Biochemical signalling molecules

This session centres on biochemical signalling molecules, offering a structured look at how cells communicate through key molecules, their receptors, signalling pathways, and the mechanisms that shape response specificity and intensity. Biochemical signalling molecules exist in various forms, each suited to different distances and communication speeds. Hormones like insulin, cortisol, and thyroxine are secreted from endocrine cells, travel in the bloodstream, and regulate metabolism, growth, and reproduction. Neurotransmitters such as acetylcholine and dopamine are released by neurons into synaptic clefts for rapid, local communication. Cytokines and chemokines modulate immune responses and direct leukocyte movement. Other types include growth factors (such as EGF), paracrine and autocrine signals, gaseous molecules like nitric oxide, and nucleotides like ATP. Peptides, amino acids, lipids, and other small molecules also act as messengers, illustrating the body’s chemical communication diversity. To interpret external signals, a cell needs molecular sensors— receptors—that specifically recognise signalling molecules. Cell-surface receptors like G-protein-coupled receptors and receptor tyrosine kinases detect hydrophilic ligands that cannot cross the plasma membrane. G-protein-coupled receptors, with seven-transmembrane domains, bind many ligands and play key roles in senses such as smell and taste, and in hormone and neurotransmitter action.