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let's say that I have this green container and inside this green container I have some air molecules now the air molecules we assume that there's some temperature there's some average kinetic energy to them but they're all going to have different velocities they're all going to be bumping around in different ways now the way that I've drawn it you might notice something on the left-hand side and I'll just draw an imaginary line here this line has no no real I guess you could say structural significance it's not like it's actually dividing it I'm just using it to visualize the left and right hand sides you see on the left hand side I have a higher concentration of my molecules higher concentration and how do you measure concentration well the number of molecules per it will really way you should do it is unit volume but we're looking at a cross-section here if I were to take a section that large look I got four molecules here looks like I have about four three to five molecules per section around that size well if I took at that size Sexton on this side I'm getting one or maybe two molecules and I'm not going to get too precise but it's clear that I have a higher concentration here I have more molecules that we have drawn it per unit area but if we're thinking in three dimensions per unit volume then we have on the right-hand side so we have a higher concentration on the left we have a lower concentration on the right lower concentration on the right and when you have this situation where you have a higher concentration and then a lower concentration we call this a concentration gradient the concentration is changing from high to low and so we call this a concentration cons and tration concentration gradient now what do we think is going to happen let's say this is this is what our situation is right when we look at it what do we think is going to happen well these all of these particles are all going to be bouncing around things can move from the left to the right things can move from the right to the left but we have more particles on the left that are likely to move to the right then we have particles on the right that are likely to move to the left remember they're all moving in different directions with these random velocities but I have more on this side and they're all bouncing around so in any given moment when we have this higher concentration on the left I have a higher chance that I'm going to have stuff go from the left to the right go from the left to the right then I do from the right to the left and so as time goes on as time goes on it's going to look something like this if we let this system stabilize if we let the system stabilize for a while it should look like this and let me see if I can do a good job a good job drawing it and I'll just draw the molecules I won't draw their actual their actual velocity vectors so if we wait a while how many molecules did I have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 so I have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 so if we let enough time go by we see that we don't we should no longer have a concentration gradient that the concentration should be fairly uniform over time so even if I were to draw that imaginary dashed line if I were to draw that imaginary dashed line I should have the same concentration on both sides so I no longer have a gradient I have no gradient and once again there's nothing magical here it's not like the the molecule said oh we are less concentrated over there I somehow have to know to move there you just have to think that these are all just randomly bouncing around if you have a higher concentration on the left there's a higher chance that you have bounces or you have things moving from the left to the right as you do from the right to left even in this situation things are still going to be moving from left to right and right to left but now that you have the same number on either side in any given moment in any given a period of time you have an equal probability of things moving from the left to the right as you do from the right to the left so you're getting to kind of this equilibrium situation sure in a given you know if you take a certain unit of time maybe that one moves from the left to the right that one moves from the left to the right that one moves from the left to the right but since you have equal concentrations on both sides you're just as likely to have the same number move from right to left and I only did this with 20 I only did this with 20 particles which a little bit of an artificially low number if we're actually talking about concentrations of air molecules or as we'll see when we think about cellular membranes if we think about different types of molecules that might be in an aqueous solution we're talking about way more than 20 molecules and so you really do think in terms of probabilistic large numbers well hey for the probability of something move from the left to the right is the same as the right to left and so you're going to have the stability right here there's a much higher probability in any given moment of something moving from left to right then right to left and that's why you see things moving from high concentration to low concentration or another way to think about it what we just observed here is we saw things diffusing down their conference down their concentration gradient so this process that we just described this is diffusion and this is diffusion and as we study different types of systems we'll see that this is actually very important to biological systems and even chemical systems because this doesn't require an ad at any extra energy to move the molecules from here to there this is going to happen probabilistically it's going to happen naturally and once again no magic just more stuff here higher chance moving from left to right then moving from right to left and I really want to make that point clear you can still move from right to left for example you might have this character maybe his maybe instead of moving in that direction completely possible completely possible that he goes from right to left it's not like everything is moving from left to right but you have a higher chance you're going to more things moving from left to right so that guy could move in that direction because there's just more stuff here they're all bouncing around and all in all different in all different random directions

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