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I’ve always been into fish.
And, if you’re into fish, you’re also going to be into fish tanks because you have
to house your fish.
5 gallon, 10 gallon, 20 gallon, 55 gallon---these are all tanks I’ve owned.
And with all these tanks, I’ve had pumps.
Pumps are important, because whether they are helping to aerate the water or bring water
through a filter, they help your fish live their best fish life.
The pumps I’ve always had plug into an electrical outlet, and they work hard drawing air or
water through some type of tubing.
You have pumps in your cells too.
Tiny, tiny little pumps because, well, your cells themselves are microscopic.
And while these pumps don’t plug into electrical outlets, many types of pumps do require an
input an energy to power them.
ATP is a great example of an energy currency used to power many of these pumps, and we
have a video on that.
And in our ATP video, we state one of our favorite pumps: the sodium potassium pump.
We also mention in that video that we might need to make a video on that pump---this is
that video.
Now, the sodium potassium pump isn’t trying to aerate your cells so while it has the word
“pump” --- it really isn’t much like a fish tank pump.
But it is requiring an input of ATP energy as we mentioned, and it is doing something
very important.
One major function is it is helping to maintain a resting membrane potential.
We should explain that.
This is an animal cell here.
Here is the extracellular part.
That means outside of the cell.
This is inside the cell here.
We’re going to mention two ions specifically.
Sodium and potassium.
These ions have positive charges and like most ions, they have trouble traveling directly
through the cell membrane.
Instead, they need a protein to travel through.
The protein could be a channel, it could be a pump…just some examples.
So back to this resting potential---this involves its electric potential.
The difference between the electrical voltage inside and outside of the cell.
Cells have their own resting potential depending on the type of cell.
At rest, generally most cells are more negative inside the cell than outside the cell.
For many cell types, when this resting potential is changed by some type of stimulus, there
is a response.
Muscle and neuron cells, for example, are excitable cells and many of their functions
depend on a change to their electric potential.
We’re not going to go into the change of a cell’s electric potential in this video.
Explore the action potential if you’re curious about that.
Instead, we want to talk about how the resting potential is maintained.
How is it more negative inside a cell versus the outside of the cell?
For a cell at rest, or if it needs to return to its resting potential, the sodium potassium
pump is pretty critical.
The sodium potassium pump initially is open on the intracellular side of the cell.
It has binding room for sodium ions, three specifically.
When these three sodium ions bind, the protein is phosphorylated.
In our ATP video, we talk a bit about what that means.
ATP, a molecule of energy currency, transfers its phosphate to the protein.
This actually changes the protein’s configuration, the protein’s shape.
This shape change causes the pump to open to the outside.
The sodium ions are pushed out to the outside, the extracellular side.
And now, due to this configuration change, its open to the extracellular side.
It has two binding sites for potassium ions.
Two potassium ions bind.
After the potassium ions bind, the phosphate group is released from the protein.
The sodium potassium pump reverts back into its original position so it will open once
again to the intracellular side.
The potassium is dropped off inside the cell, and once again there is room now for three
sodium ions.
The cycle repeats.
We should mention that this pump is moving sodium and potassium against their concentration
gradient.
What do I mean by that?
It keeps putting more sodium outside—where there is already a high concentration of sodium.
And it keeps putting more potassium inside---where there is already a high concentration of potassium.
So with this active transport, the ions are moving from low concentration to high concentration.
A great example of active transport.
So what is the result of this pump’s activity?
Well, you are moving 3 positive ions out for every 2 positive ions in.
So while this may contribute to the cell’s more negative charge inside, it’s really
important to understand that this alone is not the big reason you see a voltage difference
in the resting potential.
The result of the pump’s work means that there are specifically more potassium ions
inside the cell and more sodium ions outside of the cell.
This causes an electrochemical gradient.
That means there is not just a difference in charge but also in the actual ion types.
Remember in our diffusion video, we talk about how substances have a net movement from a
high concentration to a low concentration.
Well, if going by the chemical gradient here, those potassium ions inside the cell?
They’d like to have a net movement out.
They generally can’t travel through the membrane directly themselves as we mentioned,
but they could if there happened to be a channel.
These channels do exist.
It is common for many cells to have far more potassium leakage channels than sodium leakage
channels.
This makes the cell membrane more permeable to potassium at rest.
So while sodium would like to diffuse in, there are less leakage channels for it to
do so.
Positive potassium ions that travel out of the cell contribute to a more negative charge
inside the cell.
If really exploring resting potential, there are additional factors that contribute to
the cell’s more negative charge.
Keep in mind also because this video is focused on the sodium potassium pump, we focused on
the ions sodium and potassium.
But there are other ions that can contribute to the resting potential as well.
And now this electrochemical gradient is a perfect setup for a whole lot of things.
We already mentioned that an action potential, which makes a change to the cell’s voltage,
could result in a cellular response.
We should also mention that there are other proteins that rely on this gradient to move
critical molecules: some proteins that transport in glucose for example require that gradient
to be established.
Overall, the sodium potassium pump is one pretty remarkable protein pump.
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious!