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Cell Diffusion

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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Cell diffusion is a type of passive cell transport. In diffusion, molecules move from areas of high concentration to areas of low concentration in order to decrease the concentration gradient. Diffusion from areas of low concentration to areas of high concentration is not energetically favorable

In Biology and in Science overall diffusion is one of the basic principles that once you get it, it is usable or applicable in a bunch of different places. So let's start in with the definition then look a little bit closer and figure out why does it happen. So diffusion is called or defined as the movement of particles from area of high concentration to an area of low concentration. Now if you notice that's a really long phrase and so scientists have figured out a way to describe this area of high concentration and low concentration in just 2 words and that's concentration gradient.

In general and in Science I'll use the term gradient to refer to a difference between 2 areas. So you caould talk about a thermal gradient, it's hot outside, it's cooler inside if you and your friends are of different heights you can arrange yourselves in a height gradient, where the tallest person is on one side, the lowest person is on the other. So in general this just means that stuff moves from where there's lots of it to where there's not so much of it. And you got to be careful though it's not just the grand total quantity of things that are moving from an area of lots of things to little things it's based on concentration. The amount of stuff in any one small area compared to its immediate neighboring area. So let's take a look at how that would work, now you should know that atoms and molecules are constantly moving and vibrating kind of like little kids in the kindergarten class.

The more temperature or heat energy you provide those atoms and molecules the faster they move. It's kind of like giving your kindergarten class a whole bunch of candy and as you give the more and more candy they start moving faster and faster and bouncing around until they're out of their seats and running all over the playground. Alright so here we have say a block of dye and some water, now I have a lot of these little molecules right here and they're all vibrating. Are they vibrating in one direction? No they're vibrating all over the place so some of them are going this way some that way, some that way, some that way. So as time goes by some of them wind up over here, as they spread out and they bump into water molecules and then bounce off this way and then they bounce off that way, and this guy is moving here and that guy is moving there and this guy is moving there.

They're just randomly bouncing all over the place and this continues to happen as they spread out randomly like they're little children in the playground, they're running around all over bouncing in the walls, bouncing off of each other, the occasional yard duty is yelling at them to get on the bench, whatever. And they just bounce around all over the place until you get relatively equal distribution throughout the beaker of fluid. Now do they stop? No but every time one of these guys moves this way, one of these guys moves that way to replace him. Not on purpose is just by random statistical averaging, it's kind of like if you look at the freeway at rush-hour and you count the number of cars that are there and then you wait 10 minutes and you count the number of cars there're there it's probably roughly the same amount. Because every time gets on to the freeway another one gets off.

And so you reach this equilibrium but because the molecules haven't stopped moving, they're still bouncing around it's called a dynamic equilibrium. Now to focus in on this one last time I'm going to give you an example that you might see in one of our text books or on a test. And that's where you have 2 sides of a beaker, one with high concentration, one with low concentration again what's that called? Concentration gradient, and things move down the concentration gradient. So if I look right here, this is a membrane that's separating the 2 sides but these small red dots can pass through. Every second 10 of these dots are passed are passing through this hole randomly there's a bunch of moving all of the other directions but the only direction I care about right now is from one side to the other.

And randomly 2 are moving the other direction, so what is the net change after I subtract I see a grand total of 8 moving over here after I've subtracted the 2. So overall I'm going to see gradually a bunch of these dots on this side move over onto that side. A few of these guys will move back over but I don't really notice the effect. It's kind of like, let's see in China there are a number of people immigrating from China to the United States. Similarly I'm sure there's a few people from United States immigrating back to China or over to China but over all we see a net migration towards United States and these things just cancel each other out. So we have a lot of movement this way not so much back. And that's diffusion you can see it in all areas of Science or Biology from the communication of one cell in your brain to another cell the diffusion of neural transmitters to the diffusion of glucose from your blood stream into your cells.