A receptor is a protein or glycoprotein that receives signals by binding to signaling molecules, also known as ligands. Receptors can be found either inside a cell or on its surface. They are chemical structures composed of protein that receive and transduce signals that may be integrated into biological systems). The binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell. There are several types of cellular receptors, including:
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Cell surface receptors: These are proteins that are found on the surface of cells and span the plasma membrane. They bind to ligands that cannot themselves pass through the plasma membrane. Examples of cell surface receptors include ligand-gated ion channels, G protein-coupled receptors, and enzyme-linked hormone receptors).
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Intracellular receptors: These are receptor proteins found on the inside of the cell, typically in the cytoplasm or nucleus. In most cases, the ligands of intracellular receptors are small, hydrophobic molecules, since they must be able to cross the plasma membrane in order to reach their receptors. Examples of intracellular receptors include cytoplasmic receptors and nuclear receptors).
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Ion channel receptors: These are receptors that allow ions to pass through the plasma membrane of a cell. They are typically found in neurons and muscle cells.
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G protein-coupled receptors (GPCRs): These are a large family of cell surface receptors that share a common structure and method of signaling. The members of the GPCR family all have seven different protein segments that cross the membrane, and they transmit signals inside the cell through a type of protein called a G protein.
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Enzyme-linked receptors: These are receptors that are also enzymes. When a ligand binds to the receptor, it activates the enzyme, which then triggers a signaling pathway inside the cell.
Receptor proteins can be classified by their location, and a molecule that binds to a receptor is called a ligand).
