MODULE #1: Biology: The Study of Life

What Is Life?


If biology is the study of life, we need to determine what life is. Now to some extent, we all have an idea of what life is. If we were to ask you whether or not a rock is alive, you would easily answer “No!” On the other hand, if we were to ask you whether or not a blade of grass is alive, you would quickly answer “Yes!” Most likely, you can intuitively distinguish between living things and nonliving things.

Even though this is the case, scientists must be a little more deliberate in determining what it means to be alive. Thus, scientists have developed several criteria for life. Not all scientists agree on all of these criteria, but in general, most biology courses will list at least some of the following criteria for life:

  1. All life forms contain deoxyribonucleic (dee ahk' see rye boh noo klay' ik) acid, which is called DNA.
  2. All life forms have a method by which they extract energy from the surroundings and convert it into energy that sustains them.
  3. All life forms can sense changes in their surroundings and respond to those changes.
  4. All life forms reproduce.

If something meets all of these criteria, we can scientifically say that it is alive. If it fails to meet even one of the criteria, we say that it is not alive. Now if you're not sure exactly what each of these criteria means, don't worry. We will discuss each of them in the next few sections of this module.