 Did you think we were going to get here to the place where we actually define what qualities make up living systems? Well, the first one, one of them is that living systems exhibit this hierarchical organization because all matter exhibits hierarchical organization. I don't tend to look at that as one of the home-marks of living systems, but it does provide some structure that I think makes understanding biological processes easier. But this next one is a quality of living systems. Living systems maintain homeostasis. I'm not going to write this down for you, but you write it down. Homeostasis is the maintenance of a relatively constant internal environment. We actually won't spend a huge amount of time talking about homeostasis, but if you come with me to human physiology, if you take human anatomy and then you end up with me in human physiology, basically, physio is all about homeostasis. We learn all the mechanisms that enable humans, human bodies, to keep the internal environment relatively constant. That means our body temperature stays relatively constant. Our blood pressure stays relatively constant. The concentration of our blood remains relatively constant. The amount of glucose in our blood remains relatively constant. These are qualities. These are homeostatically maintained qualities in multicellular organisms. Single-celled organisms also maintain homeostasis. It means that you can live in an environment and not just have to change to match that environment every single time. You can actually adjust yourself and keep yourself constant no matter what's happening to you around you. This is, I could, well, I could spend a whole class talking about homeostasis, and I do, so come with me to physio, and we'll talk about this in more detail. In this class and in physio, we talk about the cell membrane. Living things have cell membranes. This was the point of debate that I had with my son who argued viruses, of course, are alive, but viruses do not have a cell membrane, which is one of the reasons why a case might be made that viruses aren't actually alive. We'll have a whole lecture just on the cell membrane because this thing, oh, so cool. The cell membrane is how we can maintain homeostasis because it separates the inside from the outside and moderates what gets in and what gets out. Some things can come in and out, other things can come in and out only sometimes, and some things you're not coming in or getting out. And that is, that's it. It's actually really profound and a powerful thing that enables life. Okay, so living systems have cell membranes. Living systems get energy, they eat, and they use energy, they move. That's a very generalized example. Guess what? We're going to spend three lectures talking about energy. What is energy and how do living systems get it? What do they do with it once they have it? And where does it ultimately come from? Fantastically, it comes from the sun. But all living critters have to be able to get it and use it. All living systems have hereditary material. In most cases, this is DNA. I'm just gonna stop a minute because I know there are viruses that only have RNA, but are there any bacteria that only have RNA? I actually can't remember the answer to that question. RNA and DNA are both examples of hereditary materials. Hereditary materials let you pass information from generation to generation, and hereditary materials are providing the instructions for building who you are. Guess what? Three lectures talking about how the hereditary material works, and I think another three talking about how heredity works. Living critters, they grow, which means their cells multiply, and they develop over time, and they make more of themselves. I'm sorry, but if I saw, like, look at those little babies. It's a good thing that I have some discipline in my life, because I guarantee you that those should live in my house. I would like that. Unless I had to feed them, in which case I don't do a good job of feeding people or animals. Okay, I'm sure that my children would step up and help. Living things grow and develop and reproduce, and populations of living critters evolve. So, evolution is a core concept in the study of biology, and guess what? We get to talk about it a lot. The awesome thing about our conversations about evolution, we get to focus on biodiversity, we get to focus on genetics and heredity, and none of it comes in conflict with faith, or any kind of spiritual belief that you may have, which is awesome. That makes me happy, but it is a quality of living systems. In this last chunk, I want to look at the consequences of evolution. Evolution is change in populations that leads to biodiversity, and I just want to take a minute to selfishly go through and look at some really cool critters.