 Before we talk about concentration gradients, we probably should talk about what is concentration? What is it? You have a sense, don't you, of something when it is concentrated or when it's not concentrated? And I don't mean you right now because I know you're concentrated. Look at this picture. These are molecules. Which side of this is concentrated and which side is not? Here's a concentrated side. Do you agree? Concentrated, not concentrated. How do we define that? Like, what does that actually mean? Well, concentration and we, again, I hope you realize that I have eliminated things that are not game changers. I, like, we don't have enough time to go through something that isn't critical. Your understanding of concentration is critical. Concentration is a measure of how much stuff is in a solution. So it's basically somehow some measure of the number of solutes in some kind of measure of the volume of solution. Okay, so what's a solute? This is a solute. The solutes are the particles. What's the solution? This whole thing is the solution. Now, you can actually, we're going to have, you can measure, we're not going to go into, like, measuring the concentration of something in the air because we're talking about, I don't know, I don't think that we need to split hairs about what is a solution. Can a solution be in a gas? I really don't know. I feel like, no, it's always a liquid. And we use different terms when we talk about the concentration of molecules in a gas. We talk about the partial pressure of those molecules. Relax. We will do that when we get to the respiratory system. We'll be talking about partial pressures of different kinds of molecules. But when we're talking, I mean, you are fluid. You and you have fluid compartments and you know that already. You're like, dude, I'm all over that extracellular fluid compartment. And the concentration of stuff in that fluid compartment is relevant. Like, we're going to be playing with this a lot. So what is a solute? Examples of solutes. Hydrogen ions would be an example. Sodium ions. I'm giving you, like, several different things. Those bicarbonate ions. Those are all examples of things. Proteins are plasma proteins. Remember how we talked about the difference? We talked about this a little bit in lecture. That the difference between the extracellular fluid plasma and the extracellular fluid interstitial fluid is the presence of these plasma proteins. And all that stuff is going to matter for what happens with these molecules and how fluid moves in your body. That's our lab this week. So the number of solutes, the number of particles in a volume of solution, that's how we're going to figure out our concentration. We can have different units for these. For example, we could actually do particles, like the number of solutes could be actually particles where you literally go in and count them. Yeah, that's going to be fun. We can do moles of particles. What is a mole? It's easy. 6.02 times 10 to the 23rd. That's all. That's 602 sextillion things. A mole of hydrogen atoms weighs the same as one paperclip. I want to show you a paperclip. Really, because you've never seen one before. I know you've never seen a paperclip before. This paperclip weighs one gram. There are 602 sextillion hydrogen ions in here if this was made out of hydrogen ions, but it isn't. But you get the idea. That is not very heavy for 602 sextillion atoms. A mole is a giant number. A mole of atoms is like a dozen atoms. Do you want a dozen cookies or do you want a mole of cookies? Dude, hook me up with a mole of cookies. If you bring me a mole of cookies, the next time I see you, A plus. Good luck. I don't think it's even possible to make a mole of cookies because that's a big number. I calculated one time, like how long it would take me to count to a mole. If I counted one number a second, go ahead and try that. It's like, it's ridiculous. And then, like, you have to count whatever. I don't need to explain all that to you. It's a big number. Because this was a mole of hydrogen ions, really, sometimes we use moles per liter to calculate concentration and guess what that is? What is a new measure of concentration called molarity? So I can talk about the molarity of a solution. And that is just how many moles of particles are in there for every liter of solution that we have. And I'm not going to get crazy in math land. Like, we're not going to do conversions and calculations. We're not going to do the math, but definitely. If I told you I have a one molar solution of something and I, right next to it, have a two molar solution of something, which one's more concentrated? Hopefully, you're like, dude, the two molar is more concentrated because it has two moles of stuff in every liter, whereas this solution only has one mole of stuff in every liter. Pretty straightforward, don't you think? Why do we care? Because of the next thing. Concentrations provide energy to make stuff happen. I'll be right back to tell you that cool story.