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Scale of cells

Even though molecules, proteins, viruses, and cells are all tiny, there are significant size differences between them. The diameter of a water molecule is roughly 0.28 nanometers. The diameter of the protein hemoglobin is roughly 5 nanometers. The diameter of the HIV virus is roughly 120 nanometers. A red blood cell is 6-8 micrometers. Created by Sal Khan.

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Video transcript

- [Voiceover] When we study science it's natural to just categorize a whole series of things as just being really really unimaginably small. So when people say, hey atomic scale, or molecular scale, or protein, or cell, you often just group that together and say oh, those are really really really really small things. But what I want to do in this video is get an appreciation that even though all the things I've just mentioned are really small, there's actually a huge difference in the sizes of those things. And hopefully that'll give us an appreciation for how complex something like a cell can be. How it can have all of this machinery. How it can actually be a living organism or part of a living organism. And so at this scale this is a this is my little rendering, my drawing of a water molecule. You have the oxygen atom in the right over here in this purplish color, and then you have two hydrogen molecules bonded to it. And this is going to be roughly 0.275 nanometers. And just to remind ourselves, a nanometer is a billionth of a meter. And just to get an appreciation of that so let me. So this is one billionth one billionth one billionth of a one billionth of a meter. That's a nanometer. And if you want to even attempt to visualize that that would be a millionth of a millimeter. So one, let me write this. One one millionth one millionth of a millimeter. And I actually like using this one because a millimeter is about as small as I can on a reasonable basis visualize. But instead of a millionth of that it goes well beyond, at least my capabilities of visualization. So that would be the diameter or the width of a water molecule. But now let's go to the next scale up. We've talked a lot about proteins and this is our friend, this is right over here is our friend hemoglobin. And this gives you a sense of scale, the width of hemoglobin is going to be about five nanometers. Or five billionths of a meter. Now that seems super small, so in some ways it's okay to categorize that into your super small part of the brain. But it's good to appreciate, this is much larger than a water molecule. If a water molecule were on this scale I predrew it, this little thing over here, that's my attempt at drawing a water molecule at this same scale. So even when you go from something like a water molecule to a protein you're already going dramatically up in size and dramatically up in complexity. And we've talked a lot about protein structure and how they can take on all of these interesting shapes, and do fairly surprising and complex things inside of biological systems. But now let's go, let's go the next scale up. And the next scale up I'm gonna go to a virus. And what I've attempted to draw here this is a fairly well known virus, this is HIV. And it's actually one of the larger viruses and its diameter is roughly 120, 120 nanometers. So if we were to draw this hemoglobin this hemoglobin protein at the same scale as we've drawn this virus this thing right over here would be the hemoglobin protein. And we wouldn't even be able to see the water molecule at this scale right over here. But this is still really really small. This is 120 billionths of a meter. So this is still this is still unimaginably small. But now let's go up to the, let's go up to the next scale. So this creepy picture right over here, this is a T-cell. This is a depiction this is a, if you want to see the whole thing, that is a T-cell right over here. This is a T-cell. And this creepy picture, this all the blue, that's the T-cell. And what you see in yellow that's the HIV virus emerging, taking advantage of this T-cell. So, that's why it's so creepy, it's using that cell's machinery to reproduce itself. But you immediately see on this picture how small the HIV virus is compared to the actual T-cell. Each of these small little things each of these small things, is an HIV virus, which we already saw is a lot bigger than something like a hemoglobin protein. And so a hemoglobin protein you wouldn't even be able to on this scale, maybe it would be a pixel, if that. And on a similar scale, is this T-cell, you have a, you have things like red blood cells. This is actually a comparison this side by side. This is using an electron, this is using an electron microscope you see a red blood cell right over here and you see a T-cell and they're roughly roughly on the same size, or at least the same order of magnitude size. And a red blood cell is going to be six to eight micrometers, micrometers wide. So this is six to eight millionths of a meter. So if we were to if we were to just take seven as the average. Seven millionths, seven seven millionths of a meter. Over here, we're talking about a millionth of a millimeter. Now we're talking about seven millionths of a meter. And just to get an appreciation for size, we already compared the virus, the HIV virus, to this cell. We're seeing it directly as a emerge from the cell. But each of these red blood cells are gonna contain roughly 280 million hemoglobin molecules. So there's gonna be 200 each of these there's gonna be 280 million of these. So 280 million, that's a million million hemoglobins in each one of these. So hopefully this starts to give you an appreciation for even though we categorize cells as these unimaginably small things, they're actually far larger they're ginormous compared to things even like even like large even proteins. And especially when you think of things on the molecular, or the atomic scale. And that's why cells are so interesting. They actually have a lot of complexity to them. But just to have an appreciation also for how small cells are even though we've just described these red blood cells and these T-cells there's these kind of worlds unto themselves. They are these incredibly complex things. If I were to draw the width of a human hair on this screen right now relative to the scale of these red blood cells, it would be about as wide as this video. So from if I were to draw a human hair it would go from there roughly to there and there's actually a lot of variance in the width of a human hair. But the width of a human hair would be just about like that. If you looked at the scale of if you looked at the scale of this picture right over here. If you looked at these scales it would be much much much bigger. And I encourage you we can think, oh okay, the width of a human hair. Pluck a hair out. Look at it. Put it on a piece of paper. It's hard to even discern the width. But we're saying that that width compared to these red blood cells would actually be your entire width of the screen. We've already said these red blood cells these T-cells, these are kind of these are worlds unto themselves when you think of it from a viral, or especially at a molecular scale.