Membrane receptors bind to external molecules called ligands and cause an internal cellular response.

Membrane receptors are important in facilitating the transport of molecules, changing of a cells internal environment or communicating between cells. There are different types of membrane receptor adapted to do different roles.

Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.

Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through. To form a channel, this type of cell-surface receptor has an extensive membrane-spanning region. In order to interact with the phospholipid fatty acid tails that form the center of the plasma membrane, many of the amino acids in the membrane-spanning region are hydrophobic in nature. Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through.

G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane. All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.

Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme or the enzyme-linked receptor has an intracellular domain that interacts directly with an enzyme. When a ligand binds to the extracellular domain, a signal is transferred through the membrane and activates the enzyme, which sets off a chain of events within the cell that eventually leads to a response.

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Key Points

• Cell-surface receptors bind to an external ligand molecule and convert an extracellular signal into an intracellular signal.

• Three general categories of cell-surface receptors include ion-channel, G-protein, and enzyme-linked protein receptors.

• Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through.

• G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein, which then interacts with either an ion channel or an enzyme in the membrane.

• Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme

Key Terms

transmembrane receptors: cell surface, membrane-anchored, or integral proteins that bind to external ligand molecules

ion channel-linked receptors: ligand binding opens a channel through the membrane that allows specific ions to pass through

G-protein-linked receptors: ligand binding activates a membrane protein called a G-protein, which then interacts with either an ion channel or an enzyme in the membrane

enzyme-linked receptors: ligand binding activates an enzyme

ligand: a chemical molecule that binds to a receptor

plasma membrane: the bilayer composed of phospholipids

signal transduction: is the transmission of molecular signals from a cell’s exterior to its interior

extracellular: outside the cell

Intracellular: inside the cell

hydrophobic: avoids water

hydrophilic: likes water

conformational: change in shape

G proteins: a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals