Question to all Murcers - What is the (scientific) definition of life ?

[Dogbert]

Ok, we're talking science here, not theology.
Forget soul, desires and all of that nonsense (lawyers have no soul).

So, how would you define life ?

Some problems in advance:

Many living creature don't use oxygen.
Viruses don't eat, they attack and duplicate.
Crystals duplicate themselves yet they aren't considered a life form.
Acid interacts with the surrounding but isn't considered a life form.

Let the brainstorming begin.

[hawke_67]

Hmm, tricky question. To qualify as "living" as far as I know something has to:

1: be able to reproduce.

2: be able to repair itself (heal).

3: grow

4: interact with its enviroment

5: extract energy from its enviroment on one way or another.

6: be able to evolve.

7: be able to adapt to it's enviroment

I know these definitions are a bit "fuzzy" but so are life.

[Wulfman]

Originally posted by Dogbert
Viruses don't eat, they attack and duplicate.

they are not a "lifeform" 'cause they cannot duplicate themself without the help of a cell, IIRC.

the question is really tricky. the "reproduce" - argumentation will fail if not applied to a whole species: a single rabbit is dead.

maybe PICERAS can be used as a basis:

P(rogram), I(mprovisation), C(ompartmentalization), E(nergy), R(egeneration), A(daptability), S(eclusion)

mfg
wulfman

PS.: everything which can die is alive

[Dogbert]

To make things worse, read this:

You've mentioned prions. What are they? How are they associated with the pathogenesis of these diseases?

Prions are the most bizarre infectious agents ever imagined. It was Stanley Prusiner of the University of California, San Francisco, who first discovered the nature of prions and suggested they are the causative agents of the spongiform encephalopathies. For this he won the prestigious Lasker Prize two years ago. Prions (pronounced pree-ons ) are proteins, rogue proteins, and nothing else. They contain no nucleic acid (DNA or RNA). They consist of a single molecule containing about 250 amino acids, termed the PrP protein. They are abnormal variants of proteins that occur normally in cells, such as human brain cells. Amazingly, abnormal PrP proteins, when they enter the body, are able to convert their normal counterparts into more of the abnormal forms. The difference between the normal and abnormal proteins does not lie in their primary structure (the sequence of their amino acids), but rather in their folding -the abnormal PrP proteins are folded in a way that allows them to resist normal protease degradation so that over time this leads to the build up of aggregates of PrP, especially in neurons in the brain. These aggregates resemble the tangles of abnormal protein found in neurons in Alzheimer s disease patients, but as in Alzheimer s disease, we do not know how the presence of these tangles causes neurologic disease.

Prions are the only "life forms" that break the great "central dogma" of biology. That is, we have come to expect that all life forms from viruses to bacteria to plants to humans to hand down the blueprints for all their progeny via their DNA (except for some viruses which carry their blueprints as an RNA genome), and we expect that the process for converting the blueprints into building blocks must involve replication of DNA, transcription of the message into RNA, and translation of the RNA s message to form proteins, the building blocks of cells, tissues, organs and whole organisms. Here we have life forms where abnormal proteins, the PrP proteins, direct the refolding of normal proteins just by direct contact.

PrPs from the various spongiform encephalopathies have been sequenced and found to differ, in some cases by very little, in some cases by quite a bit. For example, recent research has shown that the scrapie PrP protein differs from the BSE PrP protein at only seven amino acid loci, whereas the BSE PrP protein differs from the human CJD PrP at more than 30 loci. These differences explain the concept of strains and help explain why prions from one species might jump more easily into another species than another. It is difficult to find the terms to discuss prions -- for example, can we talk about mutants when there is no DNA? What would Watson and Crick think of all this?

[thop]

pretty easy to answer: if it lives, it is alive. seriously

[Dogbert]

Thop, how would you know IF it lives ?

[mdhome]

Dogbert
We seem to have this instinctive grasp of what is alive and what isnt based on our day to day existence. It could be that the current term 'alive' is too restrictive and that we have to define new terms/categories for some of the more bizarre creations in the universe. Maybe some of these categories are just as valid and as important in the grand scheme of things as living organisms (assuming that we are import). When we eventually reach out into deep space we may be surprised by what is out there.


Regards MD

[Dogbert]

Forget us, forget our perspective, try giving a scientific definition that'll seperate living matter from non living matter in the universe.
What are the characteristics of life. Try giving a check list like hawke_67 did. Kind of "if you answered yes for at least X questions - YOU ARE ALIVE !!!"

Then check yourself to be sure there isn't any non living thing sthat could pass as being alive according to your list.
IMHO you could be hospitalised if you suddenly decided that your car is alive and stopping it's engine would be murder

[mdhome]

Originally posted by Dogbert
Forget us, forget our perspective, try giving a scientific definition that'll seperate living matter from non living matter in the universe.

Thats the point, I dont think we really know what living matter is. All we have is this fuzzy definition based on our own experiences.

REgards MD

[hawke_67]

Maybe we can all be happy with the explanation that anything that reproduces itself AND preforms some sort of action to protect and/or prolong it's own existense (not sure about the spelling there), grows and evolve is alive?

[Admiral]

Thats the point, I dont think we really know what living matter is

And what if it's energy ?

The only thing we can do is keep open minded and if we come up with a relative scientific definition of life, not apply it in an "inquisitve" manner, meaning: that ain't alive because it doesn't fit the criteria of the definition we've busted our heads for so long and... well, we don't like it proving us wrong.

[Wulfman]

Originally posted by hawke_67
Maybe we can all be happy with the explanation that anything that reproduces itself AND preforms some sort of action to protect and/or prolong it's own existense (not sure about the spelling there) is alive?

what about fire?

mfg
wulfman

[KRSESQ]

Perhaps add "Cabable of Deliberate Action" to the list.

Now define "Deliberate Action."

Viruses invade healthy cells in order to reproduce. This would be "deliberate action" for our purposes. So would an ameoba adsorbing food, or Mountain Goats butting heads over a mate. Fire, on the other hand would not be considered a life form because its action is not "deliberate." Combustion is sustained by chemical reactions that are orders of magnitude simpler than the chemical processes governing even the simplest life forms (Prions, for example).

Organizational complexity might be another criteria. Fire is deceptively simple in its organization. Even the simplest recognized life forms are significantly more complex.

Kevin

[hawke_67]

Good question, what about fire? It grows, it reproduces (sparks starting new fires) so it sure can look like its alive. But it does not:

1: react to dangers (protect its excistance)

2: evolve

3: adapt to its enviroment

4: interact with its enviroment

[mdhome]

Theory of entropy states that as the universe gets older, it becomes less organised. Isnt life the only known mechanism that reverses that process?

Regards MD