English: # Ligands

1. Receptors
2. Secondary Messengers

These are examples of membrane receptors. Typically, they are proteins that are embedded in the membrane. Although there are many different ligands located outside of the cell, membrane proteins are specific, and only certain ligands will bind to each one. That is why each protein has a different ligand, and also induces a different cellular response. The response may be transcription of a gene, cell growth, or many other cellular actions.