Signals and receptors
Reception begins when a ligand binds a compatible receptor and changes its shape or activity. Large or hydrophilic ligands usually use cell-surface receptors because the membrane core blocks their passage. Many small hydrophobic ligands can enter cells and bind intracellular receptors. A circulating signal can reach many tissues while producing a response only in specific targets. Receptor location, abundance, and downstream proteins all matter.
MAKE IT CONCRETE
A peptide hormone reaches both liver and another tissue. If only liver cells express the matching receptor, only the liver can initiate that receptor-mediated pathway.
DON’T FALL FOR THIS
A hormone reaching a tissue does not guarantee a response. Also, the ligand does not have to enter the cell to deliver information.