The Meaning of "Life"

"The meaning of Life" generally refers to the philosophical question concerning questions such as "Where did we come from?", "What is the significance of our existence?", "Where are we going?" or "Why did God put me here on this planet?".  Although these questions are more than worth discussing over a cozy campfire or a mug of beer, I am not prepared to tackle them today.  Maybe in another post. 

Instead, I would like to explore the biological meaning of "LIFE", that is, "What is life?", and "How can we recognize life?", or "What does it mean for something to be alive?".

OK, so let's tackle this.  Although it may seem that "life" may seem difficult to define, the good news is that many others have also considered this exact question.  Let's start with my bookshelf.  I teach university-level introductory biology and I have five different introductory level biology textbooks by different authors.   According to four of these textbooks (and also Wikipedia), life is something that exhibits all or most of these seven properties (isn't 7 one of those magical numbers?):  (You can take a look at the Wikipedia article to see definitions of each of these).

1. Order
2. Energy Processing
3. Regulation/Homeostasis
4. Growth & Development
5. Reproduction
6. Response to the Environment
7. Adaptation

The fifth textbook does not have a section that directly defines life, but does say that all organisms are made of cells.  That the cell is the basic unit of organization by all living things is echoed by all of the textbooks.  This is significant because there are a small number of biological entities of which not everyone can agree if they are living or not (and some of these are not made of cells).  These entities include seeds, viruses, mitochondria, and chloroplasts.  I will discuss some of these things in the next few posts.

NASA and Star Trek writers are also fond of pondering these questions.  NASA of course would love to discover extraterrestrial life and is actively searching on Mars.
(Incidentally, I used to love the TV show "Life on Mars" which was not really about life on Mars, but a great fish-out-of-water cop show).  The NASA website makes an attempt to define life, but really rehashes the list I give above stating that "To qualify as a living thing, a creature must meet some variation for all these criteria".  In practically, NASA has been trying to for water since it is essential for life on earth.  Future Mars rovers will presumably have the ability to scan for simple biological molecules (nucleotides, amino acids, etc.).

Trying not to make this post too long, let's test the above criteria to a few things....

Is a crystal alive?

A crystal is something that is ordered, it can grow, and even react in response to stimuli (some crystals go cloudy or clear in different salt solutions).  A portion of a crystal can even break off and seed a new crystal.  These are a lot of check marks in favour of classifying a crystal as alive.  My gut, and most people, tell me otherwise.  Crystals are not technically alive since they don't process energy, they don't adapt, and I must admit that part about crystals responding to the environment was a bit of a stretch.

Is a car alive?

This one is somewhat more tricky.  Cars display order (but they are engineered that way), they process energy (consume fuel), and they can even maintain homeostasis (e.g. the engine has a cooling system that makes sure it doesn't overheat).  The other characteristics are where it may be more difficult fit with the definition of life.  Cars don't really grow or develop (except in the factory or on the drawing table).  They do not have mechanisms for reproduction.  (However, many lifeforms are not able to reproduce... have you ever heard of a mule?).  Some cars do respond to their environment, have you see the ones that can parallel park on their own?  Lastly, cars do not reproduce and therefore they cannot adapt from generation to generation, although engineers work hard at making this year's models better than last years model.

Once again, my guts says not alive and most of us do not consider our automobiles to be alive. (Although it does satisfy about half of the characteristics of life, which is why we may say things like "My car died." or "It still has some life in it yet."). We don't see cars driving up to the pumps to "feed themselves".



Is a maple tree or a spider alive?

These are good examples to start with since (most) everyone agrees these are living things.  I think we can check off all of #1-7 of the life list.  You may wonder how trees react to their environment since they aren't quite as responsive as animals.  However, you have probably also noticed that plants will grow or wither in response to water or sunlight.  On a molecular level, plants have pores in their leaves that can open or close for gas exchange.  They will close or open based on stimuli from the environment (such as humidity or carbon dioxide levels).  They both contain cells and genomes and all those things we associate with life.  My gut says definitely alive.


What does your gut tell you?



Clearly my gut has a lot to say about living things.  Well, there are a number of examples of things that my gut is still undecided on.  For example, when a leaf falls from a tree is it still alive?  Well, yes.  In a way.  It does have cells that carry out (most of) the 7 characteristics of life.  Although most leaves cannot reproduce, some leaves can be used to start a new plant.  Eventually, however, each of the cells in the leaf will die and no longer be living.

This post is long enough for now, so I will tackle some more difficult examples in some future posts:

• Are bacteria alive?
• Are viruses alive?
• Are seeds alive?
• Are mitochondria and chloroplasts alive?