What's Alive?

[N S]

How do you say a thing is living or non-living? I would say that it is something that tries best to maintain itself in such a condition. But what is "such" a condition?

[SebastianOakes]

The more accepted definition is, as far as I'm aware, the idea that anything living has the ability to self sustain: metabolism, growth, reproduction, adaptation to environment etc. This may be some answer to your question on the 'condition', since the organism is balancing its existence with the environment around it, rather than being a part of it. Of course this is a loose definition, mainly because no definitive list has been decided upon, other than simply describing features that life on our planet exhibits/shares. In some cases, such as with viruses, many of these conditions are met, but not enough for them to be considered 'alive' in the same sense as most life on earth, which poses some very big questions.

The origin of life is another broken aspect of our understanding. If the theories of life simply springing into life from simple chemical compounds is true, then i see no reason why life couldn't spring up elsewhere if the conditions are there, though again what is to say these other 'living' creatures/molecules/whatever bear any similarities to our definition of life at all? If we ever came across life this different, we may be incompatible in ways we can't yet comprehend.

[Civat]

I would say everthing it is.. it does not need to have a heart or pulse

[Moontanman]

http://en.wikipedia.org/wiki/Life

 

 

 

Life (cf. biota) is a characteristic that distinguishes objects that have signaling and self-sustaining processesfrom those that do not,^[1][2] either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate.^[3][4]Biology is the science concerned with the study of life.

Any contiguous living system is called an organism. Organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means.^[1][5] A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- andwater-based cellular form with complex organization and heritable genetic information.

Edited February 5, 2013 by Moontanman

[N S]

Thanks...that was clarifying. But there remains one oddity - what are viruses according to these definitions of life.

[Moontanman]

From the same link...

 

 

 

Viruses

Electron micrograph of icosahedraladenovirus

Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life,"^[42] since they possess genes, evolve by natural selection,^[43] and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and they require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.^[44][45][46]

[CharonY]

It should be noted that life is not an easily measurable property and as such precise definitions are going to be iffy. Especially as the definitions are exclusively based on what we found on earth so far.

I would take it more as a guideline rather than a strict definition. But then this is true for much of biology in general. Squishiy things do strange things, as I like to say.

[Dekan]

A flame might have some claim to be a living thing.

I mean, suppose you light a candle, then watch what happens. You see the following:

 

1. The flame starts quite small - it's "born". Then it gets bigger - ie it "grows".

2. As it grows, it increasingly absorbs the melting candle-wax, which could be regarded as the flame's "food". So the flame is "feeding".

3. The flame also absorbs oxygen from the air - a kind of "breathing".

4. When the flame attains its maximum size - its "adult" stage - it reacts to its environment. For example, it responds to stray draughts of air, by flickering or dancing about.

5. The flame can ignite other candles. If another candle wick is inserted into the flame, that wick is lit, and produces another flame. This could be regarded as "reproduction".

6. Finally, when the original flame has used up all the wax in its candle, it goes out, and "dies".

 

Thus the flame seems to exhibit many characteristics of a living being - birth, growth, feeding, breathing, reproduction, and death. Is this too facile, or could flame really qualify as a form of life?

Edited February 6, 2013 by Dekan

[imatfaal]

Dekan - that's why many people would include the concept of responding to enviromental changes through a process of evolution by natural selection as a necessary part of life