hi it's Paul Anderson most people are
familiar with a chemical gradient as
seen in diffusion so if I add some red
food coloring to this warm water the
molecules are going to quickly
distribute throughout that whole
container what's going on at the
molecular level
well those red food coloring molecules
have a certain amount of kinetic energy
they're bouncing off of each other
through two Brownian motion and they're
going to move along their chemical
gradient or their concentration gradient
from an area of high concentration to an
area of low concentration this is how
diffusion works this is how oxygen gets
into your cells and carbon dioxide gets
out and it's real visual we can see
what's going on but this is only one
half of a more complex gradient called
the electrochemical gradient and to
understand complex biological systems
like what causes a resting potential in
your cells or how excitable cells like
the neuron can fire off an action
potential we really have to understand
this electrochemical gradient what's
going on let's start with a simulation
this is a pH ET simulation I'll put a
link in the description down below well
we've got is some green food coloring on
the bottom and we're going to put it in
motion there's 50 molecules on the
bottom now we're going to allow it to
bounce around I'm going to pause for a
second and we're going to open up these
channels these channels allow that green
food coloring to move through so you
should make a prediction of what you
think is going to happen on this
simulation if we let it go so let's
watch what happens and you probably got
this right so those molecules have a
certain amount of kinetic energy this
random Brownian motion is going to move
them up from the bottom to the top and
from the top to the bottom so we started
with 50 and zero now we have roughly 25
on either side now they can still move
back and forth but that chemical
gradient that concentration gradient has
gotten much much smaller and so when we
started by opening up that channel that
chemical gradient continues to shrink as
they move that random walk from the
bottom to the top
that's diffusion and it's real visual
you can see it but let's make this
simulation a little bit more complex
imagine I put some potassium chloride on
the bottom and we dissolve it in water
so again this is an ionic compound it's
breaking apart into its ions we've got
the potassium ions which
positive charge and the chloride ions
which have a negative charge and I put
50 of each on the bottom we start them
moving and now I'm going to open up some
channels that only allow potassium to
move through so the only thing that can
move through here is going to be the
potassium ions the chloride ions can
move through again we're putting 50 of
each on the bottom so predict what you
think is going to happen if we let this
run for a while hopefully you've made a
prediction and let's see what happens so
this is probably what you thought was
going to happen we're getting some of
that potassium to go through and so you
probably got half of this problem right
so there are 50 chloride on the bottom
zero on the top because the chloride
can't come through but it's not 25 and
25 when it comes to the potassium ions
it's more like 37 to 13 so it's almost
like there's a magical force that's
pushing them in the other direction from
the top to the bottom and that is the
electrical gradient and and it's
counteracting that chemical gradient and
it's the combination of those two that
is the electrochemical gradient let's
zoom into that membrane I'll show you
what's going on so once a potassium goes
through once this potassium ion goes
through again it can go through this
channel it's got a charge and as it
moves through we have an increase in the
positive charge outside the membrane and
now we have a relative negative charge
on the inside of the membrane because
we've lost that potassium ion each time
a potassium ion goes through we're
building up a charge we've got a
positive charge on the on the outside of
this membrane and a negative charge on
the inside now if we think about these
ions for a second these are all positive
charges or like charges and you probably
know that light charges repel
in other words they don't like spending
time around each other and so there's
going to be a force that's pushing them
in the other direction and also since
they have a positive charge they're
going to be attracted to the relative
negative charge that's on the bottom and
so what happens over time and is we have
a balance now of the chemical gradient
that's pushing it in from the bottom to
the top this potassium and then we have
this electric gradient that's moving in
the other direction the combination of
those two is an electrochemical gradient
and it's reached a equilibrium or a
potential we could put use a voltmeter
and we could we could measure the
charge on either side of that and since
we have a separation of the charge now
we've got a potential now we've got a
voltage that we could actually measure
and so what's causing that resting
potential in cells is simply the
permeability of different ions moving
across that membrane now we can look at
the concentration inside and outside and
use the Nernst equation to figure out
what that voltage is actually going to
be and so the University of Arizona is
put together a little simulator I'll put
a link to that down below as well and
you could run the typical potassium
levels in a generic human cell and it'll
show you what's going on again there's
going to be more potassium on the inside
of the cell than the outside we could do
the same thing with other big ions like
sodium we'll have more sodium on the
outside and we could get a potential or
a voltage there we could do the same
thing with chloride ions as well or we
could run the Nertz or the goldman
equation rather with all of these ions
and it's going to show us what the
voltage of that generic cell is going to
be so what are we seeing we're seeing
the molecules we're seeing that chemical
gradient but what I want you to remember
is there's also that electrical gradient
the one that we can see that's based on
the charge and to really understand
complex biological systems you have to
understand both parts of that gradient
that's electrochemical gradient and I
hope that was helpful
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