Transport across a cell membrane
- Let's talk a little bit about exocytosis, which is how the cell is able to release larger molecules that might be used by the rest of the body. Other cells, or maybe it's going to be part of the extracellular matrix. And to understand how this works, it's really the reverse of endocytosis, we're going to go and produce some proteins in the endoplasmic reticulum, this is our classic example. Those proteins are then going to bud off, in their own little vesicles, which then merge with the golgi apparatus where they are further processed. So they're processed in the golgi apparatus, right over here, and then eventually they're going to bud off of the golgi apparatus in new vesicles, and those vesicles are going to make their way over to the cell's outer membrane, the plasma membrane and the membranes of the vesicles are going to merge with the membrane of the cell and in doing so, they're going to release their contents. And this is classic exocytosis, there are other cases where maybe it merges partially, releases the contents and then buds back is called the "kiss and run" method of exocytosis, but the classic one is it merges with the membrane. We can look at this, this membrane after the vesicle's membrane has merged with the plasma membrane, the membrane might look like this. So if the vesicle, let me do the vesicle's membrane in orange, if the vesicle's membrane is at an orange, well now it has merged like this and it has released its contents. It has released the protein to be used someplace else, someplace else in the body. And I want to be clear, this membrane, and I talk about it many times in many other videos, even though I've drawn it as one line right over here, this is going to be a phospholipid bilayer, so if we were to zoom in, it would look like this. It's a phospholipid bilayer. So these are some of the phospholipids that were part of the original part of the original membrane and then we also, in my little box, I get some of the ones that are part of, or that were part of the vesicle that was holding that protein. That were part of the vesicle that was holding the protein. So I really want to stress I want to really stress these lines that I'm drawing these are bilayers of phospholilpids. And all of these lines, these membranes that you see, these are all bilayers of phospholipids. Just to make sure that we are visualizing this correct. And that's what exocytosis is and one thing that I find interesting is when you first learn about it you see it diagrammed like this and you just assume, okay, well, these bubbles of these with these membranes they just randomly must float eventually to the membrane where they get merged and then they release their contents. But actually it isn't that chaotic. They actually can sit on tracks so they actually can sit on tracks. Remember we talked about the cytoskeleton which isn't drawn enough probably because it makes drawings really messy but whenever we think about a cell there's all this structure to it. There's all this structure to it, microtubules, microfilaments, intermediate filaments, all of these things over here that not only provide structure to the cell, but they can be used to transport. And these vesicles these vesicles can actually ride on these structures and you could actually have motor proteins that are using ATP to actively push the vesicle containing its contents so this is a kind of transportation. It's really like a factory to push them towards the membrane so that they can be released. So whenever I think about it, it's fascinating 'cause I always talk about these cells being a universe unto themselves. And they aren't just these blobs, they have all of these structures, there's all of these proteins that are really on an unbelievably small scale able to do these fairly intricate processes. So what I just showed you, once again, this is classic exocytosis, you'll see it when you have proteins, lipids, being produced by the cell that need to be released somehow. They're also famously used in neurons, the chemical signal, when you go from one neuron to another, you have exocytosis of neurotransmitters that will trigger the next neuron. So these are very, very important processes. Our bodies would not function properly or, wouldn't function at all, if we did not have exocytosis.