Cell-surface receptors are transmembrane proteins. They lie embedded in the plasma membrane and are made up of three main components: 

1. An external ligand-binding domain, also known as the extracellular domain or recognition domain - it is exposed to the cell’s outer surface
2. A hydrophobic domain that spans the plasma membrane – it is made up of hydrophobic protein molecules
3. An intracellular domain or coupling domain – it is in contact with the cytoplasm in the cell and often sends the signals

Each of these domains vary in terms of size and extent, depending on the type of receptor.