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Characteristics of Life
Characteristics of life include living things that are made of cells or cell products, use energy, respond to changes in the environment, maintain homeostasis and reproduce with similar offspring. These characteristics define whether something is living or not.
When people start studying what is life, my students often say "why do we need to bother having all these definitions for life because it's kind of duh obvious what is a life." And that seems true in the beginning but scientists actually had to sit there and try to narrow it down because originally when people did study life, they ascribed life to a lot of things that we now know aren't alive. For example, they used to think that fire was alive. And we know now it's not. So what are the characteristics of life?
Well one, we've discovered that all life on this planet is made up of cells or products of cells. For example, my body is made up of gazzillions of cells. Skin cells, muscle cells, nerve cells, blood cells et cetera but then things like my finger nails or hair, those aren't made of cells but they are made by cells. So that's what I mean by cell products.
Living things use up energy and materials. One of the laws of the universe is that the universe hates order and it's called entropy this love of disorder. So in order to maintain this highly organized state we need to be constantly spending energy to maintain the highly organized cells of our bodies. So one of the things that we know about life is that it has this active metabolism.
Living things respond to changes in their environment. Scientists like to call those stimulus. So when you poke me I look at you and say "hey, stop poking me." If you poke a tree, it doesn't turn and look at you but if you keep poking it ultimately it'll start growing thicker bark perhaps in that area and if it's a Venus Fly Trap and you are a fly and you're poking the little trigger hairs, it will catch you and eat you.
We maintain homeostasis, this kind of goes hand in hand with this idea about responding to changes in the environment. Homeostasis, this is the idea that we try to maintain an internal balance. For example, if I get too hot, I start to sweat in order to get rid of the excess heat. If I get too cold I start to shiver doing muscle contractions to help release some additional heat energy. And that's all to keep my body temperature same. So homeostasis is an example, or sorry, body temperature maintenance is an example of homeostasis.
Living things reproduce with similar offspring. So when I reproduce I have a mechanism of inheritance. I have a way of passing on my traits to my children. In me I use DNA, so pretty much everything on this planet uses DNA. There are some examples of viruses, although some people argue against them being alive. Some viruses were also use RNA.
Now, life evolves at the species level. Individuals don't evolve. We don't undergo genetic change over time. Unless of course you're a spiderman you get beaten. But species evolve over time in response to environmental changes. Whether it's, it gets colder so you need to evolve to have better protection against the cold or it's a new environmental change because some member of your species figured out a new genetic mechanism that's just better than was previously found in your species. That's life.