WHAT IS LIFE ?

[Freethinker]

Originally posted by: TeleMad
On a related note...there are three classes of things: living, non-living, and dead. Living things are, well, living. Examples include bacteria, plants, and animals. Dead things were once living, but have died. A good example is roadkill (it is "techincally incorrect" to call roadkill non-living).

Would it even be technically correct to call road kill "dead"? It is teaming with LIFE.

Not to pick on road kill specifically (it already is subject to prejudice), but it seems most living things are actually a composite of many other living things. This is how I started my thought journey RE "living matter". Some living organisms (such as animals) have other living organisms with-in them on a symbiotic level. Animals, in fact, can not live without some of these symbiotic organisms. Each could be considered alive in and of themselves. Commensalism in fact is positive symbioltic relationship between organisms. There is fungus around plant roots that help the plant extract minerals from the soil. We have bacteria in our intestines that are benefitial to our processing of food. OTOH they would not exist if not for us.

In some ways this would even seem to extend to sub-cellular level. Such as mitochondria. Because it contain ribosomes and it's own genetic material it can almost be defined as it's own life. It "lives" in a close commensalism relationship with the rest of the cellular structure, just as the bacteria in our intestines.

To the other extreme, we are a parasite on earth's ecosystem in (sometimes) a commensalism level.

[GAHD]

Originally posted by: TeleMad
No, the individual atoms and molecules are not alive. For a person or a tree - in fact, for all objects biologists agree are living - the smallest unit of matter that is living is a cell. Smaller units of matter - organelles, macromolecules, atoms, nucleons, quarks - are mere constituents that make up the cell(s).

So a virus(basically a complaex molecule) isn't alive?

[Bo]

A definition i quite like (taken from: http://www.hyperdictionary.com/dictionary/life)

3. (Philos) The potential principle, or force, by which the
organs of animals and plants are started and continued in
the performance of their several and co["o]perative
functions; the vital force, whether regarded as physical
or spiritual.

Wikipedia (http://en.wikipedia.org/wiki/Life) Also gives some nice definitions -Bo

[Uncle Martin]

Originally posted by: GAHD

So a virus(basically a complaex molecule) isn't alive?

A virus is often the line that Freethinker mentioned. Biologists are divided on whether or not a virus is alive, because the definition of life is, to simplify, that which metabolizes. A virus in itself does not have a metabolism until it combines with a "living" cell host. That is pretty much the extent of my knowledge on this. Sorry I can't be more informative.

[TeleMad]

FreeThinker: Commensalism in fact is positive symbioltic relationship between organisms.

No, that's mutualism. In commensalism, one organism benefits while the other neither benefits nor "suffers".

[TeleMad]

Well, here's something I wrote up in my personal notes about viruses...

Viruses
Viruses are non-cellular, infectious agents that are unable to reproduce by themselves, and must, therefore, highjack the synthetic machinery of a host cell in order to produce more copies of themselves– they are non-cellular obligatory intracellular parasites. Viruses consist of a nucleic acid core (either DNA or RNA, but never both) surrounded by a protein coat called a capsid. Some viruses, called enveloped viruses, also have a lipid membrane covering part or all of their capsid.

Two methods of “reproduction” exist for viruses: the lytic cycle and the lysogenic cycle. In the lytic cycle, the viral genes immediately overtake a host cell’s translational machinery and release of the hundreds or thousands of newly produced viruses involves the death of that single infected host cell. For these viruses, their genome – which the host cell processes – contains genes for lysozyme, an enzyme that breaks down bacterial cell walls. This gene is expressed late (after most copies of the virus have already been produced) and leads to the lysing (“splitting”) of the host cell, releasing the viral particles. On the other hand, in the lysogenic cycle the viral genes remain dormant for some time and death of the originally infected host cell does not occur. For these viruses, their genes become incorporated into the host cell’s DNA and are processed normally: they get duplicated along with the host cell’s DNA and during cell division even get passed on to progeny cells along with the host cell’s DNA. During the lysogenic cycle, viral genes spread throughout a population of host cells without causing (immediate) death to their hosts. Later, some stimulus may activate the viral genes of one or more descendant cells and lead to the lytic cycle in them (overtaking of those host cells’ translational machinery to produce hundreds or thousands of copies of viruses in each, followed by the lysing of those host cells to free the newly assembled viral particles). Regardless of the method of “reproduction”, any released viral particles are free to infect other cells with which they come into contact. For the most part, a virus outside of a cell, called a virion, is completely inactive and displays no signs of being alive (it is non-cellular and does not reproduce, metabolize, move, grow, or respond to stimuli).

Viruses straddle the line between the living world and nonliving world, and their classification into one or the other has caused division among scientists for years. Some sources state that viruses are indeed living, others state that they are not (but rather just highly-complex organizations of organic molecules), while yet others are noncommittal (referring to viruses as “quasi-living”).

According to the cell theory, viruses are not living since they are not composed of one of more cells. Those that favor the self-replication definition of life also should view viruses as inanimate since they cannot reproduce by themselves – they absolutely require the assistance of host cells. Concerning the basic characteristics of life, viruses posses hardly any – they don’t grow, develop, move, or metabolize.

However, those characteristics that viruses do possess seem to be relatively key ones. Viruses contain at least two of the four organic compounds found in all life forms (proteins and either DNA or RNA), can reproduce (although not unaided), and they can in a sense respond to their environment because they can dock to an appropriate host cell and inject their genetic material (some viruses infect cells in other manners) and their DNA or RNA is subject to genetic mutations. Thus, with reproduction (of sorts) and genetic mutation, they can evolve (but not unaided).

It appears that as of late, the consensus may be leaning towards the inclusion of viruses into the family of the living, but the issue if far from settled. For example, viruses are not now, nor have they ever been, assigned to

[Freethinker]

Originally posted by: TeleMad

No, that's mutualism. In commensalism, one organism benefits while the other neither benefits nor "suffers".

You are correct, my bad.

Thankks for correcting it.

[Freethinker]

Originally posted by: TeleMad
Viruses straddle the line between the living world and nonliving world, and their classification into one or the other has caused division among scientists for years. Some sources state that viruses are indeed living, others state that they are not (but rather just highly-complex organizations of organic molecules), while yet others are noncommittal (referring to viruses as “quasi-living”).

Sounds as much like an etymological disagreement as anything.

The very topic of this thread.

[TeleMad]

TeleMad : On a related note...there are three classes of things: living, non-living, and dead. Living things are, well, living. Examples include bacteria, plants, and animals. Dead things were once living, but have died. A good example is roadkill (it is "techincally incorrect" to call roadkill non-living). ...

Freethinker: Would it even be technically correct to call road kill "dead"? It is teaming with LIFE.

Yes, it would be to correct to call roadKILL dead. Run over an armidillo and kill it and...it's roadkill...it's dead. Doesn't take an Einstein to figure that out.