. Yes – they're both constantly moving in and out of the cell through various means (channel proteins, carrier proteins, etc), but the rate at which K+ moves in and Na+ moves out is significantly greater than the opposite. For simplified purposes you could ignore this, though. In Biology things seldom happen one-way!

  9. We have been taught that another contributing factor to the maintenance of the potential difference is the presence of highly negative anions inside the cell which obviously have a profound effect on the negativity inside the cell. Also, due to the fact they are absent on the outside of the cell, this could be another contributing factor to the potential difference. Any views on this anyone?

  10. So when you say K can go through the membrance can it go out and into the cell? Gosh this whole K Na+ thing makes me sick.

  11. The potassium and sodium ions both have specific channels for each. The potassium ions have leak channels which stay open all the time. The sodium ions don't have these leak channels. And that is why potassium is more permeable.

  12. Why did he just presume that Potassium does not exist on the outside of the cell? If K can go out easily, cant they also get IN as easily?

  13. Mr. Khan, I believe there was a minor mistake made in explanation of RMP explanation here. Specifically at ~ 6:08 where you mention potassium goes "against" its chemical gradient, via "leakage" through the cellular membrane. I believe potassium is going with its chemical gradient, as it is in higher concentration INSIDE the cell, and thus going outside of the cell during leakage makes it going WITH the gradient. If I'm mistaken, please excuse me. Thanks for your consideration.

  14. Does the potassium, in this open channels in the membrane, only go "out", or it does go "in" from outside of the cell too?

  15. but surely, due to the 'high permeability' of the membrane to K, the K can just as easily enter back in, down its conc. gradient via the same ion channel it exited/leaked out?

  16. Hi, I find your videos extremely helpful and interesting in understanding physiology. I have a question regarding this video I wonder if you can answer. The contributor to this video claims that the main cause of the inside of a cell being more negative than the outside is due to the membrane being more "leaky" to K ions than it is to Na ions. However, I'm struggling to understand this because surely positive K ions would therefore leak quickly into the cell, making it more +? Can you explain?

  17. The professor is wrong, too. Membranes aren't "highly permeable" to K+ which makes it easier for K+ to go down its concentration gradient, membranes are selectively permeable to K+ meaning there are more nongated K+ channels compared to nongated NA+ channels. Plus, intracellular neurons are more negative because of the negatively charged protein molecules (A-) which cannot cross the membrane. Inside the cell there are high concentrations of A- and K+ with low concentrations of CI- and NA+, while outside the cell there is no A-, low concentrations of KA+ with high concentrations of CI- and NA+

  18. I also want to mention there is another force acting to keep K+ in the cell even though it wants to go down its ionic concentration gradient, and that is the electrical gradient. As K+ leaves the cell the inside of the cell becomes more negative, and since K+ is positively charged, it wants to remain on the negative side (its concentration gradient). This is the basis for resting membrane potential, called electrochemical equilibrium and can be summed up by the Goldman equation

  19. As I understood, there are several competing factors:
    1. Pump moving Na⁺ out and K⁺ in, thus creating different concentrations of sodium and potassium outside and inside.
    2. Membrane not allowing Na⁺ to get inside, thus a higher number of sodium ions will exist outside than inside.
    3. Membrane allowing K⁺ to leak out of the cell, thus restoring chemical concentration outside to be almost the same as inside.
    4. Membrane allowing Na⁺ to leak into the cell when voltage difference becomes too high, thus acting as a safety feature.
    As a result, a higher number of sodium and potassium ions will exist outside than inside, thus creating the voltage that we can measure.

  20. Rather than having a wrong video and then making a correction video, why not remove both the videos and a more reliable and accurate single video. just imagine the plight of the students who have watched your initial video and not watched this correction video, they will pickup wrong concepts

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