What is Life? II

To go just a little further in depth as to what we meant last time when we discussed the characteristics of living things, I wanted to make a follow up post.  I’ve been putting this off, but it’s time to bite the bullet.

  • All living things are made up of 1 or more cells.  Cells are the basic unit of life.  Most are smaller than you can see with the naked eye, but they are composed of different parts called “organelles” that work together to allow the cell to function and reproduce.
  • All living things reproduce.  Reproduction creates offspring, which are similar, but not identical to the parent(s).  Reproduction can be sexual (two parents) or asexual (one parent).
  • All living things are based on a genetic code.  Usually that code is DNA (but sometimes RNA in the case of some viruses, which remember are technically not living), which is a molecule that tells your cells what to do in order to function.  Essentially the genetic code is a set of instructions for your cells.
  • All living things grow and develop.  Some organisms simply grow larger and prepare for reproduction.  Other organisms may develop legs or wings for movement, or teeth for chewing, or breasts for feeding their young.
  • All living things obtain and use energy.  Just as you need food, so do plants, fungi, and even bacteria.  The sum of all chemical reactions to build up and break down materials is called metabolism.
  • All living things respond to their environment.  A stimulus is a signal to which an organism responds.  When you get pollen in your nose, you sneeze.  When soil is moist and warm, a seed germinates.  When you turn on a light, roaches run away!
  • All living things maintain a stable internal environment.  No matter what goes on outside the body, all organisms keep their internal conditions stable.  The process to do this is called homeostasis.  When you get cold, your body tries to keep your internal temperature from dropping too much.  So you begin to move involuntarily.  We call this shivering.  Likewise, if you’re too hot, you’re body sweats to cool you off.

What is Life?

If Biology is the study of life, how do we define life?  Is there one characteristic that defines all life?  If you ask a room of students “how can you tell if something is alive” you’ll hear things like “poke it with a stick and see if it moves.”  Unfortunately, that isn’t right.  In fact, there is not one characteristic that defines all life.  Living things are defined by a few characteristics.

All living things:

  • are made up of 1 or more cells.
  • reproduce.
  • are based on a genetic code .
  • grow and develop.
  • obtain and use energy.
  • respond to their environment.
  • maintain a stable internal environment.

Even nonliving things can meet some of these descriptions, so it’s important to be able to tell living from nonliving. For example, a car obtains and uses energy and maintains a stable internal environment.  Some can even respond to their (external) environment!  To be defined as living an object must meet ALL of these requirements.

Technically, viruses don’t meet this description of life and many scientists don’t  consider them to be living things.  However, since they act like living things, they are studied under the umbrella of biology in a field either specifically called Virology or as a broader biology field called Microbiology.

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