we're gonna talk about what makes things alive. So there are actually eight characteristics that is shared by all living organisms. And these eight characteristics distinguish them from the non living things that surround them. And the lack of just one of these eight characteristics would render something non living. And so you guys have talked about these characteristics in your previous biology courses. And so what we're gonna do is a quick review on each of these characteristics. And so, for the first characteristic we have that all living things air composed of cells. And that's pretty straightforward, right? That's just saying that the most basic fundamental unit of life is the cell. So the second characteristic of life is that all living things have order. And of course, this means that they're not random. And in fact, what they do is they take smaller, simpler molecules, and they strategically build them into larger, more complex molecules and structures to survive. Now, the third characteristic is that living things respond to stimuli. And of course, what that means is that they can respond to specific triggers in the environment. Now, the fourth is that living things maintain home. Eo Stasis, and you've learned in the past that homeostasis refers to the ability of organisms to maintain internal conditions, despite the fact that the external conditions are constantly changing now, the fifth characteristic of life is that all living things can reproduce. And of course, that means that they can create more life. But they couldn't do that either sexually or a sexually. The sixth is that all living things have a dynamic metabolism and metabolism is referring to the sum of all chemical reactions. And dynamic is referring to the ability of this metabolism to change and be flexible under different conditions. And remember that the metabolism of an organism is focused on extracting and transforming environmentally acquired energy so that they can use that energy to survive. Now. The seventh characteristic is that all living things have heredity, and we know that the hereditary information of a cell is DNA and DNA can be passed down from older generations down to future and newer generations. Now, the last characteristic of life is that living things can evolve. And again, we've learned in the past that evolution has to do with changes to the DNA or mutations to the DNA over long periods of time and in a population, sometimes that leads to different physical traits. And sometimes those physical traits allow organisms to be better adapted and improved fitness or survival in an environment. And we'll talk more about evolution later on. In our course. Now we know viruses air not alive. But in our next video, we're going to talk about which of these characteristics above do viruses lack.

so we already know from our previous bio courses that viruses are not considered alive. And of course, that's gotta be because they lack some of the characteristics of life. But the question is, which ones do they lack? And so here, what we're gonna do is less the corresponding number of each characteristic that viruses tend toe lack. And so, for the first one, we know that viruses are not cellular, so we'll go ahead and cross that off the list and list characteristic number one as being one that viruses lack. And when we look at the typical structure of a virus, recall that they have a protein shell that's known as a caps it and the caps it encapsulate, say, nucleic acid molecule That could be either DNA or RNA. And some viruses contain a lipid layer that surrounds the caps it known as an envelope. And viruses are much smaller than even the components of cells, so they're definitely not cellular Now, for the second characteristic order, viruses are ordered, and in fact they are an assembly of macro molecules that come together in an ordered fashion, so we'll go ahead and give them a green check mark for this one now for a response to stimulate. There's not enough research and evidence out there that shows that viruses can respond to stimuli, so we'll go ahead and cross this off the list. Enlisted is one that they lack for maintaining home yo Stasis. Viruses don't have complex mechanisms like living organisms due to maintain home yo Stasis. However, many scientists say that the envelope of some viruses and even the caps it viruses can help to maintain and stabilize internal conditions. And for that reason, we'll go ahead and give him a green check mark for this one. For reproduction viruses air kind of like on this blurry line because viruses cannot reproduce sexually or a sexually. And in fact, we know that viruses can Onley reproduce by hijacking the machinery of other living Selves. And because viruses don't have the machinery themselves to reproduce, we will go ahead and cross this off. Unlisted is one that they lack now for characteristic sex. Viruses do not have a dynamic metabolism like that of living organisms, and in fact, viruses don't even have a requirement toe extract energy from the environment like living organisms do and so we'll go ahead and cross this one off. Enlisted is one that they lack now for heredity. We know that viruses can contain nucleic acids, and so they can have DNA and have the hereditary material. And because they have DNA, they can also evolve over time. And which will notice is that viruses have a collection of some of these characteristics, but they lack other characteristics, and some scientists are pushing to redefine our definitions of life. But that hasn't happened yet, and viruses are not considered alive now. And our next video, we're going to change gears a little bit and talk about how living organisms order themselves. So we'll talk about the biological levels of organization, and I'll see you guys in that next video.

living organisms are incredibly ordered and contain what's known as a biological hierarchy of organization. And we've talked about this plenty in our previous bio courses. So we're just gonna do a quick review here and in our example of the biological levels of organization, noticed that on the far left we have larger and more complex organisms and structures, and as we move from the left towards the right, we get smaller and less complex organisms and structures. Now, starting over here on the far left, we have multi cellular organisms, which includes ourselves, obviously, and humans are composed of multiple organ systems where each organ system is a collection of individual organs that work together. Now the organs are made up of a bunch of tissues, and the tissues are a collection of the same types of cells. And we've learned that cells are made up of smaller components called organelles, and the organelles are made up of macro molecules where the term here macro means large. So these air large molecules and the macro molecules include proteins, carbohydrates, lipids and nucleic acids, and moving forward in our biochemistry course, we'll talk a lot more about the macro molecules and the organelles. Now the macro molecules are made up of smaller molecules, and the molecules are made up of individual atoms that are bound together chemically. And, uh, these include the atoms that we see on the periodic table. And the way that this hierarchy works is that the lower levels are used to build the immediate higher levels and notice that we have this big red box that is going around the macro molecules, molecules and atoms levels. And what this is referring to is what's known as the biochemical unity of life. And all that saying is that all living organisms have a resemblance, and you can see that when you compare these bacteria to May. No, I'm just kidding. The biochemical unity of life does say that there is a resemblance between all living things, but that resemblance is seen at the atomic and molecular levels of life. And there's a lot of evidence at the atomic and molecular levels that support what's known as a common universal ancestor. We're all life today, is theorized to have evolved from a single universal common ancestor, and hopefully these bacteria don't look like may. But at the biochemical and molecular levels. We have a great resemblance, and that's pretty fascinating. So that concludes our lesson on the characteristics of life, and I'll see you guys in our practice videos.