Source: Video and Images Created by Amanda Soderlind
Hello. I hope you are excited to learn about facilitated diffusion today. So we're going to discuss the process of facilitated diffusion and also identify other ways that solutes can cross cell membranes.
So to start off, facilitated diffusion is a type of passive transport. And if you think back to osmosis and diffusion, passive transport is just a way to transport molecules across a cell membrane that does not require the use of ATP or cellular energy. So facilitated diffusion does not require cellular energy.
Facilitated diffusion uses transport proteins to transport molecules across a cell membrane. So in regular diffusion, we would have molecules that would just be able to pass through the cell membrane. But in facilitated diffusion the difference is we still have molecules passing through the membrane. But they're passing through these transporter proteins to get through the membrane. So they need help to get through the membrane, so it's called facilitated diffusion, because these transporter proteins are facilitating the way that these molecules are getting through the cell membrane. So that's facilitated diffusion, the use of transport proteins to move molecules across the cell membrane without the use of ATP.
So some other ways that molecules can get across a cell membrane include active transport. And there are several different types of active transport, or ways that molecules can get across a cell membrane by means of active transport. So active transport is when a molecule is moving across a cell membrane but it requires the use of ATP. So unlike facilitated diffusion, unlike diffusion and osmosis, active transport is using ATP to move molecules across a cell membrane.
So a good example of this is sodium potassium pumps. Sodium potassium pumps are moving sodium and potassium ions against their concentration gradient. So let's think back again to facilitated diffusion, diffusion osmosis. We're moving from an area of high concentration to an area of low concentration, so where there's a lot to where there are fewer. And you can see that in this example here. We have more here and less here, so naturally they're going to move down that concentration gradient. So ATP isn't needed.
But as I said, in active transport, we're moving against the concentration gradient. So we're moving from an area where there's a low concentration to where there is a high concentration. And because it's moving against what is natural, it's going to require energy from the cell to push it in that direction.
So again, back to here. It says sodium potassium pumps are an example. So sodium potassium pumps, we're moving that sodium and potassium against their concentration gradient.
And the reason for this is because by moving them against their gradient, it's producing an electrochemical gradient that allows action potentials for neurons or muscle cells to be produced, which is necessary. So when we get into how muscle cells and how neurons work, we'll talk a little bit more about the sodium potassium pumps. But just for now, you need to know that sodium potassium pumps are type of active transport, requiring cell energy to move something against its concentration gradient.
Another type of active transport besides sodium potassium pumps is endocytosis and exocytosis. So this is another way to move molecules across the plasma membrane. And endocytosis and exocytosis is used to move larger particles that wouldn't be able to move through on their own. So I have an example here of endo- and exocytosis.
Endocytosis means moving molecules into the cell, whereas exocytosis is moving them out. So those prefixes should help you remember what each process is responsible for. Endo- is moving in. Exo- is moving out or exiting out of the cell.
So we're going to pretend that this is the outside of the cell and this is the inside of the cell for each of these. So in this case, you'll see that particles are moving from the outside of the cell to the inside of the cell. So this would be an example of endocytosis. We're moving particles into the cell, whereas this would be an example of exocytosis, moving particles out of the cell.
So basically what happens in endocytosis is we have the particles on the outside of the cell. The cell membrane will start to form this little vesicle. And then that vesicle will pinch off and move those particles into the cell, whereas in exocytosis, we have a vesicle that's moving towards the plasma membrane, attaches to it, and then expels whatever the contents are out of the cell. So it's a means of moving larger particles either into or out of the cell.
So this lesson has been a brief overview on the process of facilitated diffusion, as well as active transport.
