If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Facilitated diffusion

Facilitated diffusion is a type of passive transport that uses specialized proteins, such as channel proteins and carrier proteins, to help molecules move across a cell membrane. In this process, molecules can move down their concentration gradient without requiring any energy input from the cell.

Want to join the conversation?

Video transcript

- In the first video on passive transport we talked about the most passive of passive transports and that is simple diffusion. And we talked about how small non-charged, non-polar molecules would actually have the easiest time. Things like carbon dioxide or molecular oxygen, would have the easiest time diffusing through the cellular membrane. They're small enough to kind of get through the little gaps here and then since they have no charge or polarity they're gonna be fairly indifferent as they pass through. And then we talked about in-between you have things like water molecules which are small enough to pass through the gaps but they have some polarity so they're not going to be able to get through super easily, but they will be able to seep through. And then we talked about things that would have a tough time. And that's charged particles because charged particles, and we have some ions right over here, a sodium ion, a potassium ion. Even though these are fairly small they're going to interact a lot with the phosphate heads right over here with this charge, which is gonna make it hard for them to actually penetrate through the membrane. What I wanna talk about in this video is still passive transport. And remember passive transport is about not using energy. It's about moving down the concentration gradient. But we're gonna talk about ways that passive transport can happen a little bit easier for some of these molecules over here. And that's because their transport, their passive transport, is going to be facilitated. So what we're going to talk about in this video. Let me just figure out a place where I can write it, is facilitated, faciliated diffusion. So let me write that down. "Facilitated, "Facilitated Diffusion" So last video was just straight up diffusion, now we're gonna talk about facilitating it. So what do you think, if you were trying to engineer something, that would make it easy for these types of molecules, either a water molecule or an ion, to move down its concentration gradient? What would you do? Well you might say, "Well if I didn't have "to mess with all of this, you know, "all the hydrophilic heads and then the hyrophobic tails "and then the hydrophilic heads here." Well that would make it pretty easy to move down your diffusion gradient. And that's exactly what has emerged in nature. Essentially just tunnels through the membrane. And so one form of facilitated diffusion can happen through what we call channel proteins. Let me write this in orange, for no good reason. "Channel, channel, channel proteins." "Channel proteins." And an example of a channel protein might be this one right over here. And maybe this one is specialized for being a channel for water and so we would call this, this particular one, we could call an aquaporin "Aqua, aquaporin". Which is just a channel protein for water. And so you see it has this hole on top. And let's say you had more water molecules outside the cell than you have inside the cell. And you wanted to move down its concentration gradient, or maybe you have a higher concentration of solute here, and so we're going to have osmosis occurring. So the water molecules they're more likely to come, they're more like to come from the outside to the inside than from the inside to the outside, and so you could have water molecules going there. They don't even really have to mess with the membrane, they're just gonna go through this aquaporin and then come out on the inside of the cell. And you have similar channel proteins for ions. So this might be one for ions. And so, let's say that this is a sodium, these are sodium ions right over here. They're charged, they would have trouble getting through. But this channel protein might be specific to them and allows them, it allows them, to go through. And as we'll see when you study things like neurons, we'll see that these channel proteins, especially for ions, are incredibly important for amplifying an electrical signal down, or a chemo-electrical signal, I guess I could say. And they can also be gated, they can also open and close depending on different conditions that are in different parts of the cell. So these channel proteins they could just be open. Or they could be open and closed, gated, based on different conditions. Which you can see that's actually key to what happens in nerve cells, that we'll see in future videos. Now another type of facilitated diffusion can occur through what we call carrier proteins, "carrier proteins". And I wanna be clear I'm gonna talk about carrier proteins but people are still studying exactly how they work. But the view is, okay let me just draw the membrane here. Let me draw, let me draw, a membrane. I'm gonna draw a carrier protein in the membrane. So this is a... This is a cross-section of my membrane. My phospholipid bilayer here, almost done. And then a carrier protein, and the way I'm gonna draw it isn't exactly how a carrier protein would actually look, but it will hopefully give you the right idea. So maybe it's like this, maybe it's like this. And if things wanna move down their concentration gradient. Let's say you have a higher concentration above. And I'm just gonna say some arbitrary particle has a higher concentration above than it does below. They can actually attach potentially, or kind of get into a compartment over here, and then that would trigger the carrier protein to change its shape so that, and let me see if I can draw its changed shape well. So it could change its shape. This is when it's taking stuff from above. And then when it sees that, "Hey I've got stuff here", it can change its shape to look something like this. So it could kinda flip around. Let me get the other tool. It could, whoops, really having trouble with my tools today. Alright, it could flip around like this. So before it was open to the top but now, it could flip around, and the stuff that it just collected from the top, could be dumped inside the cell. And once again this is passive transport because it's all about things moving down their concentration gradient. If there was no cellular membrane here these things would have moved in this direction. You would have had more things moving in this direction, in a given amount of time, than you would have had things going in the opposite direction. But the cellular membrane was getting in the way, but then this carrier membrane can facilitate that passive transport. It can facilitate the actual diffusion.