Exceptions of living things NOT composed of cells?

Mark nospam at nspam.com
Tue Jan 27 09:12:21 EST 2004


Larry D. Farrell wrote:
> Des O'Connor wrote:
> 
> 
>>"Mark" <nospam at nspam.com> wrote in message
>>news:6JdRb.520$X2.32765 at news.tufts.edu...
>>
>>>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
>>>
>>
>>Hi Tom
>>
>>This really is a wonderfull topic for a budding Biologist to  put some
>>serious effort into the study of. Id
>>encourage a lot or reading and thinking around this question as you will the
>>encounter many concepts
>>relating to  the majority or core biological areas. An endless feast of
>>science no less. I seem to remeber the
>>topic has been addressed several times  in Scientific American and New
>>Scientist.
>>
>>One interesting approach might be to look at the topic from an ecological
>>point of view rather than pure a biochemcial/structural/molecular
>>viewpoint.
>>
>>At one time I personally considered the  cell  was ''Natures's Wheel''  and
>>anything not bounded by a functional membrane
>>did not qualify as life , these days Im not so certain. I think we would all
>>be interested to know what conclusion you come to
>>after diving into this area.
>>
>>Best Des
>>
>>I wish you luck and joy on your graduate course.
> 
> 
> Mark's post hasn't showed up yet on my server so I am piggybacking on Des's post
> to respond to Mark.
> 
> With the "obligate symbionts," the issue is simply that we have not yet defined
> an artifical medium/environment that will allow them to grow without their
> hosts.  However, there is almost certainly such a set of conditions that would
> allow the to grow independently.  For viruses and viroids, we may actually get
> to something of the same thing one day, when we can construct an artificial
> cell, but the bottom line is that the virus/viroid still cannot replicate
> without the help of some sort of host, whether natural or artificial.
> 
> I do agree with Des that this is an excellent discussion topic, and there are
> folks that come down on both sides of the "living" argument.  My position is
> pretty obvious above.
> 
> --
> Larry D. Farrell, Ph.D.
> Professor of Microbiology
> Idaho State University

This is a good point, you are right - an artificial environment might 
one day be devised, which would be great. Especially for those who want 
to do experiments on these symbionts. However, I wonder if that is 
useful for the 'what is life' debate. Can we use entirely artificial 
conditions to support life, then say that these organisms are truely 
capable of independent living? Would we always need to add the caveat 
that they are capable of independent living in an artifical environment? 
Now, if we could find a place in the 'natural' world where the 
intracellular guys could live freely, then that would certainly help.

Much like others in this group, I find this a fascinating topic. My view 
  has come down on both sides of the argument at different times, and 
now I admit to sitting on the fence. Somedays I like the idea that 
metabolism is important, but then I wonder if that is enough since many 
creatures are not self-sustaining solely with their own metabolic 
capabilities. Humans for example! Having said that, I think that for the 
original poster in this thread, it is worth considering metabolism as a 
potential defining characteristic of life forms.

I guess a good distinction between the viruses and the intracellular 
bacteria could be reached by considering the source of the replication - 
viruses are replicated by using the host (by the host), while the 
bacteria might be considered to be replicated by themselves, but within 
the host. What do we all think about this?

mark




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