What are the types of cell signaling receptors?

Cell signaling receptors are protein molecules that bind ligands. There are two main types of cell signaling receptors based on where they are found: Internal receptors and Cell-Surface Receptors.

Internal receptors

Internal receptors are located in the cell’s cytoplasm and bind to ligand molecules internally. They are also known as cytoplasmic or intracellular receptors. Internal receptors respond to hydrophobic ligand molecules that are capable of penetrating the plasma membrane. The ligand-receptor complex promotes initiation of transcription by penetrating the nucleus and binding to specific chromosomal regulatory regions. 

Cell-surface receptors

Cell surface receptors are located on the outer surface of the cell membrane and bind to ligand molecules externally. They are also known as cell-specific proteins or markers because they are specific to different cell types. During signal transduction, cell surface receptors penetrate the plasma membrane and enter the cell where they convert an extracellular signal into an intracellular signal. 

Cell-surface receptors are broadly divided into three groups:

1. Ion channel-linked receptors, which bind a ligand and open a channel through the membrane that allows specific ions to pass through
2. G-protein-linked receptors, which bind a ligand and activate a membrane protein called a G-protein, which then interacts with either an enzyme or an ion channel in the membrane
3. Enzyme-linked receptors, which have intracellular domains associated with an enzyme. On binding to a ligand, a signal is transmitted through the membrane, activating the enzyme and triggering a chain of events within the cell that eventually leads to a response.