- [Instructor] Let's say
that you have decided to go
canoeing, and right over
here, this is a top view of
our river right here.
This is our river.
And, let's say that the
current, the river, is going
towards the right.
So, there is two different directions
that you could be canoeing in.
You could imagine someone
who is canoeing in the same
direction as the current, so
they are canoeing that way.
And then, you could imagine
another person who's canoeing
the other way.
So, someone who's canoeing upstream.
This person is canoeing downstream,
this person is canoeing upstream.
So they are going in that direction.
So, pause this video and
think about which person
is going to have to expend more energy.
Or, which person is going
to have to be more active,
and which person is
going to be more passive.
Well, yes, this wasn't an
incredibly hard question.
If you are going with the flow of current,
as the person in yellow is
here, they don't even have to
take their paddles out.
They can just take a nap,
and the boat will naturally
go with the current.
They would be, they could
be just moving passively.
While the person in blue, here,
they're gonna have to work
really, really, really hard.
They're gonna have to paddle
some, just to not even move
to the right, and then
even paddle even more,
to actually go against the current.
So, this person would
have to be very active.
And so, this is really just
a metaphor for what we're
going to talk about now.
And, that's the idea in Biology of
Active versus Passive Transport.
So, let's start with, maybe,
the more pleasant one.
In either situation.
And that is, Passive Transport.
So, Passive Transport is when something
goes with the gradient.
So what do I mean by that?
So, you could have a
concentration gradient.
So, let's say that on, let's
say I have a tube of some kind.
And let's say it's filled with water.
And dissolved in that water,
at this end of the tube,
I have a high concentration
of some molecule,
or something right over here.
While on the right-hand side
I have a low concentration.
So what are we think is going to happen?
Well, these things are just
going to naturally move around,
and over time, they're gonna
bounce their way, so that after
a little bit of time has passed,
a lot of these things are
just going to passively move to the right.
And so at some point, you might
have an equal concentration,
or roughly equal, throughout
this entire container.
And so, this movement along
your concentration gradient...
Here you're moving from
high-concentration,
to low-concentration, this
would be passive transport.
This is a concentration gradient,
that we are moving along.
Let me write that down.
This is our concentration gradient.
But, you can also have
an electrical gradient.
So, let's take a similar
type of container.
Maybe it's filled with water,
and on the left-hand side,
imagine if you have a bunch of
positive particles, or molecules.
And on the right, you have a bunch of
negative particles, or molecules.
Well, the positive ones repel each other,
so do the negative ones.
But the positives attract the negative,
and the negative attract the positive.
And so, you would think that
things would naturally move
down their electrical gradient.
The positives want to
go away from each other,
and they're drawn to the negative.
Similarly, the negatives wanna
get away from each other,
and they are drawn to the positive.
So, whether you're talking
about a concentration gradient,
or an electrical gradient...
And sometimes you have
a combination of both.
An electro-chemical gradient.
When you're moving along
with your gradient,
you don't have to use any energy.
That's known as Passive Transport.
So, no energy needed.
It's just going to happen naturally.
Now, the opposite is the
notion of Active Transport.
Active Transport, and this is when you
go against the gradient.
So an Active Transport would
be, somehow let's say you're in
this situation right over here,
somehow getting one of these particles...
Let me do it in the same color,
somehow getting one of these
particles, instead moving
to go in that direction.
It will actually go against it's gradient,
in that direction.
Or, another situation is,
imagine if you have a positive
particle right over here,
instead of it naturally just
going in that direction,
somehow you make it go against
it's gradient, and you make it go closer
to the other positive particles.
Well, this is going to
require energy to do.
And probably the most sited
example, or the most common
example that we're going
to see, in Biology class,
of Active Transport,
is what's known as a
sodium-potassium pump.
Which we will study in
detail in other videos.
But, let's say that this
thing that I'm drawing,
here in white, this is a cell membrane.
And, I'm drawing these gaps for a reason.
And, what you have on the
outside of the cell membrane,
you have some potassium
ions, on the outside, but,
you have a lot more on the inside.
So, these are all potassium
ions on the inside of your cell.
And then, so let me just
write K+, K+, K+, K+, K+,
and you'll have some
sodium ions on the inside
of your cell;
Na+, but you have a lot more
on the outside of your cell.
And, in general, the outside
of your cell is going to have
many more positive ions than the inside.
Maybe you already see where this is going.
Na+, Na+, Na+, I think we've
got the idea, Na+, Na+.
Now, if on this membrane,
alright, let's ignore this part
right over here, if I just had
a channel, right over here,
that was open only to potassium.
So, only potassium can go through.
So, only potassium can go through this
channel right over here.
What do you think is going to happen?
Well, you would have Passive Transport.
These positively charged
potassiums right over here,
they would go down their
concentration gradient.
There's more likely to have
a potassium ion just bump
in the right way, just right
over here, so that it goes
through the channel.
Because, there's just more
potassiums right on the
inside of the cell, than
there would be on the outside.
And so, this, this potassium
is going down their
concentration gradient,
from high-concentration,
to low-concentration
through this channel.
This would be Passive Transport.
Passive Transport.
But, you could imagine
there's also Active Transport,
and that Active Transport is
what pumps the sodium ions
inside the cell outside of
the cell, even thought it's
not only against it's
concentration gradient,
it's also against it's
electrical gradient.
The outside's more positive,
so you wouldn't think a
positive ion would naturally go outside.
And the outside has more
sodiums that it does inside.
But, the sodium-potassium pump
still pumps those sodiums outside.
And, as I hinted at, it
does this using energy.
So, you'll sometimes see a
sodium-potassium pump drawn
like this and, once again
I'm not gonna go into depth
on it, we have a whole video on it.
But, the general idea
is, is that the sodiums
bind over here.
And then some ATP, which
is the powerhouse of cells;
which we will study in more
depth later on in Biology,
it leverages it's energy to
change the shape of the proteins
that make up this sodium potassium pump,
to then pump these sodiums
outside of the cell.
So, it's gonna go from
this shape, and then
it's just going to, you
could view it as opening
it up that way, the real
enzymes look quite different,
but that's the general idea.
You use energy, in the form
of ATP to pump the sodiums out
against both their concentration gradient,
and their electrical gradient.
And that's why it's
called Active Transport.
