Sodium-potassium pump | Cells | MCAT | Khan Academy

Sodium-potassium pump | Cells | MCAT | Khan Academy

September 21, 2019 100 By Kody Olson


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.