If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Introduction to passive and active transport

Active transport requires energy to move substances against a concentration or electrical gradient, like a canoeist paddling upstream. Passive transport, on the other hand, does not require energy as substances move along their gradient, similar to a canoeist drifting downstream. The sodium-potassium pump is a key example of active transport. Created by Sal Khan.

Want to join the conversation?

Video transcript

- [Instructor] Let's say that you have decided to go canoeing, and right over here, this is a top view of our river right here. This is our river. And, let's say that the current, the river, is going towards the right. So, there is two different directions that you could be canoeing in. You could imagine someone who is canoeing in the same direction as the current, so they are canoeing that way. And then, you could imagine another person who's canoeing the other way. So, someone who's canoeing upstream. This person is canoeing downstream, this person is canoeing upstream. So they are going in that direction. So, pause this video and think about which person is going to have to expend more energy. Or, which person is going to have to be more active, and which person is going to be more passive. Well, yes, this wasn't an incredibly hard question. If you are going with the flow of current, as the person in yellow is here, they don't even have to take their paddles out. They can just take a nap, and the boat will naturally go with the current. They would be, they could be just moving passively. While the person in blue, here, they're gonna have to work really, really, really hard. They're gonna have to paddle some, just to not even move to the right, and then even paddle even more, to actually go against the current. So, this person would have to be very active. And so, this is really just a metaphor for what we're going to talk about now. And, that's the idea in Biology of Active versus Passive Transport. So, let's start with, maybe, the more pleasant one. In either situation. And that is, Passive Transport. So, Passive Transport is when something goes with the gradient. So what do I mean by that? So, you could have a concentration gradient. So, let's say that on, let's say I have a tube of some kind. And let's say it's filled with water. And dissolved in that water, at this end of the tube, I have a high concentration of some molecule, or something right over here. While on the right-hand side I have a low concentration. So what are we think is going to happen? Well, these things are just going to naturally move around, and over time, they're gonna bounce their way, so that after a little bit of time has passed, a lot of these things are just going to passively move to the right. And so at some point, you might have an equal concentration, or roughly equal, throughout this entire container. And so, this movement along your concentration gradient... Here you're moving from high-concentration, to low-concentration, this would be passive transport. This is a concentration gradient, that we are moving along. Let me write that down. This is our concentration gradient. But, you can also have an electrical gradient. So, let's take a similar type of container. Maybe it's filled with water, and on the left-hand side, imagine if you have a bunch of positive particles, or molecules. And on the right, you have a bunch of negative particles, or molecules. Well, the positive ones repel each other, so do the negative ones. But the positives attract the negative, and the negative attract the positive. And so, you would think that things would naturally move down their electrical gradient. The positives want to go away from each other, and they're drawn to the negative. Similarly, the negatives wanna get away from each other, and they are drawn to the positive. So, whether you're talking about a concentration gradient, or an electrical gradient... And sometimes you have a combination of both. An electro-chemical gradient. When you're moving along with your gradient, you don't have to use any energy. That's known as Passive Transport. So, no energy needed. It's just going to happen naturally. Now, the opposite is the notion of Active Transport. Active Transport, and this is when you go against the gradient. So an Active Transport would be, somehow let's say you're in this situation right over here, somehow getting one of these particles... Let me do it in the same color, somehow getting one of these particles, instead moving to go in that direction. It will actually go against it's gradient, in that direction. Or, another situation is, imagine if you have a positive particle right over here, instead of it naturally just going in that direction, somehow you make it go against it's gradient, and you make it go closer to the other positive particles. Well, this is going to require energy to do. And probably the most sited example, or the most common example that we're going to see, in Biology class, of Active Transport, is what's known as a sodium-potassium pump. Which we will study in detail in other videos. But, let's say that this thing that I'm drawing, here in white, this is a cell membrane. And, I'm drawing these gaps for a reason. And, what you have on the outside of the cell membrane, you have some potassium ions, on the outside, but, you have a lot more on the inside. So, these are all potassium ions on the inside of your cell. And then, so let me just write K+, K+, K+, K+, K+, and you'll have some sodium ions on the inside of your cell; Na+, but you have a lot more on the outside of your cell. And, in general, the outside of your cell is going to have many more positive ions than the inside. Maybe you already see where this is going. Na+, Na+, Na+, I think we've got the idea, Na+, Na+. Now, if on this membrane, alright, let's ignore this part right over here, if I just had a channel, right over here, that was open only to potassium. So, only potassium can go through. So, only potassium can go through this channel right over here. What do you think is going to happen? Well, you would have Passive Transport. These positively charged potassiums right over here, they would go down their concentration gradient. There's more likely to have a potassium ion just bump in the right way, just right over here, so that it goes through the channel. Because, there's just more potassiums right on the inside of the cell, than there would be on the outside. And so, this, this potassium is going down their concentration gradient, from high-concentration, to low-concentration through this channel. This would be Passive Transport. Passive Transport. But, you could imagine there's also Active Transport, and that Active Transport is what pumps the sodium ions inside the cell outside of the cell, even thought it's not only against it's concentration gradient, it's also against it's electrical gradient. The outside's more positive, so you wouldn't think a positive ion would naturally go outside. And the outside has more sodiums that it does inside. But, the sodium-potassium pump still pumps those sodiums outside. And, as I hinted at, it does this using energy. So, you'll sometimes see a sodium-potassium pump drawn like this and, once again I'm not gonna go into depth on it, we have a whole video on it. But, the general idea is, is that the sodiums bind over here. And then some ATP, which is the powerhouse of cells; which we will study in more depth later on in Biology, it leverages it's energy to change the shape of the proteins that make up this sodium potassium pump, to then pump these sodiums outside of the cell. So, it's gonna go from this shape, and then it's just going to, you could view it as opening it up that way, the real enzymes look quite different, but that's the general idea. You use energy, in the form of ATP to pump the sodiums out against both their concentration gradient, and their electrical gradient. And that's why it's called Active Transport.