Sodium-potassium pump | Cells | MCAT | Khan Academy
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Sodium-potassium pump | Cells | MCAT | Khan Academy


In the last video, I showed you
what a neuron looked like and we talked about the
different parts of a neuron, and I gave you the general
idea what a neuron does. It gets stimulated at the
dendrites– and the stimulation we’ll talk about
in future videos on what exactly that means– and
that that impulse, that information, that signal
gets added up. If there’s multiple stimulation
points on various dendrites, it gets added up and
if it meets some threshold level, it’s going to create
this action potential or signal that travels across the
axon and maybe stimulates other neurons or muscles because
these terminal points of the axons might be connected
to dendrites of other neurons or to muscle
cells or who knows what. But what I want to do in this
video is kind of lay the building blocks for exactly
what this signal is or how does a neuron actually transmit
this information across the axon– or really,
how does it go from the dendrite all the way
to the axon? Before I actually even talk
about that, we need to kind of lay the ground rules– or a
ground understanding of the actual voltage potential
across the membrane of a neuron. And, actually, all cells have
some voltage potential difference, but it’s especially
relevant when we talk about a neuron and its
ability to send signals. Let’s zoom in on a
neuron’s cell. I could zoom in on any point
on this cell that’s not covered by a myelin sheath. I’m going to zoom in
on its membrane. So let’s say that this is
the membrane of the neuron, just like that. That’s the membrane. This is outside the neuron
or the cell. And then this is inside the
neuron or the cell. Now, you have sodium
and potassium ions floating around. I’m going to draw sodium
like this. Sodium’s going to be a circle. So that’s sodium and their
positively charged ions have a plus one charge and then
potassium, I’ll draw them as little triangles. So let’s say that’s potassium–
symbol for potassium is K. It’s also positively charged. And you have them just
lying around. Let’s say we start off
both inside and outside of the cell. They’re all positively
charged. Sodium inside, some
sodium outside. Now it turns out that cells
have more positive charge outside of their membranes than inside of their membranes. So there’s actually a potential
difference that if the membrane wasn’t there,
negative charges would want to escape or positive charges
or positive ions would want to get in. The outside ends up being more
positive, and we’re going to talk about why. So this is an electrical
potential gradient, right? If this is less positive than
that– if I have a positive charge here, it’s going to
want to go to the less positive side. It’s going to want to
go away from the other positive charges. It’s repelled by the other
positive charges. Likewise, if I had a negative
charge here, it’d want to go the other side– or a positive
charge, I guess, would be happier being here
than over here. But the question is, how
does that happen? Because left to their own
devices, the charges would disperse so you wouldn’t have
this potential gradient. Somehow we have to put energy
into the system in order to produce this state where we
have more positive on the charge of the outside than
we do on the inside. And that’s done by sodium
potassium pumps. I’m going to draw then
a certain way. This is obviously not how the
protein actually looks, but it’ll give you a sense of how it
actually pumps things out. I’ll draw that side
of the protein. Maybe it looks like this and
you’ll have a sense of why I drew it like this. So that side of the protein or
the enzyme– and then the other side, I’ll draw
it like this. It looks something like this,
and of course the real protein doesn’t look like this. You’ve seen me show you what
proteins really look like. They look like big clusters
of things, hugely complex. Different parts of the proteins
can bond to different things and when things bond to
proteins, they change shape. But I’m doing a very simple
diagram here and what I want to show you is, this is our
sodium potassium pump in its inactivated state. And what happens in this
situation is that we have these nice places where our
sodium can bind to. So in this situation, sodium can
bind to these locations on our enzyme or on our protein. And if we just had the sodiums
bind and we didn’t have any energy going into the system,
nothing would happen. It would just stay in
this situation. The actual protein might look
like something crazy. The actual protein might be this
big cloud of protein and then your sodiums bond there,
there, and there. Maybe it’s inside the protein
somehow, but still, nothing’s going to happen just when the
sodium bonds on this side of the protein. In order for it to do anything,
in order for it to pump anything out, it uses
the energy from ATP. So we had all those videos on
respiration and I told you that ATP was the currency of
energy in the cell– well, this is something useful
for ATP to do. ATP– that’s adenosine
triphosphate– it might go to some other part of our enzyme,
but in this diagram maybe it goes to this part
of the enzyme. And this enzyme, it’s
a type of ATPase. When I say ATPase, it breaks off
a phosphate from the ATP– and that’s just by virtue
of its shape. It’s able to plunk it off. When it plunks off the
phosphate, it changes shape. So step one, we have sodium
ions– and actually, let’s keep count of them. We have three sodium– these are
the actual ratios– three sodium ions from inside the
cell or the neuron. They bond to pump, which is
really a protein that crosses our membrane. Now, step two, we
have also ATP. ATP gets broken into ADP plus
phosphate on the actual protein and that changes
the shape. So that also provides energy
to change pump’s shape. Now this is when the
pump was before. Now after, our pump might look
something like this. Let me clear out some
space right here. I’ll draw the after
pump right there. And so this is before. After the phosphate gets split
off of the ATP, it might look something like this. Instead of being in that
configuration, it opens in the other direction. So now it might look something
like this. And of course it’s carrying
these phosphate groups. They have a positive charge. It’s open like this. This side now looks like this. So now the phosphates are
released to the outside. So they’ve been pumped
to the outside. Remember, this is required
energy because it’s going against the natural gradient. You’re taking positive charge
and you’re pushing them to an environment that is even more
positive and you’re also taking it to an environment
where there’s already a lot of sodium, and you’re putting
more sodium there. So you’re going against the
charge gradient and you’re going against the
sodium gradient. But now– I guess we call it
step three– the sodium gets released outside the cell. And when this changes shape,
it’s not so good at bonding with the sodium anymore. So maybe these can become a
little bit different too, so that the sodium can’t even bond
in this configuration now that the protein has changed
shape due to the ATP. So step three, the three Na
plusses, sodium ions– are released outside. Now once it’s in this
configuration, we have all these positive ions out here. These positive ions want to get
really as far away from each other as possible. They’d actually probably be
attracted to the cell itself because the cell is less
positive on the inside. So these positive ions– and in
particular, the potassium– can bond this side of the
protein when it’s in this– I guess we could call it this
activated configuration. So now, I guess we could
call it step four. We have two sodium ions bond
to– I guess we could call it the activated pump–
or changed pump. Or maybe we could say it’s
in its open form. So they come here and when they
bond, it re-changes the shape of this protein back
to this shape, back to that open shape. Now when it goes back to the
open shape, these guys aren’t here anymore, but we have these
two guys sitting here and in this shape right here,
all of a sudden these divots– maybe they’re not divots. They’re actually things in this
big cluster of protein. They’re not as good at staying
bonded or holding onto these sodiums so these sodiums get
released into the cell. So step five, the pump– this
changes shape of pump. So pump changes shape
to original. And then once we’re in the
original, those two sodium ions released inside the cell. We’re going to see in the next
few videos why it’s useful to have those sodium ions
on the inside. You might say, well, why don’t
we just keep pumping things on the outside in order to have
a potential difference? But we’ll see these
sodium ions are actually also very useful. So what’s the net effect
that’s going on? We end up with a lot more sodium
ions on the outside and we end up with more potassium
ions on the inside, but I told you that the inside is less
positive than the outside. But these are both positive. I don’t care if I have more
potassium or sodium, but if you paid attention to the ratios
I talked about, every time we use an ATP, we’re
pumping out three sodiums and we’re only pumping in two
potassiums, right? We pumped out three sodiums and
two potassiums. Each of them have a plus-1 charge, but
every time we do this, we’re adding a net-1 charge to
the outside, right? 3 on the outside,
2 to the inside. We have a net-1 charge– we have
a plus-1 to the outside. So we’re making the outside
more positive, especially relative to the inside. And this is what creates that
potential difference. If you actually took a
voltmeter– a voltmeter measures electrical potential
difference– and you took the voltage difference between that
point and this point– or more specifically, between this
point and that point, if you were to subtract the voltage
here from the voltage there, you will get -70
millivolts, which is generally considered the resting voltage
difference, the potential difference across the membrane
of a neuron when it’s in its resting state. So in this video, I kind of laid
out the foundation of why and how a cell using ATP,
using energy, is able to maintain a potential difference
across its membrane where the outside is slightly
more positive than the inside. So we actually have a negative
potential difference if we’re comparing the inside
to the outside. Positive charge would want to
move in if they were allowed to, and negative charge would
want to move out if it was allowed to. Now there might be one
last question. You might say, well, if we just
kept adding charge out here, our voltage difference
would get really negative. This would be much more negative
than the outside. Why does it stabilize at -70? To answer that question– these
are going to come into play in a lot more detail in
future videos– you also have channels, which are really
protein structures that in their open position will allow
sodium to go through them. And there are also channels
that are in their open position, would allow potassium
to go through them. I’m drawing it in their
closed position. And we’re going to talk in the
next video about what happens when they open. But in their closed position,
they’re still a little bit leaky. And if, say, the concentration
of potassium becomes too high down here– and too high meaning
when they start to reach this threshold of -70
millivolts– or even better, when the sodium gets too high
out there, a few of them will start to leak down. When the concentration gets
really high and this is really positive just because of the
electrical potential, some of them will just be
shoved through. So it’ll keep us right around
-70 millivolts. And if we go below, maybe some
of the potassium gets leaked through the other way. So even though when these are
shut– if it becomes too ridiculous– if it goes to -80
millivolts or -90 millivolts, all of a sudden, there’d be a
huge incentive for some of this stuff to leak through their
respective channels. So that’s what allows
us to stay at that stable voltage potential. In the next video, we’re going
to see what happens to this voltage potential when the
neuron is actually stimulated.

About James Carlton

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100 thoughts on “Sodium-potassium pump | Cells | MCAT | Khan Academy

  1. So the signal is received at the dendrite right? Does this electrical potential gradient exist from the tip of the dendrites, through the soma (cell membrane), down the axon and to the axon terminals? I thought that the gradient only existed on the axon and the signal traveled down the axon when the signal gets to the axon hillock. Maybe I am missing something…can someone help?

  2. Look up Synapses, I think that should give you the general idea. Basically the action potential opens Calcium channels and this leads to Neurotransmitter (or even Hormones maybe?) being released outside the Neurone, converting the Electrical Signal into a Chemical Signal.

  3. Khan Academy has only been up since 2006, and it already has 3518 videos on YouTube, that's almost 1.5 videos a DAY. How does one man learn this much!

  4. Why would the Sodium ions want to bind to the receptor site of the (orange) protein in first place if the inside is less positive?
    Positive-Positive are not attracted to each other.

    Someone please answer that. Thank you very much.

  5. The Na would be constantly moving around (kinetics) so at some point the Na will combine with the receptor sites. Also the inside of the cell is also positive so the Na ions would still repel even if the inside of the cell is less positive. At least that is my thought/explanation

  6. I'm learning more on YouTube than in school for 2 reasons: 1) Visualization of information in an entertaining manner; and 2) Predisposition to learn (I choose what I'm interested in learning). This is the future of education.

  7. I agree, but for specific reasons. Although a good teacher will explain this equally well, Youtube has these benefits: 1) you follow the explanation at a time of your own choosing, 2) there's less to no interference by classmates, 3) you can pause, repeat at will.

  8. I really appreciate uploading this video! It is really helpful for my biology test!! Really, thank you soooooo much!

  9. First of, thank you for all the awesome videos! I had a question though, it resting potential -70mv or is it 90mv, or is it between the two?

  10. He made a few mistakes with naming Na K and K Na. He also called the Na in the second pump he drew to the left Phosphate,but he explained it very well.

  11. These videos are all truly great, but this is the second one in which you have said "sodium" when you actually meant "potassium." Please be more careful!

  12. still a little confuse. 
    what occurs during the resting state ( such as specific ions inside and outside the neuron) i know that part, but what is the voltage inside the neuron during the resting state? 

  13. sir,the video is very useful in understanding about the sodium potassium pump.But there is 1 mistake in recording,when you explain about two potassium ion  by mistake you say that these are sodium ions,overall the video was good.

  14. Thanks! the videos I was watching showed repolarization as only K flowing outside the membrane, bud didn't explain how the k returned inside to polarize the cell again. It's the Na/K pump!

  15. I thought the more positives outside wanted to balance out the less positives inside. Due the the copious mistakes I'm more confused as ever X.X

  16. What happens to a cell if Thallium (TI) binds on it instead of potassium?
    (It's known that TI got a higher affinity to the cell than K+)
    The cell gets bigger and bigger but I don't know why…captain!

  17. I actually learned two things. The sodium/potassium pump and how a volt meter works 😛 Really effective vids man. 🙂

  18. Hi! Can you describe a simple experiment, explaining the contribution of the pump to the membrane potential?

  19. You mix up your solutes many times. First you say that you pump out 3 phosphorus when you meant sodium and then you say you pump in sodium when you meant potassium. Otherwise, really appreciate the video.

  20. if you say that the main reason of negative resting potential is the high resting permeability of K+, what happens when K+ in the blood goes high? As I know, the K+ might even enter the cell but it (the cell) becomes less positive, but still remains negative, for example at – 60 or – 55 mV. how to explain this negativity in this case?

  21. This consept shouldn't take 13 minutes to review. I think a simple 3 minute video would have saved time and still demonstrated the consept.

  22. you told that there are 2 potassium ions passed into the neuron so does the no. of potassium ions inside the cell remain less throughout our life? please answer my doubt

  23. But, doesnt a neuron actually hold data other than the superposition of the stimulation waves? how is that data such as a memory of how to carryout an action stored and stimukated when in need of carrying out the action?

  24. Yes, around 10.30, you keep referring to the K ions as Sodium. Please change it to avoid future confusion!

  25. Kahn vids are usually pretty good . I had to stop watching this one after the repeated mistakes of saying sodium but writing potassium. Dude, check yourself!

  26. pls take this down, I came here to just double check a few things and ended up questioning everything I had studied for my M.CAT... turns out I was right and this video is just a mess :/

  27. Despite getting a little confused with the mix up with potassium-sodium, and sodium-phosphate… I found this video so HELPFUL! It was quite simplified and easy to follow. Thank you for your help.

  28. Sal, come on man leave the bio to someone else if you're going to make massive mistakes like that. What if a kid went to school to take a test on this information on got a bad grade after watching this video?

  29. He said sodium instead of potassium and since i hardly understand this i was going to lose my mind until i read the comments

  30. I truly appreciate your videos and I can't really express how much this has helped me -Manraj Singh Dhaliwal, a gifted student at the age of 10 right now and has an IQ of 154

    My dream which will be made true by Khan Academy: "Become a true student at MIT and become The CEO of Apple"

    I will always support and donate to Khan Academy whenever it needs help and I will truly make it a World Class education by spreading it to the whole world's classrooms because it still has not been spread to many countries

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