>>In this segment on cell membrane transport we’re going to be focusing on active transport,
and specifically we’re going to be looking at primary active transport in this segment.
So remember that primary active transport is a type of active transport, so we’re
going to be moving solutes against the gradient. And so this will just remind you that we actually
have two types of active transport, there’s primary and secondary. You’ll have an additional
video that will explain secondary active transport for you. So here are the types of active transport
then in, in what types of solute they’re involved with. And so primary active transport
involves the movement of inorganic solutes, okay, as well as ions. And the most common
ions that we’ll be talking about that use primary active transport in this course are
sodium and potassium. Those are just two examples of ions that use primary active transport—we’ll
have additional examples later on. Then there’s also secondary active transport. So secondary
active transport involves organic molecules and ions. So glucose is one example of an
organic molecule that will use secondary active transport, and sodium is another example.
But this is an ion that uses secondary active transport—sodium is probably one of the
most common ions that utilizes secondary active transport.
Okay, so our focus now is going to be on primary active transport. And so keep in mind that
this is active transport, so the solute is going to be moving from a low concentration
toward a high concentration. So that would be against a gradient, or up a gradient; all
of those would mean the same thing. So the requirements for primary active transport,
you have to have a carrier protein. And that carrier protein is going to function as an
enzyme called ATPase. If it ends in “ase,” A-S-E, it’s an enzyme. And not only does
this carrier protein function as an enzyme, it also functions as a pump. So those are
three different terms that we’ll use to describe the same thing. So active transport
involves the use of direct energy, and by direct energy what we mean is ATP. And so
when we say we’re using direct energy, we have to release the energy via hydrolysis
of ATP. So when the reaction proceeds then, the carrier protein is actually phosphorylated.
So let’s take a look at the reaction below. So here we have ATP, and here’s our carrier
protein involved which remember, also functions as an enzyme and a pump. And so in this hydrolysis
reaction, we’re losing phosphate. It requires water, and hence it’s called hydrolysis.
So we use water to do this, we lose a phosphate from the ATP, adenosine triphosphate, which
knocks it down to ADP, adenosine diphosphate, and that phosphate that’s released is now
bound to the carrier. You’ll see a close example here in just a moment. So here are
some different examples of primary active transporters. Several of these we’ll actually
be talking about throughout the semester, but again the most common one is going to
be the sodium potassium pump. And hence, that’s the example that we’re going to be talking
about in just a moment. So just to begin, it’s very important that you realize—or
that you actually memorize, I don’t use that term very often in physiology, [laughs]
but you actually have to memorize your ion concentrations for sodium and potassium in
the intracellular fluid as well as the extracellular fluid. And so we have a high concentration
of potassium in the intracellular fluid and a low concentration of sodium in the intracellular
fluid, and then just the reverse conditions out there in the extracellular fluid. Now
I’ve put here that this is a resting cell. And what I mean by a resting cell is that
this is a cell that is, is not reacting to its environment. So this cell is just chilling.
At this point it’s a couch potato, it’s not reacting to anything, okay? And so this
cell, if you notice, you have concentration gradients right, and electrical gradients
for these various ions. It’s important to have those, and we’ll discuss later about
why that’s so important.
The intracellular fluid has a negative charge. The extracellular fluid has a net positive
charge; that’s also important to commit to memory. Okay, and so now what we want to
do is think about primary active transport in the sense that okay, if this is primary
active transport, which way will sodium be moving? And so since it’s active—primary
active—we know that both solutes will move from a low concentration toward a high concentration.
So what that means is that sodium would be moving out of the cell, low concentration
toward a high concentration. Potassium would be moving into the cell. Again, against its
gradient: A low concentration toward a high concentration. All right, so let’s take
a really close example then of how all this is going to work. So we’re going to start
with the diagram in the center that’s labeled #1. So what I’d like to do is just zoom
in on this just a little bit here. And so here we are in step one. So again, always
get your bearings. Here’s your plasma membrane, this blue thing here is your—is your ATPase
pump, okay or your carrier protein. Here’s the extracellular fluid labeled for you and
the intracellular fluid. And so just to begin, we know that we have a low sodium concentration
in the intracellular fluid, and a high potassium in the intracellular fluid: Just the opposite
then in the extracellular fluid. So I always like to draw these in, because as a visual
person it makes it easier for me to see what’s going on.
So the first step in the sodium potassium pump is for three sodiums to bind to the carrier
protein. So that’s what you’re seeing here in this image, three sodiums are binding.
So this carrier or this pump illustrates specificity. Once your three sodiums bind, then we’ll
put a 1B over here, hydrolysis of ATP occurs. And when hydrolysis of ATP occurs, we talked
about a phosphate being added to the carrier protein or in other words, phosphorylating
the carrier protein. So that’s where this phosphate came from, it came from that ATP
in the hydrolysis reaction. So once that’s happened, now we’ve got a protein confirmation
change, and so that’s going to allow the release of your three sodiums. Okay, now so
three sodiums move out of the cell. Now two potassiums move into the cell. So the two
potassiums come in and they bind to the carrier protein. Once they bind to the carrier protein,
notice this phosphate, it’s going to fall off. So when the phosphate is removed, the protein changes
shape and when it changes shape, it opens to the opposite side releasing your potassiums
into the intracellular fluid. This protein will then go back to its original confirmation,
and then the process will start all over again. Okay so for every ATP that we use, three sodium
ions will move out of the cell—they’ll be pumped out of the cell, and two potassium
ions will be pumped into the intracellular fluid.