Introduction to passive and active transport | High school biology | Khan Academy

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 of the river is going towards the right.

So there are 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. Then you could imagine another person who's canoeing the other way, so someone who's canoeing upstream. This person is canoeing downstream, and this person is canoeing upstream, so they are going in that direction.

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 just moving passively, while the person in blue here, they're going to have to work really, really, really hard.

They're going to 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. 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. Passive transport is when something goes with the gradient. So what do I mean by that? You could have a concentration gradient. Let's say that I have a tube of some kind, and let's say it's filled with water.

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 do we think is going to happen? Well, these things are just going to naturally move around, and over time they're going to 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.

At some point, you might have an equal concentration, or roughly equal, throughout this entire container. 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're moving along. Let me write that down. This is our concentration gradient.

But you could 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 they attract; the positives attract the negatives, and the negatives attract the positive.

So you would think that things would naturally move down their electrical gradient. The positives want to go away from each other, and they are drawn to the negative. Similarly, the negatives want to 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 electrochemical 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 is when you go against the gradient. 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 in that same color—somehow getting one of these particles instead of moving to go in that direction, it will actually go against its gradient in that direction.

In another situation, imagine if you have a positive particle right over here. Instead of it naturally just going to that direction, somehow you make it go against its gradient and you make it go closer to the other positive particles. Well, this is going to require energy to do.

Probably the most cited 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.

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. 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.

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⁺, and Na⁺. I think you get the idea, Na⁺, and Na⁺.

Now, if on this membrane—let's ignore this part right over here—if I just had a channel right over here that was open only to potassium, then 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 would go down their concentration gradient. There's more likelihood to have a potassium ion just bump in the right way, just right over here, so that goes through the channel because there are just more potassiums out on the inside of the cell than there would be on the outside.

So this potassium is going down their concentration gradient from high concentration to low concentration through this channel. This would be 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 though it's not only against its concentration gradient, it's also against its electrical gradient. The outside's more positive, so you wouldn't think a positive ion would naturally go outside.

The outside has more sodium than it does inside, but the sodium-potassium pump still pumps those sodiums outside. As I hinted at it, it does this using energy. So you'll sometimes see a sodium-potassium pump drawn like this, and once again, I'm not going to go into depth on it; we have a whole video on it.

But the general idea is that the sodiums bind over here, and then some ATP, which is the powerhouse of cells—and we will study in more depth later on in biology—leverages its energy to change the shape of the proteins that make up the sodium-potassium pump to then pump these sodiums outside of the cell.

So it's going to 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.