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- [Voiceover] What I wanna do in this video is give ourselves a quick overview of the different ways that cells can communicate with each other. And maybe the most basic one, is just through direct contact and this can happen several different ways. One way could be, you just have neighboring cells so this is one cell right over here, and this is the neighboring cell right over here. And they actually might have gaps in their membranes. If these are plants cells we would call these gaps, gap junctions. I just drew two of them and this is obviously just a two-dimensional slice. So we're not fully appreciating the three-dimensional structure, but the basic idea is that maybe the cell on the left starts producing some molecules, especially some small molecules that are able to go through these gap junctions. And when they're able to go through the gap junctions, maybe they latch on to some proteins in the second cell that start maybe activating them in a certain way, or inhibit them in a certain way, or trigger some type of reaction. And so then you have a response in the second cell. So this would be a form of communication. Something happened in this first cell to produce these molecules. Those molecules were able to get into the second cell, and trigger a response. And as I mentioned, if these are animal cells, we would call these gaps, we'd call them gap junctions. So that right over there, that is a gap junction. And we covered this in other videos. And if we're talking about plant cells we can have very similar things happening, but we wouldn't call them gap junctions. These would be gaps, not even tunnels through the membrane, it would also be through the cell walls. If we were talking about plants, they would be plasmodesmata. And we'd talk about these more in The Structure of Cell videos. But there is other ways that you could communicate via direct contact. You could imagine maybe some cells that are floating around in the blood stream. Maybe this is one cell over here, maybe this is another cell over here. And they have complementary surface proteins. Surface proteins that are able to bind to each other. So maybe this one has a surface protein that looks like this. I'm obviously exaggerating its shape, the protein wouldn't look exactly like this inverted triangle. But this one has a surface protein like this, and I'm also exaggerating it's size relative to a cell, just so we can appreciate how they could maybe lock together. So this one has a surface protein like that. And when they bind to each other it might change the proteins in some way, and then trigger a reaction in each of these cells. That the communication then continues. If this protein changes a little bit, it might activate something. It might activate the release of some molecules, it might activate some other proteins, it might catalyze some type of reaction. We've seen this in multiple other videos, all sorts of crazy biological reactions can happen inside of cells. But it might illicit a, it could illicit a response. And actually it could even illicit a response in both cells, some type of response. They know that they are latched-on to someone else. Now direct contact you could imagine, is not the only way that you could have cell-cell signaling. You could actually have cells start to signal over a little bit of a distance, by releasing molecules into the extracellular space and eventually even into the blood stream. So, for example, let me draw a cell right over here, and maybe it's able to produce some molecules. And these molecules either, they might be lipid soluble and maybe make their way through the cellular membrane on their own. Or, maybe they are packed in nice vesicles that allow them to traverse the membrane. So when you get actually, so the membrane of the vesicle merges with the membrane of the cell. And then it allows these things to get out. And so you could imagine, if another cell has the right receptors for these, that would signal some type of a response or it will form some form of communication. So let's say that I have, actually let me draw a couple of cells. So let's say I have this cell over here, and then I have this cell over here. And let's say that this cell has the right receptor, and this cell doesn't. It could have other receptors, maybe it has receptors like that but it's not the right receptor. And so these molecules that were released by this first cell could bind not on this character, not on this surface protein, but it could bind on this character. So it could bind on this character and when it does so, this protein that's on the cellular membrane, it might change its shape, it might do all sorts of things. But that signal can be then taken somehow, it can continue on into the cell and once again, you might illicit some type of response. And we'll go into more detail in future videos on exactly how that happens, or what these responses actually might be. Now, if this is over a short distance, if this is a short distance, short distance, we would call this paracrine. This would be part of the paracrine system, or we would call this paracrine communication. Let me write that down, paracrine system, or paracrine communication, or paracrine signaling, and we would call these paracrine factors. But if it was happening over long distances, say maybe these molecules they enter into the blood stream, so they make their way into the blood stream right over here. So let me depict somehow that this is the blood stream. So this is the blood stream, and they're able to go through the blood stream over longer distances to other molecules. So maybe this one has the right receptors for those molecules. Then we'd call this the endocrine system, or this is endocrine signaling. So long distances, we would call this the endocrine system, or endocrine signaling. And we're talking about the endocrine system and endocrine signaling, these molecules which could just be, there could be all sorts of different types of molecules. They could be steroids, they could be proteins of some kind. In this case, we would call them hormones. And you've probably heard the word before and we will do a whole series of videos on hormones. But these molecules actually could even affect the cell that produced them. For example, the cell that produced it might have the right receptor. And so, if it's able to signal, if it's able to trigger a reaction in itself, so if these things are able to trigger a reaction in itself, we would call that an autocrine process. It is acting on itself. And just so you're familiar with some of the terminology, these proteins on the surface, and this would be the case especially if you have non-lipid soluble types of signaling factors, or molecules right over here. These proteins, actually even if they're on the surface or even if they're within the cell, we would call them receptors. So that right over there, that is a receptor. And the molecules themselves, these things that bind on to the receptor, we call them the ligand. In general, it's the general term for something that binds on to a receptor. And so at that point where you bind on to a receptor, whether you're talking about the paracrine process or endocrine, actually endo, I left a C out here, endocrine signaling, this process where it latches on, we would call that a signal perception. And then when this protein somehow changes its shape, or starts catalyzing a reaction, or inhibiting a reaction, we would call this the signal transduction. It's bringing the signal into the cell. And then you have, you have your actual cellular response. So hopefully you appreciate that as just a bit of an overview of how cells can signal with each other. And in future videos we'll go into a little bit more detail. Especially the endocrine system, and our understanding of hormones.