Cell surface receptors (membrane receptors, transmembrane receptors) take part in communication between the cell and the outside world. Extracellular signalling molecules (usually hormones, neurotransmitters, cytokines, growth factors or cell recognition molecules) attach to the receptor. This triggers changes in the function of the cell. The process is called signal transduction: The binding starts a chemical change on the inside of the membrane.
Receptors work like locks and keys. With the key, the lock can be locked or unlocked. If it is unlocked, the door belonging to it can be opened.
Part of the receptor sticks out of the cell membrane. The same applies to the membranes of cell organelles. A receptor's main function is to recognize and respond to a specific ligand, for example, a neurotransmitter or hormone. Some receptors respond to changes in 'transmembrane potential' (the difference in electric potential between the inside and the outside of a cell).
The middle part, inside the membrane itself, is a protein-lined pore through the membrane, or 'ion channel'. When the ligand binds to the surface, the pore becomes accessible to ions, which then pass through. In other cases, where differences in electric potential occur, the receptor changes in shape, which causes change inside the cell.
The inside (or cytoplasmic) part of the receptor interacts with the inside of the cell or organelle. There are several different kinds of receptor, each of which acts in a different way.
Coppélia
Coppélia ou La fille aux yeux d'email () is probably the best-known and most performed comedy ballet in the world. Arthur Saint-Léon and Charles Nuitter based the ballet on E. T. A. Hoffmann's 1816 fairy tale "Der Sandmann" (). Léo Delibes wrote the music. Saint-Léon choreographed the ballet.
The ballet is set in a Polish village long ago. Swanhilda and Frantz are sweethearts. Swanhilda thinks Frantz is in love with Coppélia, a strange girl in the house of a dollmaker named Dr. Coppélius. Swanhilda discovers Coppélia is a doll. Dr. Coppélius tries to bring the doll to life using Frantz's life force. Swanhilda saves her sweetheart's life, and the two live happily ever after.
"Coppélia" was in rehearsal for three years. It had many troubles. One was finding just the right dancer to portray Swanhilda. Sixteen-year-old Giuseppina Bozzacchi was finally chosen. Frantz was played by Eugenie Fiocre. The ballet was first performed on 25 May 1870 at the Théâtre Impérial de l´Opéra in Paris. "Coppélia" was a great success. It has been performed around the world.
Act I. Coppélia is set in a small village in Poland at the end of the 18th century. Swanhilda and Frantz are young peasants. They are sweethearts. Frantz however is flirting with Coppélia, a strange girl in the dark house of a dollmaker named Dr. Coppélius. Swanhilda is upset with her sweetheart's behavior.
One evening, Dr. Coppélius leaves home. He drops his key in the street. Swanhilda finds the key. She opens his door and goes into the house with her girlfriends. Frantz enters the street quietly. He puts a ladder against a window of the house. Coppelius returns and chases Frantz away.
Act II. Swanhilda and her girlfriends explore Dr. Coppélius's workroom. It is filled with life-size dolls. Swanhilda discovers Coppélia is also a doll. Coppélius enters and chases the girls away. Swanhilda hides. Frantz climbs through the window. Coppélius welcomes him. He needs a human sacrifice to bring Coppélia to life.
Coppélius give Frantz a glass of wine. The boy falls asleep. The dollmaker performs a magic spell. He tries to bring Coppélia to life using Frantz's life force. Swanhilda pretends to be Coppélia. She destroys the workroom, and saves Frantz's life. Coppélius faints as the sweethearts run away.
Act III. A new church bell is blessed. Swanhilda and Frantz (and other couples) are going to be married. They have all been given dowries by the Lord of the Manor. Dr. Coppélius enters. He is angry. He wants to be paid for the damage done to his workroom. Swanhilda offers her dowry.
The Lord of the Manor allows Swanhilda to keep her dowry. He agrees to pay for the damages. The couples enter the chapel. The celebrations start with the "Dance of the Hours". Dawn, Day, the Hour of Prayer, and other dances are performed. The married couples return. All rejoice.
Several changes were made to Hoffmann's dark fairy tale when it was adapted to the bright and gay "Coppélia". In Hoffmann's fairy tale, for example, the doll comes to life through the hero's life force. In the ballet though, the doll remains a doll. Coppélius's attempt to bring it to life is thwarted by Swanhilda.
In another example, Hoffmann's hero sees the doll in a vision. He then falls from a tower to his death. The hero of "Coppélia" however is saved by his sweetheart. A happy ending replaces a tragic ending.
The designers of the ballet chose only Dr. Coppélius, the life-sized doll, and the young man torn between his attraction to the doll and his love for a human sweetheart for their adaptation. They set the story in a simple country village. Hoffmann set his story in "the philosophically oriented bourgeois society" of the early 19th century.
The spirit of "Coppélia" is lighthearted and gay. The story of the ballet is simple. Its fantastic touches such as the dollmaker and his workshop make "Coppélia", according to Lincoln Kirstein, "a trifle presumed especially appealing to children."
"Coppélia" is one of many (if not the first) ballet about bringing a doll to life. Charles Nuitter started work on the libretto in late 1866 or early 1867. He had a young dancer named Léontine Beaugrand in mind for the part of Swanhilda. Delibes set to work writing the music.
Rehearsals started. The director of the Opéra wanted a star with a well known name to portray the ballet's heroine, Swanhilda. Beaugrand was a lovely young dancer, but her career was just starting. She had yet to have the sort of name that would draw ballet-goers to the theatre.
Adèle Grantzow, a German ballerina dancing at the Bolshoi Theatre in Moscow, was hired to play Swanhilda. Saint-Léon was working between France and Russia at this time. He could not completely devote himself to "Coppélia". Consequently, the ballet was rehearsed over a three-year period.
"Coppélia" lost its star when Grantzow was injured. She went back to Russia. Beaugrand was again passed over for the part of Swanhilda. Giuseppina Bozzacchi was a sixteen-year-old dancer in the company. It was she who was chosen to play Swanhilda.
"Coppélia" was first performed on 25 May 1870 at the Théâtre Impérial de l'Opéra. Giuseppina Bozzacchi played Swanhilda. Eugénie Fiocre played Frantz. Francois-Édouard Dauty portrayed Dr. Coppélius. A real mechanical doll was used as Coppélia. Because a woman played Frantz, a "pas de deux" for Swanhilda and Frantz was not possible. Nonetheless, the ballet was a great success.
Bozzacchi danced eighteen performances of the ballet before the theatre closed because of the Franco-Prussian War. She died of a fever during the Siege of Paris on her seventeenth birthday, 23 November 1870.
When the theatres reopened in 1871, Beaugrand played Swanhilda with great success. The part of Frantz was again played by a woman in men's clothing. Frantz would be a travesty role at the Paris Opéra until the 1950s.
"Coppélia" was the first ballet score written completely by Delibes. He studied with Adolphe Adam. Adam was the composer of the enduring romantic ballet, "Giselle", and the first to use leitmotifs in ballet. Delibes, like his teacher, used leitmotifs to identify persons and places in "Coppelia". Swanhilda has a waltz, and Frantz has two leitmotifs. Coppélius has a dry-sounding motif and the doll has an odd-sounding little motif. Bustling, lively music identifies the village square. Sinister music identifies Coppélius's workshop.