 Secondary active transport is relies on primary active transport. It still pumps things against the concentration gradient. Secondary active transport, it still requires energy. It still pumps things against the concentration gradient. However, it doesn't require direct energy. It's not direct. Now, what was our direct form of energy in the previous example? ATP is an example of direct energy. ATP is like direct money. In secondary active transport, there is someone who is pumping against a concentration gradient. So step one, someone does primary active transport. It happens somewhere and creates a concentration gradient. And then that concentration gradient is used to transport something else against its concentration gradient. What? Okay, take a deep breath and I'm going to show you how this is going to work. Remember, this is primary active transport and we have a little pump is basically creating a high sodium environment outside and a high potassium environment inside. Okay, so this pump is just like, it's going to work. It's using a direct source of ATP and it's pumping and pumping and it's creating these concentration gradients. And now that the concentration gradient has been created, and now watch this. Okay, I have to think this through. Sodium wants back in. Do you agree with that? It's a high concentration of sodium out here. So if we had a transporter, a secondary active transport transporter that led, okay, so I'm going to make it look like this so that you can agree, right? It looks like a little thing. Sodium is going to want to come in and go through. Like it wants back into the cell because this guy, the sodium potassium pump, is pumping it out using ATP to pump it out. Lots of ATP is being used to create this concentration gradient of sodium. Now along comes this pink thing that says, hey man, sodium over here, I'll let you in. I'll let you in down your concentration gradient. Sodium's like, boom, I'm in, let's do this. Sodium is just going to go down its concentration gradient. This is where the energy comes from. Sodium is going down its concentration gradient. Guess who's going to hitch a ride? I'll tell you who. Sugar. Glucose is going to hitch a ride. Glucose is going to like say, well, I'm telling you right now, there's a lot of glucose inside this cell. So glucose, really, if you're going to try to pump more glucose in here, good luck. You're going to have to do active transport in order to do it. So secondary active transport says, hey, but there's a concentration gradient for sodium. So let's let sodium in and have glucose hitch a ride. And glucose is going to hitch a ride and make it in against its concentration gradient. So let's get down sodium's concentration gradient. If the sodium concentration gradient disappeared, then glucose is out of luck. Sorry, man, you can't get in. Do you kind of get a sense of that? So somebody, you have to be using ATP to establish the concentration gradient in the first place. And then you have to let that molecule carry someone else against their concentration gradient. Kind of feel like, dude, let's do a big old, like a performance. We can pretend like we're cells in a room going in and out. I bet we'll do that in class. Okay, these are all relatively small substances. What if like you want to get an entire bacteria in or out? Yeah, that's going to be another whole story. We need a new method.