01Three stages
Reception: a ligand binds a specific receptor. Transduction: the receptor changes shape and triggers a cascade of relay molecules. Response: the cell does something — activates an enzyme, opens a channel, changes gene expression.
Receptors are specific: only cells with the right receptor respond, which is why one hormone can affect only certain tissues.
02Where receptors sit
Polar/large ligands (peptide hormones, epinephrine) can't cross the membrane, so they bind plasma membrane receptors.
Nonpolar ligands (steroid hormones like testosterone, estrogen) diffuse through and bind intracellular receptors, often acting directly as transcription factors.
03Notebook box
Reception → transduction → response.
Worked example: Why does adrenaline speed the heart but not affect skin cells? → Only cells with adrenaline receptors respond.