 So let's classify all the ways that something can get through the cell membrane. And we're going to start out by looking at, we're going to classify these things based on how much energy is required. So remember, we're looking at getting stuff in and out through the cell membrane. How are we going to pull it off? There are two techniques. One of them is passive. It's considered passive transport. And the other, active. It's considered active transport. And the difference between passive transport and active transport is 100% the amount of energy that's required to make this happen. Passive transport, this is my energy signal. Passive transport requires no energy. I say it requires no energy. It actually utilizes the energy that's already found in all the molecules in the entire universe, except for molecules that are at absolute zero, the temperature absolute zero, which is like negative 312 degrees Celsius. Like it's ridiculous. Or maybe it's negative 293 degrees Celsius. It's something 294.73 degrees Celsius. It's crazy. It's cold. And we've never been there to absolute zero. We've tried and we've gotten pretty close. But we think that at absolute zero, all matter will stop moving. Like all atoms will stop moving. And then they'll like collapse into each other into a crazy protoplasm of neutrons or something like that, which that sounds terrible. Let's not do that. So assuming you do not go to absolute zero, passive transport relies just on the random kinetic energy that all molecules already have. Think about kinetic energy of molecules. This is drawing you back to your chemistry rock star days. You know that the more kinetic energy a molecule has, the more it's moving around, the higher the temperature of that substance. So temperature is actually a measure of the kinetic energy of the molecules. So they're all moving around. Passive transport just moves molecules based on that kinetic energy. Active transport actually requires an input of energy. And I'm just going to throw this out here. I'm going to do a little signal. Often that energy in the cell comes in the form of ATP or adenosine triphosphate. And you know all about adenosine triphosphate also from your general bio days. So passive transport, we really don't have to throw in any extra energy. So how does it happen? Passive transport happens through all of them happen. All of my examples of passive transport happen through the process of diffusion. And we have a couple of different flavors. So diffusion, let's define it. Oh, I think I had a picture for you. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. I really do feel that was like the most amazing thing. No, I don't think that's my diffusion picture. Yeah, it is my diffusion picture. So ignore the semipermeable membrane in the middle. Diffusion is the movement of molecules from a high concentration to a low concentration. The semipermeable membrane in the middle is optional. Diffusion can happen through a semipermeable membrane. Remember to think about diffusion? I just said we were talking about a high concentration to a low concentration. And let's take a second to define concentration. Concentration is nothing more than the number of particles in a solution given a certain volume of the solution. The particles are called solutes. We call them particles because we're going to use, there's a couple of concentration units of concentration that are significant to us. One of them is molarity. Molarity is one way to measure a concentration of a solution and the molarity measures moles of a solute per liters of solution. So you can imagine, okay, take a number. How many moles of red dots do we have over here? Ten, right? Because here we have only one mole of red dots. Do you totally agree with that? But the volume, what's my volume over here? One liter. What's my volume over here? One liter. So we have 10 moles per liter on the left side and we have one mole per liter on the right side. I totally just made up my numbers. Numbers are irrelevant. I'm giving you an example to show you how we calculate molarity. Which means on the right side we have a one molar solution and on the left side we have a 10 molar solution. Does that work? You now can tell me which side is more concentrated. Dude, you don't even have to add numbers into the scene. It's very clear that this is the more concentrated side. So where are those red particles going to go? They're going to move down their concentration gradient. That's just saying they're going to move from an area of high concentration to an area of low concentration. Low concentration, low concentration. We have a gradient between those and molecules move down that gradient. If they're moving down the gradient, I think of it as little balls or marbles rolling down a hill. Do you have to put any energy in? Like maybe you just flick them a little bit to get them to go, but if they were like bouncing around already, they totally would roll down the hill without any help from you. Oh my God, we went backpacking today and my small child, his sleeping bag fell out of his backpack and tumbled down the concentration gradient. It was kind of a small miracle that it stopped on a tree. I was like, dude, I'm out because I'm not climbing down that concentration gradient to go get that sleeping bag, but my rock star partner was like, I got this and tromped through the forest down the concentration gradient. It doesn't take as much energy to get that concentration gradient to get the seatbelt. I mean, the sleeping bag. But then he had to turn around and come back up. You can imagine. Is that going to take more energy? Absolutely. Okay, where were we? Concentrations. We calculated concentrations. We have an idea of concentration gradient. If things are moving down their concentration gradient, that is an example of diffusion. We have a couple of flavors of diffusion, you guys. Simple diffusion. Simple diffusion happens when molecules move directly through the cell membrane. Oxygen, carbon dioxide, those are the two best examples of substances that move directly through the cell membrane. It's fast, it's easy, you don't need any help. You also have still diffusion. You can have diffusion through a channel. And we're going to talk about channels in a little bit more detail in the next one. And you can also have diffusion through a carrier. And a carrier is just going to say, hey, you're still moving down your concentration gradient because it's diffusion. But a carrier is going to do that like pack band work to help you get through the cell membrane. Both of these are examples of facilitated diffusion. They're requiring a protein helper to get through the cell membrane. Simple diffusion, dude, we can go through the cell membrane. Nobody's holding my hand, nobody's helping me. Facilitated diffusion, I need some help. I can't get through unless I have my own special tunnel to go through. Or I have a little Pac-Man guy who actually let me through. And we'll talk about channels and carriers. We're going to talk about both of those in the next section. Now, active transport, active transport, maybe I should write this down. Diffusion goes down the concentration gradient. Active transport goes against the concentration gradient, which is why it requires energy. You have to put energy in it, always requires a carrier. And we can have primary active transport or, oops, secondary active transport. And I have an entire section on active transport because the difference between primary and secondary active transport gets kind of crazy. Active transport also includes something called vesicular, vesicular transport. And that is endocytosis or exocytosis. Those processes, and we'll talk about those as well, they both also require energy. They're not carrier-based. These guys right here, I'm going to circle all my friends that require carriers. Those guys require proteins of some sort to get through because a channel isn't considered a carrier. Carriers are only open to the extracellular fluid or the intracellular fluid one at a time. All right, so I think the next thing we're talking about is channels. I'm so excited. Okay, I'll be right back to talk about channels.