Answer: a, c All water-soluble regulatory molecules bind to the cell surface receptor proteins. Binding of the appropriate ligand evokes an intracellular signal which usually regulates enzyme activity, membrane transport, or in some cases gene expression. Most cell surface receptors belong to one of three functional classes—these are ion channel receptors, catalytic receptors, and G-protein-linked receptors. Ion channel receptors are multisubunit assemblies which, with each subunit, have a multiple membrane spanning segment. Together these subunits form an ion-selected pore that can be gated by a change in transmembrane electrical potential or binding of a ligand to one of the subunits. Catalytic receptors are membrane proteins that possess enzymatic activity. The best understood receptors of this class are the tyrosine kinases. The largest family of cell surface receptors are the G-protein-linked receptors. G-proteins are a family of proteins that bind and hydrolyze GTP. The final component of single transduction by G-protein-linked cell surface receptors is the effector that generates the intracellular messenger. The two best understood effectors are adenylate cyclase, which converts ATP to cAMP, and the polyphosphoinositide-specific phospholipase C