Hypothetical types of biochemistry
Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10 17 million years old.[1][2] According to the panspermia hypothesis, microscopic life distributed by meteoroids, asteroids and other small Solar System bodies may exist throughout the universe.[3] Nonetheless, Earth is the only place in the universe known to harbor life.[4][5]
While the kinds of living beings currently known on Earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define and control their form, it may be possible that undiscovered life forms could exist that differ radically in their basic structures and biochemistry from that known to science. The possibility of extraterrestrial life being based on these alternative biochemistries is a common subject in science fiction, but is also discussed in a non fiction scientific context.
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Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certapatagonia better sweater womensin whether a statement that applies to all life on Earth will tpatagonia better sweater zapposurn out to apply to all life throughout the universe.[8] Sagan used the term carbon chauvinism for such an assumption.[9] Carl Sagan regarded silicon and germanium as conceivable alternatives to carbon;[9] but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.[10]
The most commonly proposed basis for an alternative biochemical system is the silicon atom, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.patagonia men's down sweater for sale[11]
However, silicon has spatagonia down vest closeouteveral drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility patagonia firefighter discountrequired for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms.[11] Moreover, where it does interact with other atoms, silicon creates molecules that have been described as monotonous compared with the combinatorial universe of organic macromolecules .[11] This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double bonded carbon is part of the carbonyl group, a fundamental motif of bio organic chemistry).
Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone based chemicals would be more stable than equivalent hydrocarbons in a sulfuric acid rich environment, as is found in some extraterrestrial locations.[12] Complex long chain silicone molecules are spatagonia guide pants 801till less stable than their carbon counterparts, though.
Finally, of the varpatagonia better sweater peacoat closeoutieties of molecules identified in the interstellar medium as of 1998[13] Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon based biologies, at least under the conditions prevalent on the surface of planets. Somewhat in support, in September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively .[14][15] (Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks. [14][15])
Also, even though Earth and other terrestrial planets are exceptionally silicon rich and carbon ppatagonia guide pants 803oor (the relative abundance of silicon to carbon in the Earth s crust is roughly 925:1), terrestrial life is carbon based. The fact that carbon is used instead of silicon, may be evidence that silicon is poorly suited for biochemistry on Earth like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat.[16] Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.
A. G. Cairns Smith has proposed that the first living organisms to exist on Earth were clay minerals which were probably based on silicon.[17]
In cinematic and literary science fiction, at a moment when man made machines cross from nonliving to living, it is often posited, this new form would be the first example of non carbon based life. Boranes are dangerously explosive in Earth s atmosphere, but would be more stable in a reducing environment. However, boron s low cosmic abundance makes it less likely as a base for life than carbon.
See also: Organoboron chemistry
Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds;[citation needed] the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth s crust than carbon. Metal oxide based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon based life would be unlikely. The Cronin group at Glasgow University has created lifelike cells based on tungsten polyoxometalates.[18]
Sulfur is also able to form long chain molecules, but suffers from the same high reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 years on Earth, thus predating the use of molecular oxygen.[19] Sulfur reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
Arsenic as an alternative to phosphorus[edit]Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.[20] Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis).[21] Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner,[30] and by the astrobiological committee chaired by John A. Baross.[31] Solvents discussed by the Baross committee include ammonia,[32] sulfuric acid,[33] formamide,[34] hydrocarbons,[34] and (at temperatures much lower than Earth s) liquid nitrogen, or hydrogen in the form of a supercritical fluid.[35]
Carl Sagan once described patagonia guide pants 800himself as both a carbon chauvinist and a water chauvinist;[36] however on another occasion he said he was a carbon chauvinist but not that much of a water chauvinist .[37] He considered hydrocarbons,[37] hydrpatagonia guide pants 802ofluoric acid,[38] and ammonia[37][38] as possible alternatives to water.
Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water has the unusual property of being less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all othepatagonia guide pants hemmedr compounds), thpatagonia guide pants holderen large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
Not all properties of water are necessarily advantageous for life, however.[39] For instance, water ice has a high albedo,[39] meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.[39]
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibriupatagonia guide pants 80m over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (9.2 of pressure, it can indeed exist in liquid form over a wide temperature range.
