- [Voiceover] The endoplasmic reticulum is an organelle in the cell that butts off of the nucleus. So, let's say that this is the nucleus of a cell. It has a nuclear envelope, which is a double membrane structure. And this is the endoplasmic reticulum. Notice that the space in the nuclear envelope is continuous with the lumen, or space, of the endoplasmic reticulum. So, what does the endoplasmic reticulum do? Well, the endoplasmic reticulum actually has two parts to it. It has one part that's known as the rough endoplasmic reticulum. And that has a bunch of ribosomes, those are the purple dots that you see. And then, the endoplasmic reticulum has a smooth part. That part does not have any ribosomes. Let's start with the smooth endoplasmic reticulum first. So, what does it do? The smooth endoplasmic reticulum synthesizes lipids... Including those that will end up being part of the cell membrane. And those that are secreted from the cell. For example, steroid hormones. The smooth endoplasmic reticulum

also metabolizes carbohydrates. And it also aids in the detoxification of drugs and other toxins. Let's go to the rough endoplasmic reticulum. So, the rough endoplasmic reticulum, as we mentioned, has ribosomes, which means it's the site of protein synthesis. But, we know that there are also ribosomes that are in the cytoplasm, so, what's the difference between those proteins that are translated in the cytoplasm, and those that are translated in the rough endoplasmic reticulum? So, let's split protein synthesis into two. We have those... That are made in the cytoplasm and those that are synthesized in the rough endoplasmic reticulum. So, proteins that are synthesized in the cytoplasm will end up in a number of places. They might end up in the nucleus. They might end up in mitochondria, which, I'm just gonna abbreviate like that. They might end up in peroxisomes. Or, they might just stay in the cytoplasm. Maybe that's where they belong. In contrast, proteins that are synthesized in the rough endoplasmic reticulum will either be secreted into

the extracellular environment... Or, they will end up becoming into raw proteins. In the cell membrane. Or, they might remain in the endoplasmic reticulum, Golgi apparatus, or lysosomes. And you'll see in a few moments why I'm grouping these three organelles together. And I just wanna mention as a side point that another thing that happens in the rough endoplasmic reticulum is post-translational modifications of proteins. For example, the formation of disulfide bridges in proteins, that happens in the rough endoplasmic reticulum. Okay, let's go back to the protein synthesis that happens in the rough endoplasmic reticulum. So, proteins that are secreted from the cell, or that become part of the cell membrane, follow what we call the secretory pathway. The secretory pathway describes the pathway a protein takes from when it's synthesized until it leaves the cell or becomes part of a cell membrane. But, you might be thinking, how does a protein, quote, unquote, know that it's supposed to be following the secretory pathway,

and therefore, that it should be synthesized in the rough endoplasmic reticulum as opposed to the cytoplasm? So, the answer to that question is that old proteins begin to be translated in the cytoplasm. But, those that need to follow the secretory pathway have what's called a signal sequence. That signal sequence is detected early on in translation and will cause the polypeptide that's being synthesized to be pushed in to the rough endoplasmic reticulum where translation is completed. In order for us to understand the secretory pathway, we need to talk about another organelle. That organelle is the Golgi apparatus. The Golgi apparatus is an organelle that's found near the endoplasmic reticulum. And it's basically a group of sacks that are stacked together. What happens in the Golgi apparatus? So, the Golgi apparatus, number one, modifies proteins that are made in the rough endoplasmic reticulum. Number two, the Golgi apparatus sorts and sends proteins to their proper destinations.

And number three, the Golgi apparatus synthesizes certain molecules that need to be secreted from the cell. So, let's take a look at a protein that was synthesized in the rough endoplasmic reticulum, let's say that this part had a couple of ribosomes and there was a protein made. Let's say this is the protein. So, what will happen to it? So, this protein has to end up either at the lysosome or outside of the cell, or as a protein that's part of a cell membrane. So, it'll butt off in a vesicle. Here's a vesicle butting off the endoplasmic reticulum. Of course, the protein is inside of it. And that vesicle will merge with the Golgi apparatus, and the protein will end up inside the Golgi apparatus. And this part of the Golgi apparatus is known as the cis stack. The cis stack is the part that's closest to the endoplasmic reticulum. Now, this protein that's in the Golgi apparatus will undergo modifications. It'll get transferred to the middle part

of the Golgi apparatus. The middle part is known as the medial stack. In the medial stack, it'll also be modified in different ways, and then, it'll eventually land up in this part of the Golgi apparatus. This part is known as the trans stack. The trans stack is the part that's furthest away from the endoplasmic reticulum. And from the trans stack, a vesicle will kind of butt off and that vesicle will be holding the protein in it. And from here, this protein can take a couple of different paths. One thing that might happen to it is maybe it's destined to land up in the lysosome. So, let's say this is a lysosome. So, in this case, the vesicle will move towards the lysosome, merge with it, and land up in the lysosome. I'm gonna digress for just a moment. If you recall earlier, I grouped together the endoplasmic reticulum with the Golgi apparatus, with the lysosomes, and I'm gonna add one more organelle to this group, the cell membrane. And the reason that I grouped all these organelles together

is that they're all part of the secretory pathway. Take a look at the protein we just spoke about. It was made in the endoplasmic reticulum where a vesicle butted off, then, the protein landed up in the Golgi apparatus, and then, another vesicle popped off and the protein ended up in the lysosome. So, all of these organelles have ways of transferring proteins between them. Let's say the protein we mentioned was not supposed to go to the lysosome. Maybe it was supposed to go to be secreted from the cell, or maybe, it's supposed to become a protein, that's part of the cell membrane, so, let's butt off another two vesicles. Let's say that this vesicle has in it proteins that need to be secreted from the cell. And let's say this vesicle has, on its membrane, a protein that needs to end up on the membrane of this cell. Let's see what happens to these two vesicles. So, here is the vesicle with the proteins that need to be secreted. And here's the vesicle with the protein that needs to end up in the cell membrane. So, what'll happen is the vesicle will move toward the cell membrane, merge with it,

and release the protein. The same will happen with the other vesicle, it'll move towards the cell membrane, merge with it, and that protein will end up embedded in the membrane of the cell. And, of course, at some point, these indentations will flatten out and the cell membrane will go back to its original shape.