>>So in this segment of the cell membrane transport we’re going to continue our discussion
of active transport, but now we’re specifically going to be focusing on secondary active transport.
And so once we complete this, you’ll be able to compare and contrast primary active
transport with secondary active transport. Just a reminder, okay, we’re going to be
focusing over here on secondary active transport, which is going to be involving organic molecules
such as glucose, and ions such as sodium. So in secondary active transport, one ion
moves down…its concentration gradient, meaning it moves along its gradient, right, from a
high concentration to a low concentration. And that’s going to cause another ion or
a molecule to move against its gradient. So I always envision it this way. All right?
So if we have our two hill analogy again, and we have our one solute on top of the hill
and then over here we have a different solute at the bottom of the hill. So one ion, right,
moves down its gradient. A lot of times this is sodium that moves down its gradient. And
when it moves down its gradient, that’s going to release energy. So this is an indirect
energy. We’re not using ATP; we’re not hydrolyzing ATP to do this. It’s just the
release of energy from this ion moving down its gradient that actually produces or creates
the energy to drive the other ion or molecule up its gradient. So without having the ion
going down its concentration gradient, you cannot cause the other ion to move up its
gradient, okay? So again, the most common ion we’re going to be dealing with here
is sodium. Sodium typically moves down its concentration gradient. And keep in mind that
it’s movement down this gradient, and it’s an electrochemical gradient, that releases
the energy that drives another ion or a molecule against its gradient. So then we have two
different types. We have co-transport, which is also called symport, and then you have
counter-transport, which is also called antiport. And so when you think of these names what
you’re trying to think about is what direction do the solutes move? Not thinking in terms
of their concentration gradient, but just what overall direction. So co means “together,”
right? And so does sym. So this means that both solutes move in the same direction. If
it’s counter-transport, right, or antiport means opposite. So the solutes are moving
in opposite directions.
Now here is a crucial point, okay? Again, we’re talking about their general direction
that they move. Do they both move into the cell? The principle that one ion still needs
to move along a gradient and the other one’s going to move against a gradient, that principle
still holds—but the gradients may be such that both solutes are moving in the same direction,
but they are following that rule. One’s going along a gradient, one’s going against
a gradient. Same thing for counter-transport—these solutes may be moving in opposite directions,
but one of those is still going along a gradient and the other one is still going against a
gradient. And you’ll have an opportunity to work some examples here in just a moment.
And so down below in this table, these are just some examples of secondary active transporters
that we’ll be talking about throughout the semester. So here’s an example of secondary
active transport. And so once again, kind of get your bearings here. Where—where are
we? Here’s the lumen of the intestine or the kidney. Okay, here’s some intracellular
fluid here. And then we have this carrier protein: the SGLT. So secondary active transport
does require a carrier protein. And so let’s take a look at what’s going to happen. So
sodium comes in and it binds to the carrier. So this carrier represents or illustrates
the concept of specificity. So sodium is moving from a high concentration toward a low concentration.
So we’re going to put low sodium over here. Glucose, then, is moving from a low concentration
toward a high concentration. Okay, so now I’ve got to think about this. When sodium
binds, it’s going from high to low. So that’s creating the energy to drive glucose from
low to high. All right, so when sodium binds that’s going to create your site for glucose.
When you create the site for glucose and glucose combined, the carrier changes shape, and then
sodium and glucose both get released into the intracellular fluid. So both of these
solutes are going into the intracellular fluid. However, sodium is going along its gradient,
glucose is going against. But despite all of that you would define this as symport or
co-transport. So we would call this a sodium glucose co-transport. So these types of co-transport
or counter-transport, those terms or those types of active transports only apply to secondary
active transport. So here I’ve listed the concentrations of sodium as well as some solute
“X.” Okay, that could be an organic molecule or it could be another ion. So we want to
figure out which direction is “X” going to move if we assume this is co-transport
with sodium. Okay? So co-transport automatically means it’s secondary active transport. So
that means that one ion like sodium here and it’s already showing you, sodium is going
high to low. Therefore that’s creating the energy to drive “X,” whatever it may be,
from low to high. So this is low “X” toward a high “X.” And that was the energy that
was generated or what we call indirect energy to drive “X” from low to high.
So since both sodium and “X” are moving into the cell, this would be an example of
symport. Okay, here’s our next example. So now we’re going to deal with sodium again.
However, we’re going to deal with “Y.” A new solute named “Y.” So “Y” has
a low concentration inside cell and high on the outside. So if you would like to pause
the video to figure out which direction “Y” is going to move, please do so. So if sodium
is going from high to low, we know that we’re dealing with secondary active transport. Therefore
you know your other solute has to be moving from low to high.