 Now we're going to spend some time on the rest of this lecture. We're looking at what do we find in this inside a cell? What are all the different cell parts that we have in there? And we're going to start by looking at what are the structures that separate the inside of the cell from the outside of the cell. And I want you to take a minute. Like, this is a pretty complex thought. Think about if there wasn't a separation between the inside of a cell and the outside of a cell, what would the functional consequences of that be? Like, what would happen if we didn't separate the inside of a cell from the outside of a cell? You no longer have a cell. And when we start looking at, especially in a class like human physiology, when we start looking at if we didn't have that separation, literally our cells would stop working. So there needs to be some kind of structure that defines the edge of the cell, what makes up the cell. Okay, so first of all, we have an inside and we have an outside. And outside the cell, there's fluid outside of a cell. Think about a multicellular critter. And thinking like inside your body, your cells, all of them are sitting inside fluid. They're surrounded by fluid. If they weren't surrounded by fluid, they would dry up and die. That would be a tragic state of affairs. So the fluid outside of a cell in a multicellular critter is called extracellular fluid. And I'm telling you this just to kind of define some perspective. Extracellular fluid. So extracellular fluid is all this stuff out here. See, look, I'm making, these aren't seagulls, this is water. I'm making a fluid that the cell is floating inside. And what kind of fluid do you think we're going to find inside the cell? Well, inside the cell, I'm going to put a little dot here so that you know, okay, never mind, I'm going to put a little arrow here so you know that I'm talking about the fluid inside a cell. Think about it. A cell is like a bag of soup. It actually, in fact, that's a great analogy of a cell. The bag separates the outside from the inside. And inside the bag is fluid broth with a whole bunch of like carrots and stuff floating around inside of it. Inside the cell is called intracellular fluid. Intracellular fluid. And as is often the case, I abbreviate intracellular fluid ICF. And extracellular fluid can be abbreviated ECF. You have different kinds, you have different kinds of extracellular fluid in your body. Blood plasma is an example of extracellular fluid. There are cells floating around in your blood vessels juice. The blood juice, the cells float in the juice. That is an example of extracellular fluid. Your intracellular fluid also has another name because that's always the case. We have 32 names for the same thing. Your intracellular fluid is known as cytoplasm. I'm going to write cytosol here as well. I believe cytoplasm is the juice plus the organelles, whereas cytosol is just the fluid. So technically the intracellular fluid would be the cytosol. This is a distinction that isn't hugely important to me. I want you to be aware that inside we have fluid and everything else is floating in that. The cytoplasm is inside. Extracellular fluid is outside. Who's separating inside from outside? It's important. It's this part right here. And in fact, this whole thing, you know, I can color it in. That's the cell membrane. I believe our entire next lecture is on nothing more than the cell membrane. The cell membrane separates inside from outside. The cell membrane is not impermeable. Stuff can go in and out. However, it is not fully permeable. Not everything can go in and out of the cell. And that, again, if everything could go in and out, you would have no difference from the inside and the outside. Setting up a difference between inside and outside is actually functionally valuable. You will explore that in the next lecture. Some cells have an additional structure on the outside. And I'm going to draw it like this, even though this cell does not have it. This is my example of a cell wall. Not all cells have cell walls. Plants have cell walls. Animals do not. Cell walls are super tough, boxy structures, often made out of cellulose, that provide nothing more than just structure to the cell. They make them like brick-like so that you can build cool stuff. Whereas animal cells are lacking that structure, so they tend to be more fluid and more round, less boxy. All right, those are our three cell parts that are kind of involved in defining the inside versus the outside. Now I want to take a look at the nucleus, knowing that not every cell has a nucleus. What kind of cells don't have nuclei? Prokaryotes. You guys are so smart. Okay, I'll be right back to talk about the nucleus.