MCAT Content / Plasma Membrane / Membrane Receptors

Membrane Receptors

Topic: Plasma Membrane

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, also known as transmembrane receptors, are cell surface, membrane-anchored, or integral proteins that bind to external ligand molecules. This type of receptor spans the plasma membrane and performs signal transduction, converting an extracellular signal into an intracellular signal. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect.

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



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