Sunday, November 05, 2006

What is Life?

As one takes an evening stroll, one can probably distinguish between all the living and non-living objects that one encounters. In spite of a classic book by Erwin Schrodinger, with a title that seems to ask one for a definition of life, published in 1943 [1], the scientific community has still not been able to come up with a single answer to this fundamental question that satisfies every scientist. Part of the reason for this ambiguity is because, to date, there remains a controversy over which objects should be considered as living beings [2]. For example, can a virus be considered as a living being?

But, first let's try to discuss some of the traits of living things:

1. Metabolism: A living being consumes energy from the surroundings by converting one form of energy to other forms of energy by a process called metabolism. Metabolism, the Greek word for change, designates all the chemical reactions carried out within a living organism.

2. Organization: The energy gained from metabolism helps organisms to remain far more organized than non-living things. Organization here refers to the fact that one can not reduce an organism into smaller independent parts. All living organisms are formed of the basic biological unit called the cell. Within each cell, there are membranes that divide the living world from the non-living world and within the membranes, the cellular constituents are organized hierarchically to form a live entity. All the molecular constituents within the cell serve a function. These molecules are organized into an integrative system and serve the activities of the cell as a whole. Some people even argue that keeping this organization going is the basic entity of life, and the minute an object is dead, this organization is lost. One can study independent parts (as molecular biology) or cells for that matter, but in reality, life as we know it, can not exist without being organized at various levels hierarchically.

3. Reproduction: Living things can reproduce on their own to produce new organisms of the same kind. The instructions to reproduce are also inherent within an organism and are inherited by each generation from their parents.

4. Evolution: Living things are able to evolve over time on their own according to their environment. They evolve due to the occasional errors that crop up while copying the instruction from one generation to another. These errors track changes in the environment and an organism that is better adapted to the environment survives. Darwin's central contention was that this adaptation stems from the interplay of random variation and natural selection. So, the history is as important as organization to understand the workings of the present day organism.

An object is traditionally considered to be living if it has all the above characteristics [3]. In addition, the definition is applied at a global level to a whole species and not to individual beings [4]. In other words, sterile organisms are also considered to be alive even though they may have lost the ability to reproduce.

Non-living things may have one or more of the above mentioned traits, but do not possess all the above mentioned characteristics. For example, a flame can use up energy and convert chemical energy to light and heat energy, using up energy in this process. However, it can not reproduce on it's own and neither does a flame evolve according to it's environment.

Viruses on the other hand are a little more difficult to distinguish. They can evolve and they can reproduce (albeit, inside another organism and not on their own), but they do not possess any metabolic capabilities, and hence, it may be argued, should be considered as not living. A small minority of the biologists have postulated that the abilities to reproduce and evolve are the only criteria for life, and that viruses should hence be considered alive.

Seeds also form an interesting example. Do we consider seeds as living or non living? Well, I did a google search and they are considered to be alive. They certainly have the ability to reproduce and, hence, evolve under the "right conditions". In addition, they are as organized as a living organism, but the real question was whether metabolism takes place in a seed under dry storage conditions. I was pleasantly surprised to find many papers reporting that seeds do undergo metabolism even during storage (an example is [5]), and hence, they do have all the criteria to be considered alive.

Physicists and chemists tend to argue over whether all the four properties are really required for life. While some chemists argue that metabolism is the real criterion for life, physicists argue that the level of organization in a cell is what really demarcates the difference between a living cell and a non-living cell. In fact, an algebraic information theoretic framework was developed to define the amount of information required to define an organism and the amount of organization in an organism [6].

What should be considered as living is not only an academic issue, but is equally important for space probes that look for signs of extraterrestrial life. In addition, it is equally important when one studies the origin of life from non-living entities. When does one consider that there is enough complexity in a system to call it a living cell? I will continue this post with a post on the quest for the origin of life and also, on a separate series of posts, on molecular evolution of living organisms.

References:
[1] What is Life? by Erwin Schrodinger.

[2] Chapters 1 and 2 of The Way of the Cell by Franklin Harold.

[3] Wikipedia entry on Life.

[4] Brittanica Encyclopedia.

[5] Metabolic activities of dormant seeds during dry storage. Naturwissenschaften, 59:3, 1972, 73-74.

[6] Toward a Mathematical Definition of "Life" by Gregory C Chaitin.

21 comments:

Prashanth said...

Its funny, but I've come across the same debate in the context of artificial intelligence: both in real life (I've done a little work in AI) and in fiction (I'm a huuuuuge scifi reader).

Here's the thing: if you create a robot that can learn (grow), see, hear, think, talk and manage, then is it alive? To extend to the criteria on this post, lets say the robot runs on solar energy, and is capable of creating more robots like itself. In a crude sense, it can metabolize, is organized, can reproduce, and can grow. So, is it alive?

I can talk on this subject for hours... but I'll stop now :)

Born a Libran said...

I have come across similar debates myself... but in the context of genetic algorithms and stuff (where the answer is simply that they do not undergo metabolism)...

My reply - If one can clone Dolly and Dolly is considered to be alive, then I don't see why a robot with all the above characteristics can not be considered alive... However, the robot has to be sufficiently complex to not be the sum of it's parts alone. On going through the algebraic information theoretic framework of arganization (not that I understood most of it), it is just a wee bit more complicated than robots as we know it (even if it is solar empowered, can reproduce, and can evolve).

Sakshi said...

Prashanth beat me to asking the question - where do you draw the line for calling something alive?
On a side note,this is a good article to read.
http://www.the-scientist.com/article/display/18854/
It talks about generating the artificial cell. Infact, that entire month's issue was dedicated to such articles.

Born a Libran said...

@sakshi: The article you pointed to made an interesting read. I dont think they are striking at the origin of life problem because they are using modern DNA, proteins, et. al. to make the synthetic cell. The next question is whether they are creating life - thats a little more difficult to define. I would like to see it reproduce and evolve and then see everything that it has in it before I have to judge that. Life as we know it lies in complexity - the whole has to be greater than the parts and nothing they have built so far has that characteristic.

Sakshi said...

@ BaL - yes, I agree. So far, we have not added the ability to mutate and evolve in an artificailly generated cell. But there are several groups activiely trying. So the whole point is if we do succeed, at what point do we say,that this is live?

Raindrop said...

Are there any good arguments for why life (extra terrestrial) should be carbon based, and not silicon based? I would also imagine that it's easiest for living organisms (in their non-dormant form) to be liquid based. Allows for more complexity and easier metabolism. Any arguments for why that liquid is more likely to be water, than, say methanol? Apart from the fact that water is a 'universal' solvent.

Mediocretes said...

You guys are all asking the wrong questions. What is Life? is the wrong question. The right question is: What's the Effin' Point of Life?

So we go beyond the emperical and come up with a general set of rules - a rigorous definition - that we can apply to various 'things' and classify them into two neat buckets: Living and Non-Living. Then what man? Back to the lab to kill more mice? And then what? Even more mice.

And does anyone care about the undead zombies from Evil Dead? What about them, huh?

Born a Libran said...

@raindrop: Good questions. There is no reason that life should be C-based or water-based except that life as we know it has evolved around what is available on Earth. In fact, that is why a more rigorous defn of life is needed for these space probes. They try to find "signs" of life which basically end up being metabolism, organization, and genetic hereditory matter even those the chemical components of these various processes could be vastly different from how we see it on Earth.

@mediocretes: The question you have is philosophical, but the scientific definition of life has to be established before one can go in and start looking at what keeps life going and how it could have evolved from non-living matter.

To keep it in perspective, in the 19th centuru, no one could have a single definition of water - what with its varied properties - solvent, thirst quencher, wetability, etc., there was not a single definition of water that satisfied one and all until they came to know the chemical constituents of water and H2O satisfied one and all. Maybe, what we need to understand about life is the basic elements of life rather than the actual properties life exhibits....

Anonymous said...

2008 Updated Life Definition:

Scientifically, not technically

Tomorrow's comprehension of life and its evolution

Intriguing Darwinian implications

Intriguing technological developments potential

Look at these life definitions, reflect about their implications...

-----------------------------------

Comprehensive Definitions Of Earth Life, Earth Organism, Gene, Genome And Cellular Organisms.

Earth Life: 1. a format of temporarily constrained energy, retained in temporary constrained genetic energy packages in forms of genes, genomes and organisms 2. a real virtual affair that pops in and out of existence in its matrix, which is the energy constrained in Earth's biosphere.

Earth organism: a temporary self-replicable constrained-energy genetic system that supports and maintains Earth's biosphere by maintenance of genes.

Gene: a primal Earth's organism.

Genome: a multigenes organism consisting of a cooperative commune of its member genes.

Cellular organisms: mono- or multi-celled earth organisms.

-----------------------------------

Nature, Origin And Function Of Earth Life


Nature of Earth life: a replicating construction temporarily constraining and maintaining energy.

Origin of Earth life: serendipitous energy-induced formation of Earth's primal organisms, individual independent genes.

Nature of Earth's organisms: temporary self-replicable constrained-energy genetic systems that support and maintain Earth's biosphere by maintenance of genes.

Function of Earth life: uphold and maintain as much constrained energy as possible by upholding and maintaining Earth's biosphere.


Dov Henis

http://blog.360.yahoo.com/blog-P81pQcU1dLBbHgtjQjxG_Q--?cq=1

MissPrissy93 said...
This comment has been removed by the author.
Anonymous said...

Pavlov's Smile
Ameisen Olivier, Imagination Medicine,
Placebo, God-Religion, Virtual Reality

(recapitulation of some earlier posts)


A. Anti-Depressants, like

- Ameisen Olivier's "end of my addiction"

- http://www.completehealthdallas.com/Anti-DepressantsNaturalAlternativeDallas.html

- http://www.answers.com/topic/serotonin


B. Imagination Medicine

http://www.sciencenews.org/view/feature/id/39046/title/Imagination_Medicine
Brain imaging reveals the substance of placebos. Expectation alone triggers the same neural circuits and chemicals as real drugs.

"It all boils down to expectation. If you expect pain to diminish, the brain releases natural painkillers. If you expect pain to get worse, the brain shuts off the processes that provide pain relief. Somehow, anticipation trips the same neural wires as actual treatment does.

Scientists are using imaging techniques to probe brains on placebos and watch the placebo effect in real time. Such studies show, for example, that the pleasure chemical dopamine and the brain’s natural painkillers, opioids, work oppositely depending on whether people expect pain to get better or worse. Other research shows that placebos can reduce anxiety."


C. Placebos: some background info

http://www.cerebromente.org.br/n09/mente/pavlov_i.htm
http://www.cerebromente.org.br/n09/mente/placebo1_i.htm
http://thjuland.tripod.com/placebos.html

The concept of a placebo comes from medieval times, when professional mourners were paid to stay by the bedside of. deceased person, reciting a psalm beginning "Placebo Domino..." or "I shall please the Lord." "Placebo" gradually became the word used for the paid mourner, whose grief was, in fact, false.


D. Life's Manifest

http://www.the-scientist.com/community/posts/list/112.page#578

Genes are the primal, 1st stratum, Earth's organism and genomes are 2nd stratum organisms,
multigenes consisting of cooperative communes of their member genes.

Life is a real virtual affair that pops in and out of existence in its matrix, which is the energy constrained in Earth's biosphere.


E. On Science and Religion

"Evolutionary Biology Of Culture And Religion"
http://www.the-scientist.com/community/posts/list/20/122.page#492

The concept “God” is a human virtual reality artifact, experienced only through sensory stimuli. Preoccupation with god-religious matters within a scientific frameworks contributes to corrosion and corruption of science and scientism by manifesting or implying acceptance of virtual reality as reality.

Everything is discussable scientifically. No limit. Including virtual matters and affairs. But for a scientific discussion the framework must be clearly defined. The totality of subjects that come under the classification "virtual" are not an exception. You can include in the discussion Pavlov and the modes and manners of exploiting virtuality in any area and towards any end.


F. So why Pavlov smiled in 2008?

Pavlov demonstrated effecting placebo phenomena in multicelled organisms by manipulation of their drives-reactions. Now placebo and imagination phenomena are demonstrated also in the smaller organisms, in the genes and genomes of multicelled organisms, in our primal first stratum and 2nd stratum base organisms. A very good reason to smile.

Now an interesting chain is exposed to our view, the Genes-Virtual Reality Chain, a most intriguing cultural evolution chain extending from the genesis of our genes to nowadays, throughout life, a virtual reality existence, and by virtual reality phenomena, exploitations and manipulations.


Dov Henis

(Comments From The 22nd Century)
http://blog.360.yahoo.com/blog-P81pQcU1dLBbHgtjQjxG_Q--?cq=1

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