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Amniote

From Wikipedia, the free encyclopedia

Amniotes

Temporal range:
Pennsylvanian–Present

PreꞒ

Ꞓ

(Possible Mississippian record)

From top to bottom and left to right, examples of amniotes: Edaphosaurus and a red fox (two synapsids) & king cobra and a white-headed buffalo weaver (two sauropsids).

Scientific classification

Domain:

Kingdom:

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Clade:

Amniota
Haeckel, 1866

Clades

incertae sedis

Amniotes are animals belonging to the clade Amniota, a group of tetrapod vertebrates that comprises the vast majority of terrestrial vertebrates. Amniotes evolved from amphibian ancestors and further diverged into two groups, namely the sauropsids (including all reptiles and birds) and synapsids (including mammals and extinct ancestors like "pelycosaurs" and therapsids). They are distinguished from the other living tetrapod clade — the lissamphibians (frogs/toads, salamanders, newts and caecilians) — by the development of three extraembryonic membranes (amnion for embryonic protection, chorion for gas exchange, and allantois for metabolic waste disposal or storage), thicker and more keratinized skin, and costal respiration (breathing by expanding/constricting the rib cage).

All three main features listed above, namely the presence of an amniotic buffer, water-impermeable cutes and a robust air-breathing respiratory system, are very important for amniotes to live on land as true terrestrial animals — the ability to survive and procreate in locations away from water bodies, better homeostasis in drier environments, and more efficient non-aquatic gas exchange to power terrestrial locomotions, although they might still require regular access to drinking water for rehydration like the semiaquatic amphibians do. Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs (reptiles, birds and monotremes) or nurturing fertilized eggs within the mother (marsupial and placental mammals), unlike anamniotes (fish and amphibians) that have to spawn in or closely adjacent to aquatic environments. Additional unique features are the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys, which are more complex, the presence of an astragalus for better extremity range of motion, and the complete loss of metamorphosis (which includes an egg and aquatic larval stage), gill and skin breathing, and any lateral line system.

The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from semiaquatic reptiliomorphs about 312 million years ago during the Carboniferous period. After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates, and soon diverged into the synapsids and sauropsids, whose lineages both still persist today. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago, while the oldest known sauropsid are probably Hylonomus and Paleothyris in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago). Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being.

Etymology

The term amniote comes from the amnion, which derives from Greek ἀμνίον (amnion), which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός (amnos), meaning "lamb".

Description

Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis, as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.

Adaptation for terrestrial living

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys (metanephros) and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta.

The ancestors of true amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic). The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating. Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion.

The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4-inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.

A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water.

The egg membranes

Fish and amphibian eggs have only one inner membrane, the embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.

Amniote traits

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage. There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of temporal fenestrae (openings) behind each eye. In anapsids, the ancestral condition, there are none, in synapsids (mammals and their extinct relatives) there is one, and most diapsids (including birds, crocodilians, squamates, and tuataras), have two. Turtles were traditionally classified as anapsids because they lack fenestrae, but molecular testing firmly places them in the diapsid line of descent – they therefore secondarily lost their fenestrae.

Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle.

Definition and classification

Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion, hence the name. A problem with this definition is that the trait (apomorphy) in question does not fossilize, and the status of fossil forms has to be inferred from other traits.

Amniotes

Archaeothyris, one of the most basal synapsids, first appears in the fossil records about 306 million years ago.

By the Mesozoic, 150 million years ago, sauropsids included the largest animals anywhere. Shown are some late Jurassic dinosaurs, including the early bird Archaeopteryx perched on a tree stump.

Traditional classification

Classifications of the amniotes have traditionally recognised three classes based on major traits and physiology:

Class Reptilia (reptiles)
- Subclass Anapsida ("proto-reptiles", possibly including turtles)
- Subclass Diapsida (majority of reptiles, progenitors of birds)
- Subclass Euryapsida (plesiosaurs, placodonts, and ichthyosaurs)
- Subclass Synapsida (stem or proto-mammals, progenitors of mammals)
Class Aves (birds)
- Subclass Archaeornithes (reptile-like birds, progenitors of all other birds)
- Subclass Enantiornithes (early birds with an alternative shoulder joint)
- Subclass Hesperornithes (toothed aquatic flightless birds)
- Subclass Ichthyornithes (toothed, but otherwise modern birds)
- Subclass Neornithes (all living birds)
Class Mammalia (mammals)
- Subclass Prototheria (Monotremata, egg-laying mammals)
- Subclass Theria (metatheria (such as marsupials) and eutheria (such as placental mammals))

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic—it has given rise to two other classes not included in Reptilia.

Most species described as microsaurs, formerly grouped in the extinct and prehistoric amphibian group lepospondyls, has been placed in the newer clade Recumbirostra, and shares many anatomical features with amniotes which indicates they were amniotes themselves.

Classification into monophyletic taxa

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below.

Series Amniota
- Class Synapsida
  - A series of unassigned families, corresponding to Pelycosauria †
  - Order Therapsida
    - Class Mammalia – mammals
- Class Sauropsida
  - Subclass Parareptilia †
    - Family Mesosauridae †
    - Family Millerettidae †
    - Family Bolosauridae †
    - Family Procolophonidae †
    - Order Pareiasauromorpha
      - Family Nycteroleteridae †
      - Family Pareiasauridae †
  - Subclass Eureptilia
    - Family Captorhinidae †
    - Infraclass Diapsida
      - Family Araeoscelididae †
      - Family Weigeltisauridae †
      - Order Younginiformes †
      - Infraclass Neodiapsida
        Order Testudinata
        Suborder Testudines – turtles
        Infraclass Lepidosauromorpha
        Unnamed infrasubclass
        Infraclass Ichthyosauria †
        Order Thalattosauria †
        Superorder Lepidosauriformes
        Order Sphenodontida – tuatara
        Order Squamata – lizards & snakes
        Infrasubclass Sauropterygia †
        Order Placodontia †
        Order Eosauropterygia †
        Suborder Pachypleurosauria †
        Suborder Nothosauria †
        Order Plesiosauria †
        Infraclass Archosauromorpha
        Family Trilophosauridae †
        Order Rhynchosauria †
        Order Protorosauria †
        Division Archosauriformes
        Subdivision Archosauria
        Infradivision Crurotarsi
        Order Phytosauria†
        Family Ornithosuchidae †
        Family Stagonolepididae †
        Family Rauisuchidae †
        Superfamily Poposauroidea †
        Superorder Crocodylomorpha
        Order Crocodylia – crocodilians
        Infradivision Avemetatarsalia
        Infrasubdivision Ornithodira
        Order Pterosauria †
        Family Lagerpetidae †
        Family Silesauridae †
        Superorder Dinosauria – dinosaurs
        Order Ornithischia †
        Order Saurischia
        Suborder Theropoda – theropods
        Class Aves – birds

Phylogenetic classification

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants". As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy. Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy.

Mosaic (genetics)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Mosaic_(genetics)

Mosaicism or genetic mosaicism is a condition in which a multicellular organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized egg. Mosaicism is one of several possible causes of chimerism, wherein a single organism is composed of cells with more than one distinct genotype.

Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo. Mosaicism can also result from a mutation in one cell during development, in which case the mutation will be passed on only to its daughter cells (and will be present only in certain adult cells). Somatic mosaicism is not generally inheritable as it does not generally affect germ cells.

History

In 1929, Alfred Sturtevant studied mosaicism in Drosophila, a genus of fruit fly. Muller in 1930 demonstrated that mosaicism in Drosophila is always associated with chromosomal rearrangements and Schultz in 1936 showed that in all cases studied these rearrangements were associated with heterochromatic inert regions, several hypotheses on the nature of such mosaicism were proposed. One hypothesis assumed that mosaicism appears as the result of a break and loss of chromosome segments. Curt Stern in 1935 assumed that the structural changes in the chromosomes took place as a result of somatic crossing, as a result of which mutations or small chromosomal rearrangements in somatic cells. Thus the inert region causes an increase in mutation frequency or small chromosomal rearrangements in active segments adjacent to inert regions.

In the 1930s, Stern demonstrated that genetic recombination, normal in meiosis, can also take place in mitosis. When it does, it results in somatic (body) mosaics. These organisms contain two or more genetically distinct types of tissue. The term somatic mosaicism was used by CW Cotterman in 1956 in his seminal paper on antigenic variation.

In 1944, Belgovskii proposed that mosaicism could not account for certain mosaic expressions caused by chromosomal rearrangements involving heterochromatic inert regions. The associated weakening of biochemical activity led to what he called a genetic chimera.

Types

Germline mosaicism

Germline or gonadal mosaicism is a particular form of mosaicism wherein some gametes—i.e., sperm or oocytes—carry a mutation, but the rest are normal. The cause is usually a mutation that occurred in an early stem cell that gave rise to all or part of the gametes.

Somatic mosaicism

Somatic mosaicism(also known as clonal mosaicism) occurs when the somatic cells of the body are of more than one genotype. In the more common mosaics, different genotypes arise from a single fertilized egg cell, due to mitotic errors at first or later cleavages.

Somatic mutation leading to mosaicism is prevalent in the beginning and end stages of human life. Somatic mosaics are common in embryogenesis due to retrotransposition of long interspersed nuclear element-1 (LINE-1 or L1) and Alu transposable elements. In early development, DNA from undifferentiated cell types may be more susceptible to mobile element invasion due to long, unmethylated regions in the genome. Further, the accumulation of DNA copy errors and damage over a lifetime lead to greater occurrences of mosaic tissues in aging humans. As longevity has increased dramatically over the last century, human genome may not have had time to adapt to cumulative effects of mutagenesis. Thus, cancer research has shown that somatic mutations are increasingly present throughout a lifetime and are responsible for most leukemia, lymphomas, and solid tumors.

Trisomies, monosomies, and related conditions

The most common form of mosaicism found through prenatal diagnosis involves trisomies. Although most forms of trisomy are due to problems in meiosis and affect all cells of the organism, some cases occur where the trisomy occurs in only a selection of the cells. This may be caused by a nondisjunction event in an early mitosis, resulting in a loss of a chromosome from some trisomic cells. Generally, this leads to a milder phenotype than in nonmosaic patients with the same disorder.

In rare cases, intersex conditions can be caused by mosaicism where some cells in the body have XX and others XY chromosomes (46, XX/XY). In the fruit fly Drosophila melanogaster, where a fly possessing two X chromosomes is a female and a fly possessing a single X chromosome is a sterile male, a loss of an X chromosome early in embryonic development can result in sexual mosaics, or gynandromorphs. Likewise, a loss of the Y chromosome can result in XY/X mosaic males.

An example of this is one of the milder forms of Klinefelter syndrome, called 46,XY/47,XXY mosaic wherein some of the patient's cells contain XY chromosomes, and some contain XXY chromosomes. The 46/47 annotation indicates that the XY cells have the normal number of 46 total chromosomes, and the XXY cells have a total of 47 chromosomes.

Also monosomies can present with some form of mosaicism. The only non-lethal full monosomy occurring in humans is the one causing Turner's syndrome. Around 30% of Turner's syndrome cases demonstrate mosaicism, while complete monosomy (45, X) occurs in about 50–60% of cases.

Mosaicism need not necessarily be deleterious, though. Revertant somatic mosaicism is a rare recombination event with a spontaneous correction of a mutant, pathogenic allele. In revertant mosaicism, the healthy tissue formed by mitotic recombination can outcompete the original, surrounding mutant cells in tissues such as blood and epithelia that regenerate often. In the skin disorder ichthyosis with confetti, normal skin spots appear early in life and increase in number and size over time.

Other endogenous factors can also lead to mosaicism, including mobile elements, DNA polymerase slippage, and unbalanced chromosome segregation. Exogenous factors include nicotine and UV radiation]. Somatic mosaics have been created in Drosophila using X‑ray treatment and the use of irradiation to induce somatic mutation has been a useful technique in the study of genetics.

True mosaicism should not be mistaken for the phenomenon of X-inactivation, where all cells in an organism have the same genotype, but a different copy of the X chromosome is expressed in different cells. The latter is the case in normal (XX) female mammals, although it is not always visible from the phenotype (as it is in calico cats). However, all multicellular organisms are likely to be somatic mosaics to some extent.

Gonosomal mosaicism

Gonosomal mosaicism is a type of somatic mosaicism that occurs very early in the organisms development and thus is present within both germline and somatic cells. Somatic mosaicism is not generally inheritable as it does not usually affect germ cells. In the instance of gonosomal mosaicism, organisms have the potential to pass the genetic alteration, including to potential offspring because the altered allele is present in both somatic and germline cells.

Brain cell mosaicism

A frequent type of neuronal genomic mosaicism is copy number variation. Possible sources of such variation were suggested to be incorrect repairs of DNA damage and somatic recombination.

Mitotic recombination

One basic mechanism that can produce mosaic tissue is mitotic recombination or somatic crossover. It was first discovered by Curt Stern in Drosophila in 1936. The amount of tissue that is mosaic depends on where in the tree of cell division the exchange takes place. A phenotypic character called "twin spot" seen in Drosophila is a result of mitotic recombination. However, it also depends on the allelic status of the genes undergoing recombination. Twin spot occurs only if the heterozygous genes are linked in repulsion, i.e. the trans phase. The recombination needs to occur between the centromeres of the adjacent gene. This gives an appearance of yellow patches on the wild-type background in Drosophila. another example of mitotic recombination is the Bloom's syndrome, which happens due to the mutation in the blm gene. The resulting BLM protein is defective. The defect in RecQ, a helicase, facilitates the defective unwinding of DNA during replication, thus is associated with the occurrence of this disease.

Use in experimental biology

Genetic mosaics are a particularly powerful tool when used in the commonly studied fruit fly, where specially selected strains frequently lose an X or a Y chromosome in one of the first embryonic cell divisions. These mosaics can then be used to analyze such things as courtship behavior, and female sexual attraction.

More recently, the use of a transgene incorporated into the Drosophila genome has made the system far more flexible. The flip recombinase (or FLP) is a gene from the commonly studied yeast Saccharomyces cerevisiae that recognizes "flip recombinase target" (FRT) sites, which are short sequences of DNA, and induces recombination between them. FRT sites have been inserted transgenically near the centromere of each chromosome arm of D. melanogaster. The FLP gene can then be induced selectively, commonly using either the heat shock promoter or the GAL4/UAS system. The resulting clones can be identified either negatively or positively.

In negatively marked clones, the fly is transheterozygous for a gene encoding a visible marker (commonly the green fluorescent protein) and an allele of a gene to be studied (both on chromosomes bearing FRT sites). After induction of FLP expression, cells that undergo recombination will have progeny homozygous for either the marker or the allele being studied. Therefore, the cells that do not carry the marker (which are dark) can be identified as carrying a mutation.

Using negatively marked clones is sometimes inconvenient, especially when generating very small patches of cells, where seeing a dark spot on a bright background is more difficult than a bright spot on a dark background. Creating positively marked clones is possible using the so-called MARCM ("mosaic analysis with a repressible cell marker" system, developed by Liqun Luo, a professor at Stanford University, and his postdoctoral student Tzumin Lee, who now leads a group at Janelia Farm Research Campus. This system builds on the GAL4/UAS system, which is used to express GFP in specific cells. However, a globally expressed GAL80 gene is used to repress the action of GAL4, preventing the expression of GFP. Instead of using GFP to mark the wild-type chromosome as above, GAL80 serves this purpose, so that when it is removed by mitotic recombination, GAL4 is allowed to function, and GFP turns on. This results in the cells of interest being marked brightly in a dark background.

Fluorochemical industry

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Fluorochemical_industry

The global market for chemicals from fluorine was about US$16 billion per year as of 2006. The industry was predicted to reach 2.6 million metric tons per year by 2015. The largest market is the United States. Western Europe is the second largest. Asia Pacific is the fastest growing region of production. China in particular has experienced significant growth as a fluorochemical market and is becoming a producer of them as well. Fluorite mining (the main source of fluorine) was estimated in 2003 to be a $550 million industry, extracting 4.5 million tons per year.

Mined fluorite is separated into two main grades, with about equal production of each. Acidspar is at least 97% CaF₂; metspar is much lower purity, 60–85%. (A small amount of the intermediate, ceramic, grade is also made.) Metspar is used almost exclusively for iron smelting. Acidspar is primarily converted to hydrofluoric acid (by reaction with sulfuric acid). The resultant HF is mostly used to produce organofluorides and synthetic cryolite.

Inorganic fluorides

About 3 kg (6.5 lb) of metspar grade fluorite, added directly to the batch, are used for every metric ton of steel made. The fluoride ions from CaF₂ lower the melt's temperature and viscosity (make the liquid runnier). The calcium content has a tangential benefit in removing sulfur and phosphorus, but other additives such as lime are still needed. Metspar is similarly used in cast iron production and for other iron-containing alloys.

Fluorite of the acidspar grade is used directly as an additive to ceramics and enamels, glass fibers and clouded glass, and cement, as well as in the outer coating of welding rods. Acidspar is primarily used for making hydrofluoric acid, which is a chemical intermediate for most fluorine-containing compounds. Significant direct uses of HF include pickling (cleaning) of steel, cracking of alkanes in the petrochemical industry, and etching of glass.

One third of HF (one sixth of mined fluorine) is used to make synthetic cryolite (sodium hexafluoroaluminate) and aluminium trifluoride. These compounds are used in the electrolysis of aluminium by the Hall–Héroult process. About 23 kg (51 lb) are required for every metric ton of aluminium. These compounds are also used as a flux for glass.

Fluorosilicates are the next most significant inorganic fluorides formed from HF. The most common one, that of sodium, is used for water fluoridation, as an intermediate for synthetic cryolite and silicon tetrafluoride, and for treatment of effluents in laundries.

MgF₂ and, to a lesser extent, other alkaline earth difluorides are specialty optical materials. Magnesium difluoride is widely used as an antireflection coating for spectacles and optical equipment. The compound is also a component in newly devised constructions (negative index metamaterials) which are the subject of "invisibility" research. The layered structures can curve light around objects.

Other inorganic fluorides made in large quantities include cobalt difluoride (for organofluorine synthesis), nickel difluoride (electronics), lithium fluoride (a flux), sodium fluoride (water fluoridation), potassium fluoride (flux), and ammonium fluoride (various). Sodium and potassium bifluorides are significant to the chemical industry.

Fluorocarbons

Making organic fluorides is the main use for hydrofluoric acid, consuming over 40% of it (over 20% of all mined fluorite). Within organofluorides, refrigerant gases are still the dominant segment, consuming about 80% of HF. Even though chlorofluorocarbons are widely banned, the replacement refrigerants are often other fluorinated molecules. Fluoropolymers are less than one quarter the size of refrigerant gases in terms of fluorine usage, but are growing faster. Fluorosurfactants are a small segment in mass but are significant economically because of very high prices.

Gases

Traditionally chlorofluorocarbons (CFCs) were the predominant fluorinated organic chemical. CFCs are identified by a system of numbering that explains the amount of fluorine, chlorine, carbon and hydrogen in the molecules. The term Freon has been colloquially used for CFCs and similar halogenated molecules, though strictly speaking this is just a DuPont brand name, and many other producers exist. Brand neutral terminology is to use "R" as the prefix. Prominent CFCs included R-11 (trichlorofluoromethane), R-12 (dichlorodifluoromethane), and R-114 (1,2-dichlorotetrafluoroethane).

Production of CFCs grew strongly through the 1980s, primarily for refrigeration and air conditioning but also for propellants and solvents. Since the end use of these materials is banned in most countries, this industry has shrunk dramatically. By the early 21st century, production of CFCs was less than 10% of the mid-1980s peak, with remaining use primarily as an intermediate for other chemicals. The banning of CFCs initially depressed the overall demand for fluorite but 21st century production of the source mineral has recovered to 1980s levels.

Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) now serve as replacements for CFC refrigerants; few were commercially manufactured before 1990. Currently more than 90% of fluorine used for organics goes into these two classes (in about equal amounts). Prominent HCFCs include R-22 (chlorodifluoromethane) and R-141b (1,1-dichloro-1-fluoroethane). The main HFC is R-134a (1,1,1,2-tetrafluoroethane).

A bromofluoroalkane, "Halon" (bromotrifluoromethane) is still widely used in ship and aircraft gaseous fire suppression systems. Because Halon production has been banned since 1994, systems are dependent on the pre-ban stores and on recycling.

A new type of fluorinated refrigerant planned to replace the high Global Warming Potential HFC compounds are hydrofluoroolefins (HFOs).

Fluoropolymers

Fluoropolymers are less than 0.1% of all polymers produced in terms of weight. Compared to other polymers, they are more expensive and their consumption is growing at a higher rate. As of about 2006–2007, estimates of the global fluoropolymer production varied from over 100,000 to 180,000 metric tons per year. Yearly revenue estimates ranged from over $2.5 billion to over $3.5 billion.

Polytetrafluoroethylene (PTFE) is 60–80% of the world's fluoropolymer production on a weight basis.^[16] The term Teflon is sometimes used generically for the substance, but is a Chemours Company and Dupont brand name—other PTFE producers exist and Chemours sometimes uses the Teflon brand for other materials. PTFE gets its fluorine without the need for fluorine gas: chloroform (trichloromethane) is treated with HF to make chlorodifluoromethane (R-22, an HCFC); this chemical when heated makes tetrafluoroethylene (abbreviated TFE), the monomer of PTFE.

The largest application for PTFE is in electrical insulation. It is an excellent dielectric and very chemically stable. It is also used extensively in the chemical process industry where corrosion resistance is needed: in coating pipes, in tubing, and gaskets. Another major use is architectural fabric (PTFE-coated fiberglass cloth used for stadium roofs and such). The major consumer application is non-stick cookware.

Major PTFE applications

PTFE dielectric separating core and outer metal in a specialty coaxial cable	First Teflon branded frying pan, 1961	The interior of the Tokyo Dome. The roof is PTFE-coated fiberglass and air-supported.

When stretched with a jerk, a PTFE film makes a fine-pored membrane: expanded PTFE (ePTFE). The term "Gore-Tex" is sometimes used generically for this material, but that is a specific brand name. W.L. Gore & Associates is not the only producer of ePTFE and furthermore "Gore-Tex" often refers to more complicated multi-layer membranes or laminated fabrics. ePTFE is used in rainwear, protective apparel and liquids and gas filters. PTFE can also be formed into fibers which are used in pump packing seals and bag house filters for industries with corrosive exhausts.

Other fluoropolymers tend to have similar properties to PTFE—high chemical resistance and good dielectric properties—which leads to use in the chemical process industry and electrical insulation. They are easier to work with (to form into complex shap es), but are more expensive than PTFE and have lower thermal stability. Fluorinated ethylene propylene (FEP) is the second most produced fluoropolymer. Films from two fluoropolymers serve as glass-replacements in solar cells.

Fluorinated ionomers (polymers that include charged fragments) are expensive, chemically resistant materials used as membranes in certain electrochemical cells. Nafion, developed in the 1960s, was the first example and remains the most prominent material in the class. The initial Nafion application was as a fuel cell material in spacecraft. Since then, the material has been transforming the 55 million tons per year chloralkali industry; it is replacing hazardous mercury-based cells with membrane cells that are also more energy efficient. While older technology plants continue to run, new plants typically use membrane cells. By 2002, more than a third of the global capacity for the industry was membrane-cell based. Proton exchange membrane (PEM) fuel cells may be installed into vehicles.

Fluoroelastomers are rubber-like substances that are composed of crosslinked mixtures of fluoropolymers. Viton is a prominent example. Chemical-resistant O-rings are the primary application. Fluoroelastomers tend to be more stiff than conventional elastomers, but with superior chemical and heat resistance.

Surfactants

Fluorinated surfactants are small organofluorine molecules, principally used in durable water repellent (DWR). Fluorosurfactants form a large market, over $1 billion per year as of 2006. Scotchgard is a prominent brand, with over $300 million revenue in 2000. Fluorosurfactants are expensive chemicals, comparable to pharmaceutical chemicals: $200–2000 per kilogram ($90–900 per pound).

Fluorosurfactants make a very small part of the overall surfactant market, most of which is hydrocarbon based and much cheaper. Some potential applications (e.g. low cost paints) are unable to use fluorosurfactants because of the price impact of compounding in even small amounts of fluorosurfactant. Use in paints was only about $100 million as of 2006.

DWR is a finish (very thin coating) put on fabrics that makes them lightly rain resistant, that makes water bead. First developed in the 1950s, fluorosurfactants were 90% of the DWR industry by 1990. DWR is used with garment fabrics, carpeting, and food packaging. DWR is applied to fabrics by "dip-squeeze-dry" (immersion in DWR-water bath, pressing water out, and then drying).

Fluorine gas

Economy

For countries with available data (free-market countries), about 17,000 metric tons of fluorine are produced per year by 11 companies, all G7-resident. Fluorine is relatively inexpensive, costing about $5–8 per kilogram ($2–4 per pound) when sold as uranium hexafluoride or sulfur hexafluoride. Because of difficulties in storage and handling, the price of fluorine gas is much higher. Processes demanding large amounts of fluorine gas generally vertically integrate and produce the gas onsite for direct use.

Applications

Uranium production

The largest application for elemental fluorine is the preparation of uranium hexafluoride, which is used in the production of nuclear fuels. To obtain the compound, uranium dioxide is first treated with hydrofluoric acid, to produce uranium tetrafluoride. This compound is then further fluorinated by direct exposure to fluorine gas to make the hexafluoride. Fluorine's monoisotopic natural occurrence makes it useful in uranium enrichment, because uranium hexafluoride molecules will differ in mass only because of mass differences between uranium-235 and uranium-238. These mass differences are used to separate uranium-235 and uranium-238 via diffusion and centrifugation.

Up to 7,000 metric tons per year of fluorine gas are used for this application. As of 2013, 686,500 metric tons of UF6, containing about 470,000 metric tons of depleted uranium (the remainder being fluorine), were stored at the Paducah Gaseous Diffusion Plant, the USEC's Piketon site, Ohio and the East Tennessee Technology Park (formerly known as the K-25 Site).

Dielectric

The second largest application for fluorine gas is for the production of sulfur hexafluoride, which is used as a dielectric medium in high voltage switching stations. SF₆ gas has a much higher dielectric strength than air. It is extremely inert. Many alternatives to oil-filled switchgear, contain polychlorinated biphenyls (PCBs). Sulfur hexafluoride is also used in soundproof windows, in the electronics industry, as well as niche medical and military applications. The compound can be made without using fluorine gas, but the reaction between sulfur and fluorine gas, first developed by Henri Moissan, remains the commercial practice. About 6,000 metric tons per year of fluorine gas are consumed.

Chemical vapor deposition

Several compounds made from elemental fluorine serve the electronics industry. Rhenium and tungsten hexafluorides are used for chemical vapor deposition of thin metal films onto semiconductors.

Plasma etching

Tetrafluoromethane, is used for plasma etching in semiconductor manufacturing, flat panel display production, and microelectromechanical systems fabrication. Nitrogen trifluoride is increasingly used for cleaning equipment at display manufacturing plants. Elemental fluorine, itself, is used sometimes for cleaning equipment.

Halogenation

For making niche organofluorines and fluorine-containing pharmaceuticals, direct fluorination is usually too hard to control. Preparation of intermediate strength fluorinators from fluorine gas solves this problem. The halogen fluorides ClF₃, BrF₃, and IF₅ provide gentler fluorination, with a series of strengths. They are also easier to handle. Sulfur tetrafluoride is used particularly for making fluorinated pharmaceuticals.

Rocket fuel

United States and Soviet space scientists in the early 1960s studied elemental fluorine as a possible rocket propellant because of the higher specific impulse generated when fluorine replaced oxygen in combustion. The experiments failed because fluorine proved difficult to handle, and its combustion product (typically hydrogen fluoride) was extremely toxic and corrosive.

Production of fluorine gas

Commercial producers of fluorine gas continue to use the method of electrolysis pioneered by Moissan, with some modifications in the cell design. Owing to the gas's corrosiveness, special containment materials and handling precautions are required. Chemical routes to the elemental form were published in 1986.

Electrolytic synthesis

Several thousand metric tons of elemental fluorine are produced annually by electrolysis of potassium bifluoride in hydrogen fluoride. Potassium bifluoride forms spontaneously from potassium fluoride and the hydrogen fluoride:

HF + KF → KHF₂

A mixture with the approximate composition KF•2HF melts at 70 °C (158 °F) and is electrolyzed between 70 °C and 130 °C (160–265 °F). Potassium bifluoride increases the electrical conductivity of the solution and provides the bifluoride anion, which releases fluorine at the anode (negative part of the cell). If HF alone is electrolyzed, hydrogen forms at the cathode (positive part of the cell) and the fluoride ions remain in solution. After electrolysis, potassium fluoride remains in solution.

2 HF₂⁻ → H₂↑ + F₂↑ + 2 F⁻

The modern version of the process uses steel containers as cathodes, while blocks of carbon are used as anodes. The carbon electrodes are similar to those used in the electrolysis of aluminium. An earlier version of fluorine production process, by Moissan, uses platinum group metal electrodes and carved fluorite containers. The voltage for the electrolysis is between 8 and 12 volts.

Handling

Fluorine gas may be stored in steel cylinders where the inside surface is passivated by a metal fluoride layer that resists further attack. Passivated steel will withstand fluorine provided the temperature is kept below 200 °C (400 °F). Above that temperature, nickel is required. Regulator valves are made of nickel. Fluorine piping is generally made of nickel or Monel (nickel-copper alloy). Care must be taken to passivate all surfaces frequently and to exclude any water or greases. In the laboratory, fluorine gas can be used in glass tubing provided the pressure is low and moisture is excluded, although some sources recommend systems made of nickel, Monel, and PTFE.

Chemical routes

In 1986, when preparing for a conference to celebrate the 100th anniversary of the discovery of fluorine, Karl O. Christe discovered a purely chemical preparation of fluorine gas; however, he stated in his work that the basics were known 50 years before the actual reaction. The main idea is that some metal fluoride anions do not have a neutral counterpart (or those are very unstable) and their acidifying would result in chemical oxidation, rather than formation of the expected molecules. Christe lists the following reactions as a possible way:

2 KMnO₄ + 2 KF + 10 HF + 3 H₂O₂ → 2 K₂MnF₆ + 8 H₂O + 3 O₂↑

2 K₂MnF₆ + 4 SbF₅ → 4 KSbF₆ + 2 MnF₃ + F₂↑

This synthetic route is a rare chemical preparation of elemental fluorine, a reaction not previously thought possible.

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