A series of steps that amplify a small signal into a large cellular response.

A signaling molecule that binds to a receptor protein.

A protein on a cell that binds to a ligand, initiating a cellular response.

The process of adding a phosphate group to a protein, often activating it.

Making the cellular response bigger or faster, often through a cascade of activation steps.

Molecules that relay signals from cell surface receptors to target molecules inside the cell, amplifying the signal.

Cell surface receptors: Located on the plasma membrane, bind to hydrophilic ligands. | Intracellular receptors: Located inside the cell, bind to hydrophobic ligands that can cross the membrane.

Primary messenger: The initial signaling molecule (ligand) that binds to the receptor. | Secondary messenger: Small, non-protein molecules that relay and amplify the signal inside the cell.

The receptor protein changes shape, initiating a signal transduction pathway.

The protein is often activated, leading to a cellular response.

Adenylyl cyclase converts ATP to cAMP, a secondary messenger.

cAMP activates protein kinases, leading to further phosphorylation and a cellular response.

Production of transcription factors, leading to new protein synthesis and a longer-lasting cellular response.