What is the exact definition of life?

[Alberts]

I have been learning biology about 15 years. But I cannot gain the exact answer to the question all along__ What is life?
Although this question is seemingly so easy, but in fact there is not an unified theory about it in scientific world. Can you answer to this question in clear words?
What is life?

[mith]

there isn't one.

[charles brough]

I have been learning biology about 15 years. But I cannot gain the exact answer to the question all along__ What is life?
Although this question is seemingly so easy, but in fact there is not an unified theory about it in scientific world. Can you answer to this question in clear words?
What is life?

I deal with this in my book, "Destiny & Civilization." As you indicate, there is a big problem in defining it because there is no clear demarcation between "living" and non-life. I find it in my work when I define what a "society" is and find it can be defined in a way that a life cycle can be detected, that it undergoes natural selection and that it reproduces according to processes that are similar to but not identical to biological processes.

Perhaps, "life' is anything that does have a life cycle, reproduces and is subject to natural selection.

charles
http://atheisticp-science.com

[favorina]

Simple words for a difficult question...
"the capacity for evolution is sometimes taken as the only essential property of life; this definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolize"
Best i found. In the question of 'What is life?" there can never be a perfect ans.

[JorgeLobo]

There is no definition of "life" per se - evolutionary potential not withstanding. Part of the challenge is the context - cell vs. species. This is not a question scientists typically debate. It's like the old saw re. pornography - we know it when we see it.

Have to laugh at charles' shameless/egotistical plug for his book. The question was offered in a biological context and he "deals with it" in a nonscientific text and apparently offers no answer.

[charles brough]

There is no definition of "life" per se - evolutionary potential not withstanding. Part of the challenge is the context - cell vs. species. This is not a question scientists typically debate. It's like the old saw re. pornography - we know it when we see it.

Have to laugh at charles' shameless/egotistical plug for his book. The question was offered in a biological context and he "deals with it" in a nonscientific text and apparently offers no answer.

"shameless, egotistical"??? LOL Why do you get upset? It is "general discussion" and that is what we are talking about. Are you claiming that "life" can be defined only by cellular biology on Earth? That there can be no other form of "life" in the universe? How could you know that?

In one post, it was stated that the term can never be defined perfectly. I would say that holds true of every word and everything. Nothing is perfect. But do you really claim that it is only "we know it when we see it?" To make the term "life" more accurately defined, we should not give up but open up and consider what processes it has to have. What do you think they are?

a life cycle, reproduce, natural selection?

[JorgeLobo]

lol - the guilty dog barks. Not upset - just pointed it out - sorry if your feelings were hurt.

You may not have noticed it but this is a biology forum - the context in which the discussion was offered. You noted that you had nothing to offer (tho you had written a book!). From how many disciplines would you like to say you've no answer?

[Alberts]

I formerly had thought that: life=reproduction.
But some living entities ,e.g. mules, worker bees cannot reproduce. Some nonliving things, e.g. flames, salt crystals can reproduce.Later I noticed that all living things on Earth possess protein and/or nucleic acids. So I had drawn the definition in my view:
life=(protiens+nucleic acids)/proteins/nucleic acids.
But there is a probability that extraterrestrial lives may not based on proteins and nucleic acids. Maybe they based on plastic and rubber. Do you think so?

[JorgeLobo]

Don't think we can assume there is extraterrestial life. But - if it exists- why would it be based on another elemental system. Come on - plastics and rubber? Why do you think that?

[alextemplet]

Plastic and rubber are mostly synthesized from organic compounds, so I doubt if anything not using similar biochemistry to terrestrial life would be able to come up with something like that. However, silicon has many similar properties to carbon, as is H2S similiar to H2O, so it is possible for life to exist that is not based on the chemistry we use.

[Alberts]

I have seen an interesting imagination from an magazine: There may exist a living form so small that they just live on the surface of electron; also there may exist a very large living form that our Galaxy just is an atom of their organism. Considering these, defining life become more and more difficult. Our Sun and other fixed stars, maybe just photons for other living organisms in universe. The proton, maybe just a very large galaxy for other very small living forms. Then how can we define life when we discovered them?

[JorgeLobo]

That's about as silly as anything I've seen here. Life so small it exists on electrons. Tell us - what are the data for that? Was the "magazine" a comic book?