Definition:

• Small intracellular signalling molecules produced in response to first messenger

Nature:

• Small, short-lived, non-protein

Types:

• Hydrophilic: cAMP, Ca²⁺, IP₃

• Hydrophobic: DAG, PIP₃

• Gases: NO, CO

Functions:

• Amplify signal

• Diffuse rapidly

• Activate multiple pathways

cAMP

• Produced from ATP by adenylyl cyclase

• Broken down by phosphodiesterase → AMP

Role:

• Activates protein kinase A (PKA)

• Leads to phosphorylation cascade

Ca²⁺

• Acts in GPCR and RTK pathways

• Stored in ER, organelles

• Released via IP₃ pathway

• Triggers cellular responses