So we've covered the three mechanisms of passive
transport where we have basic diffusion,
we have facilitated diffusion if the molecules
cannot pass through the membrane on their own.
They need a channel or a carrier. And we've also
covered osmosis which is specific to the movement
of water. Now, let's explore what active transport
really is. Active transport is the opposite of
passive in that it requires energy. We are moving
things against their concentration gradient.
So, what happens in this situation? Well there
is three ways that it could really happen.
You could have one molecule moving itself up
its concentration grandient. So from
low concentration to high concentration. We could
have symporters which are things going in the
same direction. We'll look at an example of that
shortly. Glucose is transported with sodium
in a symport protein and we'll see that shortly.
Antiport is when we have molecules being
transported in opposite directions of each other.
So all of these are possible secondary I mean,
all of this are possible active transport
mechanisms. So moving on we will take a look
at a primary active transport mechanism. One
that comes up in Biological sciences and
Physiology all the time. You may have heard
of it before. The sodium-potassium pump.
The sodium-potassium pump is moving sodium
against its concentration. At the same time
is moving potassium against its concentration in
the opposite direction. So what do we call that
based on our transporter types? One this way,
one that way, antiport right. So there are going
in opposite directions. So the sodium-potassium
pump is an antiporter. Sodium, the natural state of
the sodium-potassium ATPase which is what this
transmembrane protein is called, is open to the inside.
And it has three binding sites for sodium.
Sodium will bind to those three binding sites,
and then ATP will come along. This is where the
energy investment comes. We're pushing sodium
against its concentration gradient. ATP comes
along. Provides a little bit of energy.
Drops off a phosphate and leaves as a lower
energy molecule. But this investment of energy
causes the conformational change. It causes the
ATPase to open up on the other side and dump out
sodiums. Now, the next stage in the game is
going to be having the potassiums bind in to
their binding sites. As soon as potassiums
bind into their binding sites, what we'll see
is this antiporter snaps closed and drops them
off on the other side of the membrane.
So again we have the investment of ATP in order
to make all of this happen. So to summarize
this whole thing, first of all we've got the
picture up the top where we have our sodiums
and they want to go against their concentration
gradient. They cannot diffuse, they cannot do
facilitated diffusion because we're going up the
concentration gradient from low to high.
The sodiums jump in to their spots inside the
transporter. The ATP comes along, phosphorylates
the protein. It has a conformational change. And
we continue through the cycle. The conformational
change happens. The sodium pump opens up, grabs
its potassiums, snaps close, dumps them on the inside.
So each cycle we have 3 sodiums out,
2 potassiums in. 3 sodiums out, 2 potassiums in.
And it will continue like that. And this is
one of the primary mechanisms for creating
electrochemical gradients across cell membranes
or correcting for them for that matter.
We have sodiums. Each time we have one more sodium
going out than we have potassium coming in.
So we have two positive charges coming in. Three
positive charges going out. That's the electrical
part. And then sodiums are going against their
gradient so we're creating essentially some
energy difference there across the membrane
because of the active transport. Active meaning
we have to use ATP. So secondary active transport
is actually completely dependent on primary
active transport. So we've already established
that the sodium-potassium pump is pumping
sodium out of the cell and potassium in. And
we have established that this is creating
a gradient especially for sodium where we have
sodium getting higher concentration on the
outside of the cell. Lower concentration on the
inside of the cell. So it's in the perfect position
to actually move down a concentration gradient.
The only problem is it's slightly too large
and polar to fit through the cell membrane itself.
So we need one of those channel proteins or
transport proteins. And that's exactly what we
have. We're going to look at now the glucose
and sodium cotransport protein. In this situation,
again, we have sodium and it wants to move down
its concentration gradient. Glucose is actually
completely dependent on the transport of sodium.
So glucose can move against its concentration
gradient because essentially sodium is going to
grab its hand and jump into the cotransport protein.
So the protein that we can see here has a spot
for sodium and it has a spot for glucose. And
because sodium binds, glucose can bind,
and that can end up pushing the gluose against
its concentration gradient inside the cell.
Now, we call this secondary active transport
because the transport against the concentration
gradient is completely dependent on what happens
in primary active transport. The sodium gradient
is created by the primary sodium-potassium active
transport pump. Now we have sodium wanting to move
passively down its gradient but it grabs the
hand of glucose to run through its transporter
together. Glucose though is going against its
concentration gradient using the power
that was invested by ATP in the primary active
transport of the sodium-potassium pump.
So again, we have two different things going on
here. We've got the primary active transport
driving the cotransport of sodium and glucose.
An interesting thing to remember about this
sodium and glucose cotransporter is that it is,
glucose will always require sodium in order to
get into cells. And this is one of the reasons
that many of the sports drinks we consume
come with sodium and glucose. Because we must have
sodium in order to get the benefit of the glucose
if we want to have some recovery
during our exercise.