In this video, we're going to look at the idea of surface area to volume ratio, and we're going to use it to explain why small organisms, like single-celled bacteria, can rely on diffusion through their surface to exchange substances with their environment, but large multicellular organisms, like ourselves, require specialized exchange surfaces, like the lungs and intestines. to get things in and out of our body, and also requires specialized transport systems like the heart and blood vessels to transport those things around. To start, let's imagine a single-celled organism.
In order to survive, this cell has to continuously carry out loads of chemical reactions, building up and breaking down molecules. To do this, it needs to absorb resources like oxygen, glucose and amino acids from its surroundings, and then it also needs to get rid of any waste products that it produces, like carbon dioxide. How well it can do this depends on its surface area to volume ratio, which is basically a measure of how big its surface area is, so this area around the outside, compared to its volume, which is all of this space inside the organism.
And the key idea of this video is that as organisms get larger, for example comparing this single bacterial cell to a mushroom or a cow, their surface area to volume ratio decreases, which is just the technical way of saying that larger organisms have less surface area compared to their volume. Now, calculating the surface area and volume of a real organism, is actually really hard because they're weird shapes. So instead to see this idea in practice let's take these three cubes and see how their surface area to volume ratio changes as they get bigger. To calculate the surface area of a cube you just calculate the area of a single face and then multiply it by six because a cube has six faces. So for this small one by one by one cube We get the area of one face by doing 1cm times 1cm, which is just 1cm2.
And then we multiply it by 6 to get a total surface area of 6cm2. Then to calculate the volume of a cube, we just multiply the length, width, and height together. So for our green cube, that's a volume of 1cm times 1cm times 1cm.
so 1 cubic centimeter. And so the surface area to volume ratio is just the surface area of 6 to the volume of 1, which basically just means that the surface area is 6 times bigger than the volume. If we do the same thing for this medium cube though, we get a surface area of 24 square centimeters and a volume of 8 cubic centimeters, and so a ratio of 24 tae. which simplifies to 3 to 1. And if we do the same thing for the biggest cube, we'd have a surface area of 54, a volume of 27, and a ratio of 2 to 1. So if we now compare the smallest and biggest cubes, we can see that as the cubes get larger, their surface area and their volume both increase, but importantly their volume increases much more quickly. For example, the surface area only gets 9 times larger here, but the volume gets 27 times larger.
And this is why the surface area to volume ratio has dropped from 6 to 1 to 2 to 1. So now that you understand why the surface area to volume ratio decreases as organisms get larger, let's apply that knowledge to bacteria and humans. Because bacteria are tiny, they have a really high surface area to volume ratio, and this means that they can rely on diffusion across their surface to exchange everything that they need. On the other hand, because humans are so big, we have a low surface area to volume ratio, which means that we can't rely on diffusion for all of our needs.
Instead, we have to have specialized exchange surfaces, like the lungs and intestines. which effectively increases our surface-to-volume ratio. by giving us a huge extra surface on the insides of our body. For example in the lungs we have millions of alveoli which together gives a huge surface area over which we can absorb oxygen and get rid of carbon dioxide. And in the intestines we have villi which provide a massive surface area for the absorption of nutrients.
Now another important concept to bear in mind here is diffusion distances. As organisms get larger, the distance that molecules would have to diffuse to get from the outside of their body to the inside of their body increases massively. For example, to get from the outside to the middle of a bacteria is probably only something like one micrometer, whereas to get from the surface to the middle of a human would be at least five thousand times further. This is really important because it means that diffusion will be way slower for larger organisms, and so they won't be able to rely on diffusion alone to get all of the stuff that they need into their cells. To solve this, larger organisms often have transport systems, like the circulatory system, which transport molecules from the exchange surfaces, where they enter the body, around the body to whichever cells need them.
This means that the molecules then only have to diffuse a very short distance to get into the cells. So the takeaway here is that larger organisms generally have exchange surfaces to get substances in and out of their bodies, and also transport systems to transport those substances to the parts of their bodies that need them. The same ideas apply to plants.
For example, they have roots and leaves to exchange substances with the environment, and phloem and xylem tissues to transport those substances around the plant. We'll take a closer look at each of these specialised exchange surfaces and transport systems in other videos, but the aim of this video was just to explain why we need them. Also, it's important to understand that when we say large organisms, we don't just mean things like humans and cows. cows.
We're really referring to anything that's big enough to see with your naked eye. For example, even insects like mosquitoes have exchange and transport systems. Hey everyone, Amadeus here.
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