|MadSci Network: Biochemistry|
Water is the most abundant chemical in all living organisms. It is such an intrinsic part, that as you are well aware, it is the first thing we talk about when looking for life on other planets. So why is it needed and what for? You are right to suggest it acts as a solvent. A cell is like a huge (I know we consider it minuscule but in terms of the size of molecules it is huge) chemical reactor but with all the reactions inter-inked to create a massive reaction web. The products of one reaction are used in another, and the products from that reaction in another. It is therefore important that the chemicals be able to be solution and as such diffuse so that they collide with other reactants and enzymes. The 2nd component of the need for water in reactions is the nature of the chemicals involved and the reactions themselves. In your question you comment on RNA and the need for organic molecules of life to be in an aqueous solution. If we look at most of the organic molecules of life they are water soluble, but this is a bit like a chicken and egg story. They are because water was the solvent or was it the other way round. Well water was the only major solvent available at the "dawn of life" so necessity was filled. Another consideration is that all of the macromolecules of life use water in their synthesis and degradation. Proteins, Lipids and Carbohydrates monomers are all polymerised into macromolecules by dehydration reactions (the removal of water) and degraded by hydrolysis reactions (water in). So again we can see that water is an intricate part of life. We could continue to discuss this and bring in more examples but I think you can see the train f thought here, One final answer however is yes it does have to be liquid. Why? Well think about the above, it needs to allow diffusion and be able to be a reactant. Solid water does not fulfil these 2 criteria.
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