 Primary active transport pumps molecules against their concentration gradient, but requires a direct supply of ATP to make it happen. Secondary active transport also pumps things against their concentration gradient. So pumps against concentration gradient. However, this time, are you ready for this? This time, the energy comes from a concentration gradient, energy comes from another molecule's concentration gradient. And usually, that concentration gradient was established by primary active transport. Oh, seriously, let's break that down. Break it down. All right. If you can imagine, I'm going to give you an example. If I can establish a concentration gradient with sodium so that I have a high concentration of sodium in the extracellular fluid, okay, I'm going to do it again. And then I'm going to have a transporter here. Run around, run around. You tell me, how did I establish a high concentration of sodium outside of my cell? That isn't only going to happen if you have the sodium potassium pump. Do you agree with that? And that's an example of a primary active transporter. It required ATP to do it. But just the act of creating a sodium concentration gradient, now a secondary active transporter can take advantage. So now you have a sodium concentration gradient. So this buddy boy named Sigglet, Sigglet says, hey dog, you're a sodium ion. That's my sodium ion. Why don't you... I'll let you in. You want to get into the cell, don't you? Because there aren't very many sodiums inside the cell. I'll let you in. I'll let you in as long as you let glucose go in with you. And glucose would be an example of something that, let's say, has a high concentration inside. And perhaps a low concentration outside. Sodium's like, pfft, I don't care. I don't care who else you bring along. I just want to go down my concentration gradient and get in. So Sigglet is a secondary active transporter because it uses the energy in somebody else's concentration gradient to pump a molecule against its concentration gradient, which would be glucose in this example. ATP was required because the only reason why we have a high concentration of sodium in the extracellular fluid is because of the sodium-potassium pump, which was powered by ATP. Do you follow that? It's actually phenomenally brilliant. Now, we're going to talk, the last thing we're going to talk about is how we can move molecules. We can transport things completely across a line of epithelial tissue. Remind yourself, what is epithelium? I'll look it up and then I'll be right back.