 The easiest way for particles to get into or out of the cell membrane is just simple diffusion and not everyone can do this. So let's get a definition of diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration. We talked about concentration. We have a definition and we have units for concentration. We can talk about different molars of solutions and we're thinking solutions when we're talking about diffusion. We're thinking watery solutions because we're talking about cells and cells live in watery environments and they have watery guts. So we're talking about the moving of the particles, but they have to be moving because of a differing concentration in one area to the other and we call this a concentration gradient. I think of the concentration gradient as I literally think of it as a hill. So we have an area of high concentration and an area of low concentration and if you just have a little particle, that particle is going to move down the concentration gradient from the area of high concentration to the area of low concentration. I think that this is an intuitive idea. This makes sense, especially if you think of a concentration gradient as a hill. If you go, this is, if a particle diffuses, it goes down the concentration gradient. So diffusion goes down a concentration gradient. I guess I'm telling you that just so you have the words to understand the way I'm going to talk about it. Diffusion will not move something against the concentration gradient. So in order for diffusion to work, there has to be a concentration gradient present and then you only have one directional option. You have to go down the concentration gradient. Do you think that that's going to be enough to get like, are we ever going to need things inside the cell that we already have a lot of? Or are we going to need to get rid of stuff out of the cell that we have a whole bunch of outside? Oh, I don't know if that question made any sense at all, but I will give you, you probably figured out if I asked the question, that was kind of a lame question to ask. And the answer is no, diffusion is not going to get the job done for us with our cells all the time, but it is efficient and it doesn't require it. Now I'm going to say something else. No energy is required, but that's not entirely true. It's no, let's see, like no external energy, no extra energy is required, because there is energy. The movement of molecules happens because those molecules have energy. Who remembers what kind of energy the molecules have? Dude, they have they have kinetic energy. And put a note somewhere that the diffusion is fueled by the kinetic energy in the molecules. So these molecules are moving around, bouncing off of each other, bouncing off the walls, and that kinetic energy, that movement, is why they go from areas of high concentration to areas of low concentration. Some things cannot diffuse through the cell membrane. If you had to guess, if you're like, okay, I'm going to take a wild guess, what kinds of things do you think aren't going to be able to get through the cell membrane? You can push pause and think about it for a second, but there are two qualities that determine and it's messy. So there's nothing linear or boxy about this, but there are two factors that we will consider when trying to figure out if something can move through the cell membrane. The first one is, let me just make a little note, this is can it get through? Can it, that's the question that we're asking. And I'm telling you, yes, if it is small, small things can get through the cell membrane. Small things sometimes, regardless of any of their other qualities, they can still get through the cell membrane. Not always, because the other thing that must be true in order for it to get through, yeah. If it's small, it can get through. And if it is hydrophobic, that's a B, I see, or another word that we can use to describe hydrophobic is lipophilic, lipid loving. Because of the phospholipid core of the cell membrane, if something is a lipid or it's hydrophobic, it doesn't like water. It can usually sometimes, if it's not too huge, it can actually diffuse through the cell membrane. If it is hydrophilic, if it has a charge, so if it's polar or charged like an ion, most of the time it can't get through. The one exception to that is somebody who is polar, but is pretty small and that's water. Water is, we know, a polar molecule, but it's a pretty small molecule and it can get through the cell membrane on its own. Sometimes it can use a little bit of help and we'll talk about how water can get help through the cell membrane. But small things can get through and hydrophobic things can get through. And then you can imagine like, well, what if it's really big and hydrophobic? Well, it can be too big. If it's hydrophobic, it can still be too big. And what if it's really small, small, small, but hydrophilic? And we'll have to deal with some of those on a case-by-case basis. But this is how, these are the rules for diffusion. Now, I want to show you, I don't know what this is. It's a simulation. This is really cool. I'll put the link to this thing in the comments down here. But this is a place where we can actually simulate diffusion. And the thing I want to show you is, I'm going to just go ahead, you can see I can change the number of particles. And I put 30 particles on the left side of this little simulation. And then I can push this button right here and remove the divider. First thing I want you to notice, what do all those particles have? They have kinetic energy. I can actually change that over here by changing their temperature. If I increase the temperature, I can make them go faster. I can only get them up to 500 degrees Kelvin. But they should be going a little bit faster. I don't know if I can tell the difference if I don't want them to go slower. But then I can remove their firm. I can remove the divider. And when I remove the divider, you predict what is going to happen. Where are they going to go? If you look at the particle flow arrows down here at the bottom, you can see that the particles start out like, dude, they're all going to the right until they hit a moment where now they're going to the right and they're going to the left. And they're going evenly. And what would you say about the concentration at this point? You would say that we've reached, huh, we don't have a concentration gradient. Would you agree with that? Now the concentration is essentially equal on both sides of this container because we allowed diffusion to happen. I'm going to reset the divider. Oh, look, and it reset everything for me. Thank you very much. Can you identify the concentration gradient? I should have done this first. Do you agree? High concentration on the left, low concentration on the right. When we remove the divider, we have a concentration gradient and the particles can get through. Once the particles can get through, then they start moving, they start diffusing down their concentration gradient. Once the concentrations are essentially equal on both sides, do the particles stop moving? No. This is what's called, I want you to write this down, even though I am not going to write it down because I'm not going to go back to our notes. This is called dynamic equilibrium. Dynamic equilibrium is, since particles are constantly moving, dynamic equilibrium is where we look like and feel like we're staying the same, but things are still moving back and forth. There's just not a net change. This is a really good example of dynamic equilibrium. I'm going to reset the divider one more time and this time I'm going to add particles to the other side. I'm going to make sure that my temperature is the same. So there's no differences between these sides. The only difference is the number of particles. I mean the color of the particles. Now here's an important question that I have. If you had to throw a number at the left side, the blue side, what would you say the concentration is? There's 30 particles over there. Let's just say three. It's a three molar solution because there's 30 particles. What's the concentration of the right side? There's 30 particles over there. So let's just say it's three. The concentrations on both sides are equal. That works for everybody, right? Our concentrations are equal. If the concentrations are equal, is there a concentration gradient? Let's see what happens when we remove the divider. If there is no concentration gradient, then we're not going to have movement one direction or the other because our concentrations on both sides are the same. What's happening? Look at your arrows down here. We definitely have movement, don't we? We have, it almost looked like there was a net movement of red particles. Now it looks like we have dynamic equilibrium but the particles diffuse down their own concentration gradient. That's an important thing to be cozy with when we're looking at processes of diffusion. The particles only care about themselves. They're going to diffuse down their own concentration gradient and no one else's. However, the whole system is going to ultimately reach an equilibrium point and you can see that that's the case. Okay, I think that's everything I want you to know about diffusion. We shall see and next we're going to look at how this relates, like how are we going to get other stuff through the cell membrane and what are all of our strategies for getting through the cell membrane? Here's my off button.