Final answer:
In signal transduction pathways, a ligand binds to a receptor, triggering a cascade of events involving enzymes, second messengers, and protein phosphorylation, leading to cellular changes.
Step-by-step explanation:
The entity responsible for initiating a signal transduction pathway is the ligand. When a ligand binds to a cell-surface receptor, this action triggers the receptor's intracellular components to set off a signaling pathway or cascade. In this process, enzymes like kinases phosphorylate various proteins, which then change their shapes, leading to activation or inactivation. Second messengers such as calcium ions (Ca²+), cyclic AMP (cAMP), diacylglycerol (DAG), and inositol triphosphate (IP3) are small, non-protein molecules that help to transmit the signal within the cell. This cascade of events eventually results in changes such as an increase in metabolism or specific gene expression, significantly affecting the cell's environment.