What mechanism allows receptor proteins to communicate messages without letting the messenger enter the cell?

Receptor binding is the mechanism through which receptor proteins communicate messages to the inside of the cell without allowing the messenger molecule, such as a hormone or neurotransmitter, to enter. This occurs when the messenger binds to a specific site on the receptor protein located on the cell membrane. The binding triggers a conformational change in the receptor, which then initiates a series of intracellular signaling pathways. This allows the cell to respond to the external signal without the messenger needing to cross the cell membrane.

This mechanism is crucial in cell communication, particularly for substances that are too large or polar to easily diffuse through the lipid bilayer of the membrane. Instead of entering the cell, these messengers act as signal transducers, activating secondary messengers or altering the activity of specific proteins within the cell to elicit a response.

This process is distinct from reuptake action, which involves the reabsorption of signaling molecules, or covalent modification, which refers to chemical changes to molecules that may alter their function. Selective permeability describes the cell membrane's ability to control what enters and exits, but does not specifically pertain to the communication mechanism mediated by receptor proteins.