Receptor sites are proteins typically found on the surface of cells, which are capable of recognizing and bonding to specific molecules.

A cell is a self-enclosed system, and it serves as the basic unit of life. All organs are made of different types of cells. For instance, skin is made of skin cells, muscles are made of muscle cells, and blood is made of blood cells. Receptor sites can be found within the plasma membrane of a cell, which acts as a boundary between the cell’s internal and external environment.

Molecules that bind to receptor sites are known as ligands. Hormones, neurotransmitters, and drugs are examples of ligands. They are able to fit into specific receptor sites in the same way keys are able to fit into specific locks. For example, dopamine binds to dopamine receptors, and insulin binds to insulin receptors, but they cannot bind to each other’s receptors. By binding to the receptor site, ligands are able to transmit information from a cell’s external environment and to its interior.

Whenever a ligand binds to a receptor site, it alters the shape of the receptor and launches a cascade of chemical reactions known as signaling. A message from the ligand makes its way into the cell, which can induce a variety of responses, including changes in gene expression.

Membrane receptors are divided into three major classes: ion channel–linked receptors, G-protein-coupled receptors, and enzyme-linked receptors.

Ion channel–linked receptors reside on the cell membrane. They have a channel that spans the cell membrane and enables ions—atoms and molecules with a negative or positive charge—to freely move in and out of the cell. Examples of ions include sodium and calcium. When a ligand binds to the receptor site on the channel, the channel opens, allowing ions to travel through the channel in milliseconds.

G-protein-coupled receptors are the largest class of receptors. These receptors work with what is known as a G-protein. G-protein-coupled receptors help the cell respond to different substances, such as hormones, neurotransmitters, and lipids. Many medical drugs work by binding to G-protein-coupled receptors.

Enzyme-linked receptors are another kind of cell-surface receptor. They work with proteins, called enzymes, which play a major role in accelerating chemical reactions within cells. These reactions help cells assemble and dissemble material as well as grow and reproduce.