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.

  • 1
    $\begingroup$ 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. $\endgroup$ – bobthejoe Apr 4 '12 at 19:54
  • 1
    $\begingroup$ @bobthejoe: and what they would say it is then? $\endgroup$ – nico Apr 4 '12 at 19:59
  • 2
    $\begingroup$ 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. $\endgroup$ – CHM Apr 4 '12 at 20:06
  • 1
    $\begingroup$ @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. $\endgroup$ – LanceLafontaine Apr 4 '12 at 20:15
  • 2
    $\begingroup$ @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... $\endgroup$ – nico Apr 5 '12 at 6:55

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.

| improve this answer | |
  • 1
    $\begingroup$ I disagree with your conclusions. Otherwise should we say that a virus or a prion is alive? $\endgroup$ – nico Apr 5 '12 at 6:56
  • 1
    $\begingroup$ 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). $\endgroup$ – nico Apr 5 '12 at 7:00
  • $\begingroup$ @nico, I think I have pointed out that viruses do not qualify under the above definition. Likewise as mentioned in biology.stackexchange.com/a/96/389, prions are dependent on host factors. Citations will come after I finish my experiments. $\endgroup$ – bobthejoe Apr 5 '12 at 7:47
  • 1
    $\begingroup$ What about self-replicating computer software? $\endgroup$ – CHM Apr 5 '12 at 16:09
  • 2
    $\begingroup$ OK, I am not sure I necessarily agree with their views but your answer now deserves a +1. $\endgroup$ – nico Apr 6 '12 at 6:06

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.

| improve this answer | |
  • 1
    $\begingroup$ 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? $\endgroup$ – CHM Apr 5 '12 at 16:19
  • $\begingroup$ 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. $\endgroup$ – Fzs2 Apr 5 '12 at 16:33
  • $\begingroup$ 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? $\endgroup$ – CHM Apr 5 '12 at 16:37
  • $\begingroup$ "The Selfish Gene" and "The extended Phenotype" by Richard Dawkins. $\endgroup$ – Fzs2 Apr 5 '12 at 16:44
  • $\begingroup$ 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. $\endgroup$ – bobthejoe Apr 6 '12 at 4:08

Not the answer you're looking for? Browse other questions tagged or ask your own question.