Membrane Potentials – Part 1 | Circulatory system physiology | NCLEX-RN | Khan Academy
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Membrane Potentials – Part 1 | Circulatory system physiology | NCLEX-RN | Khan Academy


I’m going to draw a
little cell here for us. This cell is going
to be a typical cell, and it’s going to be
full of potassium. We know that cells love
to hold onto potassium. So let’s draw lots
of potassium in here. And the concentration
of potassium, let’s say, is something like 150
millimoles per liter. That’s a lot of potassium. And I’m going to put
brackets because brackets indicate concentration. And of course, there’s some
potassium on the outside, too. Let’s say the
concentration here is something like 5
millimoles per liter. And I have to also show you
how this concentration gradient gets set up, right? It’s not like it just
happens to be set up. It’s something that we put a
lot of energy into creating. So you get two
potassiums pumped in, and you actually kick
out three sodiums. So that’s how you get all
those potassiums in there in the first place. So now that they’re
in there, are they hanging out by themselves? The answer is definitely no. They are finding anions, little
negatively charged molecules, or atoms, to sit next to. And so the net charge is
going to be neutral, right, because every
cation has an anion. And usually these anions are
things like proteins, something that has maybe like a negative
side chain like a protein. It could be a chloride. It could be phosphate. It could be a number of things. So any one of these
anions would be fine. And actually let me draw a
couple of anions here as well. So these two potassiums
that just got welcomed into our cell, and
so this is how things look. If things are nice and
static, this is how they look. And actually, to
be quite honest, there’s also a little
anion hanging out here as well for this potassium. So now the truth is
that we have little gaps in our cell, little holes,
where we allow potassium to actually leak out. So let’s actually show
how that would look and how that would
affect what’s going on. So we have these
little channels. And they only allow
potassium through. So these channels are actually
very specific for potassium. They’re not going to
allow any anion through or any other thing out. The protein certainly
can’t get out. And so these potassiums
are kind of looking at these channels that are
there, and they’re thinking, huh, this is interesting. There’s a lot of
potassium in here. We’re going to want
to just slip out. And so these potassium just
kind of bail on the cell. They just get right outside. Now, when they do that, an
interesting thing happens. Most of them move outside. But there are some
potassiums outside as well. I said that there was this
one little fellow over here, and he could theoretically kind
of make his way in over here. He could come into this
cell if he wanted to. But the truth is, overall
on the whole, on net, you have more movement
outside than you do inside. And so I’ll just,
for the time being, erase that path
just because I want you to remember that overall
we have more potassium that’s going to move outside because
of the concentration gradient. In fact, that’s
point number one. So actually let me
write that down here. Concentration gradient is
going to make the potassium move outside, and that’s on net. So the potassium starts
moving out, right? So K out. And what happens next? Well, when it
moves outside– let me actually draw
it moving outside. So this K is now over here,
and this K is over here. And what it’s left
behind is an anion. In fact, this guy’s left
behind an anion as well. And those anions, all
by their lonesome, they start generating
a negative charge, a big, big negative charge. Actually, just a few anions
moving back and forth will create a negative charge. And these potassiums
on the outside, they’re thinking to themselves,
huh, that’s interesting. There’s a negative
charge in there. And if there’s a
negative charge in there, they’re attracted to it
because they’re thinking, well, I’m positive. This is a negative charge. I want to go back inside. And so on the one
hand– think about it. You have a concentration
gradient driving potassium out. But on the other
hand you have this, what we call, membrane
potential– in this case a negative one– a membrane
potential that gets set up because the potassium
has left behind an anion that’s actually going to
drive the potassium to want to be back inside. So you have one force, the
concentration, driving K out, and another force, the
membrane potential that gets created by
its absence, that’s going to drive it back in. So I’m going to actually
make a little space here. I’m going to show you something
that’s kind of interesting. So let’s create two curves. Let’s say we have–
actually, I don’t want to lose everything
on this slide. Let me actually just
set this up here so you can see the
last little bit of it. So let’s set up two curves. One will be for the
concentration gradient and one will be for
the membrane potential. So this is, let’s say, K out. And actually if you
followed it over time– this is time– you’d actually
see that you actually have something like that. K is actually going
to move out over time, and it’s going to, at some
point, get to an equilibrium. And if we did the exact same
thing with time on this axis right here, and let’s say
this is membrane potential. And we start at time zero and
this is also negative access. So this is going more and
more negative this way. And we start at zero for
the membrane potential, and this is at the
point where you start letting the K
kind of wander out, you get something like this. Basically looks the
same, but is kind of a parallel of what’s going
on with the concentration gradient. And when the two
equal each other, when the amount of K moving out
equals the amount of K moving in, we get to this
kind of plateau. And it turns out, it’s about
negative 92 millivolts. So that’s the point
where you really have almost no difference in
terms of the net movement of K. It’s equal. And in fact, we even
call that term– we call that the equilibrium
potential for potassium. So when you get to
that negative 92– and it differs
depending on the ion– but when you get to the
negative 92 for potassium, you’ve hit its
equilibrium potential. So let me just write that
out for K is negative 92. And again, this is
assuming that the cell is only permeable to one
thing, which is potassium. Now this actually
might still bring up a certain question in your head. You might be
thinking– and I want to make sure I address
this– well, wait a second. If potassium ions
are moving out– and that’s what I said
is happening– then at some point don’t we have
a lower concentration in here because the potassium has
actually left and a higher concentration out here because
potassium is moving outside? And technically that is correct. I mean, of course you have more
potassium ions on the outside. And I haven’t said the
volume has changed. So, yes, you would have
a higher concentration. And the same is
true for the cell. You’d have a lower
concentration technically. But realistically, I
haven’t changed the numbers. And the reason I haven’t
changed the numbers is because if you look at
the numbers, these are moles. And this is a huge
number, right? 6.02 times 10 to the 23rd,
that’s not a small number. And if you multiply
it by 5 then you get something–
this kind of works out to about– I’m going
to quickly do the math. 6 times 5 is about 30. And then you’ve got
millimoles here to consider. So about 10 to the
20 moles, right? I mean that’s an enormous
number of potassium ions. And really you just
need a handful of ions to create this negative charge. So if only a handful of ions
are moving back and forth, you’re not going to
really make a difference to that enormous
number, 10 to the 20th. So that’s why we don’t really
think of the concentrations as changing very much at all.

About James Carlton

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74 thoughts on “Membrane Potentials – Part 1 | Circulatory system physiology | NCLEX-RN | Khan Academy

  1. This is the first time anyone has explained membrane potential properly to me. The anion/cation relationship explained it perfectly. I'll be watching lots more of your videos!
    Thanks:)

  2. اشكرررك بعنف ايها الأجنبي الفلته .. وينك عننا بدال الهنوود المعفنين .. ليتك تدري بس ههههه

  3. Could you please teach us instead of these old expired lecturers at Jordanian university of since and technology… thanx a lot

  4. ارتقي الله يهديك هو وشوا هو هندي الاصل اساسا ههههه ابحث عنه وتعرف .

  5. Very nice video. Need one point to the puzzle to understand 100%. The K ion enter the cell again due to the membranpotential. Does it enter through the ion chanels or through the Na/k pump? Hope you have time to clear this out!

  6. It enters through both leak (always open) channels and the Na/K ATPase pump. There is a slight outflux of K+ due to the resting membrane potential being -70 mV (not -90 mV, K+'s equilibrium potential). So the pump has to make up for this by bringing some back in through active transport.

  7. K+ leaves because it has an equilibrium potential of -90 mV (if it was the only permeable ion in a cell, it would leave the cell due to its high concentration side and leave enough of the anions it was paired with so that the cell would have a potential of -90 mV). The normal human cell has a potential of -70 mV, notice that's more positive than potassium's equilibrium potential, so there is a small outflux of potassium as it tries to reach its own ion potential by leaving.

  8. Think it is like this:
    K thinks "There lives so damn many K inside the cell. hell with it im going out"
    But it comes a point when they dont wanna leave anymore because the negative charges (proteines in the example) attracts them back.

    Think like this…. theres like a party…. Dudes are K+ and girls are –
    Girls (-) attracts dudes but if theres too many dudes (K), most probably some of those may want leave.
    But not every dude (K) leaves they think it's worth staying, equlibrium is reached

  9. at 4:35 he drew potassium entering the cell through the plasma membrane,isnt that wrong?I thought ions can only pass through their specific channels.

  10. great. how do i get on your developing team? pre-med pursuing a degree in biochemistry and passion for a paleolithic lifestyle

  11. Hi, I have a question regarding membrane potentials. if a membrane potential goes to 0 mV would any ions be at equilibrium (that is, have no net forces)?

  12. Thank you for you amazing videos. I am also a teacher. What format/program are you using for these videos? Connie
    Does anyone else know?

  13. Helped a lot. I still want to understand the graphs with the concentration gradient of K+ and the membrane potential a little more. I think as time goes on the rate of K+ going out the membrane increases, but eventually reaches a certain point where the rate is constant, and the negative charge on the inside will also be constant. But I don't understand how would the net movement of K+ be equal from then on if the charge inside would still be negative? I'm not really sure what happens with net movement–if it's constant, negative, positive w.e. I would like some explanation on this. Also do the K+ ions want to have equal concentrations on both sides? Thanks.

  14. it s helps thankss but the concentration of Cl-  cytoplasm is very low like 50 mM so the k+ can't be coupled by it … the green molecules with negative charge can be pr , aa , HCO3-  … i'm i wrong ? 

  15. Ok so I am a little confused. I understand the concentration gradient, and the membrane potential. what I don't get is how the membrane potential would be negative if "the same amount of potassium is moving out, thats moving back in". If the amount moving in is equal to the amount moving out, wouldn't the voltage be zero, or whatever it was to start out with. I feel like there would me more potassium that left, than came back in, to make it negative. any explanation would be very helpful. thank you in advanced.

  16. I'm learning about the heart right now, and then I mean the advanced stuff. You addressed everything in one chapter of my book, but so much clearer. Thank you!

  17. quick question. once equilibrium is reached does that mean there is no movement of ions. is everything stationary? could you please clear that up for me

  18. This is the most pleasant, least boring video explaining Membrane Potentials that I could find. Thank you.

  19. Seriously my text books confuse the hell out of me. In 8 minutes I have a full understanding of this. I love this site.

  20. Thanks a lot! I'm so glad you addressed that problem at the end, it was causing me a bit of a headache O;

  21. Shouldn't this video be titled potassium equilibrium? You also have sodium equilibrium of +60mV and the combined effect of both Na+ and K+ creates a resting potential of -70mV.

  22. Why are these Na+ ions are forced to get out? What makes the membrane to be selective of only one positive ion(Na+ or K+) at a time? Is that true for negetive ions too?

  23. Pottashium can not move out of the cell because cell membrane is impermeable to K and negative proteins so as to cause the movement of K there is voltage gated channels which are activated only when stimulus is given not applicable here because we're discussing resting membrane potential- i.e the potential in absence of stimulus…the negative charge inside the cell is because of net movement of three sodium outside while only 2 K move inside thus relative negative charge develops inside due to the Na-K pump which is the only pump responsible for maintaining the RMP also it is an active pump that uses ATP as the movement of ions is AGAINST concentration gradient

  24. I don't know if this has been pointed out, but there is an error at ~7:55 where you say, "“so about ten to the twenty moles, right?” Not right. It is 10^20 ions. Other than that though, great video!

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