 So, what we want to do to try and address the issue of what is life is we want to look at six characteristics of life and we'll talk about characteristics that all living organisms share and as you think about all living organisms, don't just think about your dog or your cat or your horse, think in terms of the broad spectrum of life which would include plants and all the animals such as sponges or jellyfish, mushrooms, mold, algae, unicellular organisms. So think in terms of all living organisms and the six characteristics that they might display. So the first one what I would consider to be the hallmark characteristic of life is the idea of the cell. Some textbooks say that the cell is the fundamental unit of life and so if it's living then it must be cellular. Now consider the converse of that statement. If it's cellular must it be living? Of course the answer is no. There are many things that are cellular that aren't living, something that's just recently died for example would still be composed of cells but wouldn't necessarily be living. Of course we'll talk much more about cells in this course as we continue on but the cell is the fundamental unit of life. Think about a red blood cell or a muscle cell in your body for example. So a second characteristic of life is the concept of metabolism. Metabolism could be defined as the chemical reactions necessary for life. There are two types of chemical reactions that that are involved with metabolism and a little later on we'll talk about anabolism and catabolism. Basically they're building reactions or breaking down reactions, reactions that require energy or reactions that release energy. So metabolism is about how organisms use energy. A third characteristic of life is growth. All organisms grow and that simply means an increase in size. Even a unicellular bacterium such as a bacillus subtilis or something along those lines when it's when it's first created when it's first divided from the parent cell it's much smaller than over time that cell will grow and as it grows it increases in size. Now certainly growth is much different for something like a human being where we start out as a single celled zygote and not only do we grow but we grow and develop but all organisms from bacteria to human beings exhibit the characteristic of growth. So the fourth characteristic of life is the idea of response. All organisms can demonstrate response and a way to define that would be a reaction or react to a stimulus. So a stimulus is just a change in the environment. One of my favorite tricks to play on students in my classes is as they're writing down react to a stimulus is to take my biology textbook and drop it on the floor. Of course it makes a very loud noise and everybody jumps and I kind of snicker at the class and say see you're alive you demonstrated response but even unicellular organisms have this ability to react to stimuli. For example they may move toward a certain chemical or away from a certain chemical that would be called chemotaxis. They may move towards the light or away from the light. So all living organisms demonstrate response. Now these four characteristics are characteristics of life that we think about on the individual level meaning an individual human being an individual dog an individual of paramecium. But the last two characteristics of life that we're going to think about are characteristics that exist on the population level. And the population is defined as a group of similar organisms in a defined area. And so we could talk about the population of flathead catfish in the Yakken River for example. So when we look at characteristics of life on a population level we talk about reproduction. Many of you may have family members that are in their 70s or 80s and they've never had children of their own but certainly they've been living. And so when we talk about reproduction that's something that we think about on a population level. I've produced no offspring myself but I'm certainly alive. But if all the members of a given population ceased to reproduce then that population would go extinct. If flathead catfish in the Yakken River for example ceased reproduction then at some point in time there'd be no more flathead catfish left in the Yakken River. So reproduction is a fifth characteristic of life. The sixth characteristic of life is this idea of adaptation. And adaptation by definition takes place on the population level. Adaptation could be defined as a change in the gene pool a population. And so individuals cannot adapt because individuals don't change their their own genes. But then individuals can acclimate. Sometimes students will ask questions about that and they'll say well why do football players go to Mile High Stadium a week early so they can adapt to that higher elevation. Well the individual isn't adapting the individual is acclimating but populations adapt and that's when there's a change in the genetic makeup of that population over time. So we are recognizing six characteristics of life the cell metabolism growth response reproduction and adaptation. And this is just one way of looking at the characteristics of life. There are other ways of doing that for example another video you may look at talks about a seventh characteristic of life and that's related to regulation. It's an interesting discussion to have one might say regulation requires metabolism and response. And so response and metabolism are very closely related to regulation. But that's something for you to think about in perhaps an upcoming assignment.