Definition of Life

[science_spot]

There are many different definitions to the term "life".  Doctors use one definition to determie if you are alive or dead, or brain dead as the case may be. Your life is also the period between your birth and your death.  There is also a biological definition of life.  We would like an official definition so we all have an understanding of the expections of the word and the limits to its meaning.  As with anything in biology, it really isn't that simple.

 

So what is life? 

Is an organism alive because it can move around?  This works for animals and some bacteria, but plants don't usually move (yet they are alive).

Is an organism alive because it can communicate with others around it?  Higher animals obviously communicate with each other, and some plants do too.  How can we be sure with smaller organisms such as bacteria or yeast, which are also alive?

Is an organism alive because it can produce more of itself?  Many organisms do reproduce, but if an individual is sterile, does it cease being alive? Of course not.

 

There is no universally accepted definition of the word "life".  The most commonly used definition (by biologists) describes life as an object that has biological processes that it sustains by its own means.  Different types of life have different ways of performing these functions.  An organism could be considered alive if it sustains most or all of the following attributes:

Metabolism - converting a food source to energy

Growth - increasing in size

Organization - having structural purpose

Reproduction - production of new individuals

Homeostasis - regulating the internal environment (temperature, componet concentrations, etc)

Response to stimuli - a reaction to some action

Adaptation - change to the organism in respose to the environment

 

The definition above still leaves alot of room for debate, and the topic of viruses is generally at the top of the list.  Most people will say that a virus is not alive because it cannot reproduce by itself (a virus requires some proteins in animal cells to reproduce viral particles).  At the same time a mule is sterile, but still considered alive.  Alternatively, I could reproduce a dead human male in a petri dish (IVD), and yet the man would still not be alive.  A child that has not gone through puberty cannot reproduce, although it does have the capacity to do so in the future.  Is it still alive?  I would say so.

In addition, a virus does not increase in size.  An adult human does not either, so is it not alive?  Does the fact a human USED TO increase in size (during development) count toward meeting this definition?  Does this terminology refer to individuals only or to species as a whole?

 

With so many questions left unanswered, there is not likely to be a universal definition of biological lilfe any time soon.  However, an essay published in Science in 2002 offers a unique alternative definition, which I like quite a bit, if you make the minor change that meeting these pillars does not need to be done single-handed.  I really prefer to view a virus as alive, because it really is NOT an inanimate object.  It just requires a little help from a cell to reproduce.  this is not necessarily a flaw, as we would all agree viruses are very hardy and successful organisms.

 

I am interested to hear the thoughts of others regarding this topic. What do you think?  Are these things alive?

Virus  (You may also be interested in my post How to Kill a Virus)

A seed

Mycoplamsa

Prions

 

Tonya

AKA sunmoonstars

The Science Spot

 

[You_Are_Invited]

Oct. 7, 2009 at 5:49 AM

once again, you are channeling my son. =D

In his science lessons, we have been going over the difference between living and nonliving things. Living being classifed as changing, growing and needing food/air/water plus reproduction. Well the darling kid, with all the eloquence at his disposal, launched into a dissertation tonight about why he think the fan should be considered a living object. It uses electricity as food and it filters air, it grows (taller) and it moves and therefore changes. Of course he told this to his dad so I didn't get it on video, but of course dad was stumped and every argument he came up with to verify the fan's nonliving staus got shot full of holes.  If he put as much effort into remembering the house rules as he does science, I would have a perfect kid!

Great journal!

[science_spot]

Oct. 7, 2009 at 6:27 AM

Hey, you have the makings of a great lawyer there, too :D

Awesome story!  Your kid sounds like a charm!

(Original Poster)

[Tygermommanikki]

Oct. 7, 2009 at 9:40 AM

I remember once another student in class argued with the teacher for about 20 minutes that fire was not living. Cause if it was then firefighters weren't heros they were murderers...

Then the teacher said well does a fire eat...yes what ever is in its path trees, houses, does it grow?

So 13 years later the guy is still mad at fire fighters for murdering fires...

[science_spot]

Oct. 7, 2009 at 10:14 AM

Tyger - that is an interesting debate.  I don't have an argument against that one!

(Original Poster)

[DestMasters]

Oct. 7, 2009 at 10:45 AM

I do think a virus is alive.  Yes, it needs help to replicate but so do humans. One human can not reproduce without another human.  I know that's splitting hairs but it's all I've got. :)

Part of my thought process was this......viruses do need help for reproduction but they're also "smart" in that they can mutate to survive. The HIV virus, for example, will find ways to become immune to specific anti-viral medications so it can continue to replicate in its host. 

Nature finds a way.

[caitxrawks]

Oct. 7, 2009 at 11:41 AM

I love you =)

I definitely think a virus is alive. Maybe not in the same sense that we're alive, but it's definitely a living thing.

[M2TandM]

Oct. 7, 2009 at 1:19 PM

An interesting journal.

We teach MRS NERG at school -

Movement

Respiration

Sensitivity

Nutrition

Reproduction

Excretion

Growth

[sweetheart1985]

Oct. 7, 2009 at 4:46 PM

That's very interesting and thought provoking!

It will definitely be on my brain for a while =D

[evwsquared]

Oct. 7, 2009 at 5:26 PM

For the fire, I would argue that while it can grow, it can't reproduce. I assume that when a living thing reproduces, that offspring exists as a separate being. Yes, fires can jump to distant places by wind, etc., but they do not produce new individuals; it's kind of more like cloning themselves.

If you take the Science essay and apply it to fire, fire fails the first pillar: there is no programming to it. It fails the second pillar, as fire can only exist in one state. It fails the third pillar, in that it is not contained in any way.  Etc.

[science_spot]

Oct. 7, 2009 at 9:14 PM

evws - yes, but if I made clones of myself they would be alive.

Regarding the Science assay - I think that is an accurate interpretation.

(Original Poster)