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 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 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: