Ask A Scientist General Science Archive

name         Jason R.
status       educator
age          30s

Question -   I have two questions.  Actually, the first question that
I am going to ask has already been posted on your site; it has also been
answered.  My question is what is the scientific definition of life?  I
learned in my schooling that "life" is simply made up of one or more
cells.  Cells can do all of the things that were listed in your
answer.  On that note, is a virus scientifically accepted as being a form
of life, or is it considered to be innate?  If it is considered to be
alive, I believe Lamarck's system made need to be updated to include
proteins and such as a form of life.  As far as I know the lowest form of
life is bacteria.  From some of my readings however, I have read that
some viruses are classified as bacteria or a combination of a virus and
bacteria.  Is it that scientists still don't fully understand these
micro-organisms therefore making it difficult to classify them as a form
of life?
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I know of no acceptable definition of life in that it cannot be logically
replaced by a nonliving system. Viruses are not considered living since they
do not repire without the help of a cell.

PF
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Living organisms are made up of one or more cells, can grow and develop, reproduce, respond to stimuli, 
and have a metabolism. This list of characteristics was made by scientists after carefully considering 
what would be included, and excluded, by these characteristics. Using this list, viruses are not 
defined as living organisms. There are no viruses that are classified as bacteria. There are viruses 
that infect bacteria, just as there are other viruses that infect human cells. Incidently, prions, 
which cause mad cow disease, are also considered non-living under this definition.

Also, I believe you mean Linnaeus, who came up with our current system of classification, and not 
Lamarck, who proposed a now-discredited theory of inheritance.

Paul Mahoney 
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I suppose if you want to say that anything that is cellular is considered living because cells have 
certain characteristics, that would be the simplest definition of life.  Most beginning biology texts 
give a certain number of "characteristics of life" that include being made of one or more cells, 
capable of reproduction, responding to the environment, adapting and changing, requiring a source 
of energy, and growing and/or developing.  Other books give more or less.  Viruses don't meet all 
of the characteristics of life, consequently, there is a debate in the scientific community about 
whether to classify them as living or not.  I had a professor once who said that since they were 
capable of getting themselves reproduced, they were alive.  But they aren't cellular and they don't 
metabolize or require a source of energy.  They reproduce, but only within a host cell.  I suppose
you could say that assembly of viral subunits into a complete virion would qualify as development, 
and their nucleic acids do mutate, so they adapt and change.  I have never heard of viruses being 
classified as bacteria or vice versa, but there are organisms known as mycoplasmas and rikettisias 
that don't fall specifically into the bacteria category.  I wouldn't call them viruses though.  And 
when you speak of LaMarck, did you mean Linnaeus?
 
vanhoeck
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