Signalling chemicals vary chemically, including peptide or protein hormones that are hydrophilic and steroid hormones that are lipid-soluble, linking signal detection to a specific target-cell response.
Coverage 2013–2024 · Updated 15 Jul 2026
Animal chemical signals differ in source, distance, speed and target-cell effect. Hormones are released by endocrine cells into blood and act at distant targets, usually more slowly. Neurotransmitters are released by neurons across synapses and act rapidly on nearby target cells. Cytokines are released by immune and other cells and coordinate local or systemic responses. Ca²⁺ ions act as short-lived intracellular signals that regulate target proteins.
This objective is assessed through structured response, commonly using State / Distinguish.
State / Distinguish
Build the answer around this relationship: Protein and peptide hormones include insulin, glucagon, ADH, growth hormone and leptin.
Representative question
State one example of a steroid hormone and one example of a protein hormone.
Steroid hormone:
Protein hormone:
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.
