 Facilitated diffusion happens when a substance does diffuse down its concentration gradient. So let's make sure everybody is super clear about that. Facilitated diffusion happens down the concentration gradient. So if you look at this, it looks like my little green dots are my... Oh, it looks like we've got multiple substances here to transport, and it looks like we have high concentration in the extracellular space, and so we have low concentration in the intracellular space. So all of our substances are going to want to go down their concentration gradient into the cell. Facilitated diffusion happens because if, like this guy right here, okay, I should totally erase that, he tries to go through rejection. He tries to go through rejection. They want to get through and go down their concentration gradient, and it's... they only want to go through because of random molecular motion, but they get rejected every time they try to go through the cell membrane. So they need some help. They need a facilitator. Help me get through this mess, please. There are two types of facilitated diffusion proteins that can help make this happen. The first type is a channel. This right here is a channel. A channel is basically a tunnel. It's open on both ends, look. It's open on this end, it's open on this end, and it's just facilitating diffusion. So the molecules are only going to go down their concentration gradient, but they can go in either direction. So if the concentration gradient for some reason changes, then they can adjust and things can move in either direction. The other option is a carrier protein, and a carrier protein is different than a channel because it's open to one side at a time. So look at this thing. I love this. It's like this, open to the extracellular fluid, and then it changes shape, and now it's open to the intracellular fluid. Extra, intra, extra, intra, but never both at the same time. If it is open to both at the same time, it's not a transport, it's not a carrier protein, it's actually a channel, and so it would just be a different thing. In this case, it's going to move the substance again down the concentration gradient. So those are two examples of facilitated diffusion, keeping in mind that it doesn't require any energy to make this happen. It's just the energy of the random molecular motion. That's what fuels the movement of these particles, and they're going to do it. So if we have a way through, they're going to move. It's kind of wild, and it's kind of fast. If we want to move the substances against the concentration gradient, now we're going to have a different situation, and that's going to require energy, and it's going to require active transport.