In each organism there are different types of chemical signals that are called ligands and form complexes with specific receptors.
Each cell type is sensitive to different signals and each ligand-receptor interaction is associated with a particular function. Each cell responds to a set of signals.
The ligand-receptor complex transmits the message inside the cell and initiates a path that leads to the execution of a specific biological response. Through this complete process the signal is transduced.
The membrane receptors are varied. They can be part of ion channels, have enzymatic activity, or be associated with enzymes. There are receptors that activate an adapter protein, the G protein, which transmits the message to the next intermediary.
Certain small and / or hydrophobic molecules cross the cell membrane and bind to internal receptors. These complexes often bind to DNA and act as transcription factors.
