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Please see comments as to the appropriateness of this question on biology SE.

All known life on Earth is made up of cells. It is thus safe to say that all known life is characterized by the presence of cells. But is having cells as a basic unit a requirement for life?

I'm thus asking if life is currently existent and/or possible without biomembranes, and to what degree cells actually matter in our present understanding of life.

An all-encompassing, comprehensive and cited answer would be very much appreciated.

LanceLafontaine
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  • You're basically asking for the definition of "life". – CHM Apr 04 '12 at 18:40
  • I'm asking whether or not cells are part of the definition of life as we know it, or all potential life. Is that a bad thing? Clearly related to biology. – LanceLafontaine Apr 04 '12 at 18:56
  • And in some respects, the question is similar to this one:http://biology.stackexchange.com/questions/90/life-without-dna – LanceLafontaine Apr 04 '12 at 18:58
  • I think it's a fair question - perhaps very closely related to "life without membranes". – Rory M Apr 04 '12 at 19:41
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    Well at the moment, there isn't a question related to what is the definition of life. Meanwhile, I think that most evolution of life specialist will say that no, cells are not the basic unit. – bobthejoe Apr 04 '12 at 19:54
  • Maybe I should ask another question about the definition of life ahah! Although I feel that it'd be more of a debate than anything... – LanceLafontaine Apr 04 '12 at 19:57
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    @bobthejoe: and what they would say it is then? – nico Apr 04 '12 at 19:59
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    The impression I get is that the inquirer hasn't put much thought into his question. If you were a kid, I wouldn't mind of such a question, but you're not. It's exactly because there's no clear cut definition of life that your question is in fact not a question. – CHM Apr 04 '12 at 20:06
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    @CHM, just because a question does not have a clear-cut answer, it doesn't mean it isn't a valid question. Exploring such questions and explaining why or how is what scientific thinking is about. – LanceLafontaine Apr 04 '12 at 20:15
  • I've flagged this as off-topic. I could go on and explain the reasons, but I think it should be pretty clear. – CHM Apr 04 '12 at 20:38
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    @LanceLafontaine: I do not think lack of a clear-cut answer is a problem per se. The impossibility of giving a proper scientific answer is, though (for questions on this website, that is). Calling on the possibility of undiscovered extraterrestrial life which is not cell based is like claiming you have an invisible, incorporeal, floating dragon who spits heatless fire in your garage... – nico Apr 05 '12 at 06:55
  • @nico and CHM, I can see your points. I'll go ahead and edit the question for it to read something less fantasy/science-fiction based ahah. – LanceLafontaine Apr 05 '12 at 17:08
  • Lance, I don't think you're getting the point of my objections. This is not about you or your question: its about maintaining this stack's purpose and quality. You will not get an "all encompassing, comprehensive" answer from anyone - discussion is all that can be done about it. Before even drafting an answer, you'd have to define life. I suggest to close this question and move discussion to the chatroom. I also suggest you go talk with your professors, I'm sure they'll have things to say. Again, don't think this is not interesting, it's just inconsistent with this site's purpose. – CHM Apr 05 '12 at 20:50
  • As a point of community management and education: This question is not appropriate for THIS type of Q&A. It may be an interesting discussion and even educational, but we forgo these types of question to preserve the purpose of building a canonical knowledge base. Questions should be specific problems you might encounter in your day to day work. But the question above is really a "discussion starter" likely to solicit extended discussions, opinions, and debates. That type of activity is better left to the chat rooms and discussion forums, and not really what we do here. – Robert Cartaino Apr 06 '12 at 19:12

2 Answers2

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According to Gerry Joyce: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

From a meta-analysis of 123 definitions of life: "Life is metabolizing material informational system with ability of self-reproduction with changes (evolution), which requires energy and suitable environment."

According to Alexander Oparin: “Any system capable of replication and mutation is alive”.

At hand are some key elements in order to match these criteria. Maintaining a Darwinian cycle requires replication, mutation, and selection. Thus, we can break down the above into 5 criteria (personal communication with Gerry Joyce).

  • Life stores information
  • Life reproduces its information
  • Life alters that information
  • Life does something with that information (uses energy)
  • Life does all of this in a self-sustained manner

I would point out that the above criteria is quite different from what is currently on wikipedia described by metabolism and homeostasis. There are certainly additional criteria that certainly raise the threshold for what may be considered life. The common discussion revolves around viruses which do many of these things but not in a self-sustained way.

The question at hand then asks if cells are the minimum unit of life? What makes a cell a cell is that there is compartmentalization. The underlying reason behind this compartment is due to the necessity of tying the phenotype to the genotype. Paraphrasing using our definition of life, it links the information with the function that the information carries out. In the modern biological scheme, it keeps the proteins (phenotype) with the DNA (genotype).

The necessity of compartmentalization is negated when the phenotype is already linked with the genotype. The most frequent example is RNA where the material that carries the information is also the material that carries out its function. It tend, is reasonable to hypothesize that life can be made entirely with RNA without the need for compartmentalization (although compartmentalization certainly helps see Paegal and Joyce and Chen and Szostak).

Recent experiments by Gerry Joyce and others have been able to satisfy several of the requirements of life. The have self-replicating RNAs, that store information, that reproduce their information, that introduce alterations to their information, and do it in a self-sustained manner. What Gerry and his colleagues agree on is that their current self-replication Ribozyme system doesn't do anything particularly novel. However, by introducing a larger variety of functional elements to their ribozymes perhaps they will.

bobthejoe
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    I disagree with your conclusions. Otherwise should we say that a virus or a prion is alive? – nico Apr 05 '12 at 06:56
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    Also, please link to the sources of the papers that you mention and give relevant extracts of the authors conclusions about ribozymes being a life form (or otherwise clearly state that that is your personal interpretation of their results, which is absolutely fine, of course). – nico Apr 05 '12 at 07:00
  • @nico, I think I have pointed out that viruses do not qualify under the above definition. Likewise as mentioned in http://biology.stackexchange.com/a/96/389, prions are dependent on host factors. Citations will come after I finish my experiments. – bobthejoe Apr 05 '12 at 07:47
  • sorry I had missed the sentence about viruses – nico Apr 05 '12 at 07:53
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    What about self-replicating computer software? – CHM Apr 05 '12 at 16:09
  • @CHM intriguing. I guess we have to start considering the battle against entropy. – bobthejoe Apr 05 '12 at 19:49
  • Where are you getting those criteria from? – Preece Apr 05 '12 at 20:19
  • @Preece Gerry Joyce. – bobthejoe Apr 05 '12 at 20:47
  • @bobthejoe His website is disappointing :( – CHM Apr 05 '12 at 20:54
  • @nico, CHM, Preece. You guys better be satisfied. – bobthejoe Apr 06 '12 at 04:04
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    OK, I am not sure I necessarily agree with their views but your answer now deserves a +1. – nico Apr 06 '12 at 06:06
  • @nico: As someone who studies mathematical modeling of ecology and evolution, I see no reason not to consider viruses to be alive. They pass the duck test for it: they replicate like life, they consume resources like life, they evolve like life. True, they depend on a host to do so, but so do all parasites. For that matter, all heterotrophs, including us humans, are just as dependent on other life forms — none of us can live on just water, air and sunlight. – Ilmari Karonen Apr 06 '12 at 15:35
  • @Ilmari Karonen: there is a subtle difference between a parasite and an obligate parasite – nico Apr 06 '12 at 16:17
  • This discussion thread is begging to be closed. The chatroom isn't used, people. Why not move there and have a very interesting talk about this subject, where it belongs? – CHM Apr 06 '12 at 18:12
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Life is a physical entity that creates copies of itself,
sometimes in a slightly changed form.

That's it.

A cell is not the basic unit of a life but a large number of molecules that have bonded together to reap the benefits of specialization. Like what is happening today, humans coming together to form a society which collectively starts to function like a single living organism again.

So no, a cell is a far cry from being the basic unit of life, it is a very advanced form of life.

All it takes for life to start is for a single self replicating molecule to form, that's it.
Evolution takes over from there.

john-jones
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    Molecules aren't "alive". An intricate synergy of a mind-blowing quantity and variety of molecules can be. Would you consider H2O a self-replicating molecule, since it spontaneously undergoes dissociation to form H+ and OH-, at equilibrium? – CHM Apr 05 '12 at 16:19
  • Not all molecules are alive, but molecules that produce copies of themselves are. H2O is not alive,when it undergoes dissociation there are no extra copies of it being created. – john-jones Apr 05 '12 at 16:33
  • Agree. This is extremely interesting, but I just feel like it belongs in a forum, not a Q&A, since there's no answer to be had, only discussion. Mind giving some sources where I can read more on self-replicating molecules? – CHM Apr 05 '12 at 16:37
  • "The Selfish Gene" and "The extended Phenotype" by Richard Dawkins. – john-jones Apr 05 '12 at 16:44
  • Now that I've actually sat down and thought about it, this definition does fit; +1. H2O can't be life since it doesn't add any variation to itself. – bobthejoe Apr 06 '12 at 04:08
  • Well, that's only partially true - while H2O does not create extra copies of itself after dissociation, there can be a difference in its quantum state before and after the dissociation, which constitutes a variation. – CHM Apr 06 '12 at 17:47