IB Biology HL C2.1 Chemical signalling [HL only] Notes

Receptor proteins act as gatekeepers for chemical signalling. They have binding sites specific to signalling ligands. Only target cells with the matching receptor respond, because ligand binding changes the receptor and starts signal transduction inside the cell.

Quorum sensing is a threshold story. Autoinducers allow bacteria to detect population density thresholds. As density rises, autoinducers accumulate; once the threshold is reached, many cells change gene expression together. Vibrio fischeri bioluminescence and biofilm formation are key examples.

Animal chemical signals are not all the same kind of message. Animal signals include hormones, neurotransmitters, cytokines, and Ca2+ ions. They differ in source, distance, speed, and target-cell response. Hormones include protein/peptide, steroid, and amine classes; neurotransmitters include acetylcholine, amino acids, peptides, amines, and nitric oxide.

Distance is part of the signal logic. Neurotransmitters act locally across narrow synaptic clefts. Hormones travel through blood to distant cells with matching receptors. A hormone can circulate widely, but only cells with the right receptor respond.

Receptor location follows ligand chemistry. Hydrophilic ligands bind transmembrane receptors and use intracellular relays. Steroid and thyroid hormones cross membranes and bind cytoplasmic or nuclear receptors. Hormone-receptor complexes enter or act in the nucleus as transcription factors.

Signal transduction is the relay between receptor binding and response. Signal transduction relays, amplifies, integrates, and distributes ligand signals. Pathways use relay proteins, second messengers, phosphorylation cascades, and effectors, so one ligand can produce a coordinated cell response.

Acetylcholine provides a local signalling example. It can bind ligand-gated sodium channels. When the channel opens, sodium influx changes membrane potential and can depolarize the postsynaptic membrane.

GPCRs are seven-helix transmembrane receptors linked to G proteins. Ligand binding causes GDP-GTP exchange and activation of effector proteins. The G protein is the switch: GDP means off, GTP means active.

Epinephrine, also called adrenaline, is the named GPCR example. Epinephrine binds adrenergic GPCRs and activates G protein signalling. Adenylyl cyclase forms cAMP, activating kinase cascades for glycogen breakdown.

Insulin is the named RTK example. RTK ligand binding causes dimerisation and autophosphorylation. Insulin signalling moves GLUT4 vesicles to membranes and promotes glycogenesis, so cells take up glucose and store it as glycogen.

Oestradiol and progesterone are named intracellular-receptor examples. Oestradiol regulates female sex characteristics and hypothalamus-pituitary targets. Progesterone from corpus luteum or placenta maintains endometrium and pregnancy. Both work through intracellular receptor proteins that affect transcription in target cells.

Feedback is about what the response does to the original change. Positive feedback amplifies responses, such as ethylene in fruit ripening. Negative feedback restores stability, such as insulin lowering blood glucose. The exam trap is calling every control loop negative feedback.

A ligand only affects target cells with the matching receptor. Quorum sensing uses autoinducers and thresholds for group behaviour. Animal signals differ by source, distance, speed, target-cell response, and chemical class. Hydrophilic ligands use transmembrane receptors and relays; steroid and thyroid hormones use intracellular receptors that affect transcription. Named pathways then show the logic: acetylcholine opens sodium channels, GPCRs switch G proteins through GDP-GTP exchange, epinephrine uses cAMP and kinase cascades, insulin RTKs use dimerisation/autophosphorylation to move GLUT4 and promote glycogenesis, steroid hormones change gene expression, and feedback either amplifies or restores stability.

Use this for HL structured questions that combine signalling categories, receptor routes, named pathways, and feedback examples.