What is "alive"?

[thedarkshade]

The NASA definition of life: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

OK great, but my personal opinion is that this oversimplifies 'life'.

[noz92]

Why does the definition of life need to be complex. There are plenty of definitions built around life on Earth--stuff that we already know is alive, and any definition we give has to allow them to be considered life--but what about new forms of life, such as any form of life that has evolved independently of us (i.e. it does not share a common ancestor with us; for example alien life).

 

I think the definition of life needs to be very simple, as anything more complex would exclude things that really are "alive" from the definition of life.

[antimatter]

Life=Energy

[noz92]

Are we talking about a technical definition of life or a philosophical one?

[insane_alien]

Life=Energy

 

 

That would result in everything being alive. i think it can be agreed that somethings are not alive therefore energy cannot be considered alive.

 

Are we talking about a technical definition of life or a philosophical one?

 

Technical.

[noz92]

That's what I thought, but then I got confused by the "life=energy", which sounds more philosophical.

[antimatter]

eh, rocks don't have energy, afaik

[insane_alien]

rocks do have energy. appart from E=mc^2 you can probably get some chemical release of energy from them. and then there is the thermal energy they have.

 

or are you talking of energy as in some 'life force' that has never been observed to exist. also, if i chucked the roc at your head i'm sure you would agree it had energy,

[antimatter]

Well, right, they do have some energy, but yes, I'm talking about that vague life force that's never been proved to exist.

bash me as you please

[noz92]

Even if you assume "life energy" to exist, it is usually said to be present in nonliving things, such as water.

[antimatter]

Okay sorry, I was wrong by saying life=energy, my bad.

then I have no answer...

[lucaspa]

aren't bones crystaline? and enamel?

 

The extracellular matrix is partly the mineral hydroxyapetite, which is in a crystalline form. That's about 60% of the dry weight of the extracellular matrix. However, the organic part of the matrix is composed of proteins, 90% of which is type I collagen. It is not in "crystals".

 

And, of course, there are no crystals within the osteoblasts and osteocytes (the cells) in bone.

 

The NASA definition of life: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

 

I have a problem with that. Darwinian evolution requires that more individuals are produced each generation than the environment can support. What if the reproductive rate is so slow that this requirement is not met?

 

I like the "self-sustained chemical system" but I feel that this is too limiting. What about possible life that is energy?

 

No, I'll stick with the 4 criteria:

1. Metabolism

2. Response to stimuli

3. Growth

4. Reproduction

[thedarkshade]

No, I'll stick with the 4 criteria:

1. Metabolism

2. Response to stimuli

3. Growth

4. Reproduction

Isn't growth part of the metabolism, because IIRC growth (generally) represent the domination of anabolic processes, as a result of which energy is 'stored' in the organism?

[noz92]

Darwinian evolution requires that more individuals are produced each generation than the environment can support. What if the reproductive rate is so slow that this requirement is not met?

It says it just has to be "capable of undergoing Darwinian evolution." As long as, given the opportunity, it could evolve, I think it meets that criteria.

 

I like the "self-sustained chemical system" but I feel that this is too limiting. What about possible life that is energy?

How can life be energy?

[thedarkshade]

How can life be energy?

We did this life-energy thing couple posts before in this thread. Look them and you'll find what you mean.

[Donut.Hole]

Erm, I'm kinda rusty on this, but I recall 7 things that all living things must have to be considered "alive":

 

-Must use energy. This includes eating

-Must be able to reproduce

-Must be able to move on its own, in some way.

-Must breath, or exchange gases (even microorganisms do this)

-Must grow

-Must respond to stimuli

-Must produce some form of waste

[thedarkshade]

-Must use energy. This includes eating

-Must breath, or exchange gases (even microorganisms do this)

-Must grow

-Must produce some form of waste

You could say just 'metabolism' for all these.

[Donut.Hole]

You could say just 'metabolism' for all these.

 

Yeah, but listing everything makes everything clearer.

And besides, you're avoiding the question.

[iNow]

Erm, I'm kinda rusty on this, but I recall 7 things that all living things must have to be considered "alive":

 

-Must use energy. This includes eating

-Must be able to reproduce

-Must be able to move on its own, in some way.

-Must breath, or exchange gases (even microorganisms do this)

-Must grow

-Must respond to stimuli

-Must produce some form of waste

 

I think the list which has been repeatedly offered by lucaspa is both more accurate and accepted:

 

1. Metabolism

2. Response to stimuli

3. Growth

4. Reproduction

[halogirl]

so, to be alive it has to keep itself alive?

 

that doesn't really define 'alive' its just kind of circular.

 

 

yeah a little but, in a way it kinda makes sense. we know that cells are alive, but they are made up chemicals, so really they don't need a living thing working for them to keep them alive, but they still need those chemicals working for them to keep them alive. thus they still need something working for them. :}

[noz92]

I think the question of "what is life?" is somewhat irrelevant. We like to think ourselves special, but in reality, we are simply part of a multi-billion year chemical reaction. Where does chemistry end and biology begin? Any definition of life that you can produce was invented recently and retrospectively applied to things that already fit a subconscious definition of life that we already have (i.e. we know life when we see it). Like other concepts which we cannot come up with an exact definition for, it probably isn't real. I don't think life is necessarily different other chemicals on a literal level (it is different on a philosophical level, but philosophy is also man made).

[thedarkshade]

I don't think that the issue of life is not important. I mean, all the researches and advances done in the field of medicine has directly or indirectly to do with life. Understanding the cell, its components is actually the understanding of life itself. Now the fact that we do till a point or don't understand life is different, but life is a very important issue to us, has always been and probably will always be. In the past, when science was at a poor level there were only philosophical concepts of life. But now, that we have a good level of science, the issue is a key one.

 

And it is better to have an incomplete definition that not have one at all. And the definition of life would actually be just kinda crowning thing for all the hard work done to understand it.

[noz92]

But how does giving a definition to something that, at least as far as nature is concerned, doesn't exist (by this I mean there really is no difference between actual "life" and other "life-like" materials, at least as far as the beginning of life is concerned) important to medicine and biology? After all, we design medicines based on who will be using it (i.e. it is designed for humans), and biologists study more than just "life" (most biologists would not consider a virus alive, but isn't its study still apart of biology?

[lucaspa]

Isn't growth part of the metabolism, because IIRC growth (generally) represent the domination of anabolic processes, as a result of which energy is 'stored' in the organism?

 

Not necessarily. Anabolism could result in either 1) holding the organism constant or 2) lag behind metabolism and result in the slow degradation of the entity.

 

Also, in order to reproduce, an organism has to "grow" larger so that it can do the mitosis thing. Or budding. Or if it sexually reproducing get large enough to have separate germ cells.

 

But how does giving a definition to something that, at least as far as nature is concerned, doesn't exist (by this I mean there really is no difference between actual "life" and other "life-like" materials, at least as far as the beginning of life is concerned) important to medicine and biology? After all, we design medicines based on who will be using it (i.e. it is designed for humans), and biologists study more than just "life" (most biologists would not consider a virus alive, but isn't its study still apart of biology?

 

I think you are relying too much on the idea that biology is the "study of life". Then you play semantics and say "if biology studies it, then it must be alive". That's a non-sequitor. A living entity must have a criteria to be alive independent of the definition of biology. Biologists could have made a mistake. And yes, we intuitively know that there is a difference between life and other materials. The question is: what? In the beginning of life, protocells represent a discontinuity between non-life and life. There isn't a "life-like" intermediate. The amino acids polymerize to form proteins - which aren't alive. And then the proteins spontaneously form cells -- which are alive. Boom, instant life.

 

It says it just has to be "capable of undergoing Darwinian evolution." As long as, given the opportunity, it could evolve, I think it meets that criteria.

 

Noz, if the criteria I listed happens, then the entity could not evolve! That's the problem. The "capable of undergoing Darwinian evolution" involves several premises that might not be met. I was looking at stituations where the entity would be "alive" but wouldn't fit the criteria necessary to undergo Darwinian evolution. Therefore, if we can have alive entities that don't fit a criteria to be alive, then the criteria has problems.

 

The criteria to be "capable of undergoing Darwinian evolution" was added by the RNA World people. They did it for a very simple reason: replicating RNA molecules don't meet the other criteria. So, if your "alive" entity doesn't meet the criteria, what do you do? Well, you could admit it isn't alive. Perhaps on the way to directed protein synthesis, but not alive. OR, you could change the criteria! If you can't win by the rules, change the rules. You'll have to decide whether this tactic is good science.

[lucaspa]

But how does giving a definition to something that, at least as far as nature is concerned, doesn't exist (by this I mean there really is no difference between actual "life" and other "life-like" materials, at least as far as the beginning of life is concerned) important to medicine and biology?

 

There may be a difference. The RNA World hypothesis envisions a smooth transition from non-life to life, but the protocell hypothesis has a sharp demarcation.

 

Viruses are a little bit special. Most people consider viruses as going from non-life to life. However, the reality is backwards to that. Viruses are going from life to non-life. That is, viruses started out as bacterial parasites and have evolved to get rid of a lot of non-esssential functions. Non-essential for an obligate parasite, that is.

 

Remember, in natural selection everything has a cost as well as a benefit. Yes, having all the genes to code for ribosomes to make proteins is a benefit, but it takes energy to make the DNA, the transcription factors (proteins that need still more DNA), etc. to get the ribosomes. So, if an organisms is an obligate parasite and only lives in another cell, then those individuals that don't have to make all that stuff but can use the host ribosome has less cost than others. So there is selection pressure to go from an obligate cellular parasite -- such as the tuberculosis bacterium -- to a virus.

 

So, yes, biology studies viruses, but they are not a step from non-life to life. Rather, they are a degenerate form of life.