Let’s now move on to the final type
of carrier that we’re going to
talk about in this lecture and
that’s these solute carriers.
The prototype that we’re
going to use is an SGLT1.
These particular transporters are very
nice and in this particular example,
we’re going to have a molecule that
will enter into this particular
cell -- into this transporter and
then have it ratchet to exit.
So first, the solute binds
within the carrier itself
in a binding site and
then finally, it changes
or there’s a conformational
change that happens
that then allows it to
enter into the cell.
Now, how the solute is
released is also important,
so it not only needs to bind, but the carrier
needs to have that change occur and then
it also needs to open up on the outside
to allow it to move through the membrane.
What do you need for a
solute carrier to work well?
The big thing is you need
a concentration gradient.
So, similar to a pore and
similar to an ion channel,
you need to have the concentration
gradient developed already.
The other thing that you need is to
make sure that the transport substance,
does it transport by itself or does
it need a cotransporter molecule?
For our particular prototype, we
are going to use one that requires
a cotransporter or another transporter
that’s being brought along.
If the solute is brought
along in the same direction,
it is called the symporter
If it involves a substance in which
it will go the opposite direction,
it’s called an antiporter
or a countertransporter.
Or finally, we have things that just move
through by themselves or uniporters.
So why have a solute carrier
in the first place?
Why not use simple diffusion to move a
molecule through the cell membrane?
Well, first is it might not be
able to make it through the cell
membrane if it doesn’t have enough
of a nonpolar or lipophilic nature.
So that’s one reason.
The second is you can move
things through faster with
a solute carrier than you
can with simple diffusion.
So if you look at this
you have concentration of the
substance along the X-axis
and along the Y-axis you
have the rate of diffusion.
Focus in here on the red line,
the facilitated diffusion line.
You can see how it occurs.
At a lower concentration, you
get more rate of diffusion
with facilitated diffusion
through a solute
carrier than you do
through simple diffusion.
So it’s faster.
The one problem that can occur is that
sometimes, besides it being faster,
you can do it at a
Meaning, that you need less of the substance
available for the fast transport.
This is very important in terms of getting
things through as rapidly as possible.
The one problem with
a solute carrier
is that you only have a certain
number of solute carriers
and eventually, you can eclipse the number
of carriers that you have available.
And if you do that, you can then
have the substance spill over.
And this sometimes occurs
in places like the
kidney in what we’ll get
to in a few slides.
Now, the prototype that we want
to focus on today is the SGLT1,
which is the sodium glucose
and this is one of the
primary ways in which you’re
going to bring glucose into
the body in the GI system.
So this is usually how
it occurs if you take in
let’s say milk or some
sort of a dairy product.
You have lactose.
It gets broken down by an enzyme called
lactase into glucose and galactose.
Now, both of these particular
carbohydrates, simple carbohydrates,
are going to need a transporter
to get into the body.
SGLT1 provides that transporter.
In this case, we’re going to be moving in
one glucose molecule and two sodiums.
What do you need to
have this process work?
The first is you need to have
a concentration gradient.
So sodium needs to have a concentration
gradient to move in glucose into the body.
How does that concentration
gradient get developed?
It’s through another transporter,
the sodium-potassium ATPase,
which is located on the other side
of the enterocyte on the basolateral
membrane is creating the gradient so you
need to pull sodium through the cell
and then the sodium is providing
the driving force to move that
particular glucose or galactose
molecule through the transporter.
It’s interesting for SGLT1s you
actually need two sodiums.
For SGL2s, which is another transporter in
this particular family, you only need one.
So it depends on the transporter which
molecules are going to be brought through it,
but both of them are called
symporters or cotransporters.
Now, this allows for
glucose absorption and the
example there we’re using
with this particular SGLT1,
it’s through the small intestine to
allow us to reabsorb glucose or into
the body and that allows us to have
enough energy for ATP production.
There are other SGLT1s in the body besides
those that are located in the GI tract.
One of them is located in the
renal tubule and this allows
us to reabsorb glucose so we
don’t lose it out of the body.
And so in this case, we’re
moving glucose from the tubule
lumen space in through the
renal cell into the blood.
And in this case, we really want to make
sure we don’t lose glucose out of the body.
So it’s a very
However, if a person is diabetic and
has a high amount of blood glucose,
you could eclipse the Vmax of the
SGLT1 within the renal tubule cell,
and therefore, glucose
will be spilled over
into the urine and you
will urinate it out.
In this case, you’ll be losing glucose
in a diabetic hyperglycemic condition.