Characteristics of Life
Every fall for the past dozen years, I have begun my biology class in much the same way---with a question. How do you know if something is alive? Now this may seem like a straightforward question, but based on student responses, it isn't nearly that easy to answer. Students often tell me that something is alive if it moves, or breathes, or thinks. Well, these responses are largely all wrong. However, despite the disappointment shown on aspiring biology students' faces when they realize they can't yet distinguish between living and nonliving, there is hope. Fortunately biologists have developed a list of eight characteristics shared by all living organisms. Characteristics are traits or qualities. Here is the list of characteristics shared by living things:
Cellular organization
Reproduction
Metabolism
Homeostasis
Heredity
Response to stimuli
Growth and development
Adaptation through evolution
Now, let's take a moment and elaborate on each of these characteristics.
Characteristics of Life Defined
The first characteristic of life we listed was cellular organization. This simply means that living things are made of cells. Cells are the most basic unit of life. It doesn't matter if those cells are plants, animals, fungi, or bacteria. If something is going to be alive, it must be made of cells.
Second is reproduction. If something is alive it must be capable of reproducing. Multicellular life forms such as humans reproduce sexually, while unicellular life forms like bacteria reproduce asexually. The important thing to remember is that, in either case, living things reproduce.
Next, we come to our third characteristic, metabolism. This concept is a little difficult for some students to grasp. Metabolism is essentially a collection of chemical reactions occurring within the body (or cell). These reactions vary in form and function but promote processes such as protein synthesis, chemical digestion, cell division, or energy transformation. Because metabolism includes reactions that link to other characteristics, it is sometimes grouped with those other characteristics. However, for our purposes, we'll keep metabolism separate.