the endoplasmic reticulum is an organelle in the cell that buds 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 contiguous 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 a 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 going to 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 Rho 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 and grouping these three organelles together and I just want to 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 and proteins that happens in their 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 a pathway of 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 that question is that all 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 as being synthesized to be pushed into 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 and what happens in the Golgi apparatus so the Golgi apparatus number one modifies proteins they're made 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 it's part of a cell membrane so

it'll Buddle if Ana vesicle here's a vesicle budding 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 in this part of the Golgi apparatus is known as the Cystic the Cystic 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 will also be modified in different ways and then it will eventually land up in this part of the Golgi apparatus this part is known as a trans stack the trans stack is the part that's farthest away from the endoplasmic reticulum and from the trans stack a vesicle will kind of bud

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 going to digress for just a moment if you recall earlier I grouped together the endoplasmic reticulum with the Golgi

apparatus with the lysosomes I'm going to 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 leant 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 the 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 it's part of cell membrane so let's bud off another two vesicles let's say that this vesicle hasn't it proteins I 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 will 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 will 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