 We have transporters that function to move things against concentration gradients. And when molecules are moved against their concentration gradient, that's called active transport. And here's the deal. Active transport requires energy. So if I was in a room and I was by the door and the whole room was filled, like billions of people who are taking my class and you're all crammed in that room, if I wanted to add more people into that room, if I wanted to actively transport people in, do you agree that it's going to take a lot of energy for me to cram more people into that space? There's no question. I'm going to have to be a buff stud, which thankfully I am. Therefore, I can definitely cram you all into the space. Now, active transport, because it requires energy, it's something that, you know, you don't just do it. You do it if you're going to use it, which makes sense. Hopefully, you don't have to use too much energy to do active transport, because then you're just using all the fuel that you've ingested to carry out active transport. But when I tell you about this active transporter, this first example, you're going to be like, wow, yeah, we let you use like, I don't know what it is. I wrote it down somewhere. Somewhere I wrote down how much of our daily intake of calories is used powering this active transporter. It is a primary active transporter and it's the sodium potassium pump. Sodium potassium pump is awesome. This thing requires energy. Primary active transport requires a direct source of energy and it's usually, not always, in the form of ATP. ATP is the energy currency of most active cellular processes. ATP powers the sodium potassium pump. So it's a little transporter. It looks similar to, oh boy, glut. Here's my, oh, that's my cell membrane. Totally looks like a cell membrane, right? This is the sodium potassium pump. This time, the net action is going to pump potassium. I have to think about this every time. Potassium ions are going to get pumped into the cell. Two potassium ions and sodium ions are going to get pumped out of the cell. Three sodium ions. We're going to pump two potassiums out. Three sodiums, I mean, two potassiums in, three sodiums out. And what's that going to tell you about the concentration gradient of potassium inside the cell? There's a high concentration of potassium inside the cell. There's a high concentration of sodium outside the cell. If you're pumping something against the concentration gradient, that's going to require energy to make it happen. There's another form of active transport that doesn't require direct ATP. In fact, like, take a second to visualize. Like, what does that mean? It requires a direct source of ATP. It means that literally two potassiums, here's a potassium, two potassiums combined to my transporter, and that causes somewhat of a shape change, but you have to have ATP bind to, as well. When ATP binds to my transporter with the two potassiums, then the shape changes. If you don't have ATP, there's not enough energy to cause the shape change by itself. The ATP provides a little extra oomph to make the shape change happen. The potassium falls off because the shape change occurred. Now we have binding sites available for sodium. The sodium binds and the shape change happens again, and now we barf the sodium into the extracellular fluid. The key is that we had a direct source of energy that was required to make this happen. I'm going to contrast that with our second form of active transport, which is secondary active transport.