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The most common other proposed basis is silicon, since silicon has many similar chemical properties to carbon. Silicon has a number of handicaps as a carbon analogue, however. Because silicon atoms are much bigger, they have difficulty forming double or triple bonds. Silanes (hydrogen-silicon compounds analogous to the alkane hydrocarbons) are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating Si-O-Si bonds (known collectively as silicones) instead of Si-Si bonds are much more stable; ordinary sand is one such example. However, silicon dioxide (the analogue of carbon dioxide) is a non- soluble solid at the temperature range where liquid water is possible making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. In general, complex long-chain silicone-based molecules are still more unstable than their carbon counterparts.
Finally, of the varieties molecules identified in interstellar space as of 1998, 84 are based on carbon and 8 are based on silicon. Moreover, of the eight Si-based compounds, four also include carbon within them. This suggests a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies. The cosmic abundance of carbon to silicon is 3.5 to 1.
It is possible that silicon compounds may be biologically useful under certain exotic environmental conditions, however, either in conjunction with carbon or in a role less directly analogous to carbon. A simple real-world example is the silicate skeletal structure of diatoms.
Earth's atmosphere is approximately 80% nitrogen, but this would probably not be much use to a P-N lifeform since molecular nitrogen (N2) is very inert and energetically expensive to " fixNitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N) in the atmosphere and converted into nitrogen compounds useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide)." (certain Earth plantGreen algae land plants (embryophytes non-vascular embryophytes Hepatophyta liverworts Anthocerophyta hornworts Bryophyta mosses vascular plants (tracheophytes seedless vascular plants Lycopodiophyta clubmosses Equisetophyta horsetails Pteridophyta "true"s such as legumeThe term legume has two closely related meanings in botany, a situation encountered with many botanical common names of useful plants whereby an applied name can refer to either the plant itself, or to the edible fruit (or useful part). Thus, "legume" cans can fix nitrogen using symbioticClownfish (Amphiprion ocellaris) in their Magnificent Sea Anemone (Heteractis magnifica) home. Symbiosis (pl. symbioses) is an interaction between two organisms living together in more or less intimate association or even the merging of two dissimilar org anaerobic bacteria contained within their root nodules). A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful (Nitrogen actually forms a number of oxides with oxygen (N2O, N2O4), and all would be present in a nitrogen dioxide-rich atmosphere).
In a nitrogen dioxide atmosphere, phosphorus-nitrogen-based plant analogues could absorb nitrogen dioxide from the atmosphere and phosphorus from the ground. The nitrogen dioxide would be reduced, P-N sugar analogues being produced in the process, and waste oxygen would be released into the atmosphere. P-N animal analogues would consume the plants, use atmospheric oxygen to metabolize the P-N sugar analogues, exhaling nitrogen dioxide and depositing phosphorus (or phosphorus rich material) as solid waste.
In an ammonia atmosphere, P-N plants would absorb ammonia from the atmosphere and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and indeed from the known biochemistry of Earth; it would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide. Debate continues as several aspects of a P-N cycle biology would be energy deficient.
Unfortunately, nitrogen and phosphorus are not likely to be found in the ratios and quantity required in the real universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
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