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My question is:-
Only liquid water supports almost every living organism's metabolism, neither the vapour nor the condensed form of water does so. What is the chemical & the biological reason behind this fact?

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Because we depend on dissolved molecules, nutrients, minerals and so on. This is neither possible in solid (frozen) nor in gaseous water. Besides that, the temperatures necessary to have water either frozen or liquid are not really life promoting. –  Chris Mar 12 at 9:34
    
@Chris Just a minor correction, dissolved solutes may be present in solid form, forming a solid solution. Otherwise, the arguments are perfectly valid. –  Satwik Pasani Mar 12 at 10:01
    
Ok, I wasn't thinking about this, but I think this doesn't play a role for life. –  Chris Mar 12 at 10:05
    
@SatwikPasani.. There is no fluidity in solid "solutions". Reactions that happen in a solid state will be extremely local and all the diffusion effects (including morphogenesis) will not occur. Rigid solid microdomains are important to maintain structure but there should be an appropriate solvent that can sustain the multitude of biochemical reactions. –  WYSIWYG Mar 13 at 5:23
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2 Answers 2

A layman's view of the problem :

Let's assume that water in liquid form was not necessary and organisms could survive on solid or gaseous water. What would some of the difficulties be to sustain such a life form?

Solid Form of water

  1. How would the organism move without breaking or tearing itself apart? How would essential substances be transported from one part to another of the organism body?

Gaseous form

  1. What would be the size of the organism? Gas has a much bigger volume than water.
  2. In order to transport nutrient molecules within the a gaseous body, a huge air pressure needs to be generated. Also, how will gravity be overcome while transporting these molecules? A lot of energy will be required to maintain such pressures.

I guess nature (which always takes an optimal path) asked such questions and found that water in it's liquid form was the most suited to support life on Earth.

It need not be water alone. Other liquids may be able to support life as well. Only, we haven't discovered them yet. But whatever the substance, it's liquid form would seem to be an optimal solution for life.

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Organisms evolve to fit the environment they are in and all of earths organisms have a typical body temperature of above 0C and 100C, the freezing and boiling points of water respectively. So water at this temperature is liquid and we have evolved to carry out reactions at this temperature and use water as the solution for many reactions.

But why couldn't we have just done the same for water as a gas or a solid. Well as a gas it takes up a much larger volume, phenomenally higher and therefore we would either need to be massive or use high pressures. Additionally, containing reactions that carried out at this temperature with probably huge releases of energy is incredibly difficult. As a solid it is difficult to mobilise. As we dissolve essentially all of our bodies nutrients in water, or use the hydrophobic properties of fats to mobilise lipid soluble things, it would be incompatible with life. We rely very strongly on the ability of water to flow. Solids can't.

An interesting question is why water and not another liquid?

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This is because of the physico-chemical properties of water. It can form hydrogen bonds with itself, making sure it will not be gaseous before reaching 100°C (under normal pressure). H<sub>2</sub>S for example, which is relatively similar when looking at the atoms from which it is build, melts at -85.7°C and boils at -60.2°C. Water is also capable of dissolving a lot of other substances, it is not acidic or reactive (though it can take part in reactions), which makes it the perfect substance for the basis of life. –  Chris Mar 12 at 10:09
    
What about alcohols? –  AndroidPenguin Mar 12 at 10:11
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Lower alcohols could work, but we have the same problems here with melting/boiling points and the solubility of substances. An alcohol based life form would of course rely on other molecules, but it would still be difficult. –  Chris Mar 12 at 10:18
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