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## Biology library

### Course: Biology library > Unit 10

Lesson 2: Diffusion and osmosis# Concentration gradients

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**concentration gradient**occurs when the concentration of particles is higher in one area than another. In passive transport, particles will diffuse*down*a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced.## Want to join the conversation?

- In concentration gradient, what happens at equilibrium?(7 votes)
- Equilibrium is when the molecules are distributed evenly in an area. The concentration gradient is removed at equilibrium, but particles will continue to move across the whole area because molecules are always in constant random motion.(21 votes)

- Since Sal is talking about probabilities, is there a probability as well that the particles will end up spread very uneven? Or is this never going to happen?(6 votes)
- No, not really. Since if they're spread evenly, there's an equal chance on all sides that the particles would move to their sides. Therefore it's unlikely that the particles will end up spread unevenly. Not impossible, just unlikely.(7 votes)

- What is the goal of a concentration gradient?(6 votes)
- There is no goal in concentration gradient, this is just an effect of the constant motion that particles have.(4 votes)

- At1:27, Sal said that concentration gradient can be blocked, HOW?(4 votes)
- By putting any kind of barrier in the form of something impermeable between molecules. In that case, molecules cannot move.(5 votes)

- how do cells create concentration gradients?(4 votes)
- Well, the presence of the membrane itself helps to create a concentration gradient. Once you have a barrier it helos creating 2 different compartments.(3 votes)

- Does a concentration gradient exist regardless of membrane permeability? For example, if I have 140mM of sodium outside my cell and 14mM sodium inside my cell but the membrane is not permeable to sodium, do I still have a concentration gradient?(3 votes)
- No, since the membrane is not permeable.

Then you have two containers with different concentrations but there is no connection between those containers.(2 votes)

- Is difussion happening all the time? I mean, are the always concentration gradients? Why do they "happen" within the body all the time if that's the case?(2 votes)
- Yes, diffusion happens, because atoms are always moving (you can feel it as the temperature of something) and because they collide they have very random directions and cover so all the possible room (a little bit like water or gases). Does it answer your question or did I understand it wrong?(3 votes)

- What happens when concentration reaches equilibrium?(3 votes)
- No concentration gradient exist, and diffusion stops.(1 vote)

- When the particles are bouncing around, can they bounce of each other?(2 votes)
- As the particles are vibrating very fast, they are likely to bump into each other, this causes them to move away from each other, down, the concentration gradient.(3 votes)

- he talks about how it's just them bouncing around but my question is "what is actually causing them to bounce around"

thanks, ಠ_ಠ(3 votes)- their temperature i.e. average kinetic energy(1 vote)

## 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.