What is Receptor (cell biology)?
[[File:transmembrane receptor.svg|thumb|300px|{{color|red|'''E'''}} = outside the cell<br>{{color|green|'''P'''}} = [[plasma membrane]]<br>{{color|gold|'''I'''}} = inside the cell]] In [[cell biology]], a '''receptor''' is a special structure that can be found in [[cell membrane]]s. These are made of [[protein]] molecules such as [[glycoprotein]]s. Receptors bind (attach) to specialised [[molecule]]s. If the receptor has this molecule, it is activated, but if it does not it is deactivated. Depending on its state, a change inside the cell happens. Cell surface receptors (membrane receptors, transmembrane receptors) take part in communication between the cell and the outside world. Extracellular signalling molecules (usually [[hormones]], [[neurotransmitter]]s, [[agonist]], [[cytokine]]s, [[growth factor]]s 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 [[lock]]s and [[key]]s. With the key, the lock can be locked or unlocked. If it is unlocked, the door belonging to it can be opened. == Structure == 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 [[cytoplasm]]ic) part of the receptor interacts with the inside of the cell or organel