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 require 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 they 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 gets larger for example comparing there's a 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 one centimeter times one centimeter which is just one square centimeter and then we multiply it by six to get a total surface area of six square centimeters 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 one centimeter times one centimeter times one centimeter so one cubic centimeter and so the surface area to volume ratio is just the surface area of six to the volume of one which basically just means that the surface area is six times bigger than the volume if we do the same thing for this medium cube though we need a surface area of 24 square centimeters and a volume of eight cubic centimeters and so a ratio of 24 to eight which simplifies to three to one and if you do the same thing for the biggest cube read 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 nine times larger here but the volume gets 27 times larger and this is why the surface area to volume ratio has dropped from six to one to two to one 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 increase our surface area 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 5 centimeters which means molecules would have to diffuse 50 000 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 their 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 specialized 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 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 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