In biochemistry - a receptor is a protein that interacts with a very specific ligand (target). Often, receptors are proteins that span the outer membrane of a cell, having three major domains. The extracellular, transmembrane and intracellular domains.
+L+ outside
+++
ooooooooooooooooooooooooooo = ooooooooooooooooooooooooooooo
||||||||||||||||||||||||||| = ||||||||||||||||||||||||||||| phospholipid bilayer
||||||||||||||||||||||||||| = |||||||||||||||||||||||||||||
ooooooooooooooooooooooooooo = ooooooooooooooooooooooooooooo
#
### inside
###
#
The extracellular domain of a receptor binds with a ligand (L) on the outside of the cell. This causes a conformational change transmitted through the transmembrane portion of the protein, resulting in activation of the intracellular domain which then talks to other cytoplasmic proteins, causing a signalling cascade event. In some receptors, binding a ligand causes them to associate with adjacent receptors on the outside. This causes the inside domains to also cluster, which is then detected during a signalling event.