 We talked about the characteristics of life, but life has to actually implement the structure, homeostasis, or control of the interior of the structure. Metabolism or extracting energy from the environment, it needs to be able to grow, have recipes for that growth as an inheritance, reproduce, and then it has the capability of dying. So we can start with the structure, and life on earth is cellular, and it has a cell wall on the outside. So this implements the structure. So the cell wall forms the structure, but to get the homeostasis, it needs this cell needs to be able to exchange things from the outside to the inside. And a lot of that exchange is controlled by variations in the cell wall, so that allow certain things to pass in, or some of them maybe transport things out of the cell. But these processes might be very specific to certain chemicals. This control of what happens, what passes through the cell wall, is what allows the cell to maintain homeostasis. So that extra control is provided by a plasma membrane. And so this provides additional structure that helps control the types of things that can get in and out of the cell, and then that the plasma membrane also has proteins and enzymes embedded in it that do different processes. And so for a bacterial cell, the metabolism is controlled, this extraction of energy is controlled by enzymes in the cell membrane, and these can do things like produce a proton gradient. So you end up with hydrogen or protons on the outside and not in the middle, and the process of those protons going back through the membrane can create an energetic molecule, for example ATP, denicine triphosphate. So it basically takes a denicine diphosphate, adds a phosphate to it, and with that energetic molecule to it. So one of the things that cells do is they localize the metabolism in specific places. So then we have the aspect of growth, and if the cell is autotrophic, sort of automatically eating, it takes a biotic carbon like carbon dioxide and converts it to organic molecules, and that can happen somewhere inside the cell here, or if it's heterotrophic, as in eats different things, it basically takes in organics from the environment. So that would mean that there are transporters through the cell wall and membrane that take the organic molecules from the environment and brings them into the cell where they can, for example, react with oxygen. So there needs to also be an ability to bring molecular oxygen into the cell where these two can react and produce energy. So one of the interesting things about bacterial cells is that they can do a lot of different reactions to get energy, whereas animal cells, eukaryotic cells, have many fewer options. Some are photosynthetic, some use oxygen, and there's also this process of fermentation where you basically take organics in and it breaks them into smaller organics, maybe with a methane and another organic molecule that gives energy. So this is a process of fermentation. We'll talk a lot about the metabolisms in a later video, but one of the key aspects is that this requires a lot of different structures. They have to be organized structures. This energy allows cells to grow, right, and so basically that depends on the energy plus the incorporation of organic molecules into the cell itself. So we have organics coming in and these are converted into critical cell components. So, for example, this might be amino acids, which are the organic molecules that create proteins. And these are going to be different than the ones that are used for fermentation or to react with oxygen for energy. They have different components. So now these processes require the structure, which means that we need some form of inheritance or DNA, the recipe for how things go. And inside the cell, so for bacteria, they have chromosomes that are all sort of tangled up in the cell that are actually circular. And there are, again, enzymes that we'll talk about more later that will attach to the DNA and create the reactive component of the genetic material, the RNA, which can be converted into enzymes. So here we have a little bit of messenger RNA being produced. This messenger RNA breaks off at the end of the gene and there are structures called ribosomes, which basically read that messenger RNA and use it as the template to make enzymes. So this process of inheritance is really having a recipe to make the molecules that allow the cell to function. So it goes from the chromosome into messenger RNA to these structures called ribosomes, which basically assemble those amino acids into the enzymes that get embedded in the cell walls and the various membranes and all these systems that produce the energy. So that's how the cell functions, but of course we know that it needs to reproduce. And this is a process of cell division. So the DNA has to be copied into another strand of the DNA, and of course that happens with enzymes, proteins, and then the cell has to actually divide. And so the interior of the cell has all of these sorts of fibers in it that keep things in place and affect the cell structure. And when the cell is going to divide, basically there are a bunch of these filaments that sort of span across the cell and pinch it into two and allow the cell wall and cell membranes to join into, in this case, two circles, ideally with one copy of the chromosome in each cell. It doesn't always work. But if it doesn't work, the cell dies. So if you think about ways that the cells can die, there are of course many of them. Basically breaking the cell wall can do it, not having enough resources for energy can do it, not having the organic molecules for the critical cell components can do it. Problems and mutations in the DNA that kill parts of the recipe can cause death. So there are many different things that can cause death. So in summary, we basically have a whole series of structures and enzymes that enable all of these characteristics of life that have evolved through billions of years on Earth. And through that process of evolution, a lot of different detailed implementations of these have evolved through time giving us the diversity of life on Earth. Thanks for watching.