Cell compartmentalization and its origins
- [Voiceover] What I wanna do in this video is give an overview of the endomembrane system in eukaryotic cells. Endomembrane system. And at a very high level, the endomembrane system is all of the membranes that interact with each other inside of a cell. So what membranes are we talking about? Well, you can start off by talking about the cell membrane itself. And all of these membranes, these have bilayers of phospholipids. Sometimes my brain malfunctions and I call them bilipid layers, but these are bilayers of phospholipids. So if I were to zoom in right over here, if I were to zoom in right over there, that line, it really is a bilayer of phospholipids, so it would look like this. So you have your... hydrophilic heads pointing ouwards, and your hydrophobic tails pointing inwards. So hydrophilic heads pointing outwards, hydrophobic tails pointing inwards, and it keeps going. If we think of it from left to right, you have a layer of two, or you have a bilayer, I should say, of phospholipids. That's going to be true of the cellular membrane, that's going to be true of the outer nuclear membrane right over here. We drew this one on the video on the endoplasmic reticulum. And so over here you see these two membranes. You might say, "OK, is this a bilayer?" No, this is actually two bilayers. So this membrane right over here has a phospholipid bilayer, and this membrane over here also has a phospholipid... a phosopholipid bilayer. Let me do this in another color. This one that I'm starting to trace in magenta, that's the outer membrane of the nuclear envelope, and it's continuous with the membrane of the endoplasmic reticulum, which I'm starting to highlight right over here. And then the one that I'm highlighting in this purple color, this is the inner membrane of the nuclear envelope, and all of this is part of the endomembrane system. I've already started talking about the endoplasmic reticulum, and we go into some depth on that on the video on the endoplasmic reticulum and the Golgi apparatus, but it's also part of the endomembrane system. And the endoplasmic reticulum in particular can represent up to, or even more than, 50%... of the phospholipid membrane associated with the cell. And we've talked about what goes on in the lumen of the endoplasmic reticulum. So this area right over here... we've talked about what happens there. Proteins can get synthesized. Actually, other molecules, like lipids, can get synthesized there. And then they can go to the smooth ER, and then the place where they exit from the smooth ER, and we saw that in the video on the endoplasmic reticulum, how they can kind of butt out. We call this area, it's often called the transitional ER. So this area right over here, we would call the transitional endoplasmic reticulum. Transitional... transitional ER is this place where these proteins are being butted off, and they're butting off in vesicles. So this is the transitional ER. And all vesicles are are little small compartments that have a membrane around it, that things like a protein can be transported in. And I don't wanna beat a dead horse here, but all of these lines that I'm drawing, even though I drew it as a single line, these are phospholipid bilayers. So the membrane might be different, the phospholipid bilayers might be different when we go from one piece of the membrane to another, but they all have that same general notion of having this bilayer of phospholipids. But just as a review, these proteins, they can emerge from the transitional ER, they can make their way to the Golgi apparatus. And we've already talked about how in the Golgi apparatus these proteins can be matured. And when I say being matured, there's a bunch of enzymes in here, there's a bunch of Golgi enzymes in here, they can do all sorts of things to the proteins: tag them, they can actually add... add saccharides to them, so that they become glycoproteins. They can tag them so they can be used in the cellular membrane, or be used outside of the cellular membrane, or to be used other places in the cell. So for example, this protein right over here butted off as a vesicle, it makes its way to the Golgi apparatus, the membrane can then merge, and then dump the protein into the Golgi apparatus. From there it can be matured. It might turn into a glycoprotein, who knows what happens to it? And then it could butt off again, and then this protein that's now butted off, it could go to be embedded into the cellular membrane. The protein could be excreted from the cell, or it could go to other parts of the cell. Everything I've just talked about, those aren't the only parts of the endomembrane system. You have things like vacuoles, which are membrane-bound organelles in a cell. In plant cells, a vacuole can be used for storage, it could be used for structure. Vacuoles can get quite large, and they can actually give, kind of, the structure of the actual plant. In animal cells, you might have something called a lysosome. And a lysosome is a membrane-bound structure where, essentially, things go to, for the most part, be recycled, or to be torn apart. So maybe something got packaged from someplace. Let me do this in another color. And I drew that vesicle a little bit too big. But maybe this stuff, it needs to be destroyed, so this membrane, it can then merge with that membrane and dump its contents in here, and this has a low pH, and it can actually, kind of, break apart this stuff, and it can digest this stuff, and recycle it into its, I guess you could say, more constituent material. So all of this is part of the endomembrane system. So once again, I don't think there's an appreciation for how complex and, on a lot of levels, beautiful cells can be.
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