Proteins that span cell membranes, called transmembrane proteins, are often important for recognizing extracellular signals and stimulating a change within the cell. classify the following statements as describing transmembrane proteins that act as ligand-gated ion channels, g-protein-linked receptors, or enzyme-linked receptors. (a) activation by ligands causes a conformational change that allows ions to cross the cell membrane (b) important for action potentials in neurons. (c) activation by ligand releases a protein that binds gtp (d) important for taste. (e) activation by ligand induces a change that causes the transmembrane protein to function as an enzyme (f) important for receiving growth factors