AP Syllabus focus:
‘Signaling begins when a ligand, such as a peptide or small molecule, binds a complementary receptor protein on a target cell.’
Cells coordinate behaviour by sending chemical messages. In signal transduction, the critical first step is the ligand–receptor interaction, where the chemistry of the ligand determines which cells respond and how strongly.
Core idea: ligands start signalling by binding receptors
A ligand released from a signalling cell acts as a chemical “word” in a molecular language. Only cells with the matching receptor can “read” that word, making communication highly selective.
Ligand: a molecule that binds specifically to a receptor protein and initiates (or blocks) a signalling process in the target cell.
Ligand binding depends on complementarity: the ligand’s shape and chemical properties match a binding site on the receptor protein. If a cell lacks the receptor, it is not a target and typically shows no direct response to that ligand.
Practice Questions
FAQ
Peptide ligands are typically synthesised as larger precursor proteins and then processed.
Common steps include:
Translation on ribosomes associated with the rough ER
Proteolytic cleavage to an active peptide
Packaging into secretory vesicles
Release by regulated exocytosis in response to a stimulus
Duration depends on ligand removal and stability.
Factors include:
Enzymatic degradation in extracellular fluid
Uptake by cells (including receptor-mediated internalisation)
Chemical inactivation (e.g., oxidation or modification)
Diffusion away from the source, lowering concentration
$K_d$ (dissociation constant) describes binding affinity.
Lower $K_d$ means higher affinity because less ligand is needed to occupy receptors.
Experimentally, $K_d$ is often close to the ligand concentration that produces half-maximal receptor occupancy under defined conditions.
Yes, especially for related receptors with similar binding sites.
This can produce different effects in different tissues if each receptor triggers distinct intracellular changes. It also helps explain why the same ligand may have multiple physiological roles depending on receptor distribution.
Cells limit inappropriate self-activation by controlling:
Where ligands are released (directional secretion)
Whether receptors are present at the surface (trafficking)
Receptor sensitivity (temporary desensitisation)
Rapid ligand breakdown near the releasing cell
