<P> Like any integral membrane protein, a transmembrane receptor may be divided into three domains . </P> <P> The extracellular domain just externally from the cell or organelle . If the polypeptide chain crosses the bilayer several times, the external domain comprises loops entwined through the membrane . By definition, a receptor's main function is to recognize and respond to a type of ligand . For example, a neurotransmitter, hormone, or atomic ions may each bind to the extracellular domain as a ligand coupled to receptor . Klotho is an enzyme which effects a receptor to recognize the ligand (FGF23). </P> <P> In the majority of receptors with known structures, transmembrane alpha helices constitute most of the transmembrane component . In certain receptors, such as the nicotinic acetylcholine receptor, the transmembrane domain forms a protein pore through the membrane, or around the ion channel . Upon activation of an extracellular domain by binding of the appropriate ligand, the pore becomes accessible to ions, which then diffuse . In other receptors, the transmembrane domains undergo a conformational change upon binding, which effects intracellular conditions . In some receptors, such as members of the 7TM superfamily, the transmembrane domain includes a ligand binding pocket . Bacteriorhodopsin is an example, the detailed structure of which has been determined by crystallography . </P> <P> The intracellular (or cytoplasmic) domain of the receptor interacts with the interior of the cell or organelle, relaying the signal . There are two fundamental paths for this interaction: </P>

Which of the following is the plasma membrane receptor for a growth factor