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Endoplasmic reticulum and Golgi bodies

Structure of the endoplasmic reticulum and Golgi body (Golgi apparatus). Their role in protein secretion. Created by Sal Khan.
Video transcript
We've already talked about the process from going from DNA to messenger RNA. And we call that process transcription. And this occurs in the nucleus. And then that messenger RNA makes its way outside of the nucleus, and it attaches to a ribosome. And then it is translated into a protein. And so you could say that this part right over here, this is being facilitated by a ribosome. Or it's happening at a ribosome. With that high-level overview, I now want to think a little bit in more detail about how this actually happens, or the structure of things where this happens inside of a cell. And so I'm going to now draw the nucleus in a little bit more detail so that we can really see what's happening on its membrane. So this right over here is the nucleus. Actually, let me draw it like this. And instead of just drawing the nucleus with one single line, I'm going to draw it with two lines. Because it's actually a double bilipid membrane. So this is one bilipid layer right over here. And then this is another one right over here. And I'm obviously not drawing it to scale. I'm drawing it so you can get a sense of things. So each of these lines that I'm drawing, if I were to zoom in on this-- so if I were to zoom in on each of these lines, so let's zoom in. And if I got a box like that, you would see a bilipid layer. So a bilipid layer looks like this. You have the circle is a hydrophilic end and those lines are the fatty hydrophobic ends. So that's our bilipid layer. So that's each of these lines that I have drawn, each of them are a bilipid layer. So the question is, well, how does the mRNA-- obviously you have all this transcription going on. You have the DNA, you have the mRNA. It's all in here, this big jumble of chromatin inside the nucleus. How does it make its way outside of this double bilipid layer? And the way it makes its way out is through nuclear pores. So a nuclear pore is essentially a tunnel. And there are thousands of these. It's a tunnel through this bilipid layer. So the tunnel is made up of a bunch of proteins. So this right over here-- and this is kind of a cross section of it. But you could almost imagine it if you're thinking of it in three dimensions, you would imagine a tunnel. A protein-constructed-- a tunnel made out of proteins that goes through this double bilipid membrane. And so the mRNA can make its way out and get to a free ribosome, and then be translated into a protein. But this right over here is not the complete picture. Because when you translate a protein using a free ribosome, this is for proteins that are used inside the cell. So let me draw the entire cell right over here. This is the cell. This right over here is the cytosol of the cell. And you might be sometimes confused with the term cytosol and cytoplasm. Cytosol is all the fluid between the organelles. Cytoplasm is everything that's inside the cell. So it's the cytosol and the organelles and the stuff inside the organelles is the cytoplasm. So cytoplasm is everything inside of the cell. Cytosol is just the fluid that's between the organelles. So anyway, the free ribosome over here, this translation is good for proteins used within the cell itself. The proteins can then float around the cytosol and used in whichever way is appropriate. But how do you get protein outside of the cell, or even inside the cellular membrane? Not within it, within the cell, but embedded in the cell membrane or outside of the cell itself. And we know that cells communicate in all sorts of different ways and they produce proteins for other cells or for use in the bloodstream, or whatever it might be. And that's what we're going to focus on in this video. So contiguous with this what's called a perinuclear space right over here, so the space between these two membranes-- So you have this perinuclear space between the inner and outer nuclear membrane. Let me just label that. That's the inner nuclear membrane. That's the outer nuclear membrane. You could continue this outer nuclear membrane, and you get into these kind of flaps and folds and bulges. And this right over here is considered a separate organelle. So you get this thing that looks like this, and I'll just do it the best that I can draw it. And this right over here is called the endoplasmic reticulum. So this right here is endoplasmic reticulum, which I've always thought would be a good name for a band. And the endoplasmic reticulum is key for starting to produce and then later on package proteins that are either embedded in the cellular membrane or used outside of the cell itself. So how does that happen? Well, the endoplasmic reticulum really has two regions. It has the rough endoplasmic reticulum. And the rough endoplasmic reticulum has a bunch of ribosomes. So that's a free ribosome right over here. This is an attached ribosome. These are ribosomes that are attached to the membrane of the endoplasmic reticulum. So this region where you have attached ribosomes right over here, that is the rough endoplasmic reticulum. I'll call it the rough ER for short. Perhaps an even better name for a band. And then there's another region, which is the smooth endoplasmic reticulum. And the role that this plays in protein synthesis, or at least getting proteins ready for the outside of the cell, is you can have messenger RNA-- let me do that in that lighter green color-- you can have messenger RNA find one of these ribosomes associated with the rough endoplasmic reticulum. And as the protein is translated, it won't be translated inside the cytosol. It'll be translated on the other side of the rough endoplasmic reticulum. Or you could say on the inside of it, in the lumen of the rough endoplasmic reticulum. Let me make that a little bit-- let me draw that a little bit better. So let's say that this right over here, that right over here is the membrane of the endoplasmic reticulum. And then as a protein, or as a mRNA is being translated into protein, the ribosome can attach. And let's say that this right over here is the mRNA that is being translated. Let's say it's going in that direction right over here. Here is the membrane of the ER. So ER membrane. This right over here-- and actually, the way I've drawn it right over here, this is just one bilipid layer. So let me just draw it like this. I could do it like this. And this is actually, this bilipid layer is continuous. It's continuous with the outer nuclear membrane. So let me just make it like that so you get the picture. And then at some point in the translation process, the protein can be spit out on the inside. As it's being translated, it can be spit out on the inside of the endoplasmic reticulum. So this is the lumen. This is the ER lumen right over here. So we're inside the endoplasmic reticulum here. Here we're outside in the cytosol. So that way you get the protein now, inside the ER. Inside the endoplasmic reticulum, and it can travel through it. And at some point, it can bud off. So let's say, imagine the protein is right over here. And the smooth endoplasmic reticulum has many functions, and I won't get into all the depth of how it's involved. But at some point that protein can bud off. So let me draw a budding off protein. So let's say this is the membrane of the endoplasmic reticulum. And a protein, let's say, ends up right over here. And then it can bud out. So it could go from that to-- let me do that same color. It could go from that to that-- I think you see where this is going-- to that, and then to that. And then it could go to something like this. Now it has budded out. And when you have a protein, or really you have anything that's being transported around a cell with its own little mini membrane, we call this a vesicle. So now it'll bundle up, and now it is a vesicle. Now, this vesicle can then-- let me draw some of these vesicles holding some proteins, so let me draw that-- can then go to the Golgi apparatus, which I'll drawn in blue right over here as best as I can. So the Golgi apparatus. This is not-- obviously there could be better drawings of something like this. And then they can essentially do the reverse process, and they can attach themselves to the Golgi, oftentimes the Golgi body, named after Mr. Golgi who discovered this. And then the proteins, once they get into the inside of the Golgi body, then they essentially go into a maturation process so that they're ready for transport outside of the cell, or maybe to be embedded into the cellular membrane. So this right over here is the Golgi body, or a Golgi body or Golgi apparatus. And then once they're done with that process, then this is kind of the fully-manufactured protein ready to be used. And actually, maybe I'll make it a slightly different-- well, I'll just use that same color. This is the fully-manufactured protein. And now it can transport to the cell membrane. And that protein can either be transported outside of the cell, or it can be embedded within the membrane itself.