A biology question

[ninkon]

We're in Biology now in my grade 10 science class, and we recently covered carbon/nutrient cycles. For us to live we need Oxygen, but if we were to breath it all up then we'd die so plants us photosynthesis, taking in sunlight and CO2 to create its own nutrients, in the process emit Oxygen...sorry for the long story, short; Plants don't 'need' oxygen

 

[Doom-Slayer]

-snip-

Id have to say you absolutely right. The only reason we presume life requirs water etc is because thats what we have and what our entire planet works on. It means we have absolutely nothing else to go off.

The main problem is that since life is such an extremely complex process, starting it and saying something like "Well..lets make up a sulphur based lifeform" is nearly impossbile, since you need to drop all presumptions and start with nothing.

Since we have nothing to go off we have to presume from what we know, I could compare it to trying to imagine a new color that we cant see. You cant, since all your going to do is base it off a color you already know, and that will already have a name. We need to base ideas off something or we have nothing.

 

[Terracrete]

Slightly less short an answer:

It is less dense then air so over time it will accumulate at the upper levels of the atmosphere while your new attempts at life will be on the surface of the planet in direct exposure to the more dense oxygen gas or potentially in any liquid on the surface.

Please note that both NH4 and CO2 were critical in the development of life on Earth. However, the original question was...
"Why do we always assume that for the life to exist, the conditions on the planet have to be at least somewhat similar to those of earth?
More specifically the presence of liquid water."

From an "astronomy enthusiast" point of view, I am presuming he/she is interested in life being able to form in conditions that completely lack water or possibly have low levels of water as a means to suggest that a planet could develop life both lacking water from a physical(IE a substituted medium) and biological (could life develop without the chemical properties of water) stance.

Also of relevance is the temperance zone, this is the conditions of the distance from the nearest star to the target planet as well as the size of the planet. The significance of the Goldilocks conditions is that life required all 3 forms of H2O in other to exist, water, gas, and ice. A suitable temperature range as well as gravametric pressure is required to achieve those conditions.

 

[Nerdstar]

ah! but what about Ammonia, it can be both aqueous or gaseous, non-oxidizing/reductive (in the proper reducing envorment), and stable when exposed to high energy sources(showen in the Miller?Urey experiment).

Short answer is that Ammonia is immersible with O2, which is highly destructive to new forming life.

true, this i know but im speaking about an envirment thats condusive to ammonia based life(Ammonia is flammable and can be oxidized which means that it cant exist in such an envrioment. It would, however, as i said be stable in a reducing envriomnet with proper atmosperic condtions such as titan,

(after all if O2 could fit H2O based life would devlop)

edit: just saw your reply and left out the tidbit about these condtions being posiable under the surface of titan where they could devlop without the risk of O2 contamiantion

 

[Terracrete]

-snip-

Id have to say you absolutely right. The only reason we presume life requirs water etc is because thats what we have and what our entire planet works on. It means we have absolutely nothing else to go off.

The main problem is that since life is such an extremely complex process, starting it and saying something like "Well..lets make up a sulphur based lifeform" is nearly impossbile, since you need to drop all presumptions and start with nothing.

Since we have nothing to go off we have to presume from what we know, I could compare it to trying to imagine a new color that we cant see. You cant, since all your going to do is base it off a color you already know, and that will already have a name. We need to base ideas off something or we have nothing.

While this is correct in a purist scientist view, we have come a long way in understanding the significance that the chemicals water, NH4, CO2, etc have played in the development of life.
From this understanding it is suitable to postulate the preliminary conditions that would be suitable to life development, as well as conditions that would not be suitable to life development.

Please note that for all my above statements I have utilized the following definition for life.

An entity which demonstrates all or most of these properties: Homeostasis, Organization, Metabolism, Growth, Adaptation, Response to stimuli, and Reproduction.

 

[Agayek]

-snip-

The primary driving cause behind that is that the only proof of life we can find is on Earth-like planets. There's an infinite number of planets out there, searching is made infinitely easier by narrowing the parameters based on what we know. It's fully possible for there to be life on planets outside Earth-like conditions, but based on what we know, it's not likely.

It's basically a cost/benefit thing. We're far more likely to find extraterrestrial life sooner if we look only at Earth-like planets instead of "all planets".

Plus, we're not really sure what signs to look for on other planets. We've found some bacterium that thrives in an amonia-rich environment (I think, it was something like that anyway) not too long ago, and we could examine that and be able to expand our searches. Until we finish that though, we're gonna keep looking at Earth-like planets.

 

[Navvan]

Well there are certain things that we know are not required for life as there is life on earth that lives without them. Oxygen is not required, and there is an exception to almost every common chemical component to life. However there are two things that all known life forms possess.

Everything on our planet is carbon based
Everything on our planet needs water.

It has been theorized that silicon could be used instead of carbon. This is because both silicon and carbon share the similar molecular properties that are required for forming complex molecular structures required for life.

There is no molecules that have all the properties of water that are believed to be required for life. That of course may not be the case, and there is life that has evolved without water. However most molecules are either to reactive and thus destroy the life-information (RNA/DNA/Equivalent) or are not capable of suspending things in solution.

 

[Thaluikhain]

The OP has a very good point. There's no telling what alien life would be like.

However, when looking for something, you have to have some idea of what it looks like and/or where it might be found, otherwise you'll never recognise it when you find it.

So they assume alien life to be similar to the only life they know of, and work from there.

If life is ever discovered, whatever form it takes will shake things up massively anyway.

 

[Doom-Slayer]

While this is correct in a purist scientist view, we have come a long way in understanding the significance that the chemicals water, NH4, CO2, etc have played in the development of life.

An entity which demonstrates all or most of these properties: Homeostasis, Organization, Metabolism, Growth, Adaptation, Response to stimuli, and Reproduction.

Thats just the thing though: life on our planet. All of these things make life work so well because of the way the world is made up.

When you simplify it down, biology and chemistry basically end up as "something + something = something else". Unless scientists find no life anywhere else, I cant see why the way life works here to be a "special" case. Life in its basic sense is just a huge self sustaining chain reaction. The elements themselves that cause that chain reaction are meaningless, since the input and output dont matter, what we see in life is basically a chemical reaction that supports itself. A very complicated chemical reaciton but thats all that it is.

Makes life seem not as great as it is, but in the end all life realy is is

A -> B -> C -> A (repeat)

 

[Terracrete]

The elements themselves that cause that chain reaction are meaningless, since the input and output dont matter...

To this noted point I must say, "on the contrary". Our universe is governed with and by rules. The nature of the rules may forever be shrouded from us, however the rules exist non-the-less.

Knowing the "input" as termed above and the subsequent "output" is the basis of science. In a word prediction.
Furthermore, knowing the "input" and the subsequent "output" and in addition the reason of the corollary between the two is called extrapolation.

It is prediction and extrapolation that together drive all achievements of human endeavors.

In the context of this thread we can speculate the existence of anything we wish, different or similar to that what we know. However, it can not be in contradiction to the relationships we've established between cause and effect without the relationship having not been truly understood.

It is reasonable to say we have a strong understanding of the principles of oxidation and reduction, and that through thermodynamics along with our definition of life requiring metabolism, that under these conditions oxygen is an ideal molecule for energy exchange. That is not to say that other molecules couldn't do like-wise and in fact there are numerous molecules that are classified as oxidizers under a variety of conditions. However non have exemplified the ideal conditions like oxygen has. Likewise with water.

Oh a side note to the silicon based life. Distribution of universal mass shows that the smaller the atomic weight, the more naturally occurring the atom is. Simply put there is more carbon in the universe by orders of magnitude then there is Silicon.

 

[Doom-Slayer]

The elements themselves that cause that chain reaction are meaningless, since the input and output dont matter...

-snip-

You misunderstand. When I say the input and output dont matter, I mean they dont matter to the lifeform. Yes..you need to put in the correct input to get a specific output, but to the lifeform that is irrelevant. Oxidation and reduction like you saaid, are processes which help our bodies work. However they arent the -only possible- method of doing this.

My point remains and you in fact reinforce this, all forms of life are not just limited to OUR definition of life. Even as you spoke you references process that are limited to earth based lifeforms and process which are common -on this planet-.

My main point was this. Life is just a cycle. Organisms are machines that take in nutrients, process them, and are able to gainn these nutrients from the outside world(conscious thought) and reproduce. There is no reason at all, and no possible evidence that says these process MUST be through oxygen/carbon/hydrogen etc like our planet is based around.

Why for example couldnt you have a lifeform existing entirely through silicon/hydrogen and magnesium? With a complicated enough structure lifeformslike that could exist, and its not that scientists can dissprove or even prove something like this. Large complicated structures made from uncommon materials are impossible for us to come up with at this stage since nothing like them exist on earth, we dont have anywhere to start from.

 

[crudus]

Why to astronomers look for water? Because when you find a planet in the habitable zone you just wanna say "man, wouldn't it be awesome if we could move there"?

that answer is only half serious

Couldn't life come to existence and evolve in, let's say, a lake of sulfured acid....or even better a lake made out of some other element that we haven't discovered yet?

Also couldn't some life forms evolve in such a way that they breathe nitrogen or CO2 instead of oxygen?
Can life come to existence in a completely different environment?

Well, Sulfuric acid just sort of melts/dissolves everything and when you think of sulfuric acid it is very diluted in water anyway. The elements beyond lead are all radioactive which really does not help life at all. I am not sure why we don't use nitrogen rather than oxygen. Clearly there was something there that prevented it (nitrogen being more than 70 percent of our air).

Anyway, water is a weird molecule. First off, it has a really large range of temperatures where it is a liquid. That is helpful. Also, everything is water soluble at least to some extent which helps enzymes (which need water too) do their work. A high specific heat capacity (the amount of energy it takes to raise a gram of a substance one degree) is the highest out of all the elements and compounds which is quite important for controlling your/any internal body temperature. The weirdest thing of all water does is it gets less dense as it freezes. Nothing else does that(well, not that I have heard of). If water didn't do that, it would basically mean a lake would stay (at least) half frozen perpetually. There are other liquids that could theoretically support life, but it would be a very unstable equilibrium.

Weird how a biology question led to a chemistry answer.

<a href=http://www.pbs.org/wgbh/nova/evolution/liquid-of-life.html>source

 

[Chameliondude]

Ok, to have life develop, a transition needs to be made from chemical to biological systems, the only known method for doing this that we know of is abiogenesis (a youtube channel potholer54 explains it really well http://www.youtube.com/watch?v=v8nYTJf62sE)
This needs simple carbon molecules in a neutral solvent, which varies in temperature between 30-80 degrees centrigrade, water is both incredibly common and perfectly neutral, so is perfect, other solvents dont tend to exist in large enough quantities for abiogenesis likely to occur, and may break down over time.

 

[spartan231490]

As an astronomy enthusiast (with very little actual scientific knowledge) and a complete lack of knowledge of biology there is one thing that always puzzled me whenever i would watch one of those shows about space, and more specifically about searching for extra-terrestrial life.

Why do we always assume that for the life to exist, the conditions on the planet have to be at least somewhat similar to those of earth?
More specifically the presence of liquid water.

Now i can understand why it's safe to assume that there is no life on super cold or super hot planets, due to the fragility of the organic matter, but why is H2O considered a magical ingredient for life.

Couldn't life come to existence and evolve in, let's say, a lake of sulfured acid....or even better a lake made out of some other element that we haven't discovered yet?

Also couldn't some life forms evolve in such a way that they breathe nitrogen or CO2 instead of oxygen?
Can life come to existence in a completely different environment?


I remind you that my knowledge of biology is very very limited, so i would like someone with a bit more insight to explain those things to me

They do consider other possibilities, but as they can't predict what those conditions would be, they can't really glean anything from it. They assume that life is most likely on planets with liquid water because it's the only way they can even guess.

Also, it's not a horrible assumption. Microbial organisms survive better in a liquid environment because it's easier for a microbe to find sustenance if it is dissolved, because it will spread due to concentration gradient. Water is the only liquid we're aware of(or one of a few) that it's solid form floats, and if the solid sinks, it freezes to the bottom, making life in a liquid very difficult.

Also, water's extreme polarity makes it a great substance to build an organism(especially macroscopic ones) around, because it allows the organism to create membranes on the simple mechanic of hydrophelia vs hydrophobia, as well as control the flow of necessary substances with the same phenomena. It's a mechanic that a lot of bioligical processes are based around. It also mixes well with glucose, which is a very simple molecule that an organism can derive a lot of energy from.

There are a lot of other reasons, water is sometimes called the magic molecule, because it has so many unique properties. It's not impossible that life could form without it, but I doubt it will because of all the unique properties of water. I'm not even a bio major, these are just the properties of water that I know are unique from some study of chemistry, not even that much.

 

[AdmiralMemo]

We only know what Earth-like life looks like. So that's what they're looking for. We don't know whether other kinds of life can exist. Also, given our studies of our solar system, we know a bunch of types of places life doesn't exist (not to say can't exist... just doesn't). Therefore, we're not looking in similar places when we look out further.

 

[ShindoL Shill]

It's much simpler to look for conditions that we already know can support life rather than guessing randomly.

yeah. also, it wouldnt be good to find an awesome species with a super-advanced city inside jupiter or under the ice of pluto.
we'd want to find species we can live with/eventually possibly breed with.

 

[Esotera]

A note, acids aren't acids without water. Acid actually refers to altered water molecules (hydronium ions), and the property of certain chemicals to create these species.

There are organisms which use things other than oxygen as a final electron acceptor (the role of oxygen in respiration).

Sure things could evolve differently, but we would have no idea what compounds to even start looking for in space.

This isn't true...water just aids the dissociation of acids. The dissociation of HA to its constituent ions will continue whatever, it'll just be aided by the presence of water, or any other polar compound. And as for compounds to look for it would be those with similar chemistry to carbon, nitrogen, etc. Although for this to be advantageous something like Silicon would have to vastly outnumber Carbon.

Now i can understand why it's safe to assume that there is no life on super cold or super hot planets, due to the fragility of the organic matter, but why is H2O considered a magical ingredient for life.

Because of its incredibly polar properties; hydrogen has a positive dipole, oxygen a negative (assuming you know this chemistry). This enables hydrogen-bonding for DNA & various other compounds which is vital for most life as we know it.

Couldn't life come to existence and evolve in, let's say, a lake of sulfured acid....or even better a lake made out of some other element that we haven't discovered yet?

You should look into extremophilic organisms; they're basically microbes that have drastically altered metabolisms & biochemistry from all other life known on earth. Various strains can survive extreme heat, cold, radiation, salinity, and whatever you can think of...but they all seem to have the same basic chemistry.

 

[Dags90]

This isn't true...water just aids the dissociation of acids. The dissociation of HA to its constituent ions will continue whatever, it'll just be aided by the presence of water, or any other polar compound. And as for compounds to look for it would be those with similar chemistry to carbon, nitrogen, etc. Although for this to be advantageous something like Silicon would have to vastly outnumber Carbon.

This may be a simple mix up of terminology. But as far as Brosted-Lowry acid-base theory is concerned, I am correct. pH is determined by the concentration of hydronium ions vs. the concentration of hydroxide ions. Both of these are altered forms of water.

In the absence of water, one may still have Lewis acids and Lewis bases. But then protons don't matter, that has to do with electron donators/acceptors.

 

[Esotera]

This may be a simple mix up of terminology. But as far as Brosted-Lowry acid-base theory is concerned, I am correct. pH is determined by the concentration of hydronium ions vs. the concentration of hydroxide ions. Both of these are altered forms of water.

In the absence of water, one may still have Lewis acids and Lewis bases. But then protons don't matter, that has to do with electron donators/acceptors.

My bad, we're both correct, just using different definitions. The Bronsted-Lowry definition is probably more relevant for this discussion.

 

[FuzzyRaccoon]

Sure, the likelyhood that other intelligent life would be bipedal is extremely low (without some sort of lantern-hung explanation such as is given in both Star Trek and Mass Effect (that a mysterious race of Aliens did it)),

Actually, if I remember correctly, in Mass Effect they never said that humans or any other species other than the Keepers or the Collectors were specifically altered in any defining way. Certain missions, like the mission with those monkey-like creatures in ME1 indicate that the Protheans observed humans from Mars. Other events, like conversations with Sovereign or some catches on the Collector ship/base we learn that Reapers have been using technology to narrow the direction in which the species innovate, but it was never suggested that they were doing anything to bipedal's specifically.

They do however say that bipedal life is the most conducive for sapient life in Mass Effect, but they never actually explain why.