What is "alive"?

[stevo247]

What is the criteria that is used to determine whether something is alive or not?

[iNow]

It depends on what that "thing" is, and also the person doing the defining.

 

Any context for your question? Is this an abortion related question, have you eaten some magic mushrooms and you're curious if the couch on which you're sitting is alive, or did you perhaps just watch a movie about a plane that crashed on a mountain and the survivers just started eating each other...

[stevo247]

This is definitely not an abortion related question.

have you eaten some magic mushrooms and you're curious if the couch on which you're sitting is alive?

Not necessarily, but thanks for the memory!

 

I am interested in what the scientific community uses as their criteria for determining what constitutes a living thing. For example, I'm pretty sure that a rock would be classified as "non-living", but a unicellular organism would be considered "living". Why? What makes something "living"?

[Daecon]

Is there a mnemonic called "Mrs Gren" or something?

 

Um... movement, respiration, something, growth, reproduction, excretion, nutrition?

 

Although, isn't excretion already covered in the "respiration" part?

[Eric 5]

What is the criteria that is used to determine whether something is alive or not?

 

One criteria that could be used to determine if something is alive or not would be by observing if that thing in question is self-determined, does it have the ability to direct itself in a way that promotes it's survival. Can this thing control its enviroment in a manner that aids it in continuing to exist.

[Resha Caner]

I'm not a biologist, but this is an interesting question. I'll quietly follow along in the background.

[Eric 5]

I'm not a biologist, but this is an interesting question. I'll quietly follow along in the background.

 

 

Why not join the conversation? I am sure that you have some idea of what would be required of something to be considered alive. I would like to hear your veiw on this subject. It would help to get the ball rolling.

[YT2095]

it also depends upon the Scale you look at also, for instance a person may be freshly dead as a Macro unit, but on a cellular level be perfectly alive and viable.

[ydoaPs]

Is there a mnemonic called "Mrs Gren" or something?

 

Um... movement, respiration, something, growth, reproduction, excretion, nutrition?

 

Although, isn't excretion already covered in the "respiration" part?

 

So, plants aren't alive? Maybe "M" is "Metabolism."

[insane_alien]

plants can move. sunflowers will track the sun as it moves across the sky.

[Klaynos]

[note the following is what I remember from GCSE science about 7 years ago]

 

Plants 'move'... if you put a plant near a window the plant will lean towards it, of you then turn the plant around it will lean back the other way. This is done by hormones changing the growth so one side of the stem grows faster than the other.

 

Or maybe venus fly traps?

[jen]

Is there a mnemonic called "Mrs Gren" or something?

 

Um... movement, respiration, something, growth, reproduction, excretion, nutrition?

 

Although, isn't excretion already covered in the "respiration" part?

 

something=sensitivity

 

i was a diligent 15 yr old.

[ydoaPs]

[note the following is what I remember from GCSE science about 7 years ago]

 

Plants 'move'... if you put a plant near a window the plant will lean towards it, of you then turn the plant around it will lean back the other way. This is done by hormones changing the growth so one side of the stem grows faster than the other.

 

Or maybe venus fly traps?

 

All plants undergo phototropism?

[stevo247]

Would the possession of a membrane, and the ability to sense and respond to external stimuli, be considered one of the fundamental criteria for a living organism?

[insane_alien]

Would the possession of a membrane, and the ability to sense and respond to external stimuli, be considered one of the fundamental criteria for a living organism?

 

then the membrane separation equipment in my uni's lab is alive. it both possesses a membrane and responds to stimuli(changes in concentration and even if there is a voltage difference over the membrane.

 

i wouldn't call it alive though. so that could not be the whole story.

[thedarkshade]

What is the criteria that is used to determine whether something is alive or not?

All the enormous variety of organisms today comes from a single cell, just one! Which at some point begun to 'live'! But what conditions must be filled in order to be alive?

 

I still remember my first biology lesson in high school where we exactly talked about these conditions and we mentioned three:

 

1. A membrane

2. Energy Source

3. Reproductive Ability

 

I. A membrane is highly necessary for a lot of reasons. The must 'primitive' reason is that a membrane holds the entire protoplasm together. Then the membrane also protects the protoplasm from harmful outside factors. Them is also serves as a port where all the substances that are needed in a cell go in, and the substances that no longer are useful go out. It is very important due to selective ability that membrane has (semi-permability).

 

II. Energy is purely the fundamental condition needed to be filled so one cell (or organism) can keep living. Energy is needed to perform all the vital processes that happen in a cell and that make life possible. Of course energy is not needed in some physical processes (like diffusion or osmosis) which are passive, but energy indeed the key factor without which life just can't go on (at least no for long). After all, this is why we eat. Energy enters in different forms like carbohydrates, proteins, lipids etc.

 

III. And reproductive ability is the third main condition that makes possible the continuity of life. Due to reproduction organisms proliferate, generation after generation and so life goes on continuously as an entirety although individual organisms die all the time. It is by reproduction that life counters death.

 

Shade!

[drwilder]

I think there was a book by Michael Crichton (the Andromeda Strain??) that talked about this. A scientist took a pice of granite and said, "This is Alive. It is living breathing, walking, and talking. Only we cannot see it, because it is happening to slowly. Rock has a lifespan of 3 billion years. We have a lifespan of about 60 to 70 years. We cannot see what is happening to this rock for the same reson we cannot make out the tune of a record being played at one rotation per century. To the rock, we are like flashes in the dark."

 

Or something like that.

Anyway, I think that "alive" is a purely subjective view. We as a species may not see the rock as alive because it does not act the way we expect living things to do, but the rock may not see us as alive for the same reasons.

 

But that's just a thought.

[bombus]

The question of what is alive becomes difficult when talking about seeds. Some seeds can remain viable for thousands of years but seem 'dead' until a little water is added. There's been a whole load of study over the years to figure out exactly what makes one seed dead, and another still alive. I don't think the answer has been found yet.

[stevo247]

The comment about granite got me wondering about the “cell structure” of minerals. Apparently, the unit cell is the basic building block of a crystal:

 

“To fully describe the internal structure of a crystal, two pieces of info are needed. First, one must characterize the geometry of the unit cell and therefore the lattice. Second, one must identify the content of the unit cell, i.e. the type and position of the atoms or ions. The atomic arrangement in the unit cell is called the crystal structure. The macroscopic crystal is then obtained by a periodic repetition of the unit cell through translation.”

http://books.google.com/books?id=0GAvKQJ2JuwC&pg=RA1-PA624&lpg=RA1-PA624&dq=mineral+%22crystal+structure%22+%22unit+cell%22&source=web&ots=PvbQyuiJg8&sig=M9BkeYzYNW8UDfZScvg6YQtAaS8&hl=en#PPA34,M1

 

I find it interesting that biological membranes are also considered “liquid crystals”.

 

“Lyotropic liquid-crystalline phases are abundant in living systems, the study of which is referred to as polymorphism. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry. In particular, biological membranes and cell membranes are a form of liquid crystal.”

http://en.wikipedia.org/wiki/Liquid_crystal#Biological_liquid_crystals

 

 

Some seeds can remain viable for thousands of years but seem 'dead' until a little water is added.

 

That makes me wonder about the influence of water and swelling.

[CDarwin]

I. A membrane is highly necessary for a lot of reasons. The must 'primitive' reason is that a membrane holds the entire protoplasm together. Then the membrane also protects the protoplasm from harmful outside factors. Them is also serves as a port where all the substances that are needed in a cell go in, and the substances that no longer are useful go out. It is very important due to selective ability that membrane has (semi-permability).

 

That seems a bit too specific and limiting criterion for such a general concept as "life." Could we not imagine something that lacked a cell membrane, but yet if we saw it we would still want to call it life? If ET walked up to you and shook your hand, but you learned that he didn't have any cell membranes, would you really reject him as a lifeform?

[insane_alien]

if ET didn't have a membrane of some sort then he wouldn't be walking. flowing to you maybe, but certainly not walking.

[CDarwin]

if ET didn't have a membrane of some sort then he wouldn't be walking. flowing to you maybe, but certainly not walking.

 

Maybe he's held together by electrostatic forces. Or something. We don't know what adaptations alien lifeforms might inherit.

[insane_alien]

then every time he touched metal he would collapse into a puddle. not advantageous from an evolutionary standpoint.

 

also, swimming would be a major problem for these organisms.

[falcon9393]

http://en.wikipedia.org/wiki/Life

this article tells what life is which should include what it means to be "alive" by scientific terms of course.

personally i think anything that can think is alive thats just me though check out the wikipedia link though!

[Mr Skeptic]

I. A membrane is highly necessary for a lot of reasons. The must 'primitive' reason is that a membrane holds the entire protoplasm together. Then the membrane also protects the protoplasm from harmful outside factors. Them is also serves as a port where all the substances that are needed in a cell go in, and the substances that no longer are useful go out. It is very important due to selective ability that membrane has (semi-permability).

 

What about acellular slime molds? They have but a single cell membrane for a humongous blob of nuclei. Stuff can go about its business without a membrane, but in a natural environment it would probably dissolve. However, I could imagine an oil-based lifeform that could probably do fairly well without a membrane.