C2.1 Chemical signallingHL

HL-only. Receptors are proteins with specific binding sites for ligands (hormones, neurotransmitters, cytokines, Ca²⁺). Hydrophilic signals bind transmembrane receptors; lipid-soluble steroids bind intracellular receptors that affect gene expression. Signal transduction cascades: quorum sensing in bacteria, ligand-gated ion channels (ACh), G-protein-coupled receptors with cAMP (adrenaline), and tyrosine kinase receptors (insulin → GLUT4). Endocrine integration via oestradiol + progesterone, with positive and negative feedback loops.