 Diffusion is one of the ways that things can pass in and out of the cell. Some things can diffuse directly through the cell membrane. Other things need help. And this is where facilitated diffusion comes into the mix. Facilitated diffusion is still diffusion. And diffusion, no energy required. You do not have to do anything to make diffusion happen. All you need is a concentration gradient. Really, concentration gradient. If you have a concentration gradient, you can make molecules move, assuming they can pass through the cell membrane. So if the molecule can't, like my friend Sodium, Sodium ion cannot pass through the cell membrane, then let's facilitate that, shall we? And you can facilitate the diffusion with a protein. This is awesome. You can put a channel in the cell membrane. And we actually have Sodium channels. And they literally look like this. And Sodium can come in and pass through. And the rest of this structure right here is, look at what I'm drawing you. This is cell membrane doggies, right? There's my phospholipid bilayer. I can do it. There we go. So accurate. It's a protein. It's almost like a little gap junction that doesn't connect two cells. That would be a fantastic compare and contrast question. Compare and contrast gap junctions and channels. It's facilitated diffusion because the movement will only happen through this channel if there's a concentration gradient present. So if there was a high concentration of Sodium on the outside in the extracellular fluid and there was a low concentration of Sodium in the intracellular fluid, then as long as I have a Sodium channel that's willing to do the job, meaning it's open, then Sodium will be able to diffuse down its concentration gradient and into the intracellular fluid. There are lots of different kinds of channels. There are some channels that are called gated channels. And I'm going to draw you a gate. I don't know why I can't see my colors here. I'm not cool with that. I'm having technical issues on this side. Oh, but they are mild technical issues, so I'm not complaining. Look, I just drew a lid on the channel. That's a gate. You can close channels. If I have a gated channel, then that channel is there, but the gate is closed. On gated channels? Dude, how awesome is this? If you feel like I will let the Sodium into the party in the intracellular fluid, come on in, fellas. Open the gate. And now the Sodium can pass through. Close the gate, and then we've made the cell impermeable again. The gates can be stimulated to open like, let's say, if there's an electric shock, open the gate. So that would be a voltage-gated Sodium channel. There are mechanical gated, different kinds of channels, so a mechanical stimulus can initiate the opening or closing of a gated channel. There are even light stimulated channels that will open with a stimulus of light or temperature or pH. There's all sorts of different kinds of stimuli that can cause the opening of gated channels. Not all channels are gated. Some of them just stay closed for good. I mean, excuse me, stay open. A significant point of the channel is that it's open to both sides. When it is open, it's open to both sides. So it's not like it has stages like here open up there and then close and then open up at the bottom. It's not like that. When it's open, it's open to both sides, so it's a little tunnel. There are other kinds of transporters that enable facilitated diffusion that don't operate that way. So channels are one type of transporter or protein in the cell membrane that enables facilitated diffusion. There's another one that we're going to talk about in the next section.