Biology 101 - Part 1 - What is LIfe?

Today I want to start a little series about basic biology.  I'm sure you remember being taught about taxonomy in sixth grade biology class.  You leant by rote that everything is divided classified by kindom, phylum, class, order, family, genus, and species.  Well, that old system proved to be useful but a little over-simplified, so it was expanded into the new three-domain system, which I'll discuss in a future post.  Today I want to discuss the question, "what is life?"  

 There are different criteria to define life, some more broad than others.  This is pretty hotly debated amongst biologists in the geeky way biologists argue about such things, but a simple description of something living includes the following characteristics:

1) Homeostasis - the ability to maintain a constant internal state (biochemistry, temperature, tec.)

2) Organization - being composed of either a single sell or organized systems of multiple cells

3) Metabolism - biochemical process wherein certain compounds are used to produce energy.  The consumpsion of this engery to maintain homeostasis.  

4)  Growth - the production of more than is used up, meaning the entity does not catabalize itself.

5) Adaptation - ability to change over time due to changes in environment.  Also a fundamental property of an organism's evolution.

6)  Response to Stimuli - for example , the chromatophors on an octopus change colour when an octopus moves to a new area of a reef so that it can blend in.  Plants turn towards light, a horses flank twitches when bitten by a fly.

7) Reproduction either sexual or asexual.

Viruses somewhat confuse this system by being dormant except when they replicate themselves inside a host.  Philosophically, the Gaia hypothesis, holds that Earth herself is alive.  As you might guess, I hold this belief, and except for the criteria of reproduction, it does fit.  If you would like to read more about life, please have a look at the references for this Wikipedia article.