So now let's take a couple of examples from cells
that's representative of what's in our body.
So we have ECF which is extracellular fluid,
that's the fluid around the cell.
And ICF, which is the intracellular fluid.
So if we take first, one particular ion,
and we'll do this one at a time.
Sodium has a concentration outside the cell of around 145 mmol.
Inside the cell, it's very low, about 12.
So there's a gradient across the membrane set up here.
If we look at potassium, it is high within the cell,
about 120 mmol, versus 4 on the outside.
Calcium is what looks like to be low on both side, but in actuality,
the extracellular fluid has more calcium than the intracellular fluid.
Intracellular fluid has almost none free calcium.
And finally we have chloride,
which is high on the outside of the cell and low on the inside.
One of the things you should notice
about these various molecules is their charge.
Sodium and potassium have a positive charge, and they have a +1.
They're considered monovalent.
Calcium is a +2. It's divalent.
And then chloride is an anion or negatively charged molecule.
The other thing to keep in mind is
which direction the arrows are pointing.
You'll notice that they always point from the base
of the highest number to the area of the lowest number.
Why? Because all these molecules want to get into equilibrium.
Meaning that they'd be the same on both sides of the membrane.
That is their goal. They want to live in harmony
and be the same on both sides of the membrane.
The membrane though, is going to be somewhat selective
about which ones it allows to pass.
So we can calculate the exact voltage difference
based upon those concentrations.
Now it's not important to be able to calculate this
on a minute by minute basis.
There are only going to be numbers
in which you have to be familiar with.
So for example, you see that this potential for sodium
is around +61 mV.
So that's based upon the concentration
in the extracellular fluid versus the intracellular fluid.
And you're gonna ask how in the world
did you figure out that that was 61 mV.
You could have done it in two ways.
One of them, we could have put
a reference electrode on one side of the membrane
and then add electrode on the other side.
Or we can use a formulated calculator,
which is called the Nernst equation.
At the Nernst equation, you take 61.5, divide it by the valence,
times the log of the concentration inside the cell,
over the concentration outside the cell.
So we either directly measuring it or using the Nernst equation,
you can calculate the particular voltage that the concentration
of those two ions inside and outside the membrane would cost.
If we look at potassium,
you'll notice that it also has a voltage.
But the potassium potential comes out to be -90.
Why? Because potassium was high inside the cell and low outside.
Calcium's membrane potential, very high.
120 in this particular example.
And again, that's based upon the ion concentration
in either the measured value or the Nernst value.
Remember for the Nernst value,
this one is going to be different because it's divalent.
So you're gonna have to take 61.5 divide it by now 2,
because it has a +2 charge,
times the log of the concentration on the inside
over the concentration in the outside of the cell.
The final ion that we have here is chloride.
And chloride is a -52 mV.
And remember that it has a higher concentration
on the extracellular fluid than the intracellular fluid.
So the arrows become very important and
their particular Nernst equation value becomes important.
So we have +61, +120, -90, and minus about 52. All from the ions
that we had, or we directly measure them from the cell.
So that looks like there's four numbers here,
what is the cell really at?
Do we simply just add up all four of those and divide by four,
so we get a mean?
That would be too easy, wouldn't it?
No. What we need to do is add another factor in play.
That other factor is which channels are opened,
and which channels are closed.
Because again if you don't let the ions travel across,
they don't count.
So we have to make sure which ions move becomes more important.
So let's start with calcium.
At rest, calcium is closed. Therefore we don't actualize
that potential of 120.
Sodium channels also close at rest.
Therefore that +61, non factor.
What is a factor? Potassium. Potassium channels are open at rest.
Therefore, an excitable cell will have a negative membrane potential.
Because there's a conductant for potassium, it will be
more likely close to the potassium equilibrium, which is -90.
So there will be some cells in the body that are at a -90
if these cells are opened all the time.
Most cells that were not at -90, they are a little bit above that,
and that's because of one other factor,
and that's the sodium-potassium pump,
or the sodium-potassium ATPase
which is constantly cycling and cycling and cycling,
moving what? Potassium back into the cell and sodium out.
That is called electrogenic pump because
it creates some electrical potential across the membrane.
Calcium pump is also there and that can also be part of the issue
but it's primarily the sodium-potassium ATPase.
What we have is -70.
You also notice that I didn't talk about chloride.
Chloride channels are also closed at this condition
so they are not part of the membrane potential,
just like calcium and sodium.