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Concentration gradients

Particles move from high to low concentration areas, a process called diffusion. Each particle type has its own unique concentration gradient. This movement isn't planned, but happens randomly. Over time, particles spread evenly, balancing the probability of movement in all directions. This is a key concept in biology and chemistry. Created by Sal Khan.

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Video transcript

- In the first video where we introduced the idea of diffusion and concentration gradients, we had a container with only one type of particle in it, we had these purple particles. And in our starting scenario we had a higher concentration of the purple particles on the left-hand side than we had on the right-hand side. And so if we looked at its concentration gradient, so the concentration gradient went from high concentration on the left to low concentration on the right. And we saw what happened. Since you have more of these particles here and they're all bouncing around in different directions randomly, you have a higher probability of things moving from the left to the right than from the right to the left. You will have things move from the right to the left, but you're going to have more things, so you'll have a higher probability of things, moving from left to right. And so if you let some time pass, then they become more uniformly spread across a container. They have moved down their concentration gradient to make things more uniform. Now, what's interesting about this diagram is I've introduced a second particle, these big yellow particles. And we see that their concentration gradient is going in the other direction. So we have a low concentration, in fact we have no, on the left-hand, we have none of the yellow particles on the left-hand side, and we have a high concentration on the right-hand side. So their concentration gradient goes from right to left. And the whole point of this video is to show that each particle moves down its unique concentration gradient, assuming that it's not blocked in some way, it's going to move down its unique concentration gradient irrespective of what the other particles are going to do, for the most part. And so we see the yellow particles are going to move from high concentration, to low concentration. They're going to move, they're going to diffuse from right to left. And once again, there's no magic here. It's not like this molecule is saying, oh, I've got seven other of my friends here, it's getting too crowded, I see them, I'm claustrophobic, let me move over to the left-hand side. That's not what's going on. They're just all randomly bouncing around and when you're in the starting position, when you're exactly like this, there's no probability because of a yellow particle moving from left to right, because there aren't any yellow particles here. While there's a probability that some of these particles, in a certain amount of time, some of these yellow particles could move from right to left. And so they'll keep doing that until you get to a stable configuration where now you have an equal probability of things moving from left to right, and right to left. And that's going to be true for each of these particles. So the real takeaway, you'll hear in a biology or a chemistry class, of things moving down their concentration gradient, and you might say, and their unique concentration gradient. As you see, the yellow particles' concentration gradient goes in the other direction as the purple particles, but there's no magic to this. You just have to imagine a bunch of things just bouncing around in a bunch of different directions, and then what would just naturally happen? You would naturally have a higher probability of moving from high concentration to low concentration, than from low concentration to high concentration.