AP BIOLOGY

Detailed explanation-1: -In other cases, the receptors are inside the target cell, and the signal molecule has to enter the cell to activate them: this requires that the signal molecules be sufficiently small and hydrophobic to diffuse across the plasma membrane (Figure 15-3).

Detailed explanation-2: -Signals Travel Into Cells In order to trigger a response, these signals must be transmitted across the cell membrane. Sometimes the signal itself can cross the membrane. Other times the signal works by interacting with receptor proteins that contact both the outside and inside of the cell.

Detailed explanation-3: -When a ligand binds to a cell-surface receptor, the receptor’s intracellular domain (part inside the cell) changes in some way. Generally, it takes on a new shape, which may make it active as an enzyme or let it bind other molecules. The change in the receptor sets off a series of signaling events.