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Clarinet

The clarinet is a single-reed musical instrument in the woodwind family, with a nearly cylindrical bore and a flared bell.

Clarinets comprise a family of instruments of differing sizes and pitches. The clarinet family is the largest woodwind family, ranging from the BB♭ contrabass to the E♭ soprano. The B♭ soprano clarinet is the most common type, and is the instrument usually indicated by the word "clarinet".

German instrument maker Johann Christoph Denner is generally credited with inventing the clarinet sometime around the year 1700 by adding a register key to the chalumeau, an earlier single-reed instrument. Over time, additional keywork and airtight pads were added to improve the tone and playability. Today the clarinet is a standard fixture of the orchestra and concert band and is used in classical music, military bands, klezmer, jazz, and other styles.

Etymology
The word clarinet may have entered the English language via the French clarinette (the feminine diminutive of Old French clarin), or from Provençal clarin ("oboe"), originating from the Latin root clarus ("clear"). The word is related to Middle English clarion, a type of trumpet, the name of which derives from the same root.

The earliest mention of the word clarinette being used for the instrument dates to a 1710 order placed by the Duke of Gronsfeld for two instruments made by Jacob Denner. The English form clarinet is found as early as 1733, and the now-archaic clarionet appears from 1784 until the early 20th century.

A person who plays the clarinet is called a clarinetist (in North American English), a clarinettist (in British English), or simply a clarinet player.

Characteristics
The clarinet's cylindrical bore is the main reason for its distinctive timbre, which varies between the three main registers (the chalumeau, clarion, and altissimo). The A and B♭ clarinets have nearly the same bore and nearly identical tonal quality, although the A typically has a slightly warmer sound. The tone of the E♭ clarinet is brighter and can be heard through loud orchestral textures. The bass clarinet has a characteristically deep, mellow sound, and the alto clarinet sounds similar to the bass, though not as dark.

Range
Clarinets have the largest pitch range of common woodwinds. Nearly all soprano and piccolo clarinets have keywork enabling them to play the E below middle C as their lowest written note. The concert pitch that sounds depends on the individual instrument's transposition (this low E sounds as a concert D3 on a B♭ soprano clarinet, a whole tone lower than the written note). Some B♭ clarinets go to a written E♭$3$ to match the range of the A clarinet. Bass clarinets have keywork extending the low range to a written E♭ and some have additional keys to enable a written C$3$. Among the less common members of the clarinet family, contrabass clarinets may have keywork to written D$3$, C$3$, or B$2$; the basset clarinet and basset horn generally go to low C$3$. Defining the top end of a clarinet's range is difficult, since many advanced players can produce notes well above the highest notes commonly found in method books. G$6$ is usually the highest note encountered in classical repertoire, but fingerings as high as A$7$ exist.

The range of a clarinet can be divided into three distinct registers:
 * The low chalumeau register, from the written low E to the written B♭ above middle C (B♭$4$) (named after the instrument that was the clarinet's immediate predecessor)
 * The bridging throat tones, from written G to B♭, are sometimes treated as a separate register

The three registers have characteristically different sounds—the chalumeau is rich and dark, the clarion is brighter and sweet, like a trumpet heard from afar, and the altissimo can be piercing and sometimes shrill.
 * The middle clarion register, which spans just over an octave (from a written B above middle C (B$4$) to the C two octaves above middle C (C$6$))
 * The high altissimo register, consisting of the notes above the written C two octaves above middle C (C$6$)

Acoustics

The production of sound by a clarinet follows these steps:

The reed stays pressed against the mouthpiece until either the springiness of the reed forces it to open or a returning pressure wave 'bumps' into the reed and opens it. Each time the reed opens, a puff of air goes through the gap, after which the reed swings shut again. When played loudly, the reed can spend up to 50% of the time shut. The 'puff of air' or compression wave (at around 3% greater pressure than the surrounding air ) travels down the cylindrical tube and escapes at the point where the tube opens out. This is either at the closest open hole or at the end of the tube (see diagram: image 1).
 * 1) The mouthpiece and reed are surrounded by the player's lips, which put light, even pressure on the reed and form an airtight seal. Air is blown past the reed and down the instrument. In the same way a flag flaps in the breeze, the air rushing past the reed causes it to vibrate. As air pressure from the mouth increases, the amount the reed vibrates increases until the reed hits the mouthpiece.
 * 1) More than a 'neutral' amount of air escapes from the instrument, which creates a slight vacuum or rarefaction in the clarinet tube. This rarefaction wave travels back up the tube (image 2).
 * 2) The rarefaction is reflected off the sloping end wall of the clarinet mouthpiece. The opening between the reed and the mouthpiece makes very little difference to the reflection of the rarefaction wave. This is because the opening is very small compared to the size of the tube, so almost the entire wave is reflected back down the tube even if the reed is completely open at the time the wave hits (image 3).
 * 3) When the rarefaction wave reaches the other (open) end of the tube, air rushes in to fill the slight vacuum. A little more than a 'neutral' amount of air enters the tube and causes a compression wave to travel back up the tube (image 4). Once the compression wave reaches the mouthpiece end of the 'tube', it is reflected again back down the pipe. However at this point, either because the compression wave 'bumped' the reed or because of the natural vibration cycle of the reed, the gap opens and another 'puff' of air is sent down the pipe.
 * 4) The original compression wave, now greatly reinforced by the second 'puff' of air, sets off on another two trips down the pipe (travelling four pipe lengths in total) before the cycle is repeated again.

In addition to this primary compression wave, other waves, known as harmonics, are created. Harmonics are caused by factors including the imperfect wobbling and shaking of the reed, the reed sealing the mouthpiece opening for part of the wave cycle (which creates a flattened section of the sound wave), and imperfections (bumps and holes) in the bore. A wide variety of compression waves are created, but only some (primarily the odd harmonics) are reinforced. This in combination with the cut-off frequency (where a significant drop in resonance occurs) results in the characteristic tone of the clarinet.

The bore is cylindrical for most of the tube with an inner bore diameter between 0.575 and 0.585 in, but there is a subtle hourglass shape, with the thinnest part below the junction between the upper and lower joint. This hourglass shape, although invisible to the naked eye, helps to correct the pitch and responsiveness of the instrument. The diameter of the bore affects the instrument's sound characteristics. The bell at the bottom of the clarinet flares out to improve the tone and tuning of the lowest notes. The fixed reed and fairly uniform diameter of the clarinet result in an acoustical performance approximating that of a cylindrical stopped pipe. Recorders use a tapered internal bore to overblow at the octave when the thumb/register hole is pinched open, while the clarinet, with its cylindrical bore, overblows at the twelfth.

Most modern clarinets have "undercut" tone holes that improve intonation and sound. Undercutting means chamfering the bottom edge of tone holes inside the bore. Acoustically, this makes the tone hole function as if it were larger, but its main function is to allow the air column to follow the curve up through the tone hole (surface tension) instead of "blowing past" it under the increasingly directional frequencies of the upper registers. Covering or uncovering the tone holes varies the length of the pipe, changing the resonant frequencies of the enclosed air column and hence the pitch. The player moves between the chalumeau and clarion registers through use of the register key. The open register key stops the fundamental frequency from being reinforced, making the reed vibrate at three times the frequency, which produces a note a twelfth above the original note.

Most woodwind instruments have a second register that begins an octave above the first (with notes at twice the frequency of the lower notes). With the aid of an 'octave' or 'register' key, the notes sound an octave higher as the fingering pattern repeats. These instruments are said to overblow at the octave. The clarinet differs, since it acts as a closed-pipe system. The low chalumeau register plays fundamentals, but the clarion (second) register plays the third harmonics, a perfect twelfth higher than the fundamentals. The clarinet is therefore said to overblow at the twelfth. The first several notes of the altissimo (third) range, aided by the register key and venting with the first left-hand hole, play the fifth harmonics, a perfect twelfth plus a major sixth above the fundamentals. The fifth and seventh harmonics are also available, sounding a further sixth and fourth (a flat, diminished fifth) higher respectively; these are the notes of the altissimo register.

The lip position and pressure, shaping of the vocal tract, choice of reed and mouthpiece, amount of air pressure created, and evenness of the airflow account for most of the player's ability to control the tone of a clarinet. Their vocal tract will be shaped to resonate at frequencies associated with the tone being produced. Vibrato, a pulsating change of pitch, is rare in classical literature; however, certain performers, such as Richard Stoltzman, use vibrato in classical music. Special fingerings and lip-bending may be used to play microtonal intervals. There have also been efforts to create a quarter tone clarinet.

Materials
Clarinet bodies have been made from a variety of materials including wood, plastic, hard rubber or Ebonite, metal, and ivory. The vast majority of wooden clarinets are made from African blackwood (grenadilla), or, more uncommonly, Honduran rosewood or cocobolo. Historically other woods, particularly boxwood and ebony, were used. Since the mid-20th century, clarinets (particularly student or band models) are also made from plastics, such as acrylonitrile butadiene styrene (ABS). One of the first such blends of plastic was Resonite, a term originally trademarked by Selmer. The Greenline model by Buffet Crampon is made from a composite of resin and the African blackwood powder left over from the manufacture of wooden clarinets.

Metal soprano clarinets were popular in the late 19th century, particularly for military use. Metal is still used for the bodies of some contra-alto and contrabass clarinets and the necks and bells of nearly all alto and larger clarinets.

Mouthpieces are generally made of hard rubber, although some inexpensive mouthpieces may be made of plastic. Other materials such as glass, wood, ivory, and metal have also been used. Ligatures are often made of metal and tightened using one or more adjustment screws; other materials include plastic, string, or fabric.

Reed
The clarinet uses a single reed made from the cane of Arundo donax. Reeds may also be manufactured from synthetic materials. The ligature fastens the reed to the mouthpiece. When air is blown through the opening between the reed and the mouthpiece facing, the reed vibrates and produces the clarinet's sound.

Most players buy manufactured reeds, although many make adjustments to these reeds, and some make their own reeds from cane "blanks". Reeds come in varying degrees of hardness, generally indicated on a scale from one (soft) through five (hard). This numbering system is not standardized—reeds with the same number often vary in hardness across manufacturers and models. Reed and mouthpiece characteristics work together to determine ease of playability and tonal characteristics.

Components

The reed is attached to the mouthpiece by the ligature, and the top half-inch or so of this assembly is held in the player's mouth. In the past, string was used to bind the reed to the mouthpiece. The formation of the mouth around the mouthpiece and reed is called the embouchure. The reed is on the underside of the mouthpiece, pressing against the player's lower lip, while the top teeth normally contact the top of the mouthpiece (some players roll the upper lip under the top teeth to form what is called a 'double-lip' embouchure). Adjustments in the strength and shape of the embouchure change the tone and intonation. Players sometimes relieve the pressure on the upper teeth and inner lower lip by attaching a pad to the top of the mouthpiece or putting temporary cushioning on the lower teeth.

The mouthpiece attaches to the barrel. Tuning can be adjusted by using barrels of varying lengths or by pulling out the barrel to increase the instrument's length. On basset horns and lower clarinets, there is a curved metal neck instead of a barrel.

The main body of most clarinets has an upper joint, whose mechanism is mostly operated by the left hand, and a lower joint, mostly operated by the right hand. Some clarinets have a one-piece body. The modern soprano clarinet has numerous tone holes—seven are covered with the fingertips and the rest are operated using a set of 17 keys. The most common system of keys was named the Boehm system by its designer Hyacinthe Klosé after flute designer Theobald Boehm, but it is not the same as the Boehm system used on flutes. The other main key system is the Oehler system, which is used mostly in Germany and Austria. The related Albert system is used by some jazz, klezmer, and eastern European folk musicians. The Albert and Oehler systems are both based on the early Mueller system.

The cluster of keys at the bottom of the upper joint (protruding slightly beyond the cork of the joint) are known as the trill keys and are operated by the right hand. The entire weight of the smaller clarinets is supported by the right thumb behind the lower joint on what is called the thumb rest. Larger clarinets are supported with a neck strap or a floor peg.

Below the main body is a flared end known as the bell. The bell does not amplify the sound but improves the uniformity of the instrument's tone for the lowest notes in each register. For the other notes, the sound is produced almost entirely at the tone holes, and the bell is irrelevant. On basset horns and larger clarinets, the bell curves up and forward and is usually made of metal.

History
The clarinet has its roots in early single-reed instruments used in Ancient Greece and Ancient Egypt. The modern clarinet developed from a Baroque instrument called the chalumeau. This instrument was similar to a recorder, but with a single-reed mouthpiece and a cylindrical bore. Lacking a register key, it was played mainly in its fundamental register, with a limited range of about one and a half octaves. It had eight finger holes, like a recorder, and a written pitch range from F3 to G4. At this time, contrary to modern practice, the reed was placed in contact with the upper lip. Around the beginning of the 18th century the German instrument maker Johann Christoph Denner (or possibly his son Jacob Denner) equipped a chalumeau in the alto register with two keys, one of which enabled access to a higher register. This second register did not begin an octave above the first, as with other woodwind instruments, but started an octave and a perfect fifth higher than the first. A second key, at the top, extended the range of the first register to A4 and, together with the register key, to B♭4. Later, Denner lengthened the bell and provided it with a third key to extend the pitch range down to E3.

After Denner's innovations, other makers added keys to improve tuning and facilitate fingerings and the chalumeau fell into disuse. The clarinet of the Classical period, as used by Mozart, typically had five keys. Mozart suggested extending the clarinet downwards by four semitones to C$3$, which resulted in the basset clarinet that was about 18 cm longer, made first by Theodor Lotz. In 1791 Mozart composed the Concerto for Clarinet and Orchestra in A major for this instrument, with passages ranging down to C3. By the time of Beethoven (c. 1780–1820), the clarinet was a fixed member in the orchestra.

The number of keys was limited because their felt pads did not seal tightly. Baltic-German clarinetist and master clarinet maker Iwan Müller remedied this by countersinking the tone holes for the keys and covering the pads with soft leather. These leather pads sealed the holes better than felt, making it possible to equip the instrument with considerably more keys. In 1812 Müller presented a clarinet with seven finger holes and thirteen keys, which he called "clarinet omnitonic" since it was capable of playing in all keys. It was no longer necessary to use differently tuned clarinets for a different keys. Müller is also considered the inventor of the metal ligature and the thumb rest. During this period the typical embouchure also changed, orienting the mouthpiece with the reed facing downward. This was first recommended in 1782 and became standard by the 1830s.

In the late 1830s, German flute maker Theobald Böhm invented a ring and axle key system for the flute. This key system was first used on the clarinet between 1839 and 1843 by French clarinetist Hyacinthe Klosé in collaboration with instrument maker Louis Auguste Buffet. Their design introduced needle springs for the axles, and the ring keys simplified some complicated fingering patterns. The inventors called this the Boehm clarinet, although Böhm was not involved in its development and the system differed from the one used on the flute. Other key systems have been developed, many built around modifications to the basic Boehm system, including the Full Boehm, Mazzeo, McIntyre, the Benade NX, and the Reform Boehm system, which combined Boehm-system keywork with a German mouthpiece and bore.

The Albert clarinet was developed by Eugène Albert in 1848. This model was based on the Müller clarinet with some changes to keywork, and was also known as the "simple system". It included a "spectacle key" patented by Adolphe Sax and rollers to improve little-finger movement. After 1861, a "patent C sharp" key developed by Joseph Tyler was added to other clarinet models. Improved versions of Albert clarinets were built in Belgium and France for export to the UK and the US.

Around 1860, clarinettist Carl Baermann and instrument maker Georg Ottensteiner developed the patented Baermann/Ottensteiner clarinet. This instrument had new connecting levers, allowing multiple fingering options to operate some of the pads. The Brahms clarinetist Richard Mühlfeld used this clarinet, and the American clarinet soloist Charles Neidich has used a Baermann-Ottensteiner instrument for playing compositions by Brahms.

In the early 20th century, the German clarinetist and clarinet maker Oskar Oehler presented a clarinet using similar fingerings to the Baermann instrument, with significantly more toneholes than the Böhm model. The new clarinet was called the Oehler system clarinet or German clarinet, while the Böhm clarinet has since been called the French clarinet. The French clarinet differs from the German not only in fingering but also in sound. Richard Strauss noted that "French clarinets have a flat, nasal tone, while German ones approximate the singing voice". Among modern instruments the difference is smaller, although intonation differences persist. The use of Oehler clarinets has continued in German and Austrian orchestras.

Today the Boehm system is standard everywhere except in Germany and Austria, where the Oehler clarinet is still used. Some contemporary Dixieland players continue to use Albert system clarinets. The Reform Boehm system is also popular in the Netherlands.

Use of multiple clarinets
The modern orchestral standard of using soprano clarinets in B♭ and A has to do partly with the history of the instrument and partly with acoustics, aesthetics, and economics. Before about 1800, due to the lack of airtight pads, practical woodwinds could have only a few keys to control accidentals (notes outside their diatonic home scales). The low (chalumeau) register of the clarinet spans a twelfth (an octave plus a perfect fifth) before overblowing, so the clarinet needs keys/holes to produce all nineteen notes in this range. This involves more keywork than on instruments that "overblow" at the octave—oboes, flutes, bassoons, and saxophones need only twelve notes before overblowing. Since clarinets with few keys cannot play chromatically, they are limited to playing in closely related keys. For example, an eighteenth-century clarinet in C could play music in F, C, and G (and their relative minors) with good intonation, but with progressive difficulty and poorer intonation as the key moved away from this range. With the advent of airtight pads and improved key technology, more keys were added to woodwinds and the need for clarinets in multiple keys was reduced. The use of instruments in C, B♭, and A persisted, with each used as specified by the composer.

The lower-pitched clarinets sound "mellower" (less bright), and the C clarinet—the highest and brightest sounding of these three—fell out of favor as the other two could cover its range and their sound was considered better. While the clarinet in C began to fall out of general use around 1850, some composers continued to write C parts, e.g., Bizet's Symphony in C (1855), Tchaikovsky's Symphony No. 2 (1872), Smetana's overture to The Bartered Bride (1866) and Má Vlast (1874), Dvořák's Slavonic Dance Op. 46, No. 1 (1878), Brahms' Symphony No. 4 (1885), Mahler's Symphony No. 6 (1906), and Strauss' Der Rosenkavalier (1911).

While technical improvements and an equal-tempered scale reduced the need for two clarinets, the technical difficulty of playing in remote keys persisted, and the A has remained a standard orchestral instrument. By the late 19th century the orchestral clarinet repertoire contained so much music for clarinet in A that it has remained in use.

Classical music
The orchestra frequently includes two clarinetists, each usually equipped with a B♭ and an A clarinet, and clarinet parts commonly alternate between the instruments. In the 20th century, Igor Stravinsky, Richard Strauss, and Gustav Mahler employed many different clarinets, including the E♭ or D soprano clarinets, basset horn, bass clarinet, and/or contrabass clarinet. The practice of using different clarinets to achieve tonal variety was common in 20th-century classical music.

The E♭ clarinet, B♭ clarinet, alto clarinet, bass clarinet, and contra-alto/contrabass clarinet are commonly used in concert bands, which generally have multiple B♭ clarinets; there are commonly three or even four B♭ clarinet parts with two to three players per part.

The clarinet is widely used as a solo instrument. The clarinet evolved later than other orchestral woodwind instruments, leaving solo repertoire from the Classical period onward, but few works from the Baroque era. Many clarinet concertos and clarinet sonatas have been written to showcase the instrument, for example those by Mozart and Weber.

Many works of chamber music have been written for the clarinet. Common combinations are: Groups of clarinets playing together have become increasingly popular among clarinet enthusiasts in recent years. Common forms are:
 * Clarinet and piano
 * Clarinet trio: clarinet, piano, and another instrument (for example, a string instrument)
 * Clarinet quartet: three B♭ clarinets and bass clarinet; two B♭ clarinets, alto clarinet, and bass; and other possibilities such as the use of a basset horn, especially in European classical works
 * Clarinet quintet: a clarinet plus a string quartet or, in more contemporary music, a configuration of five clarinets
 * Wind quintet: flute, oboe, clarinet, bassoon, and horn.
 * Clarinet choir: This ensemble contains many clarinets playing together, usually including several members of the clarinet family. The homogeneity of tone across the different members of the clarinet family produces an effect with some similarities to a human choir.
 * Clarinet quartet: usually three B♭ sopranos and one B♭ bass, or two B♭, an E♭ alto clarinet, and a B♭ bass clarinet, or sometimes four B♭ sopranos

Jazz

The clarinet was a central instrument in jazz, beginning with early jazz players in the 1910s. It remained a signature instrument of the genre through much of the big band era into the 1940s. American players Alphonse Picou, Larry Shields, Jimmie Noone, Johnny Dodds, and Sidney Bechet were all prominent early jazz clarinet players. Swing performers such as Benny Goodman and Artie Shaw rose to prominence in the late 1930s.

Beginning in the 1940s, the clarinet faded from its prominent position in jazz. By that time, an interest in Dixieland, a revival of traditional New Orleans jazz, had begun. Pete Fountain was one of the best known performers in this genre. The clarinet's place in the jazz ensemble was usurped by the saxophone, which projects a more powerful sound and uses a less complicated fingering system. The clarinet did not entirely disappear from jazz—prominent players since the 1950s include Stan Hasselgård, Jimmy Giuffre, Eric Dolphy (on bass clarinet), Perry Robinson, and John Carter. In the US, the prominent players on the instrument since the 1980s have included Eddie Daniels, Don Byron, Marty Ehrlich, Ken Peplowski, and others playing in both traditional and contemporary styles.

Other genres
The clarinet is uncommon, but not unheard of, in rock music. Jerry Martini played clarinet on Sly and the Family Stone's 1968 hit, "Dance to the Music". The Beatles included a trio of clarinets in "When I'm Sixty-Four" from their Sgt. Pepper's Lonely Hearts Club Band album. A clarinet is prominently featured in what a Billboard reviewer termed a "Benny Goodman-flavored clarinet solo" in "Breakfast in America", the title song from the Supertramp album of the same name.

Clarinets feature prominently in klezmer music, which employs a distinctive style of playing. The popular Brazilian music style of choro uses the clarinet, as does Albanian saze and Greek kompania folk music, and Bulgarian wedding music. In Turkish folk music, the Albert system clarinet in G is often used, commonly called a "Turkish clarinet".

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Chojnów

Chojnów (Haynau, Silesian German: Hoyn, Silesian language: Chojnůw) is a small town in Legnica County, Lower Silesian Voivodeship, in south-western Poland. It is located on the Skora river, a tributary of the Kaczawa at an average altitude of 170 m above sea level. Chojnów is the administrative seat of the rural gmina called Gmina Chojnów, although the town is not part of its territory and forms a separate urban gmina. As of December 2021, the town has 13,002 inhabitants.

Chojnów is located 18 km west of Legnica, 26 km east from Bolesławiec and 18 km north of Złotoryja, 5 km from the A4 motorway. It has railroad connections to Bolesławiec and Legnica.

Heraldry
The Chojnów coat of arms is a blue escutcheon featuring a white castle with three towers. To the right side of the central tower is a silver crescent moon and to its left side a golden sun. In the gate of the castle is a Silesian Eagle on a yellow background. Chojnów's motto is "Friendly City".

Geography
Chojnów is located in the Central-Western part of the Lower Silesia region. The Skora (Leather) River flows through the town in a westerly direction. The city of Chojnów is 5.32 km2 in area, including 41% agricultural land.

Chojnów has a connection with the major cities of the country (road and rail) and located 5 km south of Chojnów has the A4 Autostrada. To the South of the town is the surrounding Chojnowska Plain.

History
The town is first mentioned in a Latin mediaeval document issued in Wrocław on February 26, 1253, stating, the Silesian Duke Henry III when the town is mentioned under the name Honowo. Possible the name of nearby Hainau Island. The name is of Polish origin, and in more modern records from the 19th century, the Polish name appears as Hajnów, while Haynau is the Germanized version of the original Polish name.

The settlement of Haynow was mentioned in a 1272 deed. It was already called a civitas in a 1288 document issued by the Piast duke Henry V of Legnica, and officially received town privileges in 1333 from Duke Bolesław III the Generous. It was part of the duchies of Wrocław, Głogów and Legnica of fragmented Poland and remained under the rule of the Piast dynasty until 1675. Its population was predominantly Polish. In 1292 the first castellan of Chojnów, Bronisław Budziwojowic, was mentioned. In the 14th and early 15th centuries Chojnów was granted various privileges, including staple right and gold mining right, thanks to which it flourished.

The town survived the Hussites, who burned almost the entire town center and castle, but it quickly helped recover its former glory. The largest boom Chojnów experienced was in the 16th century, however by the end of that century began to decline due to fires and epidemic, which claimed many victims in 1613. During the Thirty Years War (1618–1648), there was another outbreak in the city, it was occupied by the Austrians and Swedes and in 1642 it was also plundered by the Swedes. It remained part of the Piast-ruled Duchy of Legnica until its dissolution in 1675, when it was incorporated to Habsburg-ruled Bohemia. In the 18th century, cloth production developed and a clothmaking school was established in the town. One of two main routes connecting Warsaw and Dresden ran through the town in the 18th century and Kings Augustus II the Strong and Augustus III of Poland traveled that route numerous times. In 1740 the town was captured by Prussia and subsequently annexed in 1742. In 1804 it suffered a flood. During the Napoleonic wars there were more epidemics. In 1813 in Chojnów, Napoleon Bonaparte issued instructions regarding the reorganization of the 8th Polish Corps of Prince Józef Poniatowski. The event is commemorated by a plaque in the facade of the Piast Castle. A railway line was opened in the 19th century. Sewer, Gas lighting a Newspaper and a hospital soon followed as the towns economy improved.

The city was not spared in World War II, with 30% of the town being destroyed on February 10, 1945, when Soviet Red Army troops took the abandoned town. After World War II and the implementation of the Oder-Neisse line in 1945, the town passed to the Republic of Poland. It was repopulated by Poles, expelled from former eastern Poland annexed by the Soviet Union. In 1946 it was renamed Chojnów, a more modern version of the old Polish Hajnów. Also Greeks, refugees of the Greek Civil War, settled in Chojnów.

Population

 * 1428 – 15 inhabitants
 * 1633 – 500
 * 1657 – 180
 * 1742 – 400
 * 1788 – 20,176
 * 1801 – 2,314
 * 1890 – 8,115
 * 1910 – 10,500
 * 2005 – 14,510
 * 2011 – 14,367

Economy
Chojnów is an industrial and agricultural town. Among local products are: paper, agricultural machinery, chains, metal furniture for hospitals, equipment for the meat industry, beer, wine, leather clothing, and clothing for infants, children and adults.

Sights and nature
Among the interesting monuments of Chojnów are the 13th-century castle of the Dukes of Legnica (currently used as a museum), two old churches, the Baszta Tkaczy (Weavers' Tower) and preserved fragments of city walls.

The biggest green area in Chojnów is small forest Park Piastowski (Piast's Park), named after Piast dynasty. Wild animals that can be found in the Chojnów area are roe deer, foxes, rabbits and wild domestic animals, especially cats.

Culture and sport

Every year in the first days of June, the Days of Chojnów (Dni Chojnowa) are celebrated. The Whole-Poland bike race Masters has been organized yearly in Chojnów for the past few years.

Chojnów has a Municipal sports and recreation center formed in 2008 holding various events, festivals, reviews, exhibitions, and competitions. The regional Museum is housed in the old Piast era castle. The collections include tiles, relics, and the castle garden. Next to the Museum there is a municipal library. In śródmiejskim Park, near the Town Hall is the amphitheatre.

The local government-run weekly newspaper is Gazeta Chojnowska, which has been published since 1992. It is published biweekly. Editions have a run of 900 copies and it is one of the oldest newspapers in Poland issued without interruption. The Chojnów is the official newspaper of Chojnów with copy run of 750 copies.

Education
In Chojnów, there are two kindergartens, two elementary schools and two middle schools.
 * Mary Konopnickiej is the smallest elementary school in Chojnów, and is located in the northern part of the city, close to the train station and founded in 1962.
 * Janusz Korczak is the largest primary school in Chojnów in the southern part of the town.
 * Middle School No. (Pope John Paul II), it is situated in the north-western part of the city next to the "Small Church".
 * Gimnazjum nr 2 im. Nicolaus Copernicus is the largest high school in Chojnów.
 * Liceum Ogólnokształcące im. Nicolaus Copernicus

Religion
Chojnów is in the Catholic deanery of Chojnów and has two parishes, Immaculate Conception of the Blessed Virgin Mary and also the Holy Apostles Peter and Paul. Both parishes have active congregations. There are also two Congregations of Jehovah's witnesses.

Notable people

 * Johann Wilhelm Ritter (1776–1810), chemist and physicist
 * Georg Michaelis (1857–1936), politician, Chancellor of Germany (1917).
 * Edith Jacobson (1897–1978), German psychoanalyst
 * Oswald Lange (1912–2000), German–American aerospace engineer
 * Horst Mahler (born 1936), German lawyer, former Red Army Faction militant, now Neo-Nazi activist

Twin towns – sister cities
Chojnów is twinned with:
 * 🇫🇷 Commentry, France
 * 🇩🇪 Egelsbach, Germany
 * 🇨🇿 Mnichovo Hradiště, Czech Republic

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Chojnów


text:
Canes Venatici

Canes Venatici is one of the 88 constellations designated by the International Astronomical Union (IAU). It is a small northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for 'hunting dogs', and the constellation is often depicted in illustrations as representing the dogs of Boötes the Herdsman, a neighboring constellation.

Cor Caroli is the constellation's brightest star, with an apparent magnitude of 2.9. La Superba (Y CVn) is one of the reddest naked-eye stars and one of the brightest carbon stars. The Whirlpool Galaxy is a spiral galaxy tilted face-on to observers on Earth, and was the first galaxy whose spiral nature was discerned. In addition, quasar Ton 618 is one of the most massive black holes with the mass of 66 billion solar masses.

History
The stars of Canes Venatici are not bright. In classical times, they were listed by Ptolemy as unfigured stars below the constellation Ursa Major in his star catalogue.

In medieval times, the identification of these stars with the dogs of Boötes arose through a mistranslation: some of Boötes's stars were traditionally described as representing the club (κολλοροβος, kollorobos) of Boötes. When the Greek astronomer Ptolemy's Almagest was translated from Greek to Arabic, the translator Hunayn ibn Ishaq did not know the Greek word and rendered it as a similar-sounding compound Arabic word for a kind of weapon, writing العصا ذات الكُلاب al-'aşā dhāt al-kullāb, which means 'the staff having a hook'.

When the Arabic text was later translated into Latin, the translator, Gerard of Cremona, mistook كُلاب kullāb ('hook') for كِلاب kilāb ('dogs'). Both written words look the same in Arabic text without diacritics, leading Gerard to write it as Hastile habens canes ('spearshaft-having dogs'). In 1533, the German astronomer Peter Apian depicted Boötes as having two dogs with him.

These spurious dogs floated about the astronomical literature until Hevelius decided to make them a separate constellation in 1687. Hevelius chose the name Asterion for the northern dog and Chara for the southern dog, as Canes Venatici, 'the hunting dogs', in his star atlas.

In his star catalogue, the Czech astronomer Antonín Bečvář assigned the names Asterion to β CVn and Chara to α CVn.

Although the International Astronomical Union dropped several constellations in 1930 that were medieval and Renaissance innovations, Canes Venatici survived to become one of the 88 IAU designated constellations.

Neighbors and borders
Canes Venatici is bordered by Ursa Major to the north and west, Coma Berenices to the south, and Boötes to the east. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "CVn". The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 14 sides.

In the equatorial coordinate system, the right ascension coordinates of these borders lie between 12:06.2:0 and 14:07.3:0, while the declination coordinates are between +27.84° and +52.36°. Covering 465 square degrees, it ranks 38th of the 88 constellations in size.

Stars
Canes Venatici contains no very bright stars. The Bayer designation stars, Alpha and Beta Canum Venaticorum are only of third and fourth magnitude respectively. Flamsteed catalogued 25 stars in the constellation, labelling them 1 to 25 Canum Venaticorum (CVn); however, 1CVn turned out to be in Ursa Major, 13CVn was in Coma Berenices, and 22CVn did not exist.
 * Alpha Canum Venaticorum, also known as Cor Caroli ('heart of Charles'), is the constellation's brightest star, named by Sir Charles Scarborough in memory of King Charles I, the executed king of Britain. The English astronomer William Henry Smyth wrote in 1844 that α CVn was brighter than usual during the Restoration, as Charles II returned to England to take the throne, but gave no source for this statement, which seems to be apocryphal. Cor Caroli is a wide double star, with a primary of magnitude 2.9 and a secondary of magnitude 5.6; the primary is 110 light-years from Earth. The primary also has an unusually strong variable magnetic field.
 * Beta Canum Venaticorum, or Chara, is a yellow-hued main sequence star of magnitude 4.25, 27 light-years from Earth. Its common name comes from the word for joy. It has been listed as an astrobiologically interesting star because of its proximity and similarity to the Sun. However, no exoplanets have been discovered around it so far.
 * Y Canum Venaticorum (La Superba) is a semiregular variable star that varies between magnitudes 5.0 and 6.5 over a period of around 158 days. It is a carbon star and is deep red in color, with a spectral type of C54J(N3).
 * AM Canum Venaticorum, a very blue star of magnitude 14, is the prototype of a special class of cataclysmic variable stars, in which the companion star is a white dwarf, rather than a main sequence star. It is 143 parsecs distant from the Sun.
 * RS Canum Venaticorum is the prototype of a special class of binary stars of chromospherically active and optically variable components.
 * R Canum Venaticorum is a Mira variable that ranges between magnitudes 6.5 and 12.9 over a period of approximately 329 days.

Supervoid
The Giant Void, an extremely large void (part of the universe containing very few galaxies), is within the vicinity of this constellation. It is regarded to be the second largest void ever discovered, slightly larger than the Eridanus Supervoid and smaller than the proposed KBC Void and 1,200 times the volume of expected typical voids. It was discovered in 1988 in a deep-sky survey. Its centre is approximately 1.5 billion light-years away.

Deep-sky objects
Canes Venatici contains five Messier objects, including four galaxies. One of the more significant galaxies in Canes Venatici is the Whirlpool Galaxy (M51, NGC 5194) and NGC 5195, a small barred spiral galaxy that is seen face-on. This was the first galaxy recognised as having a spiral structure, this structure being first observed by Lord Rosse in 1845. It is a face-on spiral galaxy 37 million light-years from Earth. Widely considered to be one of the most beautiful galaxies visible, M51 has many star-forming regions and nebulae in its arms, coloring them pink and blue in contrast to the older yellow core. M 51 has a smaller companion, NGC 5195, that has very few star-forming regions and thus appears yellow. It is passing behind M 51 and may be the cause of the larger galaxy's prodigious star formation.

Other notable spiral galaxies in Canes Venatici are the Sunflower Galaxy (M63, NGC 5055), M94 (NGC 4736), and M106 (NGC 4258).
 * M63, the Sunflower Galaxy, was named for its appearance in large amateur telescopes. It is a spiral galaxy with an integrated magnitude of 9.0.
 * M94 (NGC 4736) is a small face-on spiral galaxy with approximate magnitude 8.0, about 15 million light-years from Earth.
 * NGC 4631 is a barred spiral galaxy, which is one of the largest and brightest edge-on galaxies in the sky.
 * M3 (NGC 5272) is a globular cluster 32,000 light-years from Earth. It is 18′ in diameter, and at magnitude 6.3 is bright enough to be seen with binoculars. It can even be seen with the naked eye under particularly dark skies.
 * M94, also cataloged as NGC 4736, is a face-on spiral galaxy 15 million light-years from Earth. It has very tight spiral arms and a bright core. The outskirts of the galaxy are incredibly luminous in the ultraviolet because of a ring of new stars surrounding the core 7,000 light-years in diameter. Though astronomers are not sure what has caused this ring of new stars, some hypothesize that it is from shock waves caused by a bar that is thus far invisible.

Ton 618 is a hyperluminous quasar and blazar in this constellation, near its border with the neighboring Coma Berenices. It possesses a black hole with a mass 66 billion times that of the Sun, making it one of the most massive black holes ever measured. There is also a Lyman-alpha blob.

title: Canes Venatici


text:
Chamaeleon

Chamaeleon is a small constellation in the deep southern sky. It is named after the chameleon, a kind of lizard. It was first defined in the 16th century.

History
Chamaeleon was one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman. It first appeared on a 35-cm diameter celestial globe published in 1597 (or 1598) in Amsterdam by Plancius and Jodocus Hondius. Johann Bayer was the first uranographer to put Chamaeleon in a celestial atlas. It was one of many constellations created by European explorers in the 15th and 16th centuries out of unfamiliar Southern Hemisphere stars.

Stars
There are four bright stars in Chamaeleon that form a compact diamond-shape approximately 10 degrees from the south celestial pole and about 15 degrees south of Acrux, along the axis formed by Acrux and Gamma Crucis. Alpha Chamaeleontis is a white-hued star of magnitude 4.1, 63 light-years from Earth. Beta Chamaeleontis is a blue-white hued star of magnitude 4.2, 271 light-years from Earth. Gamma Chamaeleontis is a red-hued giant star of magnitude 4.1, 413 light-years from Earth. The other bright star in Chamaeleon is Delta Chamaeleontis, a wide double star. The brighter star is Delta2 Chamaeleontis, a blue-hued star of magnitude 4.4. Delta1 Chamaeleontis, the dimmer component, is an orange-hued giant star of magnitude 5.5. They both lie about 350 light years away.

Chamaeleon is also the location of Cha 110913, a unique dwarf star or proto solar system.

Deep-sky objects
In 1999, a nearby open cluster was discovered centered on the star η Chamaeleontis. The cluster, known as either the Eta Chamaeleontis cluster or Mamajek 1, is 8 million years old, and lies 316 light years from Earth.

The constellation contains a number of molecular clouds (the Chamaeleon dark clouds) that are forming low-mass T Tauri stars. The cloud complex lies some 400 to 600 light years from Earth, and contains tens of thousands of solar masses of gas and dust. The most prominent cluster of T Tauri stars and young B-type stars are in the Chamaeleon I cloud, and are associated with the reflection nebula IC 2631.

Chamaeleon contains one planetary nebula, NGC 3195, which is fairly faint. It appears in a telescope at about the same apparent size as Jupiter.

Equivalents
In Chinese astronomy, the stars that form Chamaeleon were classified as the Little Dipper among the Southern Asterisms  by Xu Guangqi. Chamaeleon is sometimes also called the Frying Pan in Australia.

title: Chamaeleon


text:
Cholesterol

Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.

Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membranes. In vertebrates, hepatic cells typically produce the greatest amounts. In the brain, astrocytes produce cholesterol and transport it to neurons. It is absent among prokaryotes (bacteria and archaea), although there are some exceptions, such as Mycoplasma, which require cholesterol for growth. Cholesterol also serves as a precursor for the biosynthesis of steroid hormones, bile acid and vitamin D.

Elevated levels of cholesterol in the blood, especially when bound to low-density lipoprotein (LDL, often referred to as "bad cholesterol"), may increase the risk of cardiovascular disease.

François Poulletier de la Salle first identified cholesterol in solid form in gallstones in 1769. In 1815, chemist Michel Eugène Chevreul named the compound "cholesterine".

Etymology
The word cholesterol comes from Ancient Greek chole- 'bile' and stereos 'solid', followed by the chemical suffix -ol for an alcohol.

Physiology
Cholesterol is essential for all animal life. While most cells are capable of synthesizing it, the majority of cholesterol is ingested or synthesized by hepatocytes and transported in the blood to peripheral cells. The levels of cholesterol in peripheral tissues is dictated by a balance of uptake and export. Under normal conditions, brain cholesterol is separate from peripheral cholesterol, i.e., the dietary and hepatic cholesterol do not cross the blood brain barrier. Rather, astrocytes produce and distribute cholesterol in the brain.

De novo synthesis, both in astrocytes and hepatocytes, occurs by a complex 37-step process. This begins with the mevalonate or HMG-CoA reductase pathway, the target of statin drugs, which encompasses the first 18 steps. This is followed by 19 additional steps to convert the resulting lanosterol into cholesterol. A human male weighing 68 kg (150 lb) normally synthesizes about 1 gram (1,000 mg) of cholesterol per day, and his body contains about 35 g, mostly contained within the cell membranes.

Typical daily cholesterol dietary intake for a man in the United States is 307 mg. Most ingested cholesterol is esterified, which causes it to be poorly absorbed by the gut. The body also compensates for absorption of ingested cholesterol by reducing its own cholesterol synthesis. For these reasons, cholesterol in food, seven to ten hours after ingestion, has little, if any effect on concentrations of cholesterol in the blood. Surprisingly, in rats, blood cholesterol is inversely correlated with cholesterol consumption. The more cholesterol a rat eats the lower the blood cholesterol. During the first seven hours after ingestion of cholesterol, as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins (which transport all fats in the water outside cells), the concentrations increase.

Plants make cholesterol in very small amounts. In larger quantities they produce phytosterols, chemically similar substances which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption. When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism. The intake of naturally occurring phytosterols, which encompass plant sterols and stanols, ranges between ≈200–300 mg/day depending on eating habits. Specially designed vegetarian experimental diets have been produced yielding upwards of 700 mg/day.

Membranes
Cholesterol is present in varying degrees in all animal cell membranes, but is absent in prokaryotes. It is required to build and maintain membranes and modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group of each cholesterol molecule interacts with water molecules surrounding the membrane, as do the polar heads of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains, cholesterol increases membrane packing, which both alters membrane fluidity and maintains membrane integrity so that animal cells do not need to build cell walls (like plants and most bacteria). The membrane remains stable and durable without being rigid, allowing animal cells to change shape and animals to move.

The structure of the tetracyclic ring of cholesterol contributes to the fluidity of the cell membrane, as the molecule is in a trans conformation making all but the side chain of cholesterol rigid and planar. In this structural role, cholesterol also reduces the permeability of the plasma membrane to neutral solutes, hydrogen ions, and sodium ions.

Substrate presentation
Cholesterol regulates the biological process of substrate presentation and the enzymes that use substrate presentation as a mechanism of their activation. Phospholipase D2 (PLD2) is a well-defined example of an enzyme activated by substrate presentation. The enzyme is palmitoylated causing the enzyme to traffic to cholesterol dependent lipid domains sometimes called "lipid rafts". The substrate of phospholipase D is phosphatidylcholine (PC) which is unsaturated and is of low abundance in lipid rafts. PC localizes to the disordered region of the cell along with the polyunsaturated lipid phosphatidylinositol 4,5-bisphosphate (PIP2). PLD2 has a PIP2 binding domain. When PIP2 concentration in the membrane increases, PLD2 leaves the cholesterol-dependent domains and binds to PIP2 where it then gains access to its substrate PC and commences catalysis based on substrate presentation.

Signaling
Cholesterol is also implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane, which brings receptor proteins in close proximity with high concentrations of second messenger molecules. In multiple layers, cholesterol and phospholipids, both electrical insulators, can facilitate speed of transmission of electrical impulses along nerve tissue. For many neuron fibers, a myelin sheath, rich in cholesterol since it is derived from compacted layers of Schwann cell or oligodendrocyte membranes, provides insulation for more efficient conduction of impulses. Demyelination (loss of myelin) is believed to be part of the basis for multiple sclerosis.

Cholesterol binds to and affects the gating of a number of ion channels such as the nicotinic acetylcholine receptor, GABAA receptor, and the inward-rectifier potassium channel. Cholesterol also activates the estrogen-related receptor alpha (ERRα), and may be the endogenous ligand for the receptor. The constitutively active nature of the receptor may be explained by the fact that cholesterol is ubiquitous in the body. Inhibition of ERRα signaling by reduction of cholesterol production has been identified as a key mediator of the effects of statins and bisphosphonates on bone, muscle, and macrophages. On the basis of these findings, it has been suggested that the ERRα should be de-orphanized and classified as a receptor for cholesterol.

As a chemical precursor
Within cells, cholesterol is also a precursor molecule for several biochemical pathways. For example, it is the precursor molecule for the synthesis of vitamin D in the calcium metabolism and all steroid hormones, including the adrenal gland hormones cortisol and aldosterone, as well as the sex hormones progesterone, estrogens, and testosterone, and their derivatives.

Epidermis
The stratum corneum is the outermost layer of the epidermis. It is composed of terminally differentiated and enucleated corneocytes that reside within a lipid matrix, like "bricks and mortar." Together with ceramides and free fatty acids, cholesterol forms the lipid mortar, a water-impermeable barrier that prevents evaporative water loss. As a rule of thumb, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (≈50% by weight), cholesterol (≈25% by weight), and free fatty acids (≈15% by weight), with smaller quantities of other lipids also being present. Cholesterol sulfate reaches its highest concentration in the granular layer of the epidermis. Steroid sulfate sulfatase then decreases its concentration in the stratum corneum, the outermost layer of the epidermis. The relative abundance of cholesterol sulfate in the epidermis varies across different body sites with the heel of the foot having the lowest concentration.

Metabolism
Cholesterol is recycled in the body. The liver excretes cholesterol into biliary fluids, which are then stored in the gallbladder, which then excretes them in a non-esterified form (via bile) into the digestive tract. Typically, about 50% of the excreted cholesterol is reabsorbed by the small intestine back into the bloodstream.

Biosynthesis
Almost all animal tissues synthesize cholesterol from acetyl-CoA. All animal cells (exceptions exist within the invertebrates) manufacture cholesterol, for both membrane structure and other uses, with relative production rates varying by cell type and organ function. About 80% of total daily cholesterol production occurs in the liver and the intestines; other sites of higher synthesis rates include the brain, the adrenal glands, and the reproductive organs.

Synthesis within the body starts with the mevalonate pathway where two molecules of acetyl CoA condense to form acetoacetyl-CoA. This is followed by a second condensation between acetyl CoA and acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl CoA (HMG-CoA).

This molecule is then reduced to mevalonate by the enzyme HMG-CoA reductase. Production of mevalonate is the rate-limiting and irreversible step in cholesterol synthesis and is the site of action for statins (a class of cholesterol-lowering drugs).

Mevalonate is finally converted to isopentenyl pyrophosphate (IPP) through two phosphorylation steps and one decarboxylation step that requires ATP.

Three molecules of isopentenyl pyrophosphate condense to form farnesyl pyrophosphate through the action of geranyl transferase.

Two molecules of farnesyl pyrophosphate then condense to form squalene by the action of squalene synthase in the endoplasmic reticulum.

Oxidosqualene cyclase then cyclizes squalene to form lanosterol.

Finally, lanosterol is converted to cholesterol via either of two pathways, the Bloch pathway, or the Kandutsch-Russell pathway. The final 19 steps to cholesterol contain NADPH and oxygen to help oxidize methyl groups for removal of carbons, mutases to move alkene groups, and NADH to help reduce ketones.

Konrad Bloch and Feodor Lynen shared the Nobel Prize in Physiology or Medicine in 1964 for their discoveries concerning some of the mechanisms and methods of regulation of cholesterol and fatty acid metabolism.

Regulation of cholesterol synthesis
Biosynthesis of cholesterol is directly regulated by the cholesterol levels present, though the homeostatic mechanisms involved are only partly understood. A higher intake of food leads to a net decrease in endogenous production, whereas a lower intake of food has the opposite effect. The main regulatory mechanism is the sensing of intracellular cholesterol in the endoplasmic reticulum by the protein SREBP (sterol regulatory element-binding protein 1 and 2). In the presence of cholesterol, SREBP is bound to two other proteins: SCAP (SREBP cleavage-activating protein) and INSIG-1. When cholesterol levels fall, INSIG-1 dissociates from the SREBP-SCAP complex, which allows the complex to migrate to the Golgi apparatus. Here SREBP is cleaved by S1P and S2P (site-1 protease and site-2 protease), two enzymes that are activated by SCAP when cholesterol levels are low.

The cleaved SREBP then migrates to the nucleus and acts as a transcription factor to bind to the sterol regulatory element (SRE), which stimulates the transcription of many genes. Among these are the low-density lipoprotein (LDL) receptor and HMG-CoA reductase. The LDL receptor scavenges circulating LDL from the bloodstream, whereas HMG-CoA reductase leads to an increase in endogenous production of cholesterol. A large part of this signaling pathway was clarified by Dr. Michael S. Brown and Dr. Joseph L. Goldstein in the 1970s. In 1985, they received the Nobel Prize in Physiology or Medicine for their work. Their subsequent work shows how the SREBP pathway regulates the expression of many genes that control lipid formation and metabolism and body fuel allocation.

Cholesterol synthesis can also be turned off when cholesterol levels are high. HMG-CoA reductase contains both a cytosolic domain (responsible for its catalytic function) and a membrane domain. The membrane domain senses signals for its degradation. Increasing concentrations of cholesterol (and other sterols) cause a change in this domain's oligomerization state, which makes it more susceptible to destruction by the proteasome. This enzyme's activity can also be reduced by phosphorylation by an AMP-activated protein kinase. Because this kinase is activated by AMP, which is produced when ATP is hydrolyzed, it follows that cholesterol synthesis is halted when ATP levels are low.

Plasma transport and regulation of absorption
As an isolated molecule, cholesterol is only minimally soluble in water, or hydrophilic. Because of this, it dissolves in blood at exceedingly small concentrations. To be transported effectively, cholesterol is instead packaged within lipoproteins, complex discoidal particles with exterior amphiphilic proteins and lipids, whose outward-facing surfaces are water-soluble and inward-facing surfaces are lipid-soluble. This allows it to travel through the blood via emulsification. Unbound cholesterol, being amphipathic, is transported in the monolayer surface of the lipoprotein particle along with phospholipids and proteins. Cholesterol esters bound to fatty acid, on the other hand, are transported within the fatty hydrophobic core of the lipoprotein, along with triglyceride.

There are several types of lipoproteins in the blood. In order of increasing density, they are chylomicrons, very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). Lower protein/lipid ratios make for less dense lipoproteins. Cholesterol within different lipoproteins is identical, although some is carried as its native "free" alcohol form (the cholesterol-OH group facing the water surrounding the particles), while others as fatty acyl esters, known also as cholesterol esters, within the particles.

Lipoprotein particles are organized by complex apolipoproteins, typically 80–100 different proteins per particle, which can be recognized and bound by specific receptors on cell membranes, directing their lipid payload into specific cells and tissues currently ingesting these fat transport particles. These surface receptors serve as unique molecular signatures, which then help determine fat distribution delivery throughout the body.

Chylomicrons, the least dense cholesterol transport particles, contain apolipoprotein B-48, apolipoprotein C, and apolipoprotein E (the principal cholesterol carrier in the brain ) in their shells. Chylomicrons carry fats from the intestine to muscle and other tissues in need of fatty acids for energy or fat production. Unused cholesterol remains in more cholesterol-rich chylomicron remnants, and taken up from here to the bloodstream by the liver.

VLDL particles are produced by the liver from triacylglycerol and cholesterol which was not used in the synthesis of bile acids. These particles contain apolipoprotein B100 and apolipoprotein E in their shells, and can be degraded by lipoprotein lipase on the artery wall to IDL. This arterial wall cleavage allows absorption of triacylglycerol and increases the concentration of circulating cholesterol. IDL particles are then consumed in two processes: half is metabolized by HTGL and taken up by the LDL receptor on the liver cell surfaces, while the other half continues to lose triacylglycerols in the bloodstream until they become cholesterol-laden LDL particles.

LDL particles are the major blood cholesterol carriers. Each one contains approximately 1,500 molecules of cholesterol ester. LDL particle shells contain just one molecule of apolipoprotein B100, recognized by LDL receptors in peripheral tissues. Upon binding of apolipoprotein B100, many LDL receptors concentrate in clathrin-coated pits. Both LDL and its receptor form vesicles within a cell via endocytosis. These vesicles then fuse with a lysosome, where the lysosomal acid lipase enzyme hydrolyzes the cholesterol esters. The cholesterol can then be used for membrane biosynthesis or esterified and stored within the cell, so as to not interfere with the cell membranes.

LDL receptors are used up during cholesterol absorption, and its synthesis is regulated by SREBP, the same protein that controls the synthesis of cholesterol de novo, according to its presence inside the cell. A cell with abundant cholesterol will have its LDL receptor synthesis blocked, to prevent new cholesterol in LDL particles from being taken up. Conversely, LDL receptor synthesis proceeds when a cell is deficient in cholesterol.

When this process becomes unregulated, LDL particles without receptors begin to appear in the blood. These LDL particles are oxidized and taken up by macrophages, which become engorged and form foam cells. These foam cells often become trapped in the walls of blood vessels and contribute to atherosclerotic plaque formation. Differences in cholesterol homeostasis affect the development of early atherosclerosis (carotid intima-media thickness). These plaques are the main causes of heart attacks, strokes, and other serious medical problems, leading to the association of so-called LDL cholesterol (actually a lipoprotein) with "bad" cholesterol.

HDL particles are thought to transport cholesterol back to the liver, either for excretion or for other tissues that synthesize hormones, in a process known as reverse cholesterol transport (RCT). Large numbers of HDL particles correlates with better health outcomes, whereas low numbers of HDL particles is associated with atheromatous disease progression in the arteries.

Metabolism, recycling and excretion
Cholesterol is susceptible to oxidation and easily forms oxygenated derivatives called oxysterols. Three different mechanisms can form these: autoxidation, secondary oxidation to lipid peroxidation, and cholesterol-metabolizing enzyme oxidation. A great interest in oxysterols arose when they were shown to exert inhibitory actions on cholesterol biosynthesis. This finding became known as the "oxysterol hypothesis". Additional roles for oxysterols in human physiology include their participation in bile acid biosynthesis, function as transport forms of cholesterol, and regulation of gene transcription.

In biochemical experiments radiolabelled forms of cholesterol, such as tritiated-cholesterol are used. These derivatives undergo degradation upon storage and it is essential to purify cholesterol prior to use. Cholesterol can be purified using small Sephadex LH-20 columns.

Cholesterol is oxidized by the liver into a variety of bile acids. These, in turn, are conjugated with glycine, taurine, glucuronic acid, or sulfate. A mixture of conjugated and nonconjugated bile acids, along with cholesterol itself, is excreted from the liver into the bile. Approximately 95% of the bile acids are reabsorbed from the intestines, and the remainder are lost in the feces. The excretion and reabsorption of bile acids forms the basis of the enterohepatic circulation, which is essential for the digestion and absorption of dietary fats. Under certain circumstances, when more concentrated, as in the gallbladder, cholesterol crystallises and is the major constituent of most gallstones (lecithin and bilirubin gallstones also occur, but less frequently). Every day, up to 1 g of cholesterol enters the colon. This cholesterol originates from the diet, bile, and desquamated intestinal cells, and can be metabolized by the colonic bacteria. Cholesterol is converted mainly into coprostanol, a nonabsorbable sterol that is excreted in the feces.

Although cholesterol is a steroid generally associated with mammals, the human pathogen Mycobacterium tuberculosis is able to completely degrade this molecule and contains a large number of genes that are regulated by its presence. Many of these cholesterol-regulated genes are homologues of fatty acid β-oxidation genes, but have evolved in such a way as to bind large steroid substrates like cholesterol.

Dietary sources
Animal fats are complex mixtures of triglycerides, with lesser amounts of both the phospholipids and cholesterol molecules from which all animal (and human) cell membranes are constructed. Since all animal cells manufacture cholesterol, all animal-based foods contain cholesterol in varying amounts. Major dietary sources of cholesterol include red meat, egg yolks and whole eggs, liver, kidney, giblets, fish oil, and butter. Human breast milk also contains significant quantities of cholesterol.

Plant cells synthesize cholesterol as a precursor for other compounds, such as phytosterols and steroidal glycoalkaloids, with cholesterol remaining in plant foods only in minor amounts or absent. Some plant foods, such as avocado, flax seeds and peanuts, contain phytosterols, which compete with cholesterol for absorption in the intestines, and reduce the absorption of both dietary and bile cholesterol. A typical diet contributes on the order of 0.2 gram of phytosterols, which is not enough to have a significant impact on blocking cholesterol absorption. Phytosterols intake can be supplemented through the use of phytosterol-containing functional foods or dietary supplements that are recognized as having potential to reduce levels of LDL-cholesterol.

Medical guidelines and recommendations
In 2015, the scientific advisory panel of U.S. Department of Health and Human Services and U.S. Department of Agriculture for the 2015 iteration of the Dietary Guidelines for Americans dropped the previously recommended limit of consumption of dietary cholesterol to 300 mg per day with a new recommendation to "eat as little dietary cholesterol as possible" and acknowledging an association between a diet low in cholesterol and reduced risk of cardiovascular disease.

A 2013 report by the American Heart Association and the American College of Cardiology recommended focusing on healthy dietary patterns rather than specific cholesterol limits, as they are hard for clinicians and consumers to implement. They recommend the DASH and Mediterranean diet, which are low in cholesterol. A 2017 review by the American Heart Association recommends switching saturated fats for polyunsaturated fats to reduce cardiovascular disease risk.

Some supplemental guidelines have recommended doses of phytosterols in the 1.6–3.0 grams per day range (Health Canada, EFSA, ATP III, FDA). A meta-analysis demonstrated a 12% reduction in LDL-cholesterol at a mean dose of 2.1 grams per day. The benefits of a diet supplemented with phytosterols have also been questioned.

Hypercholesterolemia
According to the lipid hypothesis, elevated levels of cholesterol in the blood lead to atherosclerosis which may increase the risk of heart attack, stroke, and peripheral artery disease. Since higher blood LDL – especially higher LDL concentrations and smaller LDL particle size – contributes to this process more than the cholesterol content of the HDL particles, LDL particles are often termed "bad cholesterol". High concentrations of functional HDL, which can remove cholesterol from cells and atheromas, offer protection and are commonly referred to as "good cholesterol". These balances are mostly genetically determined, but can be changed by body composition, medications, diet, and other factors. A 2007 study demonstrated that blood total cholesterol levels have an exponential effect on cardiovascular and total mortality, with the association more pronounced in younger subjects. Because cardiovascular disease is relatively rare in the younger population, the impact of high cholesterol on health is larger in older people.

Elevated levels of the lipoprotein fractions, LDL, IDL and VLDL, rather than the total cholesterol level, correlate with the extent and progress of atherosclerosis. Conversely, the total cholesterol can be within normal limits, yet be made up primarily of small LDL and small HDL particles, under which conditions atheroma growth rates are high. A post hoc analysis of the IDEAL and the EPIC prospective studies found an association between high levels of HDL cholesterol (adjusted for apolipoprotein A-I and apolipoprotein B) and increased risk of cardiovascular disease, casting doubt on the cardioprotective role of "good cholesterol".

About one in 250 individuals can have a genetic mutation for the LDL cholesterol receptor that causes them to have familial hypercholesterolemia. Inherited high cholesterol can also include genetic mutations in the PCSK9 gene and the gene for apolipoprotein B.

Elevated cholesterol levels are treatable by a diet that reduces or eliminates saturated fat, trans fats, and high cholesterol foods, often followed by one of various hypolipidemic agents, such as statins, fibrates, cholesterol absorption inhibitors, monoclonal antibody therapy (PCSK9 inhibitors), nicotinic acid derivatives or bile acid sequestrants. There are several international guidelines on the treatment of hypercholesterolemia.

Human trials using HMG-CoA reductase inhibitors, known as statins, have repeatedly confirmed that changing lipoprotein transport patterns from unhealthy to healthier patterns significantly lowers cardiovascular disease event rates, even for people with cholesterol values currently considered low for adults. Studies have shown that reducing LDL cholesterol levels by about 38.7 mg/dL with the use of statins can reduce cardiovascular disease and stroke risk by about 21%. Studies have also found that statins reduce atheroma progression. As a result, people with a history of cardiovascular disease may derive benefit from statins irrespective of their cholesterol levels (total cholesterol below 5.0 mmol/L [193 mg/dL]), and in men without cardiovascular disease, there is benefit from lowering abnormally high cholesterol levels ("primary prevention"). Primary prevention in women was originally practiced only by extension of the findings in studies on men, since, in women, none of the large statin trials conducted prior to 2007 demonstrated a significant reduction in overall mortality or in cardiovascular endpoints. Meta-analyses have demonstrated significant reductions in all-cause and cardiovascular mortality, without significant heterogeneity by sex.

The 1987 report of National Cholesterol Education Program, Adult Treatment Panels suggests the total blood cholesterol level should be: < 200 mg/dL normal blood cholesterol, 200–239 mg/dL borderline-high, > 240 mg/dL high cholesterol. The American Heart Association provides a similar set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease: Statins are effective in lowering LDL cholesterol and widely used for primary prevention in people at high risk of cardiovascular disease, as well as in secondary prevention for those who have developed cardiovascular disease. The average global mean total Cholesterol for humans has remained at about 4.6 mmol/L (178 mg/dL) for men and women, both crude and age standardized, for nearly 40 years from 1980 to 2018, with some regional variations and reduction of total Cholesterol in Western nations.

More current testing methods determine LDL ("bad") and HDL ("good") cholesterol separately, allowing cholesterol analysis to be more nuanced. The desirable LDL level is considered to be less than 100 mg/dL (2.6 mmol/L).

Total cholesterol is defined as the sum of HDL, LDL, and VLDL. Usually, only the total, HDL, and triglycerides are measured. For cost reasons, the VLDL is usually estimated as one-fifth of the triglycerides and the LDL is estimated using the Friedewald formula (or a variant): estimated LDL = [total cholesterol] − [total HDL] − [estimated VLDL]. Direct LDL measures are used when triglycerides exceed 400 mg/dL. The estimated VLDL and LDL have more error when triglycerides are above 400 mg/dL.

In the Framingham Heart Study, each 10 mg/dL (0.6 mmol/L) increase in total cholesterol levels increased 30-year overall mortality by 5% and CVD mortality by 9%. While subjects over the age of 50 had an 11% increase in overall mortality, and a 14% increase in cardiovascular disease mortality per 1 mg/dL (0.06 mmol/L) year drop in total cholesterol levels. The researchers attributed this phenomenon to a different correlation, whereby the disease itself increases risk of death, as well as changes a myriad of factors, such as weight loss and the inability to eat, which lower serum cholesterol. This effect was also shown in men of all ages and women over 50 in the Vorarlberg Health Monitoring and Promotion Programme. These groups were more likely to die of cancer, liver diseases, and mental diseases with very low total cholesterol, of 186 mg/dL (10.3 mmol/L) and lower. This result indicates the low-cholesterol effect occurs even among younger respondents, contradicting the previous assessment among cohorts of older people that this is a marker for frailty occurring with age.

Hypocholesterolemia
Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, but some studies suggest a link with depression, cancer, and cerebral hemorrhage. In general, the low cholesterol levels seem to be a consequence, rather than a cause, of an underlying illness. A genetic defect in cholesterol synthesis causes Smith–Lemli–Opitz syndrome, which is often associated with low plasma cholesterol levels. Hyperthyroidism, or any other endocrine disturbance which causes upregulation of the LDL receptor, may result in hypocholesterolemia.

Testing
The American Heart Association recommends testing cholesterol every 4–6 years for people aged 20 years or older. A separate set of American Heart Association guidelines issued in 2013 indicates that people taking statin medications should have their cholesterol tested 4–12 weeks after their first dose and then every 3–12 months thereafter. For men ages 45 to 65 and women ages 55 to 65, a cholesterol test should occur every 1–2 years, and for seniors over age 65, an annual test should be performed.

A blood sample after 12-hours of fasting is taken by a healthcare professional from an arm vein to measure a lipid profile for a) total cholesterol, b) HDL cholesterol, c) LDL cholesterol, and d) triglycerides. Results may be expressed as "calculated", indicating a calculation of total cholesterol, HDL, and triglycerides.

Cholesterol is tested to determine for "normal" or "desirable" levels if a person has a total cholesterol of 5.2 mmol/L or less (200 mg/dL), an HDL value of more than 1 mmol/L (40 mg/dL, "the higher, the better"), an LDL value of less than 2.6 mmol/L (100 mg/dL), and a triglycerides level of less than 1.7 mmol/L (150 mg/dL). Blood cholesterol in people with lifestyle, aging, or cardiovascular risk factors, such as diabetes mellitus, hypertension, family history of coronary artery disease, or angina, are evaluated at different levels.

Cholesteric liquid crystals
Some cholesterol derivatives (among other simple cholesteric lipids) are known to generate the liquid crystalline "cholesteric phase". The cholesteric phase is, in fact, a chiral nematic phase, and it changes colour when its temperature changes. This makes cholesterol derivatives useful for indicating temperature in liquid-crystal display thermometers and in temperature-sensitive paints.

Stereoisomers
Cholesterol has 256 stereoisomers that arise from its eight stereocenters, although only two of the stereoisomers have biochemical significance (nat-cholesterol and ent-cholesterol, for natural and enantiomer, respectively), and only one occurs naturally (nat-cholesterol).

title: Cholesterol