Final answer:
Extracellular signal molecules bind to cell-surface receptors, like enzyme-linked receptors, to convert external signals into internal cellular responses without the ligand entering the cell. Small hydrophobic ligands can penetrate the cell membrane and bind to intracellular receptors that then interact with DNA.
Step-by-step explanation:
Extracellular Signal Molecules and their Receptors
Extracellular signal molecules can bind to either cell-surface receptors or intracellular enzymes/receptors. Cell-surface receptors, such as enzyme-linked receptors, span the plasma membrane and convert external signals into internal actions without the ligand entering the cell. The ligand-binding event activates an enzyme that initiates a cascade of cellular events. Conversely, small hydrophobic ligands can cross the plasma membrane and bind to internal receptors that can move to the nucleus and interact with DNA.
One example of a cell-surface receptor is the tyrosine kinase receptor, which is associated with an enzyme that transfers phosphate groups from ATP to tyrosine residues on the receptor, amplifying the signal within the cytoplasm. Enzyme-linked receptors normally have large extracellular domains and a single alpha-helix that spans the membrane. Upon ligand binding, they activate and catalyze a range of intracellular responses.