Alternative biochemistry is the speculative biochemistry of alien life forms that differ radically from those on Earth. It includes biochemistries that use atoms other than carbon to construct primary cellular structures and/or use solvents besides water. Theories about extraterrestrial life based on alternative biochemistries are common in science fiction.

Atoms other than carbon
The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group.
But silicon has a number of handicaps as a carbon alternative. Because silicon atoms are much bigger, having a larger mass and atomic radius, they have difficulty forming double or triple covalent bonds, which are important for a biochemical system. 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 sulphuric-acid-rich environment, as is found in some extraterrestrial locations.

Nitrogen and phosphorus biochemistry
Arsenic, which is chemically similar to phosphorus, while poisonous for most Earth life, is incorporated into the biochemistry of some organisms.

Other exotic biochemical elements
Physicists have noted that, while photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of solar radiation than that received on Earth. Ordinarily fungi derived their energy from decomposing other biomass, rather than by converting radiation into energy for itself.

Alternative energy sources
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. It is sometimes assumed that water is the only suitable chemical to fill this role. 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. There are other chemicals with similar properties that have sometimes been proposed as alternatives.

Non-water solvents
Ammonia is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.
However, ammonia does have some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons, science questions how well ammonia could hold prebiotic molecules together in order to allow the emergence of a self-reproducing system. Ammonia is also combustible and oxidizable and could not exist sustainably in a biosphere that oxidizes it. It would, however, be stable in a reducing environment.
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life. Terrestrial life usually exists within the melting point and boiling point of water at normal pressure, between 0°C (273 K) and 100°C (373 K); at normal pressure ammonia's melting and boiling points are between −78°C (195 K) and −33°C (240 K). Such extremely cold temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation out of solution due to high melting points. Ammonia could be a liquid at normal temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77°C (196 K) and boils at 98°C (371 K).
Ammonia and ammonia-water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based "habitability zone". Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.

Ammonia
Hydrogen fluoride, like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is -84°C and its boiling point is 19.54°C; the difference between the two is little more than 100°C. HF also makes hydrogen bonds with its neighbor molecules as do water and ammonia. All of these things make HF a candidate to host life on other planets.
Not much research has been done on liquid HF in regards to its ability to dissolve and react with non-polar molecules. It is possible that the biota in an HF ocean could use the fluorine as an electron acceptor to photosynthesize energy.

Hydrogen fluoride
Other solvents sometimes proposed include methanol, hydrogen sulfide and hydrogen chloride. The latter two suffer from a relatively low cosmic abundance of sulfur and chlorine, which tend to be bound up in solid minerals. A mixture of hydrocarbons, such as the methane/ethane lakes detected on Titan by the Cassini spacecraft, could act as a solvent over a wide range of temperatures but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen.

Other solvents
In 2007 V. N. Tsytovich and colleagues discovered the possibility of life-like behaviors being exhibited by dust particles suspended in a plasma, similar to conditions in interstellar space. Computer models showed that when the dust became charged the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures, and evolving into more stable forms. Similar forms of life were speculated on in Fred Hoyle's classic novel The Black Cloud.

Interstellar dust-based life
In the realm of science fiction there have occasionally been forms of life proposed that, while often highly speculative and unsupported by rigorous theoretical examination, are nevertheless interesting and in some cases even somewhat plausible.

In fiction
An example of silicon based life forms takes place in the novel Sentenced to Prism by Alan Dean Foster in which the protagonist Evan Orgell finds himself trapped on a planet whose entire ecosystem is mostly silicon-based.
Perhaps the most extreme example in science fiction is James White's Sector General: a series of novels and short stories about multienvironment hospital for the strangest lifeforms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species metabolise directly hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All of the life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc) with four letter codes. Humans from Earth share the DBDG specification with small furry beings called Nidians.
The Spiderman villain, Sandman is an example of a silicon based organism in comicbooks.
One of the major sentient species in Terry Pratchett's Discworld universe are the silicon-based Trolls.
Pratchett has also written the science fiction novel The Dark Side of the Sun which features a range of extraordinary lifeforms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.
Fred Hoyle's classic novel The Black Cloud features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular terrestrial life interact. Outside of science-fiction, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth. Inorganic dust-based life has been speculated upon based on recent computer simulations. and sprout apendages at will, they excel at 'closer-then-melee-range combat, primarily "meme-toxins" against other Amorphs.
A more humorous example comes from the Hitchhiker's Guide to the Galaxy, where the Hooloovoo are a hyperintelligent shade of the colour blue.

Novels, short stories and comics
A well-known example of a non–carbon-based lifeform in science fiction is the Horta in the original Star Trek episode "Devil in the Dark". A highly intelligent silicon-based creature made almost entirely of pure rock, it tunnels through rock as easily as humans move through air. The entire species dies out every 50,000 years save for one who tends all the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by murdering the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal lifeforms with an alternative biochemistry. The Tholians of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise episode "In a Mirror, Darkly" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOS's third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.
Later on, in Star Trek: The Next Generation, the Crystalline Entity appeared in one episode, Silicon Avatar and was referenced in another, Datalore. This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it.
In another episode, Home Soil, intelligent crystals that formed a "microbrain" were discovered during a terraforming mission, and they described the humans they encountered as "ugly bags of mostly water."
"The Disease", an episode of Star Trek:Voyager featured some artificially-engineered silicon-based parasites, and an Enterprise episode, "Observer Effect", also presented a lethal silicon-based virus. In another Voyager episode, "Hope and Fear", a xenon-based lifeform was mentioned. In the Enterprise episode The Communicator, an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different to terrestrial organisms that it is not red and iron is toxic to it.

Star Trek
In the Star Wars movie The Empire Strikes Back, two lifeforms were encountered by the characters that were non-carbon based entities. Although details of their physiology were not mentioned on screen, the Space slug, (a giant worm-like creature that lived on asteroids in the freezing vacuum of space), and the Mynock, (pesky bat-like vermin that would attach themselves to spaceship hulls and chew through power conduits to feed off the raw energy), are said to be silicon-based organisms in expanded universe sources.

Star Wars
In the the movie Alien the science officer Ash notes that the facehugger creature replaces its cells with polarised silicon in order to give it "prolonged resistance to adverse environmental conditions". Both stages of the alien cycle also have a highly corrosive blood, normally understood to be some kind of acid, which would be inconsistent with any known terrestrial biochemistry.

Alien
In the movie The Monolith Monsters, a silicon meteor reproduces itself, draining silicates from everything it touch. It needs water to start its cycle and contains molecular structures typical of many kinds of rocks, mixed together. A geologist says that its structure is nearly impossible. The meteor is killed by salt water, that can stop the cycle.
In "Firewalker", a second-season episode of The X-Files, a silicon-based plant that infects humans parasitically through its spores is discovered living deep in a volcano.
Also from The X-Files, the first-season episode 'Ice' deal with an ammonia-based vermiform parasite.
A key plot point in the comedy Evolution involves nitrogen-based life forms, and using selenium-based shampoo to poison them (with the bonus of a product placement for Head & Shoulders).
In the Stargate SG-1 fourth season episode "Scorched Earth", a Human society known as the Enkarans are threatened on their new homeworld by an alien ship that is terraforming the planet to be suitable for the sulfur-based Gadmeer species.

Computer and video games

Astrobiology
Carbon-based life
Extraterrestrial life
Non-cellular life
Alternative biology
Iron-sulfur world theory
Carbon chauvinism