 Endomembrane system. What is it? Well, we have heard of the digestive system, respiratory system, but what is this endomembrane system now? Some sliger is made up of some kind of membrane. Well, unlike other systems of the body like the digestive system or the respiratory system, which are made of different organs, this system is made of various organelles. Now by organelles, we mean the tiny compartments that we see inside a cell and each of these compartments are responsible for performing a particular task inside the cell. And talking about organelles, not all the organelles that you see inside this cell is a part of endomembrane system, but only a selected few. Why is that? Well, the simple answer is that the membrane of few of these organelles interact with each other in order to complete a particular task. And typically that task is either modifying packaging or transport of lipids and proteins. We will see how that is done in a minute. But which are those organelles that are a part of the endomembrane system? Now endo we know means inside. So here we talk about the membranes which are inside the cell or membrane of the organelles. But you should know that the cell membrane which typically is not inside the cell but is the one that makes up the cell is also a part of the endomembrane system. We will see how it takes an active part in the system. And apart from the cell membrane, the nuclear membrane, the nuclear membrane is also considered a part of the endomembrane system. But here we are going to concentrate on the organelles. So focusing on the organelles, we have the endoplasmic reticulum, the golgi apparatus, the lysosomes and the vacuoles. Well, vacuoles are not found in all the cells. That's why I have not made it here. But we will discuss about that as well. And endoplasmic reticulum, we have both rough and smooth, rough that has the ribosomes attached to it and smooth that do not have the ribosomes attached to it. So let's just say these are the smooth and the plasma reticulum. We will discuss about each of these organelles in detail. But before that, I would like to show you how they function, what they actually do that gave them this particular name endomembrane system. So let's say this is a cell that secretes an enzyme. Now enzyme is a protein, right? So ribosomes, these tiny dots that you see here are the ones that initiates the formation of protein, or I should say they make up the polypeptide chain. Now as they are closely attached to the endoplasmic reticulum, after they make those polypeptide chain, they leave it in the lumen. The inside of this endoplasmic reticulum is called the lumen. So they leave those polypeptide chains in the lumen of the endoplasmic reticulum. And then the endoplasmic reticulum does the folding of the protein, giving it a functional structure and sometimes also does a few modifications. And what happens after that? Those proteins are released from these endoplasmic reticulum to go to the next top pitch, which is the Golgi apparatus. Now how do they send it? They form tiny vesicles at the end of the endoplasmic reticulum. The membrane forms those vesicles and those vesicle pinches out from the endoplasmic reticulum and goes and fuses with the membrane of the Golgi apparatus. And similarly in the Golgi apparatus, these polypeptide chains undergo further modifications. And as you can see, a vesicle is formed at the end of the Golgi apparatus that holds those proteins and those vesicle will be sent to go and fuse with the cell membrane and finally it is released out from the cell. So here you saw how those membranes work very closely in order to perform a particular task that is modifying, packaging and transport of cell products. And those products can be left inside the cell or if required, those are secreted out from the cell. And again, sometimes the vesicles that are formed from these Golgi apparatus, they don't leave the cell, instead they stay inside forming either lysosomes or vacuoles. We will discuss the function of each of these different organelles in detail. So let us first start with the endoplasmic reticulum. Now from the name you can tell that endoplasmic means something that is inside the endoplasm or cytoplasm and reticulum means a network or a mesh-like structure. So the endoplasmic reticulum are network-like structures that are lying in the cytoplasm. Now it is of two types. The one that has ribosomes attached on its outer membrane, giving it a rough appearance, we call it the rough endoplasmic reticulum. And the one that do not have ribosomes attached to it, we call it the smooth endoplasmic reticulum because it has a smooth membrane without any bumps on the outer surface. Now talking about the rough endoplasmic reticulum, it is situated very close to the nucleus. In fact it is the continuation of the nuclear membrane. Yes, the nuclear membrane extends and make multiple folds in order to give out this structure. They are flattened sacks that looks like tiny tubules. And each of these are called cisterna or plural we can call them together as cisterny. The inner portion of the cisterna we call it the lumen. And the portion outside is the extra lumen or the cytoplasm. Now this is true for both smooth endoplasmic reticulum and rough endoplasmic reticulum. Now the difference comes when the ribosomes are attached onto the cisterny, making them the rough endoplasmic reticulum. Now what difference does this ribosome make? These ribosomes are the protein synthesizing factories. So they produce proteins which are nascent, which are non-functional. And through a signal system they leave it onto the lumen of the endoplasmic reticulum. What happens then? Well inside the endoplasmic reticulum the folding of this protein takes place. The protein folds into a functional protein and sometimes it is even modified by adding let's say a glucose making it a glycoprotein. And then when it is ready the endoplasmic reticulum buds out and the protein is sent out of the endoplasmic reticulum. So you saw the membrane of the endoplasmic reticulum it pinched off from this cisterna in order to form a vesicle. Now where are they going? To the next organelle which is the Golgi apparatus. Now the proteins in these vesicles are mostly designed to go out from the cell. But what about the proteins which are required inside the cell? Well for that there are ribosomes which are scattered in the cytoplasm that are not attached to the endoplasmic reticulum. And those ribosomes those free-floating ribosomes are the one that produces protein which are utilized by the cell. Alright now coming back to the rough endoplasmic reticulum. Since we saw that they play a very very important role in the formation of functional proteins so they are found in abundance in those cells that secretes protein. Let's say that secretes enzymes or proteinaceous hormones. For example we have the chief cell that secretes the digestive enzyme in the stomach and enzymes are mostly proteins we know that right. So if you look closely you will see that this cell has abundance of rough endoplasmic reticulum. These are the RER. So here we learned that the rough endoplasmic reticulum are the ones that are very essential for modification of protein and are found in abundance in protein secreting cells. Okay now let's move on to the smooth endoplasmic reticulum. They may look smooth and simple compared to the RER but they are equally important because they are the ones that produces another very important component of the cell which is the lipids that includes the fats, waxes, sterols, fat soluble vitamins, monoglycerides and all of those. Now lipids are so important because they store energy, they help send signaling, cell signaling and are the structural component of the cell membrane. So the membrane you see that makes up a cell is possible because your smooth endoplasmic reticulum are making lipids and the cell membrane are nothing but a bilayer of lipids. Now all cells will have SER but the steroid hormone producing cell, the secretory cells will have an abundance of these. That means the cells of the ovary, testes, adrenal cortex will have abundance of smooth endoplasmic reticulum. They will have more SER compared to RER and not just this. The smooth endoplasmic reticulum which are found in the muscle cells they are responsible for storing calcium in them. So let's make the smooth endoplasmic reticulum. Let's say these are the smooth endoplasmic reticulum. They will have an abundance of calcium stored in them and the reason is they are very important for muscle contraction. So every time there will be a nerve impulse it will stimulate the calcium release from this endoplasmic reticulum that will help in the muscle cells to contract. So the next time you hit your gym and do all those weight lifting remember who is doing their job in the cellular level. So the next function of SER is calcium storage and apart from this two major function they also have another very very important role to play which is detoxification of drugs. The SER in the liver cells are seen to detoxify the drugs that we take. It makes those drug water soluble so that it passes out through our urine which is such a vital role inside your body. Now you must have observed someone who was initially prescribed a low dose of medicine and gradually maybe after a month or a year the dose was increased maybe from 10 grams to 20 grams. Now why does that happen? Well the simple answer is initially the SER of the body was not capable to detoxify 10 grams of medicine so the medicine got enough time in the body to work but as time passed the SER became potent they could easily detoxify 10 grams now and detoxification means sending it out of the body through urine right and if the medicine is sent out it didn't get enough time to do what it was supposed to do right so the dosage will have to be increased. So no matter what the drug is whether it's good or bad for the body the SER is responsible for detoxifying it. So we saw the three vital roles of SER. Now let's move on to another important organelle which lies just below these endoplasmic reticulum which is the Golgi apparatus. The Golgi body as we have already seen is just below the rough endoplasmic reticulum and the smooth endoplasmic reticulum and it receives whatever is packed in vesicles from both these endoplasmic reticulum processes it and sends it out. We will see how it is done but before that let's talk about the structure of Golgi apparatus or the Golgi body. The Golgi just like the ER are made up of multiple folds of membrane forming cisternis and as you can see the cisternas are stacked parallel to each other and the cisterna which is towards the endoplasmic reticulum we call it the cis phase and the cisterna which is towards the cell membrane. Let's look at it here the one which is nearer to the cell membrane we call it the trans phase. Now why do we give them different names? Because this cisterna is responsible for accepting the vesicles that comes from the ER and the one which is at the end towards the cell membrane is responsible for sending vesicles out from the Golgi body. So what happens inside this Golgi? Well we know that proteins come in vesicles from the RER to the Golgi and Golgi depending on the demand converts that protein into phosphor protein lipoprotein or glycoprotein and not just that it also tags those protein to the location where it needs to go just like how we put address labels on packages and every cisterna takes active part in modifying those protein and finally it buds out from the trans phase of the Golgi apparatus or the Golgi body and same is the case with the lipids that comes from the smooth endoplasmic reticulum it is modified it is tagged and finally packaged and sent out from the trans phase of the Golgi. Now let's say this is the vesicle that came out from the trans phase of Golgi. Now if this vesicle contains secretory proteins and lipids it will be secreted out of the cell and if it contains phospholipid which makes up the cell membrane it will go and fuse with the cell membrane helping it grow because our cell membrane is a bilayer of phospholipid but these vesicles sometimes they fall in love with the cell in which they are formed and they refuse to go out and they end up forming something called lysosomes or sometimes vacuoles. So these two cell organelles are also formed from the Golgi body. Alright now earlier we discussed that the RER is found more in cells that secretes protein and SER is found more in cells that secretes lipids but the Golgi body is important for both of them to function therefore the Golgi is found in all kind of cells whether it be protein secreting or lipid secreting and we can call them the post office of the cell because it receives packages from the endoplasmic reticulum, modifies them, packages them, sorts them into different vesicles and dispatches them to wherever it needs to go. So this was all about Golgi. In our future video we will look at the structure and function of lysosomes and vacuoles.