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Endocytosis, phagocytosis, and pinocytosis

Phagocytosis is the process by which a cell engulfs and internalizes a large particle, such as a bacterium, by extending its membrane around it. Pinocytosis is similar, but instead the cell engulfs droplets of extracellular fluid, taking in any dissolved substances within. Both phagocytosis and pinocytosis are forms of endocytosis.

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Video transcript

- [Voiceover] We have other videos where we talk about how small molecules, or ions, might be able to go through a cell's membrane in different ways, whether actively or passively, maybe facilitated in some way. What we wanna talk about in this video is how we can do this for larger things. We're gonna focus on here is bulk, bulk transport. Transport. So this first example, you could imagine this cell with this mauve or purple-colored membrane is engulfing this big green thing which is maybe a bacteria or something. And so you see that the membrane... Let me make it very clear. This is inside. This is inside the cell. This is outside. Outside the cell. And you can see the cellular membrane starts to wrap around this, I guess we can think of this a bacteria, then it fully wraps around and then that membrane that was wrapping around the bacteria pinches off and now the bacteria is inside of the cell, and it's wrapped by this membrane. And this process where you're engulfing these large things, we call this Phagocytosis. So this is phago... Phagocytosis. And the prefix I guess you'd say, phago comes from the Greek for "to eat". So this is literally about cell eating. And in many cases, this thing that is now in here, you could view this as the cell's food, this compartment that is holding this, in this case bacteria, is gonna transport it maybe to a lysosome, so it can be processed and digested in some way. We would call this big compartment, this membrane-bound compartment, we would call this a food vacuole. Food. Food vacuole. Now this scenario down here is similar but different. Over here I have the cell which is I see part of its membrane and it's in magenta right over here. We can see The Phospholipid Bilayer . That's why I drew two lines for the membrane. And instead of engulfing a large particle or bacteria, it's just engulfing some fluid. It's engulfing some fluid, so you see it's starting to wrap around this section of fluid, wraps even more around this section of fluid, and then the membrane that was around it completely pinches off and goes into the cell. And I'm drawing all of these things in two dimension, but this would actually be happening in three dimension. So this wouldn't just be a circle, this would right over here would be a sphere. And this thing that has been pinched off and is now inside the cell, we call this a vesicle, which is just a general term for these membrane-bound compartments inside of cells. And this process where the cell has essentially drunk a bunch of fluid and the stuff that happens to be in the fluid, we call this Pinocytosis. Pinocytosis. Pinocytosis. And pino comes from the Greek word "to drink". And I'm always fascinated by word roots. And I'm not a linguistic expert here, but it's neat because even in languages I'm familiar with like Hindi and Urdu, the word pina means "to drink", so maybe it's even related to the word pani which is in those words in those languages. I know all of these have a shared linguistic root, so it's always fascinating to see these linguistic connections. So this is Pinocytosis where the cell is drinking so to speak, but it's also getting the other stuff that's in that fluid. This is Phagocytosis, the cell is eating. And these are both special cases of I guess the more general term of engulfing in this way which is called Endocytosis. Endocytosis. So Phagocytosis is a form of Endocytosis, and Pinocytosis is a form of Endocytosis. Now the next question you might say is: OK I can get that this happens, this can be observed under a microscope, but how does it happen? How does the cell wrap around and pinch around? And like I say in a lot of videos, people think that we understand some of it, but this is not fully fully understood. There is views that well the cytoskeleton must be involved in some way. It has to create space here for this thing to be able to pinch off and move in that direction. It maybe will help actually the cell's membrane wrap around in some way, but these are all areas of active research. How does this Endocytosis actually occur? How does the cell know what to consume I guess you could say, and then how does the membrane actually behave in this way to do it, to do it well?