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===Biochemistry===
{{Main|Biochemistry|Hypothetical types of biochemistry|Water and life}}
All [[life on Earth]] is based upon 26 [[chemical element]]s. However, about 95% of this life is built upon only six of these elements: [[carbon]], [[hydrogen]], [[nitrogen]], [[oxygen]], [[phosphorus]] and [[sulfur]], abbreviated [[CHNOPS]]. These six elements form the basic building blocks of virtually all life on Earth, while most of the remaining elements are found in only trace amounts.<ref>{{cite book | first1=Lucas John | last1=Mix | year=2009 | title=Life in space: astrobiology for everyone | page=76 | publisher=Harvard University Press | isbn=0-674-03321-3 | url=http://books.google.com/books?id=A60E-v4Sr-gC&pg=PA76 | accessdate=2011-08-08 }}</ref>
Life on Earth requires [[water]] as the [[solvent]] in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth.<ref>{{cite journal | first=Norman R. | last=Pace | date=January 20, 2001 | title=The universal nature of biochemistry | journal=Proceedings of the National Academy of Sciences, U.S.A. | volume=98 | issue=3 | pages=805–808 | doi=10.1073/pnas.98.3.805 | pmid=11158550 | bibcode=2001PNAS...98..805P | pmc=33372}}</ref> [[Terrestrial planet]]s such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.<ref>{{cite journal | last1=Bond | first1=Jade C. | last2=O'Brien | first2=David P. | last3=Lauretta | first3=Dante S. | title=The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations | journal=The Astrophysical Journal | volume=715 | issue=2 | pages=1050–1070 | month=June | year=2010 | doi=10.1088/0004-637X/715/2/1050 | bibcode=2010ApJ...715.1050B |arxiv = 1004.0971 }}</ref> The combination of carbon, hydrogen and oxygen in the chemical form of [[carbohydrate]]s (e.g. [[sugar]]) can be a source of chemical [[energy]] on which life depends, and can provide structural elements for life (such as [[ribose]], in the molecules [[DNA]] and [[RNA]], and [[cellulose]] in plants). [[Plant]]s derive energy through the conversion of light energy into chemical energy via [[photosynthesis]]. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially [[oxidized]] ([[carbon oxide]]s) states. [[Nitrogen]] is needed as a reduced [[ammonia]] derivative in all [[protein]]s, [[sulfur]] as a derivative of [[hydrogen sulfide]] in some necessary proteins, and [[phosphorus]] oxidized to [[phosphate]]s in genetic material and in energy transfer.
Pure water is useful because it has a neutral [[pH]] due to its continued dissociation between [[hydroxide]] and [[hydronium]] [[ionic bond|ions]]. As a result, it can dissolve both positive [[cations|metallic ions]] and negative [[anions|non-metallic ions]] with equal ability. Furthermore, the fact that organic molecules can be either [[hydrophobic]] (repelled by water) or [[hydrophilic]] (soluble in water) creates the ability of organic compounds to orient themselves to form water-enclosing [[biological membrane|membranes]]. Additionally, the [[hydrogen bond]]s between water molecules give it an ability to store [[latent heat|energy]] with [[evaporation]], which upon [[condensation]] is released. This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.
Carbon is fundamental to terrestrial life for its immense flexibility in creating [[covalent bond|covalent chemical bonds]] with a variety of non-metallic elements, principally [[nitrogen]], [[oxygen]] and [[hydrogen]]. [[Carbon dioxide]] and water together enable the storage of solar energy in [[sugars]], such as [[glucose]]. The [[oxidation]] of glucose releases biochemical energy needed to fuel all other biochemical reactions.
The ability to form [[organic acid]]s (–COOH) and [[amine]] [[Base (chemistry)|bases]] (–NH<sub>2</sub>) gives rise to the possibility of [[neutralization (chemistry)|neutralization]] dehydrating reactions to build long [[polymer]] [[peptides]] and [[Catalyst|catalytic]] [[proteins]] from [[monomer]] [[amino acids]]. When combined with [[phosphate]]s, these acids can build the information-storing molecule of inheritance, [[DNA]], and the principal energy transfer molecule of cellular life, [[Adenosine triphosphate|ATP]].
Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for [[life]].
Life forms based in [[ammonia]] (rather than water) have been suggested, though this solution appears less optimal than water.<ref>{{cite web| url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html| publisher=daviddarling.info| title=Ammonia-based life}}</ref>
From a chemical perspective, life is fundamentally a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the [[Plasma (physics)|plasma]] of a star, though it would be highly unconventional.<ref name="Darlingvariety">{{cite web| url=http://www.daviddarling.info/encyclopedia/E/etlifevar.html| publisher=daviddarling.info| title=extraterrestrial life, variety of}}</ref>
Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, a [[NASA]] scientist suggested that the color of [[photosynthesis|photosynthesizing pigments]] of hypothetical life on [[extrasolar planet]]s might not be green.<ref>{{cite news | first = Rob Gutro | title = NASA Predicts Non-Green Plants on Other Planets | date = 4 November 2007 | publisher = NASA | url = http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html | work = Goddard Space Flight Center | accessdate = 2011-02-18}}</ref>
===Evolution and morphology===
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