Membrane receptors are specialized proteins that take part in communication between the cell and the outside world. The membrane protein receptor can be subdivided into three parts or domains. The portion of the protein in the E zone is extracellular (outside of the cell). The P zone is the plasma membrane and the I zone is intracellular (inside of the cell). A glucagon hormone is a molecule that stimulates a reaction inside liver cells. Based on the diagram, where do you think a glucagon hormone most likely binds to send its signal to the cell?