 Active transport is defined as a transport that basically requires energy, okay, active transport requires energy to move molecules against, that says against, against, I think I did spell that correctly, against the concentration gradient. Now, active transport requires a transport protein, some kind of molecule that's embedded in the cell membrane that's going to do the work of the transport action. If you think about a channel, a channel is a tunnel, and so a channel is never going to be able to do active transport because it doesn't do anything. Now, a transporter that opens to one side and then the other side can. So look at this. We're going to look at a process called primary active transport, which basically requires a direct source of energy. And I want you to think about this for a second. Diffusion down a concentration gradient requires energy, but the energy comes from passive diffusion. The random molecular motion that all those molecules have, that provides the energy to move the molecules to spread them out until we reach equilibrium. When we're pumping, when we're actively moving molecules against their concentration gradient, that is not going to happen by random movement. It's like, here's my little analogy. Are you ready? It's like imagine you're in a classroom with me. Isn't it fun? And the door, there's a door. There's only one door into this classroom. And there's like all of you guys are in there. So let's just say there's 100 people in the classroom. All the seats are taken. There's some people standing on the walls because you know I'm pretty popular. And so of course you want to come into the classroom and see me, except I'm not letting you out because the walls are impermeable to you. You can't get through the walls and I've got a bouncer at the door. So good luck. You're stuck. You get to listen to me and you have to stay now. In addition to that, I want to get more people in this very crowded room. I suppose if I offered them cookies, they might come. If I offered them money, they might come. Or they might come if I like go out and manhandle them, grab them, tackle them, tie them in a knot and throw them in my classroom. All three of those require energy. Do you agree with that? It's not free. I can't, now if the room was totally empty and I had a plate full of cookies, they'd come in on their own. They'd be like, cool, there's nobody in here and there's free cookies, like dude, I'm in. And maybe a pile of money, that'd be awesome too. But otherwise, I'm going to have to like force you to come into my very crowded room because it's really crowded. Even though I'm going to have to like feed you cookies and force you. And all of that is going to require energy. Did you follow my little analogy? Because here's the energy. Energy in your cell is in the form of ATP. So this little transporter is only going to work to bring sodium out, this is my sodium, and potassium in. It's only going to work if I have cellular energy. It's money, it's cookies, it's candy, it's ice cream, right there. Dude, come in, potassium, get out, get the sodium against your concentration gradients and I'll give you a little hookup of ATP here. It'll be worth your time. The ATP provides the energy to pump against the concentration gradient. Whoa, direct source of energy. We're going to contrast this to secondary active transport because it doesn't require the direct source of energy.