Exceptions of living things NOT composed of cells?

[Mark]

Larry D. Farrell wrote:
> Scott Coutts wrote:
> 
> 
>>Tom Williams wrote:
>>
>>>I'm an undergraduate student who's curious about
>>
>> > something mentioned in an anatomy lecture:
>>
>>>My professor said that it was possible to have
>>
>> > life without a cell, or cells.
>>
>>>How can this be??? Can somebody please explain
>>
>> > how, or even if this is true?
>>
>>Hi Tom,
>>
>>It depends on how he defines 'life'. This always instigates a big
>>discussion on what 'life' actually means.
>>
>>He's probably talking about anything able to replicate, so then he's
>>including viruses. Viruses are able to replicate, but as far as I'm
>>concerned, this is not 'life'... I dont regard viruses as 'living'. If
>>you're interested, you might like to look up viriods and prions -
>>they're able to replicate, but they're smaller than viruses... They're
>>single molecules!
>>
>>Scott.
> 
> 
> However, none of these are capable of replicating independtly of other
> organisms, which is pretty much a requirement for "life" as it is
> generally defined.  (Note, of course, that most eukaryotes need another
> organism in the form of a mate to initiate the process of replication but,
> once begun, the process does not depend specifically on the other organism
> providing on-going functions for the process to continue to completion.)
> Viruses and viroids are essentially inert unless placed into (or at least
> in very close proximity to) host cells that provide the essential
> functions and materials needed for replication, while prions produce
> copies of themselves only if provided with properly folded copies of the
> prion protein, usually provided by living host cells.
> 
> --
> Larry D. Farrell, Ph.D.
> Professor of Microbiology
> Idaho State University
> 
> 

Of course, we start to get into muddy water with this kind of definition 
when we consider endosymbiotic bacteria such as Buchnera, which I think 
(I might be wrong) is an obligate symbiont - can't live independently of 
the host (aphids in this case). I sometimes wonder if this is really all 
that different to a virus - both seem to grow and reproduce, but only 
given a cellular environment to do it in. Someone was suggested to me 
that the cellular environment that is so essential to these creatures 
might be analogous to the terrestrial environment that we depend upon - 
once outside of it we are not all that good at reproducing or surviving. 
Not too sure about that myself though. Thoughts?

mark