Any given cell in a multicellular organism is bombarded with many signals. However, it responds to only some of them, because no cell makes receptors for all signals. A receptor protein recognizes its signal very specifically, in much the same way that a membrane transport protein recognizes and binds to the substance it transports. This specificity ensures that only those cells that make a specific receptor will respond to a given signal.
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Cells make specific receptor proteins that recognize only the signals to which they respond.
The binding of a chemical signal (called a ligand) with its receptor is reversible and is measured by a dissociation constant.
Intracellular receptors are located inside the cell, where they interact with physical signals such as light or with chemical signals that diffuse across the cell membrane.