Endocytic Pathways - Online Tutor, Practice Problems & Exam Prep

Endocytosis involves three main pathways: pinocytosis (cell drinking), phagocytosis (cell eating), and receptor-mediated endocytosis (RME). Pinocytosis brings in fluids, while phagocytosis engulfs large particles, forming phagosomes that fuse with lysosomes for degradation. RME selectively internalizes specific molecules, often using clathrin or caveolae. Endosomes, including early and late types, sort and transport materials, facilitating recycling or degradation. For instance, LDL uptake exemplifies RME, where LDL receptors bind cholesterol, triggering internalization and influencing blood cholesterol levels.

Types of Endocytosis

Hi. In this video, we're going to be talking about endocytic pathways. There are three types of endocytic pathways, and each one has in common the fact they bring some type of fluid or molecule into the cell.

The first is pinocytosis. You've heard about this in your Bio 101 class. That's known as cell drinking, so it brings fluid into the cell. Then you have phagocytosis, which is cell eating, and so that brings large molecules into the cell. And so, phagosomes, which are the entities that take in these large molecules, end up fusing with lysosomes to degrade most of these molecules. So, that's kind of like, you know, you eat, your esophagus takes your food to your stomach to degrade. Well, you know, you phagocytosed something, a phagosome carries it to the lysosome, and the lysosome degrades it. It's the same thing.

Then you have receptor mediated endocytosis. Now, this is very specific, so the rest of the days like fluid, just any fluid around, large molecules, whatever large molecules are around. But receptor mediated endocytosis is very specific, and that brings only specific molecules into the cell. How it happens is it uses clathrin-coated pits for the most part. And these bind receptors that select specific cargo for entry. We hear a lot about cholesterols and LDLs and HDLs and things, and I'm not going to go into how that entire process works, but receptor mediated endocytosis is the way that your cells take in these LDLs that regulate your cholesterol.

Sometimes, receptor mediated endocytosis can use caveolae, which are small invaginations of the membrane, and that also brings specific molecules into the cell. So these are the two ways that receptor mediated endocytosis works.

So let me back out here, and you can see the three different ways. You have phagocytosis bringing some type of large molecule into the cell, into a phagosome. You have panocytosis, which mainly brings fluid into a vesicle into the cell. And then you have receptor mediated endocytosis, which brings really specific molecules only into the cell. So, with that, let's now move on.

Endosomal Sorting

Hello, everyone. In this lesson, we're going to be talking about endosomes and their functions. Okay. So we previously learned about bulk transport and the way that some substances enter into the cell via invagination of the cellular membrane and then the formation of a vesicle holding that stuff or whatever was imported into the cell. And that was in our last lesson. Now we're gonna talk about specific specialized vesicles and their jobs once the cargo has entered into the cell. So we're gonna be talking about endosomes and their abilities in endosomal sorting. Endosomes are going to be large organelle sorting hubs, and these are going to be specialized vesicles that hold a variety of different materials that enter and exit the cell, and basically the job of the endosomes is to sort the important things and throw away the not important things and to take the cargo to its next destination or send it on its way. And we're gonna talk about 2 types of endosomes. We're gonna talk about early and late endosomes. So early endosomes are obviously going to happen first and these are going to be the endosomes or specialized vesicles that are most closely associated with the plasma membrane. So whenever the plasma membrane does bulk or the bringing in of material into the cell via a vesicle, that vesicle is going to then fuse with an early endosome and then the early endosome is going to begin sorting the contents of that vesicle. They are slightly acidic in comparison to the cytosol but not terribly acidic and they are going to sort the incoming material and then they're going to send it to its next location. Now, late endosomes are a little bit different. So early endosomes, whenever they find something that needs to be destroyed or degraded or recycled, they're going to send it to the lysosome, which is going to be the organelle that breaks cellular material down. So the early endosomes, once they've tagged something for the destination of the lysosome, then they're going to give it to the late endosomes. The late endosomes are much more acidic and they are going to begin breaking things down and sorting things that are going to be transported to the lysosome. I don't know that you need to know all of that. You guys will probably learn more about this in more depth in other biology lessons, but I just wanted to give you an overview of the different types of endosomes, but I'm not really sure you're gonna need to know all of those different types and their functions. Now, we're also going to talk about Multivesicular Bodies, which if you wanted to shorten this name, it's MVBs, and they are going to be when invaginations form inside of an endosome. And you might be thinking, what would be the point of that? Right? An invagination is utilized to form a vesicle which then fuses with an endosome. Why would an endosome want to make an invagination of itself? Well, what if that endosome is carrying these cell membrane proteins or receptors that need to be degraded? Well, they are going to have to take those proteins inside of the endosome, so there's going to have to be an invagination of that section of the plasma membrane of the endosome to bring those receptors and those proteins from that membrane into the inside of the endosome. I hope that makes sense. We're gonna look at this diagram right now and see if we can go over these different types of endosomes and MVBs. Okay? Alright. So, let's have a look at our diagram here. And this is going to be a diagram of a cell and the different endosomes, vesicles, and MVBs that we just talked about. So, first off, let's start with the membrane-bound protein. Perhaps this cell wants to transport or remove or do something with the membrane-bound protein, so it's gonna have to take it out of the membrane. You can see an invagination of the plasma membrane actually happening right here. So that membrane is dipping down into the cytosol, and then we're going to form a vesicle. So this guy here is going to be a vesicle, and then that vesicle is going to go on and fuse with the endosome. Now I want you to notice something. That membrane-bound protein has been tagged with this little blue particle called ubiquitin. Whenever a protein is tagged with ubiquitin, that means that that protein is destined for destruction. So where is it gonna go? It's gonna go to the lysosome, right? So that means that it needs to go into a late endosome. So whenever the invagination happens, that membrane-bound protein is taken into a vesicle, then that vesicle is going to fuse with an endosome, and then that endosome is going to become a late endosome and take that membrane-bound protein to the lysosome. But remember that with membrane-bound proteins, the endosome is gonna have to do something important. It's going to have to make a multivesicular body or an MVB because we need that membrane-bound protein inside of the endosome, not on the outer edge of the endosome. So you can actually see another invagination happening in the endosome itself right here. You can see that membrane-bound protein actually kind of dipping down into the endosome, and then you can see there are actually what look like tiny vesicles inside of the endosome. Those are gonna be our MVBs. So now the membrane-bound protein is inside of the late endosome, and the late endosome is going to fuse with the lysosome over here, and you're going to see that these MVBs are going to enter into the lysosome so that those proteins on the MVBs can be degraded. So that's going to be an overview of how endocytosis and endosomes, MVBs, and lysosomes work together to destroy certain molecules or to move certain molecules around. Now let's go down and instead of talking about endosomes utilized for the distraction of cellular components, we're going to talk about some of the other things that they do like recycling and transportation. So endosomes are going to be slightly acidic, like I said. Early endosomes are less acidic than late endosomes, and late endosomes are less acidic than lysosomes. And they are going to act as compartments that are going to make the receptors that they hold release their cargo. So the change in pH is going to change the conformation of the proteins that they hold inside of them and those receptor proteins are then going to release whatever cargo or whatever molecule they are bound to. This is important especially if that molecule that is bound to the receptor needs to go somewhere else inside of the cell. So basically think about it as there's a receptor on the plasma membrane, it binds to something, but now what does it do with it? Well, it invaginates into a vesicle then it goes into an endosome and because the endosome is going to be acidic, that cargo is going to be released and then that cargo can be transported somewhere else and the receptor is put back into the plasma membrane. What's happening in recycling endosomes. Recycling endosomes are going to carry those receptors back to the plasma membrane and this is so the receptors aren't destroyed because they're still functional and they can just go right back into the plasma membrane and try to capture another cargo. And then the cargo can travel to the lysosome if it is a toxic agent or some sort of agent that needs to be destroyed or it can go to another organelle wherever it is needed. Maybe it's some type of nutrient, some needed molecule that needs to be transported to a different organelle. Recycling endosomes are really good at that. Now, we're also gonna talk about transcytosis. Transcytosis, I believe. Basically, the transportation of molecules via an endosome. So transcytosis is going to be the process that occurs when the cargo remains attached to the receptor. So even though endosomes are slightly acidic, these particular receptors don't change their conformation and they don't let go of their cargo and this is going to allow the receptor and the cargo to travel to a different cellular location. Generally, it's from one side of the cell to the other side of the cell and that cargo remains bound to the receptors and ends up wherever the receptor ends up. This is important especially if you're transporting things to different areas of the cell and you want to keep the cargo with the cellular receptor. And a lot of the time cells are going to want to do that. So down here we're gonna see this image of the transcytosis happening and the recycling endosomes doing their job. So, you can see an invagination in the plasma membrane occurring right here, and you can see that this particular receptor has bound to its little black circle, which is going to be its cargo. So then we can see that that vesicle is going to fuse with the endosome and you can see that the cargo is inside of the endosome and because the endosome is acidic, we're going to see that this cargo is going to be released and then it's going to end up floating inside of the endosome. And that then would have another option, and then we'd have an endosome carrying the cargo and then it goes wherever it needs to go. Maybe it's going to go to the Golgi body and it can be used there. Maybe it's going to go to the mitochondria and it can be used there. And then you can see that the receptor is actually going back to the plasma membrane, and this is the recycling endosome as you can see right here. This is the recycling endosome that is returning that receptor back to the plasma membrane so it can do its job and then this one right here is going to go off to another organelle. So that's going to be how the recycling endosome works and how those receptors get back into the plasma membrane. Now what if we took this other trajectory? What if we took this trajectory right here? This is going to be the transcytosis pathway. So if the receptor did not release its cargo, so let's say it did not do this, if it did not release its cargo then what's going to happen? Well, then it's going to travel down in its endosome or its little specialized vesicle and it's going to travel down and then it's going to attach to the other side of the plasma membrane where the endosome will fuse with the plasma membrane. This is basically transporting the cargo and its receptor from one side to the other or from the plasma membrane to another organelle. That's also possible as well. So that is going to be the process of transcytosis, which is the other option for these particular endosomes depending on the type of receptor and cargo that they are working with. This is very very important especially when you have cells that have, specific jobs happening on different sides of the cell. So for example, if you guys look up what a, stomach cell endothelium cell looks like, you'll see that one side of the cell has one job and one side of the cell has the other job. So they are constantly transporting different receptors and different proteins via transcytosis, so they won't move things like that. Also, the intestinal epithelial cells do this as well. Okay. So I know we went over a lot there. I hope it wasn't too terribly confusing. Just remember that endosomes are very important. They can be utilized for recycling, different receptors. They can be utilized for transcytosis, and they can also be utilized for sorting and the recycling of different cellular components by fusing with the lysosome. Okay, everyone, let's go on to our next topic.

Cholesterol LDL Uptake

Okay. In this video, we're going to be talking about cholesterol or, LDL uptake. So this is going to be an example that you hear about a lot, especially of receptor-mediated endocytosis. And so, I'm going to just try to walk you through and explain how this happens so that you understand all the different sorting mechanisms and ways that this occurs. So, receptor mediated and ways that this occurs. So, receptor mediated endocytosis is responsible for taking up cholesterol from the bloodstream. Now, LDLs or low-density lipoproteins are the main form of cholesterol in different plasma membranes, but especially in cells that need to take up that cholesterol. And so, when the cell decides, oh, I need more cholesterol, what it does is it makes these LDL receptors and places them in the plasma membrane. Right? That makes sense. Needs more? Well, it has to have a receptor for it, so it makes those receptors. And eventually, those receptors diffuse throughout the membrane, so they have, or around the entire cell, and eventually they come in contact with clathrin-coated pits. Now, they're already kind of preassembled, they're really close to this clathrin-coated pit. And so, when the LDL binds the receptors bind the LDL, this allows for adapter proteins to interact very quickly. The clathrin-coated pit then quickly forms, and LDL is internalized. And so, you can imagine that people with mutations in this receptor have either a really great time, like are very easily able to accept a lot of cholesterol or a hard time taking in a lot of cholesterol. So, for instance, high blood cholesterol can run in families because they have a mutation in this LDL receptor that allows, that prevents the uptake of cholesterol into the cell, and so it gets left in the blood, so you end up with this high blood cholesterol. So, here we have a diagram of LDL. So, here is LDL, this triangle thing, and you have your LDL receptor. Then, whenever it binds, this triggers clathrin coated pit formation, and the clathrin will eventually leave where the LDL receptor, actually go into the cell, and do cholesterol things. So you can imagine if that there's a mutation here, the LDL is not going to bind, and that's going to result in LDL being kept in the bloodstream and resulting in high blood cholesterol. So that's a great example of receptor-mediated endocytosis and cholesterol. So with that, let's now move on.

Which of the following is not an endocytic pathway?

• A. Receptor-mediate endocytosis
• B. Pinocytosis
• C. Fluid uptake
• D. Phagocytosis

Which vesicle is responsible for carrying receptors back to the plasma membrane?

• A. Early endosomes
• B. Late endosomes
• C. Recycling endosomes
• D. Multivesicular bodies

High blood cholesterol develops when what happens to the LDL receptor?

• A. Too much LDL receptor is produced
• B. Too little LDL receptor is produced
• C. LDL binds the LDL receptor
• D. The cell takes in too much LDL in clathrin coated pits

Here’s what students ask on this topic:

What are the three main types of endocytic pathways?

The three main types of endocytic pathways are pinocytosis, phagocytosis, and receptor-mediated endocytosis (RME). Pinocytosis, also known as cell drinking, involves the uptake of fluids into the cell. Phagocytosis, or cell eating, engulfs large particles or molecules, forming phagosomes that fuse with lysosomes for degradation. Receptor-mediated endocytosis (RME) selectively internalizes specific molecules using receptors and often involves clathrin-coated pits or caveolae. Each pathway serves a distinct function in cellular intake and processing of external materials.

How does receptor-mediated endocytosis (RME) work?

Receptor-mediated endocytosis (RME) is a highly specific process where cells internalize specific molecules. It begins with the binding of target molecules (ligands) to specific receptors on the cell surface. These receptors are often located in clathrin-coated pits. Upon ligand binding, the clathrin-coated pit invaginates, forming a vesicle that internalizes the receptor-ligand complex. The vesicle then uncoats and fuses with early endosomes for sorting. This process is crucial for the uptake of essential molecules like cholesterol, which is internalized via LDL receptors.

What roles do early and late endosomes play in endocytosis?

Early and late endosomes are key players in the endocytic pathway. Early endosomes are located near the plasma membrane and are slightly acidic. They sort internalized materials, directing them to various destinations such as recycling endosomes or late endosomes. Late endosomes, which are more acidic, receive materials tagged for degradation and transport them to lysosomes. This sorting and transport system ensures that cellular components are either recycled or degraded efficiently, maintaining cellular homeostasis.

What is the significance of clathrin-coated pits in endocytosis?

Clathrin-coated pits are specialized regions of the plasma membrane involved in receptor-mediated endocytosis (RME). These pits are coated with clathrin, a protein that helps in the formation of vesicles. When specific ligands bind to their receptors in these pits, the clathrin coat facilitates the invagination and budding off of the vesicle into the cytoplasm. This mechanism is crucial for the selective uptake of molecules like cholesterol, ensuring that cells acquire necessary substances efficiently and specifically.

How does LDL uptake exemplify receptor-mediated endocytosis?

LDL uptake is a classic example of receptor-mediated endocytosis (RME). Cells express LDL receptors on their surface to bind low-density lipoproteins (LDL), which carry cholesterol. Upon binding, the LDL-receptor complex is internalized via clathrin-coated pits, forming a vesicle. This vesicle fuses with early endosomes, where LDL is released and eventually transported to lysosomes for cholesterol release. Mutations in LDL receptors can impair this process, leading to high blood cholesterol levels, illustrating the importance of RME in cholesterol regulation.