Biology_ Cell Size

Welcome to section 9.1. In this section, we're going to talk about the idea of cell size and why it's so important to cells. We're also going to talk about how cells go about this process of cell division. So to start off with, we already have learned that cells are microscopic. We know that they are small. So we've got this general idea down that cells have some reason, it seems, to be small. But we also know that there seems to be limits. You know, there are things like viruses that are much smaller. And we know these viruses kind of behave differently, that they're not able to do the things that a cell can do. So there seems to be some lower limit where a cell can be too small. So there seems to be this middle range for cells that's a pretty small cell. But overall, it is kind of this Goldilocks amount where you can't get smaller than that. You don't want to get bigger than this amount, or else it seems to cause problems for that cell. And problems biologically typically means death. So I want to go over why that happens. So when we look at a cell, there's two big ideas we have to grasp. Inside the cell, you've got the cytoplasm, where the metabolism, the chemical reactions are occurring. So this is where we have to get in enough nutrients for them to act upon doing things like cellular respiration. We also produce waste, and we have to have an ability to get rid of that waste. So we don't essentially like choke on our own waste, which would be about as fun as it sounds. We also have to get substances from one side of the cell to the other, so that we can distribute them so the cell can maintain homeostasis. So this volume idea that we're going to discuss is really going to be referring to the cell's cytoplasm, its innards. And so that's going to need stuff to metabolically and homeostatically function. We're also going to have this idea of surface area though, and this will be essentially the cell or plasma membrane, whatever you want to call it. And so this is going to be what allows things to get in and out. So if we think about this in terms of like a stadium, you have a bunch of people that need to get to their seats inside that stadium so that they can watch or do whatever they need to do. You also need to get enough food in there for the people, enough drinks. You need to have an ability to get rid of the waste, so essentially the restrooms, the trash. And so the surface area would be a lot like the doors, the entranceways, the exitways, the loading docks, all the ways we can get stuff into and out of the stadium. So you can start to realize that it's important that these match up. You don't want to have where the stadium's huge with all these seats, but there's only a few doors, so we can't possibly get enough people, enough food in, and we can't get enough people or enough waste out. This is going to cause problems, which for a cell, we've already said, means the cell would likely die because it can't get enough nutrients, it can't get rid of its waste fast enough, it's not going to be able to cope. But if you get it imbalanced the other way, we now have such a small amount of volume, so we have so few seats, so even though we have a ton of doors, we can't get enough people in there to sustain ourselves. You still have to be able to get enough people in there to make money, to be able to, you know, sell the food, to be able to produce enough that it can continue to go on. And so if you have where there's not enough volume, you can't metabolically take care of what you need to. So right in between here, we get a nice ratio of surface area to volume, where the surface area is enough to provide and get rid of everything that the cytoplasm needs, and the cytoplasm's capable of carrying out the processes of life. So our needs are basically transport, waste out, nutrients in, and we also need to communicate across the cell itself. Now I want you to realize why small means a good surface area to volume ratio. So it's easy to see that it's fast if you have a relatively low amount of volume, and you have a reasonably high amount of surface area to get stuff in and out for that volume. But why is it that a small cell does a better job of that? Why is it that a small cell somehow has more surface area relative to its volume? To demonstrate this, think of where you have this cube. So in this case, I have a picture of a Rubik's cube. If I were to cut this directly in half, wow, that's horrible. But if I were to do that, then ultimately you should see the volume will be half of what it was before. So our volume is going to decrease by one half. Now this should make sense. You have an eight ounce glass of water, you split it up, you now have two four ounce glasses of water. The problem with surface area, if you try to apply that, is if I were to go through and physically cut a cube in half, you're going to notice here in this little picture of one being rotated, that when we go through and split it, we now expose some new surface area. So if you were to cut a cube in half where your knife goes through, you can now pull it apart, and that area your knife passed through is now exposed. It's now surface area. So yes, we reduced the surface area, but we also added a little bit too. So it didn't actually decrease by half. What's going to happen instead is the surface area is going to decrease by a third. And so this means that as a cell gets smaller and smaller, the volume decreases faster than the surface area. So essentially that ratio of surface area to volume will favor surface area as you get smaller. The opposite is true as well. As you get bigger and bigger, volume increases faster. So this surface area to volume ratio will favor volume as you get bigger. So this is why we've got this sweet spot where we need to make sure we're not too big, where we have too much volume to support, or we're not too small, where we have too little volume to support the cell itself, to let it be alive. Now you might think, well how do they manage this? It's not that difficult. If a cell wants to do what I talked about the cube doing, split in half, it can do so. We call that cell division, and that's what we're going to cover next.