Answer the following question given this paragraph: 

The target cell then needs to be able to recognize the signal. Chemical signals are received by the target cell on receptor proteins. Most receptor proteins are found associated with the plasma membrane, but some are also found inside the cell. Receptor proteins are very specific for only one particular signal molecule, much like a lock that recognizes only one key. Therefore, a cell has lots of receptor proteins to recognize the large number of cell signal molecules. There are three stages to sending and receiving a cell "message:" reception, transduction, and response.


Q: Chemical signals are received by the target cell on what?


A:
receptor proteins