Second Messenger: Small molecules generated within cells upon activation of receptors. They help transmit and amplify signals during signaling pathways.
Protein Kinase: An enzyme that adds phosphate groups to proteins, often used as part of signal transduction cascades in response to extracellular signals.
Signal Amplification: The process by which one signal can produce many secondary messenger molecules or activate many enzymes, resulting in amplification of the initial signal's effect.